According to information shared by the Russian Legal Information Agency, the Investigative Committee’s department for the Moscow Region has launched a preliminary probe into the case.

Construction of the GLONASS satellite navigation system control and support center began in June 2010 on the site used by TsNIImash, the head research company of Russia’s federal space agency. The center was supposed to hold equipment for collecting and processing the data supplied by the GLONASS global network.

The construction was financed by a federal program, with 1.050 billion ($33.22 million) allocated for the project. By the end of 2010, it came to light that construction costs had been overstated, Izvestia reports. An expert appraisal revealed that the contractor had rigged the costs. The government did not allocate additional funds, so construction was suspended in December 2011 when the Federal GLONASS Program for 2002-2011 ended. The construction of the building has never been completed.

In November 2012, the general designer of GLONASS, Yuri Urlichich, was dismissed from his post as a result of the scandal.

This first FOC satellite is functionally identical to the first four in-orbit validation (IOV) satellites already in orbit, but has been built by a separate industrial team. Like the other 21 FOC satellites so far procured by ESA, the satellite’s prime contractor is OHB System AG, and the navigation payload was produced by Surrey Satellite Technology Ltd. in Guildford, UK.

Thermal vacuum testing at the European Space Research and Technology Centre (ESTEC) will simulate temperature extremes the satellites must endure in the airlessness of space throughout their 12-year working lifetimes. Without any moderating atmosphere, temperatures can shift hundreds of degrees from sunlight to shadow.

Other activities on the schedule include shaker and acoustic noise testing — simulating the vibration and noise of launch — as well as electromagnetic compatibility and antenna testing, placing the satellite in chambers shielded from all external radio signals to reproduce infinite space and check that its various antennas and electrical systems are interoperable without harmful interference.

“The Galileo FOC satellites provide the same capabilities as the previous IOV satellites, but with improved performance, such as higher transmit power,” explained Giuliano Gatti, the head of the Galileo Space Segment Procurement Office. “They are to all intents a new design that requires a full checkout before getting the green light for launch.”

The second FOC flight model is due to arrive at ESTEC in early June, and the third in the middle of July. The first two satellites are to be placed in orbit on board a Soyuz launcher, with a scheduled lift-off from Kourou in French Guyana this fall, with two more due to follow by the end of the year.

The first four Galileo IOV satellites, launched in 2011 and 2012, were provided by EADS Astrium with Thales Alenia Space Italy responsible for integrating the satellites and Astrium in Portsmouth, UK, providing the navigation payloads. They provided their first navigation fix in March 2013.

The definition, development and in-orbit validation phases of the Galileo programme are being carried out by ESA and co-funded with the European Commission (EC).

The subsequent FOC phase is managed and funded by the EC. The commission has delegated the role of design and procurement agent to ESA for the FOC phase. At the same time as the satellites are being assembled on a production-line basis, ground stations are also being established on European territories around the globe.

Photo credit: Pat Corkery, United Launch Alliance.

GPS Leaves This Earth

A t 5:38 p.m. Eastern Daylight Time (21:38 UTC) on May 15,  the fourth GPS IIF satellite, Space Vehicle Number (SVN) 66 built by Boeing, ascended towards orbit aboard a United Launch Alliance Atlas V rocket at from Cape Canaveral Air Force Station, Florida.

“The GPS constellation remains healthy and continues to meet and exceed the performance standards to which the satellites were built. Our goal is to deliver sustained, reliable GPS capabilities to America’s warfighters, our allies, and civil users around the world, and this is done by maintaining GPS performance, fielding new capabilities and developing more robust modernized capabilities for the future,” said Colonel Bernie Gruber, director of the U.S. Air Force Space and Missile Systems Center’s GPS Directorate.

The new capabilities of the IIF satellites will provide greater navigational accuracy through improvements in atomic clock technology; a more robust signal for commercial aviation and safety-of-life applications, known as the new third civil signal (L5); and a 12-year design life providing long-term service. These upgrades deliver improved anti-jam capabilities for warfighters and improved security for military and civil users around the world, the Air Force said in a statement.

The IIF-4 satellite is expected to complete testing in August, after which it will be utilized as a reserve or backup satellite. It becomes the fourth satellite in a 12-strong network of GPS IIF spacecraft manufactured by Boeing as lead contractor, the first of which was boosted into orbit in May 2010. The Air Force expects the first of the next-generation GPS IIIA satellites to enter service sometime in 2014.

System Briefs

GLONASS. The GLONASS 747 M-series satellite launched on April 26 has maneuvered into an orbital slot near GLONASS 728, the operational satellite in Plane 1, slot 2. 747 will presumably serve as a reserve until it replaces 728, unless another Plane 1 satellite expires first. The next Russian launch, a GLONASS-M trio, is scheduled for July 1. There are currently 24 operational GLONASS satellites.

IRNSS. The first Indian Regional Navigation Satellite System satellite is expected to rise at the end of June. The IRNSS plans to orbit of seven: three geostationary and four geosynchronous, providing regional coverage via navigation signals in the L5 and S bands.

GLONASS-M satellite No. 47 was launched from the Plestesk Cosmodrome on April 26 at 05:23:41 UTC by a Soyuz 2-1b rocket.

“At 12.55 [08:55 UTC] the GLONASS-M spacecraft was taken under management by the Titov Main Test and Space Systems Control Centre. The spacecraft is installed [in orbit] and has maintained stable telemetry, and its onboard systems are operating normally,” said Colonel Alexei Zolotukhin of the Russian Aerospace Defence Forces.

The satellite, also known as Kosmos 2485 and GLONASS 747, was placed in orbital plane 1 and is drifting to its designated slot.

Initial two-line element set for the satellite:

1 39155U 13019A   13117.72709898  .00000014  00000-0  00000+0 0    85
2 39155 064.8833 235.0937 0113505 116.3660 245.7001 02.09126432    35

ISS Reshetnev, the manufacturer of the satellite, reported that the first communication session confirmed that the spacecraft is operating as designed, its mechanical systems deployed, and Sun and Earth acquisition was completed successfully.

According to the CEO – Chief Designer of ISS Reshetnev, Nicholas Testoedova, this GLONASS-M satellite will be a reserve. The following year, after the completion of the examinations and tests, it will replace one of the older exhausted units.

After reaching a designated orbit, likely slot 2 in orbital plane 1, the satellite will complete several weeks of commissioning and testing before entering regular service. There are currently 24 operational GLONASS satellites. The GLONASS-M satellite is the second generation GLONASS satellite.

BeiDou. Construction of the second phase of BeiDou has been completed; further launches for the third phase – constellation completion – are on hold until tests of the existing 14-satellite constellation are complete, according to Xiancheng Ding, Senior Advisor, China Satellite Navigation Office. As of December 27, 2012, BeiDou achieved full operational capability for most of the Asia-Pacific region. The full constellation is now expected to be completed by 2020.

Other accomplishments include releasing the BeiDou Interface Control Document and manufacture of BeiDou chips for end-user applications. By the end of June, some manufacturers will release BeiDou chips in China, Ding said.

Also in December, BeiDou introduced a new logo (at right).

Yuanxi Yang (China National Administration of GNSS and Applications) presented statistics showing that BeiDou+GPS provides greater accuracy than GPS alone. For instance, the RMS of BeiDou+GPS kinematic positioning by using differential carrier phase is about 20 percent better than that of GPS alone, Yang said.

By itself, existing BeiDou constellation system accuracy is better than 10 meters, timing better than 20 nanoseconds, and velocity accuracy is better than 0.2 meters/second.

In all, BeiDou is composed of 14 satellites: five GEO, five IGSO, and four MEO. The full constellation (by 2020)  will consist of 35 satellites: 5 GEO and 30 non-GEO (a mixture of MEO and IGSO satellites).

GPS. Keynote speaker David A. Turner (U.S. Department of State) shared his time with surprise GLONASS speaker Sergey Revnivykh (International Committee on GNSS, ICG). In his GNSS Policy and Program Update, Turner provided the dates by which three new civil signals will be on 24 GPS satellites.

• The L2C signal is a developmental signal broadcasting from 10 GPS Satellites. It began launching in 2005 with GPS Block IIR(M) satellites, and is expected to be available on 24 satellites around 2018.
• The L5 signal is a developmental signal broadcasting from three GPS satellites. It began launching in 2010 with Block IIF satellites, and is expected to be available on 24 GPS satellites around 2021.
• The L1C signal begins launching in 2015 with GPS III; available on 24 GPS satellites around 2026.

“We have an increasing number of signals, increasing capability, and increasing level of service as we continue to evolve the constellation,” Turner said.

GLONASS. The next GLONASS satellite will be launched this Friday, April 26, Revnivykh said. This will be a GLONASS-M satellite, number 47. The first launch of a new generation GLONASS K satellite is scheduled for 2015.

Revnivykh stressed GLONASS’ role as a global utility. “We consider international cooperation is essential for all GNSS, and we consider GLONASS an essential part of the international multi-GNSS system,” he said. He stressed the importance of compatibility and interoperability as key to this policy.

In 2012, GLONASS performed with an average accuracy better than formally required, he said. GLONASS is in worldwide use, and positioning has improved by a factor of 10, from 35 meters to about 3 meters since the first satellites were launched. Using both GPS + GLONASS provides 1.5 times better high-precision measurements, Revnivykh said.

The new GLONASS program for 2020 for GLONASS sustainment, development, and use includes GLONASS M, K1, and K2 satellites; the positioning accuracy objective is to go from the current 2.8 meters to 0.6 meters.

Aviation. Chris Hegarty (MITRE) presented an FAA Navigation Programs Overview on behalf of the scheduled speaker Deborah Lawrence (FAA) who was unable to attend. He noted that United Airlines has begun GBAS operations in Houston.

In answer to a funding question, he said, “The sequestration is not expected to have a positive effect on schedule, but the presented timeline for APNT is the FAA’s current best estimate. Congress has some tough decisions before them, and I wouldn’t want to speculate on potential schedule impacts. In the words of Yogi Berra, predicting is hard, especially when it involves the future.”

Korean SBAS. Changdon Kee (Seoul National University) shared plans for a Korean SBAS. In South Korea, LPV availability is 49.4% compared to 90.6% in Japan. “Korea needs its own system,” Kee said.

Phase 3 of the SBAS development could start by the end of September, depending on funding. It will include open service multifunctional GEO satellites interoperable with other SBASs. A pseudolite demonstration system will be completed in 2014, clearing the way for the beginning of Phase 3.

In all, the system will include five reference stations, two master stations, two ground uplink stations, and two GEO satellites (the main GEO by 2018 and a backup by 2020).

The Korean SBAS open service system will provide GPS L1 augmentation, begin operation in 2020, and support aviation, land and maritime users. A test operation system will provide GPS L1 and L5 augmentation. The system is expected to be fully operational by 2021, with service available throughout Asia.

Japan’s QZSS. Hiroyuki Noda (Office of National Space Policy, Japan) said three more satellites for this augmentation system will be launched by the end of the decade, with the service beginning in 2018. In September 2012, the Japan cabinet made the commitment to accelerate development of the system. The first satellite, launched in 2010 (QZS-1, aka Michibiki) is performing as expected.

QZSS is expected to improve positioning availability from 90% to 99.8% in Japan. QZSS will not only improve positioning in the Asia-Pacific region, but is expected to improve the capacity to respond to natural disasters, Noda said.

Satellites expected to be launched in support of various Global Navigation Satellite Systems are the following:

GPS
May 15: Block IIF-4, SVN66, launch window: 17:39-17:58 UTC
November: Block IIF-5

GLONASS
April 26: Single GLONASS-M or -K satellite from Plesetsk
June 28: Three GLONASS-M satellites from Baikonur

Galileo
October: FOC-1 launch (two satellites)

Indian Regional Navigation Satellite System (IRNSS)
June (This is the first launch for an expected constellation of seven satellites, some of which will be geostationary. The constellation will provide continuous regional coverage for positioning, navigation and timing services.)

The Astro APS sensor is part of the satellite’s altitude and orbit control system. It will help ensure that the GLONASS satellites maintain an accurate attitude in space.

The sensor will be installed on a GLONASS-M satellite under a 2011 agreement signed between Reshetnev ISS and Jena-Optronik GmbH. It has a single box design with low mass and low power consumption. If tests are successful, it will be integrated on other ISS spacecraft.

GLONASS 743, recently moved from orbital slot 2 to orbital slot 8, was set healthy on March 5 at 07:28 Moscow Time according to NAGU 017-130305. Although the NAGU states that Moscow Time is three hours ahead of UTC (and this is the time difference normally used for GLONASS as stipulated in the GLONASS ICD), officially, it is actually four hours and has been since the switch to year-round daylight saving time on 27 March 2011. In this case, the NAGU appears to be in error since GLONASS 743 was actually set healthy at 03:28 UTC and not at 04:28 UTC. This is confirmed by Roscomos monitoring and by the navigation data collected by stations of the International GNSS Service (IGS).

There are once again 24 healthy GLONASS satellites on orbit.

For those keeping track of frequency channel changes, GLONASS 743 was switched from frequency channel 6 to channel -6 on 1 March some minutes before 10:45 UTC and back to channel 6 on 2 March, again some minutes before 10:45 UTC as determined from IGS navigation files. Although a NAGU was issued for the first frequency change (stating that it occurred at “1344 MT (UTC+0300)”), no NAGU has been issued to document the second frequency shift although the set-healthy NAGU does give the frequency channel as 6.

Meanwhile, in other GLONASS news, a single GLONASS-M satellite (Block 47s) is to be launched from the Plesetsk Cosmodrome on April 26 at 05:23:41 UTC according to the NASA Forum blog.

By Ningyan Guo, Staffan Backén, and Dennis Akos

The authors designed a full-constellation GNSS receiver, using a cost-effective, readily available, flexible front-end, wide enough to capture the frequency from 1555 MHz to 1607 MHz, more than 50MHz. This spectrum width takes into account BeiDou E2, Galileo E1, GPS L1, and GLONASS G1. In the course of their development, the authors used an external OCXO oscillator as the reference clock and reconfigured the platform, developing their own custom wide-band firmware.

The development of the Galileo and BeiDou constellations will make many more GNSS satellite measurements be available in the near future. Multiple constellations offer wide-area signal coverage and enhanced signal redundancy. Therefore, a wide-band multi-constellation receiver can typically improve GNSS navigation performance in terms of accuracy, continuity, availability, and reliability. Establishing such a wide-band multi-constellation receiver was the motivation for this research.

A typical GNSS receiver consists of three parts: RF front-end, signal demodulation, and generation of navigation information. The RF front-end mainly focuses on amplifying the input RF signals, down-converting them to an intermediate frequency (IF), and filtering out-of-band signals. Traditional hardware-based receivers commonly use application-specific integrated circuit (ASIC) units to fulfill signal demodulation and transfer the range and carrier phase measurements to the navigation generating part, which is generally implemented in software. Conversely, software-based receivers typically implement these two functions through software. In comparison to a hardware-based receiver, a software receiver provides more flexibility and supplies more complex signal processing algorithms. Therefore, software receivers are increasingly popular for research and development.

The frequency coverage range, amplifier performance, filters, and mixer properties of the RF front-end will determine the whole realization of the GNSS receiver. A variety of RF front-end implementations have emerged during the past decade. Real down-conversion multi-stage IF front-end architecture typically amplifies filters and mixes RF signals through several stages in order to get the baseband signals. However, real down-conversion can bring image-folding and rejection. To avoid these drawbacks, complex down-conversion appears to resolve much of these problems. Therefore, a complex down-conversion multi-stage IF front-end has been developed. But it requires a high-cost, high-power supply, and is larger for a multi-stage IF front-end. This shortcoming is overcome by a direct down-conversion architecture. This front-end has lower cost; but there are several disadvantages with direct down-conversion, such as DC offset and I/Q mismatch. DC offset is caused by local oscillation (LO) leakage reflected from the front-end circuit, the antenna, and the receiver external environment.

A comparison of current traditional RF front-ends and different RF front-end implementation types led us to the conclusion that one model of a universal software radio peripheral, the USRP N210, would make an appropriate RF front end option. USRP N210 utilizes a low-IF complex direct down-conversion architecture that has several favorable properties, enabling developers to build a wide range of RF reception systems with relatively low cost and effort. It also offers high-speed signal processing. Most importantly, the source code of USRP firmware is open to all users, enabling researchers to rapidly design and implement powerful, flexible, reconfigurable software radio systems. Therefore, we chose the USRP N210 as our reception device to develop our wide-band multi-constellation GNSS receiver, shown in Figure 1.

Figure 1. Custom wide-band multi-constellation software receiver architecture based on universal software radio peripheral (USRP).

USRP Front-End Architecture

The USRP N210 front-end has wider band-width and radio frequency coverage in contrast with other traditional front-ends as shown by the comparison in Table 1. It has the potential to implement multiple frequencies and multiple-constellation GNSS signal reception. Moreover, it performs higher quantization, and the onboard Ethernet interface offers high-speed data transfer.

Table 1. GNSS front-ends comparison.

USRP N210 is based on the direct low-IF complex down-conversion receiver architecture that is a combination of the traditional analog complex down-conversion implemented on daughter boards and the digital signal conditioning conducted in the motherboard. Some studies have shown that the low-IF complex down-conversion receiver architecture overcomes some of the well-known issues associated with real down-conversion super heterodyne receiver architecture and direct IF down-conversion receiver architecture, such as high cost, image-folding, DC offset, and I/Q mismatch.

The low-IF receiver architecture effectively lessens the DC offset by having an LO frequency after analog complex down-conversion. The first step uses a direct complex down-conversion scheme to transform the input RF signal into a low-IF signal. The filters located after the mixer are centered at the low-IF to filter out the unwanted signals. The second step is to further down-covert the low-IF signal to baseband, or digital complex down-conversion.

Similar to the first stage, a digital half band filter has been developed to filter out-of-band interference. Therefore, direct down-conversion instead of multi-stage IF down-conversion overcomes the cost problem; in the meantime, the signal is down-converted to low-IF instead of base-band frequency as in the direct down-conversion receiver, so the problem of the DC offset is also avoided in the low-IF receiver. These advantages make the USRP N210 platform an attractive option as GNSS receiver front-end.

Figure 2 shows an example GNSS signal-streaming path schematic on a USRP N210 platform with a DBSRX2 daughter board. Figure 3 shows a photograph of internal structure of a USRP N210 platform.

Figure 2 GNSS signal streaming on USRP N210 + DBSRX2 circuit.

Figure 3. USRP N210 internal structure.

The USRP N210 platform includes a main board and a daughterboard. In the main board, 14-bit high precision analog-digital converters (ADCs) and digital-analog converters (DACs) permit wide-band signals covering a high dynamic range. The core of the main board is a high-speed field-programmable gate array (FPGA) that allows high-speed signal processing. The FPGA configuration implements down-conversion of the baseband signals to a zero center frequency, decimates the sampled signals, filtering out-of-band components, and finally transmits them through a packet router to the Ethernet port. The onboard numerically controlled oscillator generates the digital sinusoid used by the digital down-conversion process. A cascaded integrator-comb (CIC) filter serves as decimator to down-sample the signal.

