Gps system guide
Global Positioning System (GPS) : ConservationTools
Global Positioning System (GPS) is a space-based navigation system that uses a constellation of satellites to determine the location of the receiving unit on Earth. GPS satellites circle the globe in a precise orbit, transmitting coded radio signals; at least four of their signals can reach any given point on Earth at one time. These signals can pass through clouds, glass, and plastic; the signals weaken when passing through solid objects such as buildings and cannot pass through objects that contain high levels of metals. For land conservation purposes, it is important to note that a GPS unit will not receive satellite signals when under thick forest canopies, underground, or underwater.
Although GPS was originally developed in the 1980s for military purposes, today the technology provides positioning, velocity, and navigation information for a wide range of users.
GPS helps conservation organizations and municipalities manage land by recording positional data in the form of points (e.g., location of a tree or property corner), lines (e.g., a trail), or areas (e.g., a lake). By importing the data into Geographical Information System (GIS) software, users can create maps of this data.
To use GPS effectively, an organization must invest in some combination of equipment and software, and either train or hire staff to operate it.
Mapping Landscape Features
GPS enables users to map the location of a wide variety of features in the field, such as mature forest, specimen trees, invasive species, soil erosion, fire-disturbed areas, riparian buffers, and waterways. Users can also map human-made features like trails, benches, buildings, roads, driveways, and fences. Once mapped, these features can be easily located with a GPS device on return visits to the property.
Linking Photos to GPS Coordinates
Users can take digital photos in the field and link them to GPS coordinates in the GIS database. This allows users to establish a visual record of important features and their precise locations. By comparing photos of the same location taken at different times, users can notice changes to the property. (This can be particularly helpful for monitoring easements and identifying potential violations.)
Documenting Property Boundries
GPS allows users to document the coordinates of property boundaries. In the past, surveyors used landmarks (which can be destroyed or moved over time) to define boundaries. Since GPS uses exact coordinates rather than relational landmarks, it produces measurements that remain accurate no matter what happens to the surrounding land or physical objects used as landmarks. (Note that accurate surveying of property boundaries necessitates the use of survey-grade equipment; see the heading “Survey-Grade” below. Also, depending on the purpose of the survey, the law may require the work to be completed by a licensed surveyor.)
Documenting Other Boundries
Land trusts can use GPS to document boundaries between areas subject to different levels of restrictions under a conservation easement, for example, the border between an area that is to remain in a largely wild state and an area where farming is permitted.
A GPS receiver is the electronic unit that receives satellite signals and produces positional data, which can then be analyzed using mapping software. GPS devices vary greatly in price and quality. Since technology is always improving, it is best to research current receivers before making a purchase. Some companies rent receivers, which gives organizations the chance to test different receivers before making an investment.
Types of GPS Recievers
These units are the least expensive. Designed for outdoor recreation activities like hiking and camping, they provide basic latitude and longitude coordinates while plotting points of interest and straight-line routes. Most are accurate within five to 10 meters. Users can only identify points with a short name and ID number; additional attributes must be recorded manually and entered into GIS software. Basic units generally cost between $200 and $500.
These units are more sophisticated. They allow for enhanced data collection, greater map detail, and more precise navigation. When enabled with WAAS (Wide Area Augmentation System), they are accurate within three meters; with the use of differential GPS, accuracy can be as close as one meter. Some can also receive additional signals from GLONASS satellites for even greater accuracy. Most come with base maps installed; more detailed maps are available for purchase. Units may have additional features like touch screens or built-in cameras. Garmin, Trimble, and Magellan are the leading companies offering map-grade units; Trimble offers the most advanced (and expensive) units. Units can cost from $500 to thousands of dollars depending on level of accuracy and other features.
Smartphones and tablets are another option. With built-in software or downloadable GPS applications like MotionX, GPSLogger, and GPX Viewer (recommended by multiple land trusts), they can perform some of the same functions as a commercial-grade handheld GPS. An internet search reveals a wide variety of GPS applications for both Apple and Android devices, and most of them only cost a few dollars.
Used by surveyors for precise measurements, these units are extremely accurate, sometimes to within a centimeter. They can cost tens of thousands of dollars and require extensive training and expertise, making them impractical for most land trusts and municipalities.
A recent development in GPS technology for land trusts is the use of unmanned aerial vehicles (UAVs, also known as drones) to collect data-enabled aerial imagery of properties. Since UAVs receive GPS signals, each video image they collect is linked to a specific location. Platforms like Survae allow land trusts to use this data in a variety of ways, from creating GIS map layers to monitoring easements over time. They also allow users to create customized routes, which UAVs can fly at specified elevations. Land trusts may hire licensed pilots to conduct the flights, eliminating the need to train staff or purchase UAVs themselves.
There are several factors to consider when purchasing a unit:
- Accuracy. Different receivers provide different levels of accuracy.
- GIS Data Integration. Some units convert GPS points to specific GIS or Google Earth formats, which may or may not be compatible with certain mapping software.
- Attributes. Many GIS users have found that accurate attribute collection is just as crucial as location acquisition. Only the more advanced map-grade GPS units allow users to collect and input detailed attribute information.
- Device Compatibility. Some GPS units can wirelessly synch with digital cameras, rangefinders, or other field devices to receive additional data; this might require additional hardware or software. Other models have built-in cameras.
- Memory Capacity. Units that use auxiliary memory cards in addition to internal storage allow users to purchase morel memory capacity in the future, if necessary. Some unit/software combinations allow users with an internet connection to upload data directly from the unit to the cloud for storage.
- Durability. Some units are designed for use in rugged environments, with features like water-resistant screens, push-button controls, and protective outer casing.
- Battery. Some units use standard batteries, while others use internal rechargeable batteries. A device that includes a sleep mode or battery-save function will extend battery life while in the field.
For general information about choosing a GPS receiver, see:
“Tips on Selecting the Right GPS Receiver, for Your Job” (Resource Analysis)
“Recreational Versus Professional GPS: What’s the Difference?” (Esri)
“Maps & Geospatial: Global Positioning System (GPS)” (Penn State)
For product comparisons and reviews, see:
“Best Handheld GPS Review” (Outdoor Gear Lab)
“Handheld GPS for Surveyors” (Land Surveyors United)
“Handheld GPS Buyers Guide” and “Garmin Handheld GPS Comparison Chart” (GPS Tracklog)
There are many GPS software packages available on the market. Many are free. These are some of the most prominent applications.
Installed on GPS Unit
ESRI software for GPS units that allows mobile field mapping and data collection. Purchase required; free trial available.
Trimble GPS Pathfinder
Office software that supports all aspects of GIS data collection and maintenance for most Trimble GPS receivers. Purchase required.
Previously known as MapSource, a free mapping software compatible with most Garmin GPS units.
Installed on Computer or Other Device
Free virtual globe, map, and geographic information program. Google Earth Pro is available for desktop; in early 2017, Google Earth for Chrome was released as a non-desktop version. Some GPS units have a direct download to Google Earth. See the user’s guide for more information.
Free application developed by the Minnesota Department of Natural Resources that allows users to transfer data between GPS receivers and GIS software. Users can upload or download waypoints, tracks, and routes; calculate shape attributes; hyperlink images; and more. It is compatible with most Garmin GPS units and NMEA-output units from other companies.
MapGuide Open Source
Free platform that enables users to create their own maps and mapping applications. Includes an XML database for storing and managing content, and supports most geospatial file formats, databases, and standards.
Free GPS mapping software with a variety of features that allow users to geotag photos, blend different map types, and visualize multiple routes simultaneously.
Organizations should create protocols for collecting data, focusing on consistency and accurate documentation. GPS users should always record the source of and expected precision of data, which allows it to be combined with other data and ensures that future users will know exactly how it was collected. If not documented appropriately, data might be unusable in the future.
The admissibility of GPS data in a court of law varies depending on who collected the data and how they collected it. A court will accept a property boundary determined by a licensed surveyor using a survey-grade GPS. It is less certain how a court might view geographic information recorded by land trust staff using a non-survey-grade GPS unit, for example, the boundary between areas that have different levels of protection in a case where a newly constructed building appears to encroach on an area in which it is not permitted.
For more information about legal risks and implications, organizations should discuss GPS practices with their legal counsel.
See “Stewardship Tools: Who’s Using What?”, published in LTA’s Saving Land magazine, to see comments from different land trusts about their preferred GPS units and applications (as of 2016).
See the website GPS Tracklog for GPS-related news, reviews, buyer’s guides, and more.
See the website GIS Lounge for information about GIS mapping software, GPS tutorials, and more.
 Differential GPS overcomes GPS errors by using a series of base stations in combination with satellite data.
 The Russian version of GPS; its constellation of 24 satellites provides global coverage.
 “Attribute” refers to non-spatial information about a feature. For example, attributes of a stream could include its name, length, and sediment load.
GPS units vs smartphone apps - Car navigation systems
Do you need a dedicated GPS screen on your car dashboard, or is a smartphone app smart enough to get you from A to B these days?
This guide will help you decide which type of device suits you best, and what to look for when you're shopping for reliable car navigation.
With a good quality GPS unit, you should be able to "reach your destination" without having sworn once at that smarmy, know-it-all, robot-voiced son of a... send me down a dead-end street, why don't you?
Want to know how we get our review results? Check out how we test GPS devices.
How car navigation devices work
A GPS navigation device consists of:
- global positioning system (or GPS) receiver that picks up satellite signals to determine your exact position
- screen displaying maps and route instructions
- loudspeaker for verbal instructions
- computer processor to calculate routes, distances and times
- map database, including points of interest such as schools, police stations, car parks, petrol stations and hospitals (most car navigation systems available in Australia use the same mapping data, so the difference between models is how they let you use and display the data).
GPS apps for smartphones
If you're willing to spend $20 to $90, you can get an app on your phone that will work just like a car navigation device, with safety camera warning, trip planning and advanced lane guidance.
Most people know about Google Maps, which provides basic information on where you are and how to get from A to B.
Android and iPhone users have access to the free Google Maps app, which shows your current position and offers voice direction. Apple has also released their own version for iOS devices that's simply called Maps.
Maps are delivered in one of two ways. Apps either:
- Gradually download portions of the map as you enter new areas. This requires an active 3/4G connection.
- Download an entire state/country map in one hit over WiFi.
Most apps that use 3/4G connectivity, gradually download portions of the map as you require them, to save space on your smartphone storage. Though handy, this puts you at the mercy of mobile networks which are prone to dropouts depending on your carrier and location. Drive away from an urban centre and things can become spotty. Head into a tunnel and the mobile network may die altogether.
Some let you download an entire map over WiFi, so you can get around without worrying about your 3/4G connection. The downside is that a country or state map will take up a large chunk of storage on your phone. Plus, while almost all GPS apps support 3/4G, far fewer give you the option to download maps for use in offline mode.
Plus, if you plan to use the app often and for more than a few minutes, you'll need a car charger, as GPS can quickly drain a phone's battery.
Dedicated GPS units don't face these issues, as they don't require internet connectivity to access maps and satellites.
How you use GPS systems and apps
It doesn't take long to get your head around GPS units and their app equivalents. Using satellite signals, the system keeps track of your position and guides you along the plotted route with visual instructions on the display and verbal instructions via a computer-generated voice. All you need to do is:
- Type in where you want to go and your device plots a route, calculates the travel distance and estimated time of arrival, and displays the route on a map.
- You can usually choose between the fastest or the shortest route – which aren't necessarily the same – or specifically exclude toll roads or highways.
- Using satellite signals, the system keeps track of your position and guides you along the plotted route with visual instructions on the display and verbal instructions via a computer-generated voice.
Portable or integrated?
Portable units sit in a cradle with a suction cap that can be attached to the windscreen. They plug into the cigarette lighter, but also have a battery that provides a few hours of operation. They're installed within moments, and can therefore easily be moved from car to car. Smartphones mount in the same way.
Integrated systems are usually connected to the car's electronics, and can overcome some of the limitations of portable units. For example, they can use speed information to keep calculating your position when there's no satellite signal (such as in a tunnel).
Smartphone GPS laws
Since smartphone's capabilities extend beyond navigation, they fall under different laws to GPS units. However, these rules differ across states and territories:
- ACT: Smartphone must be securely mounted in a bracket. Drivers and riders can legally touch the phone when it's securely mounted.
- New South Wales: Smartphone must be securely mounted in a bracket without obscuring your field of view. Drivers can legally touch the phone when it's securely mounted. Learner, P1 and P2 licence holders are not permitted to use a mobile phone at all while driving or riding, which includes GPS functions.
- Northern Territory: Smartphone must be securely mounted in a bracket. Drivers can legally touch the phone when it is securely mounted (under an exemption from ARR 300).
- Queensland: Though there are laws regulating smartphone usage, GPS apps aren't specifically mentioned. In this instance, it's best to err on the safe side and avoid using your smartphone as a GPS in Queensland. Dedicated GPS units are legal.
- South Australia: Smartphone must be securely mounted in a bracket. Driver's can't touch the smartphone while operating the vehicle. Learner and P1 licence holders are not permitted to use a mobile phone at all while driving, which includes GPS functions.
- Tasmania: Smartphone must be securely mounted in a bracket. Driver cannot touch the smartphone while operating the vehicle.
- Western Australia: Smartphone must be securely mounted in a bracket. Driver cannot touch the smartphone while operating the vehicle.
Most states that allow smartphone GPS apps permit interaction using voice commands. There are several in-car mount kits available ranging from simple cradles to hold the phone, to devices with an embedded GPS chip to enhance the phone's inbuilt GPS performance.
How much should I pay?
- GPS navigation devices range from $89 to $449
- GPS smartphone apps range from $0 to $70
What to look for in a car navigation system
- Data entry: Check how easy it is to enter addresses and routing preferences.
- Display: This should be large and glare-free, and show the information you want to see – such as a two- or three-dimensional map view, distance to the next turn, current street name, time of arrival, and distance to destination.
- Installation and portability: Check how easy the system is to attach, but also how easy it is to remove and carry or stow away.
Most units offer these necessary features (but it's worth checking to make sure):
- battery for use away from the car for a short amount of time (no more than a couple of hours)
- advanced lane guidance tells you when to move to the exit lane and displays complex multi-lane manoeuvres clearly
- SD memory card slot
- USB PC connection
- option to store your home address
- option to select a location on the displayed map
- option to calculate fastest or shortest route
- option to exclude highways and toll roads
- display car speed, distance to destination, and estimated time of arrival
- display street name and distance to the next turn
- option of two- or three-dimensional map display
- volume control for voice instructions
- points of interest: schools, police stations, car parks, fast-food outlets, post offices, petrol stations, airports, railway stations, hospitals
- option to search for a point of interest.
- advanced lane guidance tells you when to move to the exit lane and displays complex multi-lane manoeuvres clearly.
If you're multilingual and prone to wandering around obscure neighbourhoods and foreign lands, some units offer the following handy features:
- Walking option: The system can plot a route for pedestrians – for example, ignoring one-way streets, or using walkways through parks.
- Languages: You can select a number of other languages for the display and voice instructions (such as French, Spanish, German, Italian, Swedish, Danish and Dutch).
For more information see our car navigation system reviews.
What to look for in a GPS app
- Points of Interest (POI) are a great way to quickly find landmarks such as hospitals, police stations, shopping centres, tourist attractions, and so on. Some apps also allow you to phone these places by selecting the phone number on the screen. While the free apps Google Maps, Apple Maps and Nokia Drive do have some POI categories, they largely depend on a search function rather than the category selections available on other apps.
- Advanced lane guidance tells you when to move to the exit lane and displays complex multi-lane manoeuvres clearly.
- Routing options can exclude toll roads, unsealed roads or highways from the calculated route.
- Walking mode allows you to plot a route for pedestrians, such as taking a short-cut through a park. This is a great feature for a mobile phone and arguably more important to have than on a dedicated device that is less likely to be in your pocket.
- Speed alert warns you when you exceed the speed limit for the road you're currently on. However, speed limit data can sometimes be inaccurate so don't rely solely on the GPS for this information – check the speed limit signs.
- Text to speech (TTS) announces the street name so you know when to turn without having to glance at the screen. Some apps provide a choice of more than one voice (though no guarantees of correct pronunciation).
- School zone alerts when you're approaching a school. Some units show a speed alert only.
- Full-route display – shown as a line on the map from your existing position through to your destination.
- In-app purchasing allows you to start off with a basic car navigation app and add features and functions you may want later on.
- Remote trip planning (also known as A-B routing) allows you to plan and run through your driving route in virtual mode before leaving.
- Trip log records your travels and saves the information to post online in an application such as Google Earth. Although common on dedicated car GPS devices, this feature is also beginning to appear on car GPS apps as well.
- Traffic information usually available as an in-app purchase, with live traffic information delivered to you as you drive.
- Liveview is a fairly new feature with photographic representation of the street to help you more effectively determine where you are.
So do you really need a dedicated GPS device or will an app do?
If you travel in an area with good mobile reception and have a full drivers licence then a Car GPS app on your smartphone such as Google Maps, Apple Maps for iPhone and Maps for Android is a great option. The data download will not be an issue as long as you're on a reasonable data plan (anything over 1GB per month will be fine).
Learner, P1 and P2 drivers and riders must not use any function of a mobile phone while driving or riding – not even hands-free or on speaker. So if you're not fully-licenced driver, your only option is a dedicated car GPS, end of story.
If you travel a lot in marginal reception areas or if you don’t own a smartphone (there are a lot of people who don’t own or want a smartphone) then a dedicated car GPS is a very good option – they've never been more affordable or feature packed.
Many also give you live traffic information using their own antenna, but some may require you to connect the car GPS to your mobile phone for the data updates.
How does the Global Positioning System (GPS) work ?
Article by Darren Griffin
When I first wrote this article back in 2002, consumer grade GPS was very new, very expensive and very rare! Consequently most of those who chose to invest in GPS hardware had a vested interest in discovering how this marvel of technology worked. Back in 2001 when map based GPS navigation first arrived, new users could not believe that the system was fee free with no service plan and no contract, what was the catch they all asked? And so the seed of an idea that became this explanation was born.
6+ years on GPS is mainstream, a commodity item that is no longer amazing or to be marveled at. We just open the box, switch on and use it with little thought to the technology that drives it. But it is still worth explaining how a small black box sat on your dashboard or held in your hand can know where you are anywhere on the surface of the planet to an accuracy of about 10m for consumer grade and 10mm for survey grade devices! That device on your dash is receiving a signal from a satellite orbiting above you at an altitude of over 11,000 miles! Not bad for a device that is not connected to a 2m dish!
Background - Navstar
The Global Positioning System (GPS) network we all use is called Navstar and is paid for and operated by the US Department of Defence (DoD). This Global Navigation Satellite System (GNSS) is currently the only fully operational system but Russia has GLONASS, China has COMPASS and the EU has GALILEO each at varying stages of development or testing.
As a military system, Navstar was originally designed and reserved for the sole use of the military but civilian users were allowed access in 1983. Back then, accuracy for civilian users was intentionally degraded to +/- 100m using a system known as Selective Availability (SA) but this was eliminated in May 2000.
The Satellite Network
The GPS satellites transmit signals to a GPS receiver. These receivers passively receive satellite signals; they do not transmit and require an unobstructed view of the sky, so they can only be used effectively outdoors. Early receivers did not perform well within forested areas or near tall buildings but later receiver designs such as SiRFStarIII, MTK etc have overcome this and improved performance and sensitivity markedly. GPS operations depend on a very accurate time reference, which is provided by atomic clocks on board the satellites.
The Navstar GPS Constellation
Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are further away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.
There are at least 24 operational GPS satellites at all times plus a number of spares. The satellites, operated by the US DoD, orbit with a period of 12 hours (two orbits per day) at a height of about 11,500 miles traveling at 9,000mph (3.9km/s or 14,000kph). Ground stations are used to precisely track each satellite's orbit.
Here is an interesting comparison. The GPS signals are transmitted at a power equivalent to a 50 watt domestic light bulb. Those signal have to pass through space and our atmosphere before reaching your satnav after a journey of 11,500 miles. Compare that with a TV signal, transmitted from a large tower 10 - 20 miles away at most, at a power level of 5-10,000 watts. And compare the size of your TV's roof mounted antenna with that of your GPS, often hidden inside the case itself. A wonder then that it works as well as it does and when the occasional hiccup occurs you will at least understand the reasons why.
Signals from multiple satellites are required to calculate a position
How Position is DeterminedA GPS receiver "knows" the location of the satellites because that information is included in the transmitted Ephemeris data (see below). By estimating how far away a satellite is, the receiver also "knows" it is located somewhere on the surface of an imaginary sphere centred at the satellite. It then determines the sizes of several spheres, one for each satellite and therefore knows the receiver is located where these spheres intersect.
GPS AccuracyThe accuracy of a position determined with GPS depends on the type of receiver. Most consumer GPS units have an accuracy of about +/-10m. Other types of receivers use a method called Differential GPS (DGPS) to obtain much higher accuracy. DGPS requires an additional receiver fixed at a known location nearby. Observations made by the stationary receiver are used to correct positions recorded by the roving units, producing an accuracy greater than 1 meter.
How Is The Signal Timed?All GPS satellites have several atomic clocks. The signal that is sent out is a random sequence, each part of which is different from every other, called pseudo-random code. This random sequence is repeated continuously. All GPS receivers know this sequence and repeat it internally. Therefore, satellites and the receivers must be in synch. The receiver picks up the satellite's transmission and compares the incoming signal to its own internal signal. By comparing how much the satellite signal is lagging, the travel time becomes known.
What does the signal consist of?
GPS satellites transmit two radio signals. These are designated as L1 and L2. A Civilian GPS uses the L1 signal frequency (1575.42 MHz) in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass, plastic etc but will not travel through solid objects such as buildings and mountains.
The GPS signal contains three different bits of information — a pseudo random code, almanac data and ephemeris data.
- The pseudo random code is simply an I. D. code that identifies which satellite is transmitting information. You can often view this number on your GPS unit's satellite information page, the number attached to each signal bar identifies which satellites it's receiving a signal from.
- Almanac data is data that describes the orbital courses of the satellites. Every satellite will broadcast almanac data for EVERY satellite. Your GPS receiver uses this data to determine which satellites it expects to see in the local sky. It can then determine which satellites it should track. With Almanac data the receiver can concentrate on those satellites it can see and forget about those that would be over the horizon and out of view. Almanac data is not precise and can be valid for many months.
- Ephemeris data is data that tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite will broadcast its OWN ephemeris data showing the orbital information for that satellite only. Because ephemeris data is very precise orbital and clock correction data necessary for precise positioning, its validity is much shorter. It is broadcast in three six second blocks repeated every 30 seconds. The data is considered valid for up to 4 hours but different manufacturers consider it valid for different periods with some treating it as stale after only 2 hours.
Cold Starts & Warm Starts Explained
Often manufacturers and reviews will refer to Factory, Cold and Warm Start times. Understanding the above, these can be simply explained as follows:
- Factory Start
- All data is considered invalid.
- Cold Start
- Almanac data is current but Ephemeris is not or has expired.
- Warm Start
- Both Almanac and Ephemeris data is current.
To compute a PVT (position velocity time) solution the receiver will look for satellites based on where it 'thinks' it is roughly located and the almanac if current. If it finds one or more of the satellites it expects to see it will lock onto that satellite and begin downloading ephemeris data. Once data from three satellites has been received an accurate positional fix is calculated.
If you are moving whilst trying to obtain a fix this process may take much longer than it would if you were stationary. Your receiver must complete reception of ephemeris data without error, this data is transmitted in three packets. Should any one packet not be received completely without error then it must start over again. Clearly doing this whilst moving leads to much higher error rates and longer fix times. Considerably less than a second of interruption is enough to mean the receiver will have to wait for the next transmission.
If you are attempting a lock having re-located more than a couple of hundred miles since your last fix then the ephemeris data will in most cases no longer be valid. The receiver will be looking for satellites in the sky above that cannot be seen because of your re-location. In this case the receiver will initiate a factory start and begin downloading both almanac and ephemeris data. This will extend the initial time to lock considerably. This is why your GPS is so slow to calculate a fix when you switch it on in your hire-car at the airport!
QuickFix is a feature provided by some manufacturers/devices. To understand what QuickFix is you need to understand in detail how a GPS calculates your position.
For the initial position calculation your GPS chipset needs to find at least 4 satellites with a strong enough signal (28 dBHz or more) and it must keep those satellites and the signal strength for approximately one minute in order that it can download the data from the satellites that is essential for calculating your position (this it the ephemeris data explained earlier).
If at any time the GPS receiver loses the signal of any satellite or the signal drops below 28 dBHz then it has to start all over again and track that satellite for another minute. In a real life scenario for example, you may be driving between high buildings (urban canyons, see below) and the received GPS signal keeps changing all the time.
The QuickFix file you download from the internet is part of a solution from your GPS chip manufacturer. SiRF call their solution Instant Fix (I Edition) or A-GPS (assisted GPS). The file contains specially prepared ephemeris data that is valid for 7 days that your GPS chip uses instead of the the data received from satellites for calculating your first fix.
This allows the chip to skip the "download ephemeris from satellite" step and instead to start calculating your position immediately after powering on. This takes around 5-15 seconds on average. The signal strength required for downloading ephemeris data from satellites is 28dbHZ whereas the signal strength required for calculating your position once your GPS has received the ephemeris data is much lower at only 15 dBHz.
So a valid QuickFix file allows your device to calculate your position in 5-15 sec rather than the minute it would otherwise take (if stationary), and lowers the minimal signal strength required for calculating your position from 28 dBHz to 15 dBHz.
If at any time your GPS chipset finds the Quickfix ephemeris data is invalid or very old it defaults to calculating your position the traditional way, i.e. tracking a minimum of 4 satellites with 28dbHz signal continuously for around a minute.
Sources of GPS signal errorFactors that can degrade the GPS signal and thus affect accuracy include the following:
There are many causes for position errors or low signal
- Ionosphere and troposphere delays The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
- Signal multi path This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
- Receiver clock errors A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
- Orbital errors Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
- Number of satellites visible The more satellites a GPS receiver can "see," the better the accuracy.
- Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
- Satellite geometry/shading This refers to the relative position of the satellites at any given time.
- Ideal satellite geometry exits when the satellites are located at wide angles relative to each other.
- Poor geometry results when the satellites are located in a line or in a tight grouping.
- Intentional degradation of the satellite signal Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. DoD. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.
Some Satellite Facts
Here are some other interesting facts about the GPS satellites:
- There are some 2,500 satellites of all types and purpose orbiting the earth.
- There are over 8,000 foreign objects orbiting the earth consisting of items like nose cones and panels from old satellites, an astronaut's glove, spanner and more!
- The first GPS satellite was launched in 1978.
- A full constellation of 24 satellites was achieved in 1994.
- Each satellite is built to last about 10 years.
- Replacements are constantly being built and launched into orbit.
- A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
- Transmitter power is a mere 50 watts or less.
For more information about satellites and GPS satellites in particular, visit NASA's web site where you will find a GPS Satellite tracker applet similar to below that allows you to track all of the 2,500 plus satellites that currently orbit our planet but more specifically you can track the Navstar network of satellites and see which ones are currently flying over your location.
2500 Satellites orbit the Earth
GPS in the IFR System: A Guide from the Ground Up
Despite the proliferation of GPS in IFR applications during the past few years, many instrument pilots are afraid of anything that happens after they hit the power switch. Jeremy Jankowski explores some of the information that every pilot who uses GPS in the IFR system should know, including the importance of RAIM, what goes on behind the scenes during a GPS approach, and some of the ways to utilize GPS to its maximum potential in some less-than-ideal situations.
By Jeremy Jankowski | July 7, 2002
Jeremy Jankowski works as a "freight dog," running cancelled bank checks around in Piper Lances and Senecas in the Southeast, and has recently been named a "Master CFI" by the National Association of Flight Instructors.
Previously, he's flown a variety of turbine- and piston-engine aircraft as both a co-pilot and a flight instructor from coast to coast with Purdue University in West Lafayette, Ind., and has also spent time working in the dispatch department of nationally recognized UTFlight, the corporate flight department of United Technologies Corporation.
Jeremy used to work as a computer technician in Southeastern Michigan but doesn't miss it a bit.
Aviation changes so rapidly that it's easy to make yourself feel like an old-timer without spending a lot of time as a pilot. I jumped in right before the alphabet airspace arrived a few years back, and so I have always been able to tell those who came after me: "We didn't have any of those Class B and C airspace categories; we had ARSAs and PCAs and TCAs." We didn't use METARs back then. Most of us didn't use GPS, either, and LORAN, even in its best form, pales in comparison to the GPS technologies available today, which will probably be far superseded by what is available a year from now! Using VORs and (ick!) NDBs to get around seems to be like the proverbial "walking through snow, uphill, both ways," in aviation.
Surprisingly, many in the aviation community have been more than a little afraid of the GPS movement, and it's not terribly surprising. Without a doubt, the FAA's "phase-in" program, the buzzwords, the TSOs, and the computer-like logic (not to mention typing with a rotary knob) have made the transition from a device that was relatively simple (but gave little situational awareness without extensive practice, like a VOR or NDB), to one that has, literally, hundreds of functions.
Any individual who has been charged with showing me how to use any one of these elusive units has invariably said something to the effect of, "You will never learn all of the functions that this thing is capable of." Unfortunately, it's true. I could explain to you how an ADF works in an hour (though, in practice, it will take much longer than that for almost any student to really understand how an ADF works), but explaining all of the functions of a modern GPS system would take weeks and several trips to the manual for the both of us.
What is imperative to users of the GPS system, however, is a basic understanding of how the system is set up, and the caveats and pitfalls of improper use. Unfortunately, I've found a great number of pilots, flight instructors, and even commercial operators who don't quite understand the GPS as well as they think they do.
I used to teach and evaluate students in Frasca FTDs at one of the major university flight programs. One of the most prominent things I noticed with students is a consistent fear of the "magic box." On the final examinations I gave, I'd often provide the student with the choice of executing the partial-panel GPS or partial-panel NDB approach. Surprisingly, to the best of my knowledge, nobody decided to use the GPS. Upon inquiry, after they'd stumble through the NDB, they'd typically indicate that they found the GPS difficult to understand. I also found that most of the important items in its use had been glossed over by their instructors. Whether this is due to the CFI's apathy or misunderstanding (I hope mostly the latter), the fact remains that there is a generation of flight instructors teaching the next generation of pilots how to use a device that nobody taught them how to use.
How Does GPS Work?
A GPS Receiver Triangulating Position from Three Satellites
A GPS receiver works on the principle of a very precise clock. Through transmission of a signal, which essentially contains the time that the transmission was sent from a satellite, the receiver can determine the distance to the satellite. It knows how fast the signal travels (the speed of light), it knows what time it was sent (because the satellite told it), and it knows what time it arrived (with its internal clock). It does the math, and determines the distance that satellite is from the receiver's current location. The satellites themselves follow a predictable orbit, and therefore can reliably relate their current position in the transmission as well.
With only one satellite, all the receiver is capable of doing is determining that it is a particular distance away from the first satellite. Unfortunately, the sphere around the satellite at that distance could still place the aircraft east, west, north, or south of the satellite, without another way of fixing position (you could even be on the other side of the satellite, headed for the moon, for all it knows!). With several more satellites, however, your receiver can fix your position three-dimensionally with reasonable accuracy (for enroute operations, a minimum of four satellites).
Have You Ever Seen the RAIM?
Let's say, for the sake of argument, that the internal clock on one of the GPS satellites becomes incorrect. It doesn't take much 0.0001 seconds off equates to a position error of roughly 16 nm! Unfortunately, if there aren't enough satellites, your receiver might go on believing the broken satellite, and fix your position incorrectly by several miles. It's not a good thing to be going into Aspen, Colo., without being able to fix your position more accurately.
