The System: L5 Arrives

April 29, 2009 By: GPS World Staff GPS World

L5 Arrives

Anew era in GPS modernization has begun. On March 24, the GPS IIR-20(M) satellite known as SVN49 rose and joined the constellation. The satellite carries a demonstration payload to transmit the new civil GPS L5 signal on 1176.45 MHz.

The L5 demonstration payload, developed by Lockheed Martin and subcontractors, started transmissions on April 10, at 11:58 UTC. