L5 gps


В 2018 году в смартфонах появятся сверхточные чипы глобальной навигации / Geektimes

Компания Broadcom порадовала новостью о разработке первого на коммерческом рынке GNSS-ресивера двойной частоты (L1 и L5) — чипа BCM47755, который будет доступен для производителей телефонов в 2018 году. Первые образцы чипа готовы, а сейчас компания готовится наладить массовое производство.

В сегодняшних ресиверах точность приёма сигнала GPS всего 5 метров, что иногда приводит к неловким ситуациям. Например, GPS-навигатор в автомобиле может неправильно определить, когда вы уже проехали поворот, и дать неправильную рекомендацию. Новые чипы обеспечивают точность 30 см. Что не менее важно, эти ресиверы будут лучше ловить сигнал в сложных условиях, например, на городских улицах поблизости от высоких зданий. И напоследок, они потребляют вдвое меньше энергии, чем микросхемы нынешнего поколения.

BCM47755 уже включён в дизайн нескольких моделей смартфонов, предназначенных для выпуска в 2018 года, но Broadcom не говорит, каких именно. Ресивер способен одновременно принимать следующие сигналы систем глобальной навигации (GNSS):

  • GPS L1 C/A
  • GLONASS L1
  • BeiDou (BDS) B1
  • QZSS L1
  • Galileo (GAL) E1
  • GPS L5
  • Galileo E5a
  • QZSS L5
Помимо GPS, поддерживаются и европейская Galileo, и японская QZSS, и российский ГЛОНАСС.

За счёт чего удалось улучшить качество приёма в городе? Дело в том, что все спутники GPSS, даже самого старого поколения, передают сигнал L1, который содержит координаты спутника, точное время и идентификатор. Однако новое поколение спутников передают не только L1, но и более сложный сигнал L5 на другой частоте, которая отличается от стандартного сигнала L1. До недавнего времени на орбите было недостаточно спутников L5, чтобы их действительно можно было использовать на практике. Но в 2015 и 2016 году запустили достаточно таких спутников, и сейчас их около 30, учитывая те, которые висят только над Японией и Австралией. Но всё-таки сейчас даже в узком окне неба в городских условиях можно наконец-то увидеть шесть или семь таких спутников, говорит представитель Broadcom. Поэтому сейчас наступил момент, когда можно выпускать ресивер нового поколения с повышенной точностью, работающий с сигналом L5 (спутники следующего поколения и вовсе обеспечат сантиметровую точность).

Микросхема BCM47755 сначала фиксируется на спутнике по сигналу L1, а потом уточняет рассчитанную позицию по сигналу L5. Частота последнего более предпочтительна для сложных городских условий, потому что этот сигнал менее подвержен искажениям от многочисленных отражений.

В городе ресивер получает одновременно и сигнал напрямую от спутника, и его отражения от зданий. То есть он получает несколько одинаковых сигналов немного в разное время, за счёт чего формируется своеобразный сигнальный блоб. Ресивер ищет сигнал максимальной силы, чтобы зафиксировать время приёма, но если сигналы частично накладываются друг на друга, то вычисления получаются не очень точными. Так вот, сигналы L5 настолько короткие, что практически невероятно, чтобы отражения смешались с оригинальным сигналом. Чип Broadcom дополнительно использует фазу несущего сигнала, чтобы ещё более увеличить точность, объясняет журнал IEEE Spectrum.

На самом деле на рынке уже есть системы, которые используют сигнал L5 и повышенную точность GNSS, но это обычно промышленные системы, они используются, например, в нефтедобыче. Чип BCM47755 станет первой массовой микросхемой, которая одновременно принимает и L1, и L5.

На диаграмме показывается количество спутников нового поколения, передающих сигнал L5 и схематично объясняется, зачем ресиверу принимать сигнал на двух частотах L1 и L5

В новой микросхеме Broadcom реализовано несколько инноваций, в том числе новая архитектура с применением дизайна big.LITTLE от ARM. Это двухпроцессорная архитектура, где у одного CPU меньшая производительность и меньшее энергопотребление, а другой процессор больше и мощнее. В данном случае это процессоры Cortex M-0 и Cortex M-4.

Дополнительную информацию о BCM47755 расскажут на конференции ION GNSS+ 2017, которая состоится 27 сентября 2017 года.

geektimes.ru

GPS.gov: New Civil Signals

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A major focus of the GPS modernization program is the addition of new navigation signals to the satellite constellation.

The government is in the process of fielding three new signals designed for civilian use: L2C, L5, and L1C. The legacy civil signal, called L1 C/A or C/A at L1, will continue broadcasting, for a total of four civil GPS signals. Users must upgrade their equipment to benefit from the new signals.

The new civil signals are phasing in incrementally as the Air Force launches new GPS satellites to replace older ones. Most of the new signals will be of limited use until they are broadcast from 18 to 24 satellites.

Second Civil Signal: L2C

L2C is the second civilian GPS signal, designed specifically to meet commercial needs.

Its name refers to the radio frequency used by the signal (1227 MHz, or L2) and the fact that it is for civilian use. There are also two military signals at the L2 frequency.

When combined with L1 C/A in a dual-frequency receiver, L2C enables ionospheric correction, a technique that boosts accuracy. Civilians with dual-frequency GPS receivers enjoy the same accuracy as the military (or better).

For professional users with existing dual-frequency operations, L2C enables faster signal acquisition, enhanced reliability, and greater operating range.

L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it easier to receive under trees and even indoors.

The Commerce Department estimates L2C could generate $5.8 billion in economic productivity benefits through 2030.

The first GPS satellite featuring L2C launched in 2005. Every GPS satellite fielded since then has included an L2C transmitter.

In April 2014, the Air Force began broadcasting civil navigation (CNAV) messages on the L2C signal. However, L2C remains pre-operational and should be employed at the user's own risk until it is declared operational.

Related Links:

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Third Civil Signal: L5

L5 is the third civilian GPS signal, designed to meet demanding requirements for safety-of-life transportation and other high-performance applications.

Its name refers to the U.S. designation for the radio frequency used by the signal (1176 MHz).

L5 is broadcast in a radio band reserved exclusively for aviation safety services. It features higher power, greater bandwidth, and an advanced signal design.

Future aircraft will use L5 in combination with L1 C/A to improve accuracy (via ionospheric correction) and robustness (via signal redundancy).

In addition to enhancing safety, L5 use will increase capacity and fuel efficiency within U.S. airspace, railroads, waterways, and highways.

Beyond transportation, L5 will provide users worldwide with the most advanced civilian GPS signal. When used in combination with L1 C/A and L2C, L5 will provide a highly robust service. Through a technique called trilaning, the use of three GPS frequencies may enable sub-meter accuracy without augmentations, and very long range operations with augmentations.

In 2009, the Air Force successfully broadcast an experimental L5 signal on the GPS IIR-20(M) satellite. The first GPS IIF satellite with a full L5 transmitter launched in May 2010.

In April 2014, the Air Force began broadcasting civil navigation (CNAV) messages on the L5 signal. However, L5 remains pre-operational and should be employed at the user's own risk until it is declared operational.

Related Links:

  • CNAV Message
  • Jun 2010: News Release on First L5 Transmission from GPS IIF Satellite (af.mil)
  • Apr 2009: News Release on L5 Demo Signal from GPS IIR-20(M) Satellite (af.mil)

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Fourth Civil Signal: L1C

L1C is the fourth civilian GPS signal, designed to enable interoperability between GPS and international satellite navigation systems.