The signals are filtered by a half pass filter for rejecting the out-of-band signals. A Gigabit Ethernet interface effectively enables the delivery of signals out of the USRP N210, up to 25MHz of RF bandwidth. In the daughterboard, first the RF signals are amplified, then the signals are mixed by a local onboard oscillator according to a complex down-conversion scheme. Finally, a band-pass filter is used remove the out-of-band signals.

Several available daughter boards can perform signal conditioning and tuning implementation. It is important to choose an appropriate daughter board, given the requirements for the data collection.

A support driver called Universal Hardware Driver (UHD) for the USRP hardware, under Linux, Windows and Mac OS X, is an open-source driver that contains many convenient assembly tools. To boot and configure the whole system, the on-board microprocessor digital signal processor (DSP) needs firmware, and the FPGA requires images. Firmware and FPGA images are downloaded into the USRP platform based on utilizations provided by the UHD. Regarding the source of firmware and FPGA images, there are two methods to obtain them:

•   directly use the binary release firmware and images posted on the web site of the company;
•   build (and potentially modify) the provided source code.

USRP Testing and Implementation

Some essential testing based on the original configuration of the USRP N210 platform provided an understanding of its architecture, which was necessary to reconfigure its firmware and to set up the wide-band, multi-constellation GNSS receiver. We collected some real GPS L1 data with the USRP N210 as RF front-end. When we processed these GPS L1 data using a software-defined radio (SDR), we encountered a major issue related to tracking, described in the following section.

Onboard Oscillator Testing. A major problem with the USRP N210 is that its internal temperature-controlled crystal oscillator (TCXO) is not stable in terms of frequency. To evaluate this issue, we recorded some real GPS L1 data and processed the data with our software receiver. As shown in Figure 4, this issue results in the loss of GPS carrier tracking loop at 3.18 seconds, when the carrier loop bandwidth is 25Hz.

Figure 4. GPS carrier loop loss of lock.

Consequently, we adjusted the carrier loop bandwidth up to 100Hz; then GPS carrier tracking is locked at the same timing (3.18s), shown in Figure 5, but there is an almost 200 Hz jump in less than 5 milliseconds.

Figure 5. GPS carrier loop lock tracking.

As noted earlier, the daughter card of the USRP N210 platform utilizes direct IF complex down-conversion to tune GNSS RF signals. The oscillator of the daughter board generates a sinusoid signal that serves as mixer to down-convert input GNSS RF signals to a low IF signal. Figure 6 illustrates the daughter card implementation. The drawback of this architecture is that it may bring in an extra frequency shift by the unstable oscillator. The configuration of the daughter-card oscillator is implemented by an internal TCXO clock, which is on the motherboard. Unfortunately, the internal TCXO clock has coarse resolution in terms of frequency adjustments. This extra frequency offset multiplies the corresponding factor that eventually provides mixer functionality to the daughter card. This approach can directly lead to a large frequency offset to the mixer, which is brought into the IF signals.

Figure 6. Daughter-card tuning implementation.

Finally, when we conduct the tracking operation through the software receiver, this large frequency offset is beyond the lock range of a narrow, typically desirable, GNSS carrier tracking loop, as shown in Figure 4.

In general, a TCXO is preferred when size and power are critical to the application. An oven-controlled crystal oscillator (OCXO) is a more robust product in terms of frequency stability with varying temperature. Therefore, for the USRP N210 onboard oscillator issue, it is favorable to use a high-quality external OCXO as the basic reference clock when using USRP N210 for GNSS applications.

Front-End Daughter-Card Options. A variety of daughter-card options exist to amplify, mix, and filter RF signals. Table 2 lists comparison results of three daughter cards (BURX, DBSRX and DBSRX2) to supply some guidance to researchers when they are faced with choosing the correct daughter-board.

Table 2. Front-end daughter-card options.

The three daughter cards have diverse properties, such as the primary ASIC, frequency coverage range, filter bandwidth and adjustable gain. BURX gives wider radio frequency coverage than DBSRX and DBSRX2. DBSRX2 offers the widest filter bandwidth among the three options.

To better compare the performance of the three daughter cards, we conducted another three experiments. In the first, we directly connected the RF port with a terminator on the USRP N210 platform to evaluate the noise figure on the three daughter cards. From Figure 7, we can draw some conclusions:

• BURX has a better sensitivity than DBSRX and DBSRX2 when the gain is set below 30dB.
• DBSRX2 observes feedback oscillation when the gain set is higher than 70dB.

Figure 7. Noise performance comparisons of three daughter cards.

The second experimental setup configuration used a USRP N210 platform, an external OCXO oscillator to provide stable reference clock, and a GPS simulator to evaluate the C/N₀ performance of the three daughter boards. The input RF signals are identical, as they come from the same configuration of the GPS simulator. Figure 8 illustrates the C/N₀ performance comparison based on this experimental configuration. The figure shows that BURX performs best, with DBSRX2 just slightly behind, while DBSRX has a noise figure penalty of 4dB.

Figure 8. C/N₀ performance comparisons of three daughter cards.

In the third experiment, we added an external amplifier to increase the signal-to-noise ratio (SNR). From Figure 9, we see that the BURX, DBSRX and DBSRX2 have the same C/N₀ performance, effectively validating the above conclusion. Thus, an external amplifier is recommended when using the DBSRX or DBSRX2 daughter boards.

Figure 9. C/N₀ performance comparisons of three daughter cards with an external amplifier.

The purpose of these experiments was to find a suitable daughter board for collecting wide-band multi-constellation GNSS RF signals. The important qualities of an appropriate wide-band multi-constellation GNSS receiver are:

• high sensitivity;
• wide filter bandwidth; and
• wide frequency range.

After a comparison of the three daughter boards, we found that the BURX has a better noise figure than the DBSRX or DBSRX2. The overall performance of the BURX and DBSRX2 are similar however. Using an external amplifier effectively decreases the required gain on all three daughter cards, which correspondingly reduces the effect of the internal thermal noise and enhances the signal noise ratio. As a result, when collecting real wide-band multi-constellation GNSS RF signals, it is preferable to use an external amplifier.

To consider recording GNSS signals across a 50MHz band, DBSRX2 provides the wider filter bandwidth among the three daughter-card options, and thus we selected it as a suitable daughter card.

Custom Wide-band Firmware Development. When initially implementing the wideband multi-constellation GNSS reception devices based on the USRP N210 platform, we found a shortcoming in the default configuration of this architecture, whose maximum bandwidth is 25MHz. It is not wide enough to record 50MHz multi-constellation GNSS signals (BeiDou E2, GPS L1, Galileo E1, and GlonassG1). A 50MHz sampling rate (in some cases as much as 80 MHz) is needed to demodulate the GNSS satellites’ signals.

Meanwhile since the initiation of the research, the USRP manufacturer developed and released a 50MHz firmware. To highlight our efforts, we further modified the USRP N210 default configuration to increase the bandwidth up to 100MHz, which has the potential to synchronously record multi-constellation multi-frequency GNSS signals (Galileo E5a and E5b, GPS L5 and L2) for further investigation of other multi-constellation applications, such as ionospheric dispersion within wideband GNSS signals, or multi-constellation GNSS radio frequency compatibility and interoperability.

Apart from reprogramming the host driver, we focused on reconfiguring the FPGA firmware. With the aid of anatomizing signal flow in the FPGA, we obtained a particular realization method of augmenting its bandwidth. Figure 10 shows the signal flow in the FPGA of the USRP N210 architecture.

Figure 10. Signal flow in the FPGA of the USRP N210 platform.

The ADC produces 14-bit sampled data. After the digital down-conversion implementation in the FPGA, 16-bit complex I/Q sample data are available for the packet transmitting step. According to the induction document of the USRP N210 platform, VITA Radio Transport Protocol functions as an overall framework in the FPGA to provide data transmission and to implement an infrastructure that maintains sample-accurate alignment of signal data. After significant processing in the VITA chain, 36-bit data is finally given to the packet router. The main function of the packet router is to transfer sample data without any data transformation. Finally, through the Gigabit Ethernet port, the host PC receives the complex sample data.

In an effort to widen the bandwidth of the USRP N210 platform, the bit depth needs to be reduced, which cuts 16-bit complex I/Q sample data to a smaller length, such as 8-bit, 4-bit, or even 2-bit, to solve the problem. By analyzing Figure 10, to fulfill the project’s demanding requirements, modification to the data should be performed after ADC sampling, but before the digital down-conversion. We directly extract the 4-bit most significant bits (MSBs) from the ADC sampling data and combined eight 4-bit MSB into a new 16-bit complex I/Q sample, and gave this custom sample data to the packet router, increasing the bandwidth to 100 MHz.

Wide-Band Receiver Performance Analysis. The custom USRP N210-based wide-band multi-constellation GNSS data reception experiment is set up as shown in Figure 11.

Figure 11. Wide-band multi-constellation GNSS data recording system.

A wide-band antenna collected the raw GNSS data, including GPS, GLONASS, Galileo, and BeiDou. An external amplifier was included to decrease the overall noise figure. An OCXO clock was used as the reference clock of the USRP N210 system. After we found the times when Galileo and BeiDou satellites were visible from our location, we first tested the antenna and external amplifier using a commercial receiver, which provided a reference position. Then we used 1582MHz as the reception center frequency and issued the corresponding command on the host computer to start collecting the raw wide-band GNSS signals. By processing the raw wide-band GNSS data through our software receiver, we obtained the acquisition results from all constellations shown in Figure 12; and tracking results displayed in Figure 13.

Figure 12. Acquisition results for all constellations.

Figure 13. Tracking results for all constellations.

We could not do the full-constellation position solution because Galileo was not broadcasting navigation data at the time of the collection and the ICD for BeiDou had not yet been released. Therefore, respectively using GPS and GLONASS tracking results, we provided the position solution and timing information that are illustrated in Figure 14 and in Figure 15.

Figure 14. GPS position solution and timing information.

Figure 15. GLONASS position solution.

Conclusions

By processing raw wide-band multi-constellation GNSS signals through our software receiver, we successfully acquired and tracked satellites from the four constellations. In addition, since we achieved 100MHz bandwidth, we can also simultaneously capture modernized GPS and Galileo signals (L5 and L2; E5a and E5b, 1105–1205 MHz).

In future work, a longer raw wide-band GNSS data set will be recorded and used to determine the user position leveraging all constellations. Also an urban collection test will be done to assess/demonstrate that multiple constellations can effectively improve the reliability and continuity of GNSS navigation.

Acknowledgment

The first author’s visiting stay to conduct her research at University of Colorado is funded by China Scholarship Council, File No. 2010602084.

This article is based on a paper presented at the Institute of Navigation International Technical Conference 2013 in San Diego, California.

Manufacturers

The USRP N210 is manufactured by Ettus Research. The core of the main board is a high-speed Xilinx Spartan 3A DSP FPGA. Ettus Research provides a support driver called Universal Hardware Driver (UHD) for the USRP hardware. A wide-band Trimble antenna was used in the final experiment.

Ningyan Guo is a Ph.D. candidate at Beihang University, China. She is currently a visiting scholar at the University of Colorado at Boulder.

Staffan Backén is a postdoctoral researcher at University of Colorado at Boulder. He received a Ph.D. in in electrical engineering from Luleå University of Technology, Sweden.

Dennis Akos completed a Ph.D. in electrical engineering at Ohio University. He is an associate professor in the Aerospace Engineering Sciences Department at the University of Colorado at Boulder with visiting appointments at Luleå University of Technology and Stanford University