As with any other facet of aviation, GPS comes with its own set of alphabet soup. Among the most important items in GPS use is the concept of RAIM, or Receiver Autonomous Integrity Monitoring. RAIM's function is to utilize an additional satellite to identify a discrepancy between the satellites that are being used, thus ensuring that one satellite's slight error is detected and a potential disaster in position accuracy is averted. In other words, your GPS receiver can detect a disagreement between the positions that the satellites indicate, and toss the incorrect one out of the equation. The catch is that the receiver requires more visible satellites to execute this function. Since it is of the utmost importance that your receiver be accurate during the approach phase, the FAA requires that you acquire RAIM before executing an approach.
The existence of RAIM is one of several reasons that handheld GPS units are not permitted to be utilized for IFR operations. Though arguably a rare occurrence, the FAA is concerned that the handheld units could be miles off course without any indication of a problem. We'll get into the other requirements later.
Some Other Buzzwords
I don't care what type of GPS you're using, it's got all of the next functions available in some form. However, one of the principle problems in working with the abundance of information available is that it can be difficult to isolate what's really useful in a tight situation, especially to new users of the system. Here's a breakdown of the best things to look for on your particular unit:
The Relationship Between Track, Desired Track, and Bearing
Track (abbreviated TRK or TK, generally): The single most useful (and neglected) function of the GPS receiver. Essentially, the track display provides the pilot with the aircraft's precise track along the ground. This isn't heading, folks, this is the actual direction the aircraft is moving, corrected for wind, deviation, and variation, the whole nine yards. The best part is that you don't need to put in a flight plan, execute an approach, or push many buttons to use it.
Desired Track (abbreviated DTK): The course line to your next waypoint that you told the GPS you wanted to go. Basically, if VOR A is directly south of VOR B, then your desired track would be 360°. This is often confused with bearing, described next.
Bearing (abbreviated BRG): The direction you'd need to go to get to your next waypoint from where you are now. If VOR A were directly south of VOR B, as described above, then your DTK is 360°, whether you're on it or not. However, if you're east of the DTK (the course), your bearing would be some number less than 360° (like 340°, for instance). If you turned left until your track was 340°, you'd fly directly to the station.
It's important to note the difference between DTK and BRG. If you're flying airways from VOR to VOR, you want to stay on your DTK. In the above example, if you wanted to get back on the 180° radial of VOR B from the position where the bearing to VOR B is 340°, you'd need to turn left to a heading of 330° or even 320°, depending on how far off course you are, how quickly you want to return, and how much wind there is. A common error is to make the track line up with the bearing, which will work fine, so long as you don't mind being off course! The classic ADF "homing error" is the result of consistently changing the track to match the bearing, which is not the course you actually want to fly.
Distance: This may seem like a no-brainer, but it can get confusing, because DME distances are based on slant distance. For example, if the aircraft is 12,000 feet (roughly 2 nm) above the elevation of the station, when the GPS says you're 5 nm from the station, the DME will read 5.4 nm, because DME measures the distance between the aircraft (which is 2 nm in the air) and the station (on the ground). This error gets larger as you get closer to the station or higher from the elevation of the station; for instance, when you cross over the station, the DME will read 2 nm! GPS distances do not have these errors built into them, so you will not pass over a GPS waypoint until the distance reads zero, although the GPS may sequence to the next waypoint earlier. You may use GPS as a substitute for DME fixes, but you should be cautious due to the differences between the distances given by GPS and those by DME. More on that in a minute.
OBS or Non-Sequential Mode: When you enter a series of waypoints in your GPS, it assumes that, as you cross each one, it should automatically switch to the next waypoint to reduce your workload. This is a great feature, unless you need to do something like a hold or a procedure turn. In these instances, you will cross a particular waypoint more than once before you want to activate the next waypoint. As a result, every GPS has a method of turning off the auto-sequencing function. Typically, the "holding" mode is activated by a big pushbutton on the unit itself or on an auxiliary panel for easy activation. One more thing: When you get to your missed-approach point (MAP), the GPS will not automatically sequence to the missed-approach segment. You must activate the missed-approach segment by taking the unit out of the OBS mode, which may require a significant amount of user intervention on some models, just at the moment you should be following the missed-approach procedure very carefully!
Moving Map: There are GPSs out there that don't provide a moving map display, but they're becoming scarce, and those are typically used as receivers to feed data to a larger display package. The information provided by any particular moving map display varies by manufacturer, but there are generally a few common ways of using the map. The Track-Up display rotates the map around to your direction of flight, making what is in front of you in the airplane at the top of the map. The Desired Track Up display will put the direction you're supposed to go at the top of the map, but this can be dangerous, since you could feasibly fly off course enough for your aircraft to depart the map display. Finally, the North-Up display places north at the top of the map (and east at the right, south at the bottom, and west at the left), more like a chart.
GPS Approaches and the Phase-in Program
Open up a book of instrument charts, and you'll find that there are a variety of types of GPS approaches. Don't be intimidated, though; if you have an IFR-certified GPS, you can probably shoot the approach, no matter what its title.
When the FAA began authorizing GPS for use in instrument approach procedures (IAPs), it decided that authorizing the use of GPS as the sole source of navigation was not prudent before reliability of the system had been established. At first, the GPS had to be supported by another means of navigation; then, the aircraft was required to have other means of navigation installed and operational, though not necessarily monitored. Now, GPS approaches may be executed without any reference to any other navigational system in fact, many GPS approaches exist now that cannot be executed through any other means, giving many airports the opportunity to have an IAP without incurring the costs of ground-based navigational equipment.
GPS approaches can typically be subdivided into three types: the overlay approach, the GPS-only approach, and the area-navigation approach.
(Click charts for hi-res versions)
GPS Overlay Approach
GPS Only Approach
Overlay approaches were the first GPS approaches to be created, and allow you to mirror a previously established IAP without utilizing the traditional navigational equipment at all (VOR, NDB, etc.) These approaches are found as "VOR or GPS," for example, in the title of the IAP.
A popular "gotcha" comes from the overlay approaches that involve a DME from a station while heading toward the airport. For instance, a VOR approach where your MAP is 10 DME from the station can be confusing when using the GPS, since the GPS will always count down to the next waypoint (see chart). In this scenario, your MAP, contrary to the approach diagram, will occur at 0.0 nm (from the MAP). The only way to make the GPS display the distance from the VOR is to activate the OBS/Non-Sequential Mode. However, as you'll see, this will preclude the GPS from ensuring its accuracy through RAIM, and would not be a legal or safe way to execute the approach.
GPS-only approaches are just that they can only be executed with a properly certified IFR GPS system. No other type of equipment may be used (or is needed) to execute these approaches. They are identified by a title like "GPS RWY 22" (see chart).
Area-navigation approaches, or RNAV approaches, also stand alone without reference to any specific ground-based stations. One thing that makes these types of approaches confusing is that, previously, an "RNAV" approach implied use of a special piece of avionics, such as the Bendix/King KNS80. Now, however, the FAA has decided that an RNAV approach requires only a certain level of precision, no matter what type of navigational system the aircraft is using. The Required Navigational Performance, or RNP, typically required is 0.3, which means a CDI sensitivity of 0.3 nm from the center of the indicator to the edge of the case. You'll see how that comes into play a little further into the article.
Why the fuss? Well, many larger aircraft use multi-sensor navigational systems, such as a Flight Management System (FMS). The concept behind these systems is to augment any particular navigational source (VOR, DME, GPS, LORAN, Omega, Inertial, etc.) with as many other sources as possible. In other words, it compares the GPS position with positions of any VOR stations in range, any DME values it can find, etc. As a result, the failure of any one particular system does not necessarily prohibit it from achieving the required degree of accuracy. It is possible that these aircraft, even without GPS, could execute an RNAV (though not a GPS-only) approach. The RNAV approaches are identified by the title "RNAV (GPS) RWY 36L," and there's typically a note about an RNP or GPS required (see chart).
What's next? The FAA is currently in the process of implementing the Wide-Area Augmentation System, or WAAS. Through largely separated ground-based stations (fake, or pseudo-satellites), GPS accuracy can be greatly improved down to an error as small as 7 meters (23 feet). With that degree of accuracy, the FAA will begin developing precision-approach procedures utilizing GPS receivers, so that hundreds of airports that may never have anticipated precision approach capability may have it soon.
Shooting a GPS Approach
Before you can go out and start blasting through terminal airspace with your own "magic box," you'll need to have a basic understanding of how to use the particular model of GPS that you have. At the bare minimum, you should know how to go direct to a waypoint, enter a flight plan, load an approach, and use the moving map. These steps can vary greatly between models of GPS.
What does not vary is the GPS logic used to shoot an approach. GPS units will automatically step up the sensitivity of the CDI during an approach to achieve the required degree of accuracy to shoot the approach. During enroute operations, each dot on the CDI corresponds to 1 nm off course, meaning, if it reads off the scale, you are more than 5 nm off course!
If you have loaded an approach, the first step is to Arm/Enable that approach. This procedure can also differ depending on units. Some I have used will do it automatically, unless you say otherwise (such as the KLN90B, which has a toggle-switch installation), whereas others will prompt you to enable the approach (such as the Apollo or II Morrow GX50 series). In either case, once you've enabled the approach, the first sensitivity change will occur when you come within 30 nm of the destination airport. At this point, the CDI will start to change sensitivity incrementally toward 1.0 nm (full scale), or 0.2 nm per dot on the CDI.
CDI Sensitivity Changes During GPS Approach
At 2 nm from the FAF, the GPS receiver will run a quick check to ensure that it has enough satellites to ensure RAIM during the approach. If it believes that the accuracy can be guaranteed, it will activate the approach, which should trigger a message or annunciation on the panel to alert you that the approach has been "activated." If this RAIM check fails, a message will alert you to this. If the approach does not activate, you may not shoot the approach.
After activation, the CDI will progressively step up sensitivity again, this time to 0.3 nm for a full-scale deflection (approximately 360 feet per dot). It will achieve this sensitivity by the time you cross the FAF.
It's important to mention that this change in CDI sensitivity is precisely why you must "load" and "activate" an approach rather than just typing in the names of the fixes on the approach plate. Without the approach being armed and activated, the CDI sensitivity could stay at +/- 5 nm, which means you would have to keep the CDI deflected less than one half of one dot from the center to ensure obstacle clearance! So, it is not acceptable to shoot an approach unless it is in the database and you've loaded it properly.
Another thing to keep in mind is that, during the changes in sensitivity, a common mistake is to overcorrect for any deviations from course. For instance, let's say that at 2.5 nm from the FAF, you're one dot (1 nm) off course, so you put in a heading correction of thirty degrees. After you cross the two mile mark, the approach activates, so the sensitivity starts to increase. As you move toward your course, the sensitivity increases, so the needle doesn't move. Keep in mind that you are getting closer to being on course; however, many pilots who don't understand this change in sensitivity assume that they need a larger correction angle ("geez, this is one heck of a crosswind!"), and start putting in a very large heading change to compensate. All of the sudden, the sensitivity stops changing, and before you know, it the needle is pegged on the other side of the dial.
Also worth discussing is the RAIM check that the GPS performs. By itself, the GPS needs five satellites to guarantee accuracy of the system during the approach. However, all IFR-approved GPS systems have a sensor connected to the encoding altimeter this gives the GPS information about the aircraft's altitude, thus giving one positive fix on the aircraft's location. As a result, the GPS will only require four satellites to achieve RAIM and execute the approach. This is called Baro-Aiding. This will only work if you set the internal altimeter (which the GPS should prompt you to do when you get near the terminal environment). I've met at least three people from completely different facets of flying who believed that you only needed to set the altimeter if you're utilizing the vertical navigation functions of the GPS not true!
If, at any point during the approach, your GPS loses its capability to achieve RAIM, then you must not descend to the MDA for the approach. If you already have started down, you should execute a missed approach immediately. You should still overfly the MAP to guarantee clearance with obstacles, but you should begin your climb to a safe altitude immediately. Note that once the approach has been activated, the GPS receiver will give up to five minutes without the necessary satellites before giving an annunciation, to give itself the opportunity to reestablish communication with the satellites. If a warning pops up, you could already be considerably off course.
As discussed previously, the GPS will not automatically sequence past the MAP to the missed-approach segment without user interaction. Typically, this involves taking the GPS out of the OBS mode and putting it back into sequencing mode, but for some models, the user will have to manually alter the flight plan to go to the next waypoint. There is typically more than one way to achieve the desired result, but, either way, low, slow, and busy is not a good time to be giving all of your attention to the GPS. Point yourself in the right direction, start climbing, clean it up, do anything else you need to do, and then worry about sequencing the GPS.
Potpourri of Pitfalls
One classic dilemma with GPS is the fascination factor. During critical phases of flight, the GPS seems to demand the most attention, with blinking annunciators, inquiries for input, and constantly changing information. Unfortunately, these phases of flight are the most important times to look for traffic when in VMC. Also, since many aircraft have the GPS retrofitted on the other side of the panel from the pilot, the pilot's attention can be diverted away from the most important instruments on the panel. On the bright side, you'll know your exact latitude and longitude after you lose control of the airplane in this manner. The best idea is to enter information into the GPS in stages and well ahead of the time that you need it.
Consider the study done by Diane Damos, Richard John, and Elizabeth Lyall in 1999, which studied the amount of time crews spent looking for traffic in highly automated and computer-dependant cockpits, versus non-sophisticated versions of the same aircraft. They found that the pilots in automated aircraft (which, arguably, should lessen pilot workload!) spent significantly less time looking for traffic during approach and landing. Why? Because they were concentrating on setting up the computerized cockpit to do what it should do properly, rather than looking around outside.
Most importantly, don't ever let yourself become so wrapped up in the GPS that you lose control of the airplane. It's happened more than once that I've flown with students who get lost or confused in the myriad of functions and forget that you "navigate" after you "aviate." If you find yourself asking, "Why is it doing that?!?" turn it off, turn it on again, and get yourself back to the beginning. Remember: The GPS doesn't care if it makes it home safely.
Here's food for thought you could shoot an entire approach using only the GPS. I don't mean in lieu of a VOR; I'm saying that you don't need any of your instruments at all. Of course, I'd never recommend that you ignore other instrument indications (and please don't try this without a safety pilot on a beautiful day), but it's worth noting that the GPS can provide everything you need: heading (actually, track, which is even better), airspeed (indirectly through groundspeed indications), altitude (through the encoding altimeter input), and course information. To prove the point, I took an appropriately rated safety pilot up with me in a Cessna 172R with the KLN90B, covered up the airspeed indicator, the attitude indicator, the altimeter, the turn and bank indicator, the directional gyro, and the vertical speed indicator. I did not utilize any other radios, and permitted myself only the inclinometer and the tachometer in addition to the GPS. The result? A passable (although not entirely smooth) GPS approach under the most bizarre of circumstances. I call it a "no panel" approach.
It's pretty unlikely that you'll be faced with such an emergency, but with all that information available, how bad can a more traditional vacuum failure be? Think about trying to shoot an NDB approach, partial panel, with a heavy crosswind, to minimums. Bring back bitter memories? It should be a snap with a GPS receiver, because if your inbound course is 90°, just make your track 90°. That's it. No need to think about wind correction angles, no worrying about leading the needle or pushing the needle or any other such nonsense. If you're north of course, make the track 95° for a nice correction angle. What could be simpler?
The real benefit to this is that track information is available without any input of the pilot. You don't need to enter a flight plan, load an approach, or navigate any bizarre menus. There are typically several pages that display track information, to boot. The only catch is a lag during turns of a significant bank angle, but with practice and patience, you have something that's a lot easier to handle than the whiskey compass!
Other Neat Stuff
I couldn't begin to list everything that's available with these units. The nearest airport, VOR, intersection, etc., is generally a few button pushes away from you with any newer model. The airport information pages, while they shouldn't be used solely for navigation, can be a great way to find information about an airport in an emergency, or to find a nearby FSS or radar facility. Some units have flight timers, or stop/start timers, or countdown timers (set it up for the time inbound from FAF to MAP, start it, and wait until it says "0," rather than trying to remember that bizarre time from the approach plate). It can also tell you your estimated time enroute, time of arrival, and time of departure, calculate the current winds aloft, and provide a plethora of other information.
GPS has already become the most remarkable advance in aircraft technology since the advent of the VOR, and as the system continues to prove itself, along with lowering costs and improved receiver interfaces, it will undoubtedly become the norm rather than the exception. If you're working on your instrument rating, try to find a way to incorporate a little GPS work into your course of study. If you have the rating and are thinking about getting a GPS unit for your own aircraft, or renting one that already does, grab hold of a good CFI and give yourself the opportunity to learn how to use the GPS as another tool in all of your instrument flying. When the soup is thick, the instruments are shaky, and the vertigo starts to set in, you'll be glad it's there (but only if you know how to use it!)
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ESR's Guide to Hacking With GPS
This is a gentle introduction to writing GPS-aware applications — or, How I Stopped Worrying And Learned To Love Spherical Trigonometry. It will explain the capabilities and limitations of GPS hardware and the gpsd software that you need to know about when writing location-aware applications.
We'll go from general to specific, beginning with an overview of how GPS works and ending with architectural suggestions about how to use gpsd to make your application location-aware.
First, the basics of how GPS works. It depends on the fact that satellite orbits can be modelled accurately. A GPS receiver is a combination of a radio receiver and computer that receives timing signals and orbit information from GPS satellites, and in particular can compute exactly where each satellite will be at any given time with respect to the fixed Earth. (For those of you who enjoy such details, what they actually predict is each satellite's position with respect to a coordinate system known as WGS84 which closely fits the mean sea level of Earth.)
Although the term GPS properly refers only to the system operated by the United States Air Force (also called NAVSTAR, it is sometimes used loosely to refer to similar systems. We use the accepted term Global Navigation Satellite System (GNSS) to refer to all satellite-based navigation systems. The main systems other than GPS are GLONASS (Russia), Galileo (EU) and BeidDou (China). Because the gpsd project originated when GPS was the overwhelmingly dominant paradigm, many parts of the documentation and code refer to GPS when GNSS is more accurate. Our intent is to adjust the documentation and to leave the code alone because there is a real cost to changing code.
This paragraph and much of the following text is specific to GPS/NAVSTAR, but the principles largely apply to other systems. There are presently about 30 dedicated Navstar satellites (full coverage can be achieved with 24), twenty thousand km (twelve thousand miles), up in high-inclination orbits, so that each one's trajectory wraps around the Earth like a ball of yarn as the planet spins beneath them. The inclinations are tuned to guarantee that about twelve will be visible at any given time from anywhere on Earth (coverage falls off a little at high latitudes).
You can look at a very nifty simulation of Navstar satellite orbits. (Requires Java, also includes GLONASS, the Russian military equivalent).
Each satellite broadcasts identification pulses, each one including the clock time it was sent. A GPS receiver, picking up these pulses, and knowing the speed of light, can recover its 4-dimensional location (Latitude, Longitude, Altitude, and Time). Ideally, you would need to solve four simultaneous equations with the four unknowns, so would need four visible satellites. This is known as a 3D fix. Computing the GPS's exact position with respect to the satellites becomes a relatively simple if tedious exercise in spherical trigonometry (which, fortunately, the GPS's firmware does for you). Note that the GPS reciever does not need to know its own time or location to begin with.
An excellent introduction is available at: How GPS Receivers Work, see page 4 in particular.
That's the theory. In practice, the system has important limits. Anything, natural or artificial, that messes with the signal timings will degrade the accuracy of your position fix. Until it was abolished by Presidential decree in 2000, the most important limit was artificial, the so-called 'Selective Availability' feature. The satellites were programmed to introduce patterned timing jitter into the signals. The U.S. military knew the pattern, but nobody else did (or, at least, nobody who was admitting it).
In Sep 2007 the U.S. government announced that the future generation of GPS satellites, known as GPS III, will be without the SA feature. Doing this will make the policy decision of 2000 permanent. The important limits are on accuracy are now due to physics. One is a variable amount of signal lag produced as the GPS signals pass through the ionosphere and troposphere, which partly refracts radio waves. This can be largely compensated for by a technique called "Differential GPS" (DGPS). Ground-based reference stations are established in well-surveyed locations, and compare measured ranges (pseudoranges to be precise) with their calculated values. These errors account for unknown propagation delays, clock errors, and any other unmodeled errors. The reference stations can then tell nearby GPS receivers the required corrections, which are then applied to observed pseudoranges before computing a position. This information may be broadcast via radio (called "Ground Based Augmentation Systems"), or via satellites (called "Space Based Augmentation Systems"). See DGPS and friends for details on how this works.
In practice, the most important limit on accuracy is the actual visibility of satellites. A timing signal has to be fairly strong and clear, with little noise or distortion, before a GPS can use it. The frequencies GPS has to use in order to punch through the ionosphere with minimal attenuation (unlike conventional radio and TV signals) don't cope well with solid barriers. Thus, GPS tends to work poorly if at all inside buildings. Tall trees and tall buildings can mess it up, blocking line of sight to satellites that aren't nearly directly overhead.
Accuracy also falls off a bit when you're in motion. This isn't a physical effect, but mostly due to the fact that computation always takes a little time; by the time the GPS figures out where you are, you're not there any more.
Another limit, implicit in the geometry, is that GPS is relatively poor at getting your precise altitude. When you can get a signal lock on four satellites, a modern GPS will give you longitude and latitude within about 10 meters or yards, down to 2 with DGPS correction. Vertical uncertainty will be much higher, as much as fifty meters.
People who really obsess about GPS accuracy quote it not as a single figure but as a probability-of-error: e.g., you're within 10 meters 95% of the time and 2 meters 50% of the time.
DGPS requires out-of-band communication with a service providing GPS signal correction information to make the GPS positioning more accurate. There are two ways of communicating this in real-time, GBAS and SBAS. An example of GBAS is the National Differential GPS system (NDGPS) in the US, transmitting corrections around 300 kHz. There are similar systems all over the worldwide, usually run by maritime navigation authorities. A disadvantage of these systems is that consumer GPSes do not listen on these frequencies, so this information is not really available to them. Another example is the system being installed at airports worldwide, to enable precision landing. Coverage for both these system types is between 30km and 40km.
Examples of SBAS include WAAS (US), EGNOS (Europe), GAGAN (India), and MSAS and QZSS (Japan). These systems are almost identical in their operation. They provide DGPS corrections with in-band communication — geo-stationary satellites broadcasting GPS signal correction information on the same frequency and format as the GPS satellites. The system makes GPSes more accurate, and adds integrity checks making it possible to detect when the GPS location is totally wrong. Unlike GBAS, your GPS will generally use these systems automatically whenever it can see the satellites.
SBAS data starts out as normal DGPS stations observing the errors. That data gets processed and interpolated into a grid which models ionospheric and tropospheric delay over the SBAS coverage area. The GPSes then interpolate into that grid to get an estimate of lag for their current position. Accuracy will vary based on how close a GPS is to a DGPS station.
SBAS systems have wider coverage areas than GBAS, but still not worldwide; as can be seen from the list above, each country has a system covering its area of interest. See SBAS Service Areas.
Note that DGPS improves accuracy in both position and time, the two are intrinsically related.
From a software designer's point of view, a GPS receiver is an oracle that tells you its location whenever it can get line-of-sight to four satellites. Our next topic is how it gets that information to a computer in a form your application can use.
Almost all GPSes are serial devices that use either RS-232C or USB to communicate with the host machine. Most track a standard called NMEA 0183 which prescribes both electrical signal levels and a data encoding. The protocol is bidirectional, but designed in the expectation that most of the traffic will be GPS-to-computer, with commands going in the computer-to-GPS direction rare.
The modern trend in GPSes is away from RS232C and towards USB. USB GPSes keep the NMEA data protocol but discard the NMEA link layer. Under Linux, USB GPSes use the usbserial module and look like serial ports. Part of gpsd's job is to hide this stuff; applications don't have to be aware of NMEA or the link layer, they just query gpsd for information.
The good news about NMEA
The basic design of the NMEA data protocol is very simple. The GPS throws ASCII text lines called 'sentences', each beginning with a '$' and terminated by CR/LF, at the host machine. Usually the host gets one update a second, but the GPS has the option of sending more frequently when it detects a change of position or velocity or status. The standard prescribes a serial encoding at 4800bps, 8 bits, one stop bit, no parity; although most consumers recievers use 9600bps.
Here are some sample NMEA sentences:$GPGGA,212734,4740.0569,N,12219.6612,W,1,08,74.00,73.9,M,638.000000,M,,*6D $GPRMC,212734,A,4740.0569,N,12219.6612,W,0.000000,0.000000,020403,18.936255,E*60 $GPGSA,A,3,17,06,23,15,16,18,10,30,,,,,152.00,74.00,133.00*3F $GPGGA,212735,4740.0569,N,12219.6612,W,1,08,74.00,74.1,M,638.000000,M,,*63 $GPRMC,212735,A,4740.0569,N,12219.6612,W,0.000000,0.000000,020403,18.936255,E*61
Each sentence consists of a comma-separated fields. The first field is always a message type and the last a checksum that can be used to check for data corruption. Interpreting NMEA sentences is not complicated. Modulo a few glitches like 2-digit year numbers, the NMEA standard does a pretty good job of specifying a message set for GPSes that want to convey data to computers.
More good news: you should never have to deal with this level — gpsd's purpose is to insulate you from it.
The bad news about NMEA
That's the good news. Now for the bad news, which comes in five pieces:
First, the NMEA standard does not specify a command repertoire for the opposite direction. Thus, functions like changing the GPS's update frequency or selecting the subset of sentences for it to send are often not supported at all, and when they are it's all by sentences that are vendor-specific.
This used to be more of a problem than it is today. Early GPSes tended to have elaborate facilities for accepting lists of waypoints and sending back course information to help you navigate to them. Modern high-end units still do, but the GPSes designed for connecting to computers are increasingly designed on the assumption that the host computer will do all the waypoint geometry itself and the GPSes only job is to deliver periodic position and velocity readings. Thus, they tend to have no control codes at all. This makes them laudably stupid.
Second, vendors don't stick to the NMEA-prescribed 4800bps data rate. This is understandable; 4800 is very slow by today's standards, and by boosting bits per second they can deliver information that's fresher by a few milliseconds (which might make a difference if, say, you're using a GPS-enabled autopilot to land an aircraft). Some GPSes feature data rates upwards of 38400bps. However, this actually does little good unless the application polls the GPS at a rate faster than 1Hz rather than waiting for it, as most GPS receivers cannot be told to ship updates faster than once per second — and the polling commands (when they exist at all) are proprietary. And the fact that GPSes don't have a single data rate graven in stone brings back all the well-known baud-mismatch configuration problems we thought we'd left behind in the 1980s.
The third problem with NMEA is that it's not universal. A decreasing but still significant percentage of GPSes use proprietary binary protocols. For example, there was a GPS chipset called "Zodiac" made by Rockwell International, that used to be very widely OEMed by GPS makers. It spoke NMEA, but had irritating limitations in that mode like not being able to accept DGPS corrections. It preferred a tight-packed binary protocol. There haven't been any new Zodiac-based designs in a few years, but a lot of Zodiac-based GPSes (like the DeLorme EarthMates made before they switched over to a SiRF chipset in 2003) are still around.
2004's equivalent of the Zodiac is the SiRF-II chipset, which seems to be nearly ubiquitous in inexpensive GPS receivers. It too speaks a binary protocol, but only if you ask it to; it's fully capable in NMEA mode. Which is where it boots up. The idea seems to be that you can switch to binary to improve your bits-per-second in latency-critical applications.
The fourth problem with NMEA is that it doesn't deliver all the information that the GPS has in one atomic message. In particular, you can't get altitude (delivered in the GPGGA sentence only) and speed (delivered in GPRMC and GPVTG) at the same time. This is annoying; ideally, you want your position/velocity/time oracle to deliver one observation tuple conveying all seven degrees of freedom (t, x, y, z, vx, vy, vz) and their error estimates.
Learning more about NMEA
The final irritation about NMEA is that it's expensive to buy a description. The National Marine Electronics Association is a trade group of electronics dealers, and they want $250 for a copy of their standard. Numerous Web sources have reverse-engineered or abstracted bits of it; the NMEA page piously warns that In most cases they are very old versions or incorrect interpretations and should not be depended upon for accuracy, then mutters darkly about copyright violations.
Here is the best compendium I know of. I have never seen a copy of the official NMEA standard. Fortunately, it isn't necessary for even gpsd developers to know most of it, since most modern GPS receivers only emit about a half-dozen of the eighty or so NMEA sentences. RMC, GGA, GSA, GSV (and now possibly GBS) are all you are ever likely to need to know about.
After you've read about those sentences, it can be instructive to run gpsd in a mode something like this:./gpsd -N -n -D 2 /dev/ttyUSB0
Watching the output for thirty seconds or so will give you a good feel for what your GPS has to say, and how often it says it.
If a GPS receiver has to figure out its postition without outside assistance, it is said to be in autonomous mode.
The time required for a GPS to get a fix (Time To First Fix (TTFF)) can vary from under 15 seconds up to just under 30 minutes (actually, 29 plus calculation time). The main factors affecting this latency are (a) whether it has an almanac available, (b) whether it has satellite ephemerides available, and (c) whether it has recent fix available. Of course the quality of signal at your location matters as well.
If a GPS has not been on for several months, then it has no current almanac available. It was to wait to download one before it can generate a fix. This can take just under 15 mins. This is sometimes called an cold start.
While the almanac download takes 15 minutes, you have to be there for the start of it, otherwise you have to wait for the next cycle. So if you are unlucky and just miss the start of one, it could take just under 29 minutes to obtain, and on average closer to 22 min.
If a GPS has not been on for a day (four to six hours) then it has a valid almanac but no valid satellite ephemerides, and must download at least four before it can generate an accurate fix. This is sometimes called a warm start. Each satellite has its own ephemeris that must be downloaded if a current copy is not fresh.
GPSes store ephemerides is non-volatile memory, either internal flash storage or battery-backed SRAM. Thus, a GPS does not need to have been on continuously to have ephemerides available, but it will consider old data to be invalid after a while. In normal operation the GPS occasionally gets refreshes of ephemeris and almanac data from the satellites it's listening to.
For both an cold start and a warm start, if the sat signal is momentarily lost, the process may have to restart and you'll get more delay.
If a GPS has been on recently, in the current location, then this is sometimes called hot start and an accurate fix can be generated quite quickly. This will usually be a few seconds for a modern GPS.
If the GPS receiver can download the almanac or ephemerides, it can proceed quickly to a hot start. This is uncommon for GPSes that are connected to laptops, but is usual for GPS in mbile phones. A-GPS is variously expanded as Augmented, Assisted, or Aided.
Many GPSes are designed to power down or go to standby mode when DTR or its USB equivalent goes low (under Linux, this happens when you close the port). An important category of exceptions is USB SiRF-II GPSes; these don't seem to power down on DTR low, but instead go to a low-power standby mode for the 8/10s of every second that they're not shipping packets.
Powering down on DTR low can be a valuable power-saving measure if the GPS is (say) running off of laptop batteries in a navigation or wardriving system. Thus, you don't want to keep your GPS device open when you don't actually need information from it.
Unfortunately, this rule can collide with one of the persistent problems with GPSes — though they can update a previous fix quickly (in 0.1sec or less), they can take a long time to acquire a first fix when they power up.
When a GPS receiver boots up, it has to suck radio waves for a while just to figure out what satellites might be available in line of sight. The speed at which it can do this is inversely proportional to the number of GPS channels it can sample simultaneously. Older one- or two-channel units could take several minutes at this. In 2004, even low-end GPS receivers have twelve channels and can thus cock a separate ear for as many satellites as they're ever likely to see. Even so, it's not uncommon for them to take 30 or 40 seconds after a cold boot to get a fix.
One of the things gpsd does for applications is handle this power-management issue. When no clients are active, gpsd will automatically close the GPS device, re-opening it only when another client connects to the daemon.
For more details on programming with gpsd, see the FAQ.How does GPS work? This is where the orbital simulation came from. Wikipedia on GPS Good introduction with much more on the history of the system. GPS Hackery Chris Kuethe's page has links to many interesting resources. GPS interfaces and software Linux and open-source resources for working with GPSes.