Its name refers to the radio frequency used by the signal (1575 MHz, or L1) and the fact that it is for civilian use. There are also two military signals at L1, as well as the legacy C/A signal. L1C should not be confused with L1 C/A.

L1C features a Multiplexed Binary Offset Carrier (MBOC) modulation scheme that enables international cooperation while protecting U.S. national security interests. The design will improve mobile GPS reception in cities and other challenging environments.

The United States and Europe originally developed L1C as a common civil signal for GPS and Galileo. Japan's Quasi-Zenith Satellite System (QZSS) and China's BeiDou system are also adopting L1C-like signals.

The United States will launch its first L1C signal with GPS III. L1C will broadcast at the same frequency as the original L1 C/A signal, which will be retained for backwards compatibility.

Related Links:

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Codeless/Semi-Codeless Transition Plan

Once L2C and L5 are fully operational, their features will obviate the need for codeless or semi-codeless GPS receivers, which many GPS professionals use today to attain very high accuracy. Such receivers work by exploiting characteristics of the encrypted military P(Y) signal at the L2 frequency to achieve dual-frequency capability.

The U.S. government encourages all users of codeless/semi-codeless GPS technology to start their planning for transition to the modernized civil signals.Learn more

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www.gps.gov

Innovation: GPS L5 First Light : GPS World

A Preliminary Analysis of SVN49’s Demonstration Signal

By Michael Meurer, Stefan Erker, Steffen Thölert, Oliver Montenbruck, André Hauschild, and Richard B. Langley

Great excitement surrounds the activation of a new transmitter from a satellite — an occasion dubbed first light. Research groups around the globe joined the GPS Wing in monitoring and analyzing the first L5 signals from space. We describe the equipment and procedures used to capture and analyze SVN49’s signals and give an assessment of their characteristics.

INNOVATION INSIGHTS with Richard Langley

ON APRIL 10, a new type of radio signal was transmitted from space. I am referring, of course, to the L5 demonstration signal from the Block IIR-M satellite SVN49, launched on March 24. The L5 signal, the second of two new civil GPS signals, will be standard on the next generation of GPS satellites — the Block IIFs — and its frequency band was duly registered with the International Telecommunication Union (ITU) back in 2002. But satellite operators only have seven years after filing a frequency application to start transmitting signals from the designated orbit, and delays in launching the first Block IIF satellite meant that GPS could lose the allocation. The GPS Wing and its contractors determined that the best way to secure the L5 frequency was to add an L5 demonstration payload to one of the remaining modernized Block IIR satellites. And so SVN49 made history with the inaugural broadcast of L5 with just a few months to spare before the clock ran out on the ITU filing.

Great excitement always surrounds the first photons captured by a new telescope or other detectors of electromagnetic signals. Or when a transmitter is activated for the first time. Just as we do for the dawning of a new day, we call this occasion first light. Research groups around the globe joined the GPS Wing in monitoring and analyzing the first L5 signals from space, including a group of scientists and engineers from Germany and Canada. This month the group describes the equipment and procedures used to capture and analyze SVN49’s signals and gives an assessment of their characteristics.

“Innovation” features discussions about advances in GPS technology and applications as well as fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering, University of New Brunswick. To contact him, see “Contributing Editors.”

A key feature of GPS modernization is the addition of the L5 civil signal to the suite of signals transmitted by the satellites. The introduction of such a signal on a different carrier frequency than that used by the legacy L1 GPS signal was proposed in the 1995 reports by the U.S. National Research Council and the National Academy of Public Administration on the future of GPS. The reports argued that an unencrypted signal on a second frequency would offer civil users the benefit of ionospheric delay correction, wide-lane carrier-phase ambiguity resolution, improved interference rejection, and faster accuracy recovery in multipath environments.

Studies showed that it would be possible to add a civil signal on the L2 frequency without compromising the military signal. High-precision (and accuracy) civil users had been using the L2 frequency — initially designated for military use only — ever since the first GPS satellites were launched, and through clever (though suboptimum) tracking techniques even after the L2 signals were encrypted. An unencrypted signal on L2 would bring these users a more robust signal as well as affording all civil users the benefits of a second frequency. But unlike the L1 signal, the L2 signal is situated in a part of the radio spectrum not officially protected from interference by other users of the spectrum. So such a second civil signal could not be used for safety-of-life applications such as navigating aircraft.

So, in Vice President Al Gore’s statement of March 30, 1998, on the enhancement of GPS for civil users, the decision to deploy two new civil signals was announced: the civil signal on L2, now known as L2C, and a signal on a new frequency, which became known as L5. Some readers might wonder why this new signal was not designated L3 or L4. Those designations had already been assigned to signals associated with other payloads on the GPS satellites.

Although the Gore announcement proposed to introduce both of the new civil signals with the launch of the Block IIF satellites, the addition of the L2C signal to the legacy signals was deemed a relatively straightforward task and the decision was made to modify the last eight Block IIR satellites for the provision of L2C. The first modernized Block IIR satellite was launched on September 26, 2005, and seven of these satellites are now in orbit.

The frequency selected for the L5 signal, 1176.45 MHz, is in a protected aeronautical radionavigation services (ARNS) band. This frequency, as with frequencies used by all satellite operators, had to be coordinated with the International Telecommunication Union-Radiocommunication Sector (ITU-R). The ITU-R registers frequencies essentially on a first-come, first-served basis, but a user must actually transmit signals on the assigned frequency from the designated satellite orbit type within seven years from the date of filing with ITU-R. This meant that L5 signals had to be transmitted before August 26, 2009, to avoid the potential claim of the frequency by a different country. A decision was made to modify an existing Block IIR-M satellite to carry an L5 demonstration payload. The L5 demo payload, which was developed by Lockheed Martin and its subcontractors, was added to space vehicle number (SVN) 49. SVN49 was launched on March 24, 2009, the seventh modernized Block IIR satellite to be placed in orbit. Also known as PRN1, from the primary pseudorandom noise (PRN) codes assigned to the satellite, the satellite began L5 transmissions on April 10, at 11:58 UTC, and so satisfied the ITU-R filing requirement with a few months to spare.

The L5 Signal Structure

The structure of the future full L5 signal will differ significantly from the legacy L1 signal or even the modernized L2C signal. It is fully described in the Navstar GPS L5 interface document, IS-GPS-705. We present just a brief overview of the signal here.

Two-Component Signal. The full L5 signal will offer two signal components: one with and one without a superimposed navigation data message. The two signal components — in-phase (I) and quadrature (Q) — have equal power. Both will have a minimum received power of –157 dBW. Each component is modulated with a different, but synchronized, L5 PRN code. The in-phase component (the I- or data channel) is further modulated with a 100-symbol per second (sps) symbol stream carrying the navigation message data, and the quadrature component (the Q- or data-free channel, also called the pilot channel) is modulated only with a PRN code. Different, nearly orthogonal PRN codes (referred to as I5 and Q5) are used in the two components to prevent tracking biases by making each component completely independent of the other, except for the underlying carrier phase.

Another novel aspect of the L5 signal design is the use of Neuman-Hoffman (NH) synchronization codes.

Code Structure. As previously mentioned, the I5 and Q5 channels are modulated with different PRN codes. These codes differ significantly from the C/A-, P-, and L2C-codes used on L1 and L2 both in length and chipping rate.

The natural code chipping-rate frequency of 10.23 MHz as provided by the SV atomic frequency standards satisfies a number of requirements for a modernized signal within the bandwidth constraints — increased bandwidth efficiency, improved signal accuracy, immunity to waveform distortion, and improved rejection of narrowband interference. The bandwidth constraints include rejection of out-of-band interference. Accordingly, a 10.23 megachip per second (Mcps) chipping rate, 10 times that of the C/A- and L2C-codes, was adopted for the L5 PRN codes.

Improved Cross-Correlation. There is a trade off between code period and the capability to do direct acquisition. A longer code period provides better cross-correlation properties, but takes longer to search. However, one can speed up an acquisition to some extent with lower code cross-correlation levels.

The L1 C/A-code period is 1023 chips, or 1 millisecond. The desire to maintain that epoch rate of 1 kHz with the 10.23 Mcps chipping rate results in a code period of 10,230 chips. For both the I5 and Q5 ranging codes, the 1-millisecond sequences are the modulo-2 sum of two sub-sequences referred to as XA and XB with lengths of 8,190 and 8,191 chips, respectively. The same XA sequence is used for both I5 and IQ, whereas the XB sequence for I5 is different from that for Q5. The XB sequences are selectively advanced to produce different 1-millisecond-long code sequences. In this way, a large number of unique codes can be generated. Thirty-seven primary code pairs have been designated, of which 32 are reserved for use by GPS satellites (PRNs 1–32). An additional 173 pairs have been defined (PRNs 38–210). PRN sequences 38 through 63 are reserved for satellites.

Demo Signal Verification

The L5 signal transmitted by SVN49 contains only the dataless quadrature component modulated with the PRN63 Q5 sequence. Furthermore, the transmitted L5 signal power and the satellite antenna radiation pattern are different from those expected for the L5 signals to be transmitted by the Block IIF satellites as described in the L5 interface specification.

Over the past few weeks, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt or DLR) has monitored SVN49 using its GNSS verification and analysis facility. The core element of the facility is a 30-meter dish antenna at Weilheim, near Munich, Germany, and is shown in FIGURE 1. The antenna, which is based on a shaped Cassegrain system, has a 30-meter-diameter parabolic reflector and a hyperbolic sub-reflector with a diameter of 4 meters. The L-Band gain of this high-gain antenna is around 50 dB, with a beam width of less than 0.5°. The position accuracy in both azimuth and elevation directions is 0.001°. The antenna’s maximum slewing speeds are 1.5° per second in azimuth and 1.0° per second in elevation angle, allowing it to easily track MEO satellites.

FIGURE 1. GNSS verification and analysis facility with 30-meter high-gain antenna at Weilheim, Germany.

In September 2005, DLR’s Institute of Communications and Navigation established an independent monitoring station for the analysis of GNSS signals using this powerful instrument. For the new challenge, the antenna was adapted to the requirements in the navigation field. A newly developed broadband circularly polarized feed and a new receiving chain including an online calibration system were installed at the antenna during preparations for the GIOVE-B in-orbit test campaign in the spring of 2008.

During this time, intensive work on the system calibration was performed using well-known signals from radio “stars” and EGNOS satellites for the antenna gain determination, and sophisticated calibration methods for the receiving system. The calibration provides an absolute measurement uncertainty significantly less than 1 dB.

Due to the distance of the antenna location from the institute at Oberpfaffenhofen (around 40 kilometers), it was necessary to perform all measurement and calibration procedures during the measurement campaigns under remote control. A software tool was developed that can control any component of the setup remotely. In addition, this tool is able to perform a completely autonomous operation of the whole system by a pre-definable sequence over any period of time. Additional details about the GNSS verification and analysis facility and the calibration techniques used can be found in the literature cited in Further Reading.

A detailed signal-in-space (SIS) analysis of the new L5 signal transmitted by SVN49 was conducted by recording several passes with the GNSS verification and analysis facility. A high elevation-angle transit of SVN49 every night allows a long observation time for each satellite pass. To ensure precise tracking of the satellite with the high-gain antenna, we used the latest two-line element sets from the U.S. Air Force Space Command.

The first signals transmitted by the satellite on the L5 frequency were captured during the pass on April 10. Compared to later measurements, the power of the L5 payload signal was measured with a lower output level on this first pass. This points to a power “fade in,” which is a common procedure in commissioning a new satellite payload. A controlled and slow heating of the payload elements avoids possible damage caused by the out-gassing of the power amplifiers, for example.

The SIS analyses that we performed using the high-gain antenna will be described for one example satellite pass recorded on April 29. During this pass, the satellite reached an elevation angle of around 80° and was visible for about seven hours (see FIGURE 2). A set of spectral snapshots as well as time sample records for the L1, L2, and L5 signals were processed and adjusted with the corresponding calibration values during a post-processing stage.

FIGURE 2. Skyplot of SVN49 pass at Weilheim, Germany, on April 29, 2009.

Time and Frequency. A first view of the captured spectrum snapshot in FIGURE 3 shows the L5 signal and its typical binary phase-shift-keyed (BPSK) spectral shape. The signal is significantly band limited by the used front-end filters of the satellite’s L5 payload. This ensures the required spectral separation from the adjacent L2 signal of the satellite, as the L5 signal must not interfere with the operational L2 frequency. Overlaying the theoretical spectral mask of the L5 BPSK signal, we note a slight asymmetry of the spectral shape. The two side lobes differ around 2.5 dB in their peak power level (see Figure 3). Spectral asymmetries of that kind typically result from frequency selectivity in the RF transmitter chains in satellite payloads, including the amplifiers and antennas.

FIGURE  3. L5 spectrum plot from data recorded on April 29.

FIGURE 4 shows a temporal snapshot of the L5 signal after wiping off the Doppler frequency shift due to satellite orbital motion. Figure 4 (left) depicts a snapshot of 10 microseconds for the I and Q channels. It can be seen, that in compliance with the requirements of the L5 signal explained in the introduction of this article, the signal is a bi-level signal with a chipping rate of 10.23 Mcps. Plotting the normalized histogram of the L5 signal, one obtains the normalized I/Q probability density function (PDF) diagram of the L5 signal shown in Figure 4 (right). The constellation diagram shows a remaining deformation of the Q component after Doppler removal. Although the L5 signal transmitted by the test payload only contains the dataless Q5 component, a non-negligible contribution can be seen in the I channel. This slight distortion may stem from a nonlinear and frequency-dependent amplification of the Q baseband signal leading to crosstalk between the Q and I channels.

FIGURE  4. (left) L5 I and Q time samples; (right) L5 I/Q probability density function (PDF).

Signal Code Sequence. With the use of the high-gain antenna, it is possible to look in detail at the transmitted L5 code chips. The signal is raised high above the noise floor and, after Doppler wipe off, allows us to compare the received code sequence with the theoretical code sequence for the PRN63 Q channel. FIGURE 5 shows an example for the first 10 microseconds of the code — both for the measured L5 signal and the expected theoretical code. The analysis performed also for several full code periods shows that the demo payload’s Q5 code structure is in full compliance with the “theoretical” code described in the official signal interface document.

FIGURE  5. Comparison of measured and theoretical code sequences.

Power of Received Signals. The GNSS verification and analysis facility is fully calibrated, allowing highly accurate absolute measurements of GNSS signal power levels. We have used the system to evaluate the SVN49 signal power levels as received on the ground. FIGURE 6 shows the different signals transmitted in the L1, L2, and L5 frequency bands in terms of the received power per square meter versus elevation angle of the SV during its pass. It can be seen that there is a significant elevation-angle dependency of the L5 received power (about 18 dB between low and high elevation angles) compared to L1 and L2 (with a variation of about 3 dB). In this measurement, the combined power of the I and Q channels is plotted for the signals. So this means that the L1 and L2 signal measurements include the power of the C/A-, P(Y)-, and M-codes. Such a strong elevation-angle dependency is not typical of signals radiated by GPS satellites. However, the L5 signal is radiated using the legacy L1/L2 Block IIR-M satellite antenna, which is to the authors’ knowledge not optimized for the L5 frequency.

FIGURE  6. Absolute received power for SVN49 L1, L2, and L5 signals on April 29, 2009.

In the spectrogram plot of FIGURE 7, which was generated by plotting all recorded L5 spectra versus elevation angle, the impact of this elevation-angle dependency of the received power can be detailed for the complete frequency range. The side lobes of the BPSK signal are only clearly visible in the spectrogram at higher elevation angles.

FIGURE  7. Spectrogram for L5 signal received on April 29, 2009.

Signal Tracking

In parallel with the detailed signal validation using the high-gain antenna and vector signal analyzer, an effort has also been made to track the new GPS L5 signal using conventional correlating GNSS receivers. Given the relevance of L5/E5 signals for future aeronautical applications and the ongoing transmission of such signals from the GIOVE satellites, a growing number of commercial receiver manufacturers have announced receivers supporting this frequency band. However, due to the special nature of the SVN49 test signal (pilot only, with different PRN code designations on L1 and L5) some modifications to receiver software are required to properly track the first GPS L5 signal. In particular, the use of different PRN code designations employed for L1/L2 (PRN1) and L5 (PRN63) is clearly non-standard and requires suitably adapted receiver software, which was provided by the makers of the two receiver types we selected for our test campaign.

Receiver type N is a highly configurable test receiver for L1 and L5/E5a signals developed as part of the Galileo program. It offers a total of 16 tracking channels, which are implemented in a field-programmable gate array and can thus be flexibly adapted for tracking of civil GPS, satellite-based augmentation systems, and the GIOVE-A and -B signals in their respective frequency bands. Receiver type J, in contrast, represents the latest generation of geodetic grade multi-constellation receivers. It uses an advanced application-specific integrated circuit with 216 tracking channels supporting all types of non-military navigation signals in the L1/E1, L2, and L5/E5a bands. Both receivers have been used for some time prior to the launch of SVN49 to track GPS and GIOVE satellites from stations at the University of New Brunswick (UNB) in Canada and at DLR in Germany.

The first measurements of GPS L5 were successfully collected on April 10 with a type N receiver at UNB. While these measurements confirmed the capability to properly track SVN49 in the L5 band, they already revealed a distinct aspect of the GPS L5 test signal that potential users must be aware of. The signal is much weaker at low elevation angles than the L1 signal. Normal carrier-to-noise-density ratios (C/N0) are only achieved at elevation angles of about 60° and higher. On the other hand, the measured C/N0 near zenith may even outperform that of L1 and L2 tracking with sufficient L5 antenna gain. For illustration, FIGURE 8 compares the measured C/N0 values of GPS and GIOVE-A/B signals as obtained with receiver type J and a geodetic antenna at DLR, Oberpfaffenhofen.

FIGURE  8. Comparison of the relative signals strength (expressed as carrier-to-noise-density ratio, C/N0) for GPS (left) and GIOVE-A/B signals (right). The signals are described by their respective RINEX 3.00 data format identifiers, which reflect the type of measurement (S=signal strength), the frequency band (1=L1/E1, 2=L2, 5=L5/E5a) and the signal attribute (C=C/A or L2C, W=P(Y) semicodeless, X=pilot and data).

While not officially confirmed so far, the abnormal variation of the L5 signal strength can best be attributed to a non-standard gain pattern of the satellite transmitter antenna. Apparently, the existing Block IIR-M satellite antenna “farm” has been used to transmit the L5 signal, which results in more directivity than that of the L1 and L2 signals. This results in a weaker signal for receivers further away from the antenna boresight axis, or, equivalently, stations observing the satellite at low elevation angles. Even though the achieved C/N0 of the GPS L5 test signal is lower than that of the direct L1 C/A-code and L2 L2C-code tracking for most of a tracking arc, the signal quality still exceeds that of the semicodeless P(Y)-code tracking on L1 and L2. This makes the signal a valuable basis for experimentation in aviation applications or triple-frequency processing.

To assess the quality of the raw GPS measurements, we made use of the so-called multipath combination of pseudorange and carrier-phase measurements:

The combination is essentially the difference between the pseudorange (P C5) and carrier-phase measurement (ΦL5) on the L5 frequency, and therefore measures the sum of the pseudorange multipath (M) and noise (ε). Due to the opposite sign of ionospheric path delays on code and phase measurements, an ionospheric correction is used in the multipath combination, which requires phase measurements on a second frequency (in this case L1). The individual carrier-phase biases are, furthermore, aggregated into a common bias (b). Other than in a traditional zero-baseline test, the multipath combination neither requires a second receiver nor a second satellite transmitting the same signal in space. It is therefore best suited for studying the tracking performance of the new GPS L5 test signal.

Results for receiver types N and J obtained at DLR, Oberpfaffenhofen, are shown in FIGURE 9 for a sample, high-elevation angle tracking pass. Despite obvious differences that can be related to the specific multipath environment and code-smoothing strategies for the two receivers, a high quality is obtained in both cases. For the central three-hour interval, during which the L5 signal was received with normal signal strength, the achieved tracking accuracy clearly outperforms that of the L1 C/A-code signal for the given receivers. For further comparison, FIGURE 10 shows sample results of GIOVE-B E5a tracking with receiver type J. Again, the GPS L5 signal at medium- to high-elevation angles is fully competitive and a notable degradation is only evident when the signal strength is well below the values to be expected in the future operational system.

FIGURE  9. Pseudorange multipath and receiver noise of SVN49 (PRN G01) L5 tracking for a selected pass over Oberpfaffenhofen, Germany, on April 29-30, 2009. Top: receiver type J with geodetic antenna. Bottom: receiver type N with a Galileo antenna. The satellite exceeded an elevation angle of 50° between 20:30 and 23:30 with a peak elevation angle of 80° near 22:00.

FIGURE 10. Pseudorange multipath and receiver noise of GIOVE-B L5 tracking for a high pass over Oberpfaffenhofen, Germany, on April 17, 2009, using receiver type J.

Legacy Signal Anomaly. While the GPS L5 signal transmission by SVN49 is clearly designated as experimental, the legacy signals (that is, the C/A- and P(Y)-code on L1 as well as L2C- and P(Y)-code on L2) were expected to achieve the same level of performance as observed on other satellites of the existing constellation. This is not the case, however, in the L1 band where both the C/A-code measurements and the semicodeless P(Y)-code pseudoranges exhibit a systematic, elevation-angle-dependent bias. This bias is not specific to any of our test receivers and can be similarly observed in heritage receivers employed at the stations of the International GNSS Service (IGS). As an example, FIGURE 11 illustrates the variation of the C/A-code error for high-elevation angle passes of SVN49 over western Canada and Germany. The bias varies between approximately -0.5 meters near the horizon and 1meter near zenith.

The cause of the bias is unclear but resides apparently in the design of the transmitter antenna or signal generation chain. It is exclusively seen on SVN49 and not on other GPS (or GIOVE) satellites, which excludes a possible problem of the receiver antenna or environment. Furthermore, data collected at UNB using the UNBJ IGS station a few days after launch clearly demonstrate that the elevation-angle-dependent L1 bias existed well before L5 signal activation and therefore might not be related to the signal generator. It is unclear to what extent the L1 signal bias can be corrected on the spacecraft and how it will affect the declaration of SVN49 as a fully healthy satellite.

 

FIGURE 11. Pseudorange errors of SVN49 L1 C/A code tracking for high-elevation-angle passes using a type A receiver at IGS station DRAO in Penticton, Canada (top), and a type J receiver at Oberpfaffenhofen (bottom). The satellite achieved peak elevation angles of about 70° and 80°, respectively, at the two sites.

Conclusions

Tracking and analysis of SVN49’s L5 signal using both the 30-meter dish and code-correlating receivers reveals that it possesses improved signal characteristics with respect to the legacy signals, in particular with regard to its bandwidth, and therefore will allow even more accurate and reliable positioning when the signal is deployed on the future Block IIF constellation.

Acknowledgments

We thank NovAtel and JAVAD GNSS for supplying special firmware, Sébastien Carcanague at UNB, and DLR colleagues at Weilheim for their help. The L5 signal description comes from the Innovation article by A.J. Van Dierendonck and C. Hegarty, September 2000 issue of GPS World.

Manufacturers

Receiver N is the NovAtel (www.novatel.com) EuroPak-15a. Receiver J is the JAVAD GNSS (www.javad.com) Triumph Delta-G2T. Receiver A is an Allen Osborne Associates (AOA) Benchmark ACT (www.itt.com). Space Engineering (www.space.it) Galileo Experimental Sensor Station antenna, Trimble (www.trimble.com) Zephyr Geodetic II antenna, and AOA D/MT antennas were used.

MICHAEL MEURER received a Ph.D. in electrical engineering from the University of Kaiserslautern, Germany. He is director of the Department for Navigation in the Institute for Communications and Navigation of the German Aerospace Center (DLR).

STEFAN ERKER received his diploma degree in information technology from the Technical University of Kaiserslautern and works at DLR’s Institute for Communications and Navigation.

STEFFEN THÖLERT received his diploma degree in electrical engineering from the University of Magdeburg and works at DLR.

OLIVER MONTENBRUCK works at DLR’s German Space Operations Center, Oberpfaffenhofen, where he is head of the GPS Technology and Navigation Group. He holds a Dr.rer.nat degree in physics.

ANDRÉ HAUSCHILD received his diploma degree in mechanical engineering from the Technical University of Braunschweig, Germany, and is a Ph.D. candidate at DLR’s German Space Operations Center.

Further Reading

L5 Signal DetailsInterface Specification, IS-GPS-705 (IRN-705-003), Navstar GPS Space Segment/User Segment L5 Interfaces, ARINC Engineering Services, LLC, El Segundo, California, September 22, 2005.“The New L5 Civil GPS Signal” by A.J. Van Dierendonck and C. Hegarty in GPS World, Vol. 11, No.9, September 2000, pp. 64–72.

DLR’s GNSS Verification and Analysis Facility“GNSS Signal Verification: Spectral and Temporal Analysis of GIOVE B and BEIDOU Signals” by S. Thölert, S. Erker, M. Cuntz, M. Meurer, U. Grunert, and J. Furthner, presented at Navitec 2008, the 4th ESA Workshop on Satellite Navigation User Equipment Technologies, Noordwijk, The Netherlands, December 10–12, 2008.“GNSS Signal Verification with a High Gain Antenna – Calibration Strategies and High Quality Signal Assessment” by S. Thölert, S. Erker, and M. Meurer in Proceedings of ITM 2009, the 2009 International Technical Meeting of The Institute of Navigation, Anaheim, California, January 26–28, 2009, pp. 289-300.

Nonlinearities in Microwave Signal Components“Frequency-independent and Frequency Dependent Nonlinear Models of TWT Amplifiers” by A. Saleh in IEEE Transactions on Communications, Vol. 29, November 1981, pp. 1715–1720.“Analysis of GIOVE-A L1-Signals” by S. Graf and C. Günther in Proceedings of ION GNSS 2006, the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation, Fort Worth, Texas, September 26-29, 2006, pp. 1560–1566.

Commercial GNSS Receivers Used for L5 Signal Acquisition“Triumph Technology” by J. Ashjaee presented at the 5th Allsat Open Conference, Hannover, Germany, June 19, 2008.“A Dual-frequency L1/E5a Galileo Test Receiver” by N. Gerein, M. Olynik, M. Clayton, J. Auld, and T. Murfin in Proceedings of the European Navigation Conference – GNSS 2005, Munich, Germany, July 19-22, 2005.

The Multipath Observable“TEQC: The Multi-Purpose Toolkit for GPS/GLONASS Data” by L.H. Estey and C.M. Meertens in GPS Solutions, Vol. 3, No. 1, 1999, pp. 42–49.

1995 Reports on the Future of GPSThe Global Positioning System: Charting the Future: Charting the Future by a panel of the National Academy of Public Administration and by a committee of the National Research Council, National Academy of Public Administration, Washington, D.C., 1995, ISBN 0-9646874-1-0.The Global Positioning System: A Shared National Asset, Recommendations for Technical Improvements and Enhancements by the National Research Council Committee on the Future of the Global Positioning System, National Academy Press, Washington, D.C., 1995, ISBN 0-309-05283-1.

The Seminal Article on the Benefits of Three GPS Signal Frequencies“The Promise of a Third Frequency” by R.R. Hatch in GPS World, Vol. 7, No. 5, May 1996, pp. 55–58.

gpsworld.com

Simulation and Performance Evaluations of the New GPS L5 and L1 Signals

Department of Electronic Engineering, International Islamic University, H-10, Islamabad 44000, Pakistan

Received 27 June 2016; Revised 19 November 2016; Accepted 5 December 2016; Published 17 January 2017

Copyright © 2017 Tahir Saleem et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The Global Positioning System (GPS) signals are used for navigation and positioning purposes by a diverse set of users. As a part of GPS modernization effort L5 has been recently introduced for better accuracy and availability service. This paper intends to study and simulate the GPS L1/L5 signal in order to fulfill the following two objectives. The first aim is to point out some important features/differences between current L1 (whose characteristics have been fairly known and documented) and new L5 GPS signal for performance evaluation purpose. The second aim is to facilitate receiver development, which will be designed and assembled later for the actual acquisition of GPS data. Simulation has been carried out for evaluation of correlation properties and link budgeting for both L1 and L5 signals. The necessary programming is performed in Matlab.

1. Introduction

The GPS (Global Positioning System) is satellite system operated by the United States of America (USA) defense department. Its services (Location, Navigation, and Time) can be accessed all the time by anyone having necessary GPS receiver. The GPS system has total 32 satellites out of which 24 satellites are operational. These operational satellites are arranged in 6 orbits. GPS satellites being connected to ground stations revolve around the earth with a distance of 20,000 km from the surface of earth.

Initially GPS had started its operation with two signals L1 and L2. L1 is transmitted at 1575.42 MHz frequency and L2 at 1227.60 MHz. These GPS signals include two ranging codes. C/A (Carrier Acquisition) code and P (Y) or Precision code. The first code is used for civilian purpose, while the second one is restricted to military use only. These ranges codes are utilized for measurement of distance to the satellite as well as identifying uniquely the navigation message [1].

Although the GPS system has almost reached its full operational capability, due to the increasing demand for better service and advances in technology, modernization and implementation of a new GPS system have recently started. The addition of L5 GPS signal is one of the modernization efforts being taken by US Department of Defense. L5 signal which is being transmitted on 1176.45 MHz frequency is also known as Safety of Life signal in the GPS community. With higher transmission power and improved signal design as compared to other GPS signals (L1 or L2) it is believed that L5 will enhance the existing performance of the GPS system. Due to wide bandwidth and comparatively longer spreading codes, the L5 signal is expected to give a high processing gain. For L5 signal transmission the Aeronautical Radio Navigation Service frequency band has been reserved which is easily accessible around the world. One of the unique features in proposed L5 signal is the inclusion of both separate carrier and quadrature data modulation component. Separate PN codes are used for the modulation of both components with PN chip clock rates of 10.23 Mcps and periods of 10230 chips or 1 ms [2–4].

In this paper we tried to point out some important features like power levels, data encoding, correlation (auto and cross), and power budget analysis between L1 and L5 signal. Both L1 and L5 signals have been simulated and comparison is done on the basis of obtained results.

2. Performance Evaluation Parameters
2.1. Autocorrelation

In satellite navigation applications, autocorrelation function has great importance. It basically refers to the integration and multiplication of a signal with its delayed copy. The general formula for the autocorrelation function as given in [1, 5] can be written aswhere represents signal with time for th satellite, is time period, and is delay in time.

The autocorrelation properties are utilized to detect a GPS signal in a noisy environment. The C/A codes of GPS signals exhibit greater autocorrelation peak and low cross-correlation. For better detection of a weak signal, it is necessary that autocorrelation peak of the weak signal must be greater than the cross-correlation peak of the strong signal. As C/A codes are near to orthogonal therefore cross-correlation value will approach to a smaller value. The autocorrelation function of a maximum length C/A code consists of an infinite sequence of triangular function, as shown in Figures 1 and 2 for both L1 and L5, respectively. The peaks in the figures show high correlation value, which can be defined mathematically for L1 and L5 as follows:where is C/A code with time for th satellite, is single C/A chipping period (L1 = 977.5 nSec and L5 = 97.75 nSec), and τ is delay in time.

Figure 1: L1 autocorrelation function.

Figure 2: L5 autocorrelation function.

2.2. Power Budget Analysis

To check suitability of reflected L1 and L5 GPS signal with the aim of its utilization in remote sensing, power budget analysis is needed. Power budget analysis of reflected GPS signals has been elaborated in [6–8] where the reflected signals were used for passive imaging and target detection. The analysis is first accomplished for L1 and then for L5 signal.

Let represent the power of transmitter (GPS satellite), gain of transmitter, and target cross section, shows range (distance) from GPS satellites to target, and is range from target to receiver; then the received power can be calculated as [9]where represents receiving antenna effective area calculated by following mathematical formula when GPS signal wavelength and receiver gain is known: The receiver antenna SNR (Signal to Noise ratio) can be computed aswhere represent noise of receiver while is wavelength of GPS signal.

Due to wide bandwidth, addition of Neuman-Hoffman codes in modulation, and comparatively longer spreading codes, the L5 signal is expected, given a high processing gain which is evident from Figure 6. SNR comparison plots for L1 and L5 shown in Figures 5 and 6 are in Section 3.

2.3. Power Level

The L1 signal has a minimum signal strength of −158.5 dBW while L5 has −154.9 dBW. It means that L5 is 3.6 dB better level as compared to L1. Some other important difference parameters between L1 and L5 are summarized in Table 1.

Table 1: L1 and L5 comparison parameters.

3. Results and Simulation

Simulation is carried out in Matlab environment, for the autocorrelation of one set of C/A code for a satellite broadcasting L1 signal, and the result is plotted in Figure 3. The amplitude peak value around 1500 in the diagram represents autocorrelation of C/A code.

Figure 3: The autocorrelation of L1 signal.

Similarly Figure 4 depicts the autocorrelation of the simulated L5 signal with autocorrelation value more than 4000. It is evident from the diagram that L5 autocorrelation value is greater than L1. Hence L5 provides better detection capability as compared to L1 signal. The secondary peaks in Figure 3 of the autocorrelation are significantly less than higher peak. Both Figures 3 and 4 clearly show that the cross-correlation values are very small, which enables the GPS satellites to broadcast signals simultaneously at the same frequency using different C/A codes. From both figures it is evident that the secondary peaks in the autocorrelation diagram are significantly lower than the higher peak. This higher peak value helps the receiver in acquisition and tracking of the GPS signal.

Figure 4: The autocorrelation of L5 signal.

Figure 5: SNR versus range plot without processing gain.

Figure 6: SNR versus range plot with processing gain.

The simulation result of SNR versus range for reflected L1 and L5 GPS signals shown in Figure 5 was carried out for 0 to 1000 m range with target cross section of 10 m2. SNR is calculated for different values of the range. At a range of 100 m SNR values of L1 and L5 signals are, respectively, −50 dB and −30 dB, which are very low and detection of target is almost impossible in both cases. It is evident from Figure 5 that the SNR is very poor even at short distances; hence tracking of the GPS signals is almost impossible.

For further SNR improvement the GPS signals were correlated for a longer period of time consequently better processing gain was achieved. The simulation results of SNR with processing gain of 43 dB for GPS L1 and around 50 dB for L5 are shown in Figure 6. It is worth mentioning that the L5 reflected GPS has 7 dB more processing gain when compared with L1 signal.

Since correlation peaks of L5 signal are much larger than L1 signal, as shown in Figures 3 and 4, it can be deduced that L5 signals have improved signal reliability and are more resistant to false acquisition problems. Moreover from the results it can be observed that, for same acquisition time, noise floor of L5 signal is lower than L1 and therefore acquisition peaks are more prominent. This low noise floor decreases, susceptibility to wave form distortion, and offers better accuracy and processing gain than L1 signal.

4. Conclusion

In this paper performance evaluation for the newly introduced L5 and L1 GPS signals have been performed. Different evaluation parameters are considered and analyzed. Among these parameters simulation is carried out for correlation and power budget analysis. The results are recorded above. From the results it can be deduced that L5 has superior detection characteristics as compared to L1 due to greater bandwidth, high correlation peaks, and better SNR. Hence better results can be achieved with L5 GPS signal as compared to L1 signal.

Competing Interests

The authors declare that they have no competing interests regarding the publication of this paper.

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Исследования GPS/ГЛОНАСС - приемников Trimble

Прорыв в науке и технике второй половины XX века способствовал стремительному развитию всех видов человеческой деятельности. После запуска в СССР первого искусственного спутника земли в 1957 году перед человечеством впервые открылись перспективы использования спутниковой навигации. Изначально Глобальные Навигационные Спутниковые Системы (ГНСС) разрабатывались министерствами обороны СССР и США. Перспективы систем были огромны. Вскоре был придуман уникальный метод измерения фазы несущего сигнала, который изначально не рассматривался создателями системы. Эта технология позволила выполнять измерения координат с точностью до нескольких миллиметров.

В настоящее время ГНСС представлены в виде двух спутниковых группировок: ГЛОНАСС (Россия) и GPS NAVSTAR (более распространенное пользовательское название GPS) (США), которые регулярно модернизируются с целью повышения точности и надежности. Данные ГНСС широко применяются для определения местоположения с высокой точностью для неограниченного числа пользователей в любой точке земного шар в любое время. Появилась возможность совместного использования систем позиционирования GPS и ГЛОНАСС с помощью новых мультисистемных приемников. Эти приемники поддерживают новые сигналы вещания L2C (GPS), а также новые частоты L5 (GPS) и L3 (ГЛОНАСС). L2C представляет собой псевдослучайный код, передаваемый на L2 частоте и доступный в мирных целях. В настоящее время функционирует 10 спутников, транслирующих его. Полная группировка из 24-х американских спутников, вещающих новый сигнал L2C, появится в 2018 году. Все это позволяет повысить скорость и качество работы геодезистов. Кроме того, современное программное обеспечение дает возможность комбинировать режимы съемок, позволяя использовать или исключать из решения необходимые спутники и сигналы.

Внедрение технологии RTK (Real Time Kinematic - кинематика в режиме реального времени) позволило вывести спутниковые технологии на эволюционно новый уровень, поэтому в последнее время RTK-технология приобретает большую популярность среди геодезистов. Основной причиной популярности RTK-технологии является введение исключения неоднозначности фазовых измерений методом «On-The-Fly» (OTF – «на лету»), который обеспечивает высокую точность независимо от динамики движения пользователя.

Возникает вопрос, увеличивается ли эффективность определения местоположения от количества спутников и передаваемых ими сигналов, и насколько ГЛОНАСС упрощает проведение спутниковой съемки.

Для ответа на этот вопрос специалисты компании «Эффективные технологии» провели эксперимент, целью которого было проанализировать точность определения координат контрольных точек, а также стабильность измерений, выполненных в разных режимах: статика (GPS, GPS+ГЛОНАСС), кинематика (GPS, GPS L2C, GPS+ ГЛОНАСС).

Для исследования был выбран полигон с различными условиями обзора небосвода, на котором располагались три точки:

Т1 – 92% открытости небосвода  

Т2 – 89% открытости небосвода  

Т3 – 69% открытости небосвода  

Рис.1 Схема расположения контрольных точек

Измерения производились на экспериментальной базовой сети ООО «Эффективные технологии» в городе Санкт-Петербург, состоящей из трех опорных пунктов.

Выполнение экспериментальной части проходило в три этапа:• подготовительный этап,• эксперимент №1,• эксперимент №2.

На подготовительном этапе с помощью тахеометра Trimble M3 (2"; DR) была создана геодезическая сеть исходных точек в условной (местной) системе координат.

В рамках первого эксперимента было определено положение пунктов при помощи спутникового оборудования Trimble R8 GNSS в статическом режиме относительно двух разноудалённых базовых станций. Результаты, полученные на первом и втором этапах, сравнивались между собой.

В рамках второго эксперимента координаты пунктов были получены в режиме кинематики. Значения, полученные в результате экспериментов, сравнивались между собой.

Подготовительный этап

Координаты исходных точек были вычислены при помощи электронного тахеометра Trimble M3 (2”; DR). С каждой точки было выполнено пять полных циклов измерений круговыми приемами, при этом применялась трехштативная система. Данные наблюдений были обработаны в программном обеспечении Trimble Business Center Advanced. Полученные результаты представлены в таблицах 1.1 и 1.2.

Таблица 1.1 Ведомость уравненных плоских координат.

Имя Восток Y (Метр) Восток Y Ошибка (Метр) Север X (Метр) Север X Ошибка (Метр) Отметка (Метр) Отметка ошибка (Метр)
Т1 (исх.) 0.000 - 0.000 - 0.000 -
Т2 0.000 0.000 54.623 0.002 0.158 0.001
Т3 68.130 0.002 15.392 0.003 0.279 0.001

Таблица 1.2 Компоненты эллипса ошибок.

Имя Большая полуось (Метр) Малая полуось (Метр)
2 0.003 0.000
3 0.003 0.003

 

В дальнейшем приращения координат контрольных точек, определенные электронным тахеометром, принимались за эталонные и использовались для оценки точности спутниковых измерений в режиме статика.

Эксперимент №1

Для оценки возможности использования спутникового геодезического оборудования при выполнении высокоточных геодезических определений взаимного положение контрольных пунктов, закрепленных на подготовительном этапе, были выбраны две базовые точки EFT (удаление 100 м) и Fertoing (удаление 20 км).

Наблюдения на пунктах выполнялись синхронно с использованием пяти комплектов приемников: Trimble R8 GNSS model 3 - 3шт, Trimble R7 GNSS - 1шт, Trimble NetR9 - 1шт. Измерения на пунктах выполнялись с дискретностью один раз в 15 секунд в течение 1 часа и маской угла возвышения 10.

Обработка синхронных спутниковых наблюдений на пунктах EFT, Fertoing, T1, T2, T3 осуществлялась с помощью пакета программного обеспечения Trimble Business Center Advanced. Обработка результатов и сравнение выполнялись в четырех комбинациях: 1) EFT - GPS+ГЛОНАСС, 2) Fertoing -GPS+ГЛОНАСС, 3) EFT - GPS, 4) Fertoing - GPS.  Обработка полигона с дополнительной базовой точкой EFT представлена на рис. 2.

Рис. 2. Обработка полигона с дополнительной базовой точкой EFT.

Вычислялись длины базовых линий, и выполнялось сравнение результатов, полученных спутниковым методом и электронным тахеометром.

Отклонения длин базовых линий от эталонной длины представлены в таблицах 2.1 и 2.2, приведенных ниже.

Таблица 2.1 Сравнение длин базовых линий. В решении участвовала базовая станция EFT.  

Eft GPS+ГЛОНАСС, м GPS, м БАЗИС, м ∆ G+R, м ∆ G, м
1-2 54.615 54.615 54.623 0.008 0.008
1-3 69.851 69.853 69.847 0.004 0.006
2-3 78.629 78.650 78.618 0.011 0.032

Таблица 2.2 Сравнение длин базовых линий, м. В решении участвовала базовая станция Fertoing.

Fertoing GPS+ГЛОНАСС, м GPS, м БАЗИС, м ∆ G+R, м ∆ G, м
1-2 54.615 54.615 54.623 0.008 0.008
1-3 69.852 69.854 69.847 0.005 0.007
2-3 78.631 78.642 78.618 0.013 0.024

Из полученных результатов можно сделать вывод: при выполнении статической съемки в благоприятных условиях, сигналы спутников системы ГЛОНАСС незначительно влияют на точность определения координат.

Максимальная разница между эталонными длинами, полученными электронным тахеометром, и длинами векторов, вычисленных статическим методом, по направлениям 1-2 и 1-3 не превышает 2 мм, причем двадцатикилометровое удаление базового приемника практически не влияет на результат. С направлением 2-3 ситуация совершенно другая ввиду того, что точка Т3 находилась в самых сложных условиях наблюдений. Максимальная разница составила 21 мм.

Эксперимент №2

В рамках второго эксперимента были выполнены измерения методом непрерывной кинематики (RTK). На закрепленных пунктах Т1, Т2, Т3 были установлены роверные приемники Trimble R8 model 3. На рис.3 представлен приемник Trimble R8 model 3, расположенный на контрольной точке Т3.

Измерения производились следующим образом: базовые станции с выделенным IP адресом были подключены к Интернет и вещали поправки через выделенный канал. Передача поправок от базовой станции к роверным приемникам осуществлялась по GPRS каналу по протоколу CMR+. Измерения на пунктах выполнялись с дискретностью один раз в 15 секунд в течение одного часа для каждого режима. Наблюдения на пунктах выполнялись синхронно и поочередно в следующих комбинациях:

EFT 1. GPS+ГЛОНАСС 2. GPS L2C 3. GPS

Fertoing 1. GPS+ГЛОНАСС 2. GPS L2C 3. GPS

Таблица 3.1 Количество наблюдаемых спутников на пунктах.  

Пункт GPS ГЛОНАСС
Т1 10 8
Т2 9 7
Т3 7 6

В результате съемки на каждом пункте было получено множество координат определяемых пунктов. На рис.4 представлено множество координат, полученных на контрольной точке Т1.

Рис. 4. Множество координат, полученных на контрольной точке Т1.

Для оценки точности измерений можно применять разные критерии. Количественной характеристикой погрешности определения местоположения, связанной с особенностями пространственного положения навигационного космического аппарата (НКА) и антенны приемника, служит GDOP (геометрический фактор ухудшения точности).

GDOP2=PDOP2+TDOP2, где PDOP – снижение точности по местоположению, а TDOP – снижение точности по времени.

Минимальное значение GDOP=1.5 достигается в случае, когда потребитель находится в центре правильного тетраэдра. Соответственно, для наземного потребителя, с учетом кривизны земной поверхности, минимальное значение GDOP=1.732 достигается тогда, когда один НКА находится в зените, а три других равномерно расположены в горизонтальной плоскости, т.е. когда объем тетраэдра максимален.

Однако такая геометрия рабочего созвездия не оптимальна с точки зрения атмосферных ошибок, поэтому при используемых на практике углах маски более 10° минимальное значение GDOP≈2. Перед выполнением съемки в настройках контроллера были указаны значения PDOP равные 7 (согласно ГКИНП (ОНТА) - 02-262-02), при которых значения координат, полученные при большем или равном показателе PDOP, не записывались. Однако на протяжении всей съемки значения PDOP не превышали 7.

Критерием оценки точности является средняя квадратической ошибка (СКО). Квадрат средней квадратической ошибки равен математическому ожиданию квадрата истинной ошибки результата измерения. В данном случае было принято считать измерения равноточными. Вычисление СКО проводилось по следующей формуле:

где l – измеренное значение величины, x – истинное значение, n – количество измеренных величин. Результат вычисления СКО на каждом пункте в каждом режиме измерений представлен в таблице 3.2.

Таблица 3.2 Значение СКО, м.

1 EFT GPS+GLANASS GPS+L2C GPS
Y 0.003 0.012 0.008
X 0.008 0.006 0.004
H 0.010 0.011 0.011
Fertoing GPS+GLANASS GPS+L2C GPS
Y 0.006 0.010 0.016
X 0.013 0.016 0.020
H 0.009 0.021 0.016
2 EFT GPS+GLANASS GPS+L2C GPS
Y 0.007 0.007 0.005
X 0.004 0.007 0.004
H 0.010 0.012 0.015
Fertoing GPS+GLANASS GPS+L2C GPS
Y 0.011 0.014 0.016
X 0.014 0.018 0.015
H 0.020 0.015 0.031
3 EFT GPS+GLANASS GPS+L2C GPS
Y 0.008 0.019 0.020
X 0.035 0.029 0.037
H 0.029 0.045 0.081
Fertoing GPS+GLANASS GPS+L2C GPS
Y 0.030 0.029 0.033
X 0.053 0.047 0.057
H 0.032 0.050 0.061

Руководствуясь ГКИНП - 02-033-79, из полученных результатов можно сделать вывод: точность определения плановых и высотных координат удовлетворяет топографической съемке в масштабе 1:500, причем точность определения координат не зависит от режима съемки и от расстояния до базового приемника.

При этом находящийся в самых сложных условиях наблюдений пункт Т3 (69% открытости небесвода) имеет удовлетворительный показатель СКО.

Важным моментом при использовании RTK является скорость выполнения инициализации и ее стабильность. Время выполнения инициализации на каждом пункте при каждом режиме измерений представлено в таблице 3.3.

Таблица 3.3 Время выполнения инициализации.  

1 EFT GPS+GLONASS GPS+L2C GPS
  2 сек 10 сек 15 сек
Fert GPS+GLONASS GPS+L2C GPS
  2 сек 2 сек 37 сек
2 EFT GPS+GLONASS GPS+L2C GPS
  2 сек 2 сек 10 сек
Fert GPS+GLONASS GPS+L2C GPS
  2 сек 5 сек 35 сек, 23 сек
3 EFT GPS+GLONASS GPS+L2C GPS
  5 сек 10 сек 24 сек, 12 сек
Fert GPS+GLONASS GPS+L2C GPS
  5 сек 15 сек 120 сек, 43 сек, 31 сек

На первой контрольной точке инициализация стабильно сохранялась. При использовании режима GPS+ГЛОНАСС инициализация выполнялась в течение 2 сек. Наблюдалась высокая скорость инициализации при использовании нового сигнала L2C. При использовании только GPS, решение неоднозначности выполнялось в течение 37 сек при работе от удаленной базовой станции Fertoing, что на 22 сек дольше, чем от базовой станции EFT.

На второй контрольной точке с помощью режимов съемки GPS+ГЛОНАСС и L2C инициализация выполнялась в течение 2 сек и сохранялась на протяжении всей съемки. При использовании режима GPS ожидание выполнения инициализации составило 35 сек. В течение съемки была потеря инициализации один раз. На повторную инициализацию потребовалось 23 сек. Следует заметить, что точка Т2 имеет 89% открытости небосвода.

На контрольной точке T3, находящейся в условиях ограниченной видимости небесвода, инициализация стабильно сохранялась при использовании режима GPS+ГЛОНАСС и L2C. Однако инициализация не выполнялась мгновенно, ожидание составляло 5-10 сек. При использовании только режима GPS, съемка была осложнена постоянными срывами инициализации. Первый раз инициализация выполнялась 2 мин. За один час инициализация была потеряна два раза. Время инициализаций при повторных запусках составило 43 и 31 сек.

На основании выше изложенного можно сделать следующие выводы:• Использование спутников ГЛОНАСС никак не повиляет на увеличение точности в благоприятных условиях. Однако в городской застройке спутники ГЛОНАСС обеспечивают получение более стабильных результатов и уменьшение времени инициализации.• Если обратить внимание на таблицы оценки точности, то видно, что данные по L2C и ГЛОНАСС отличаются не более чем на 1см. Прием сигнала L2C и использование спутников ГЛОНАСС открывает дополнительные возможности увеличения точности измерений в сложных условиях и сокращения времени инициализации.

Новые технологии позволяют отслеживать спутники в более сложных условиях. На небосводе теперь постоянно наблюдается около 15-20 спутников. Из их множества в решении участвуют спутники с наилучшей геометрией расположения.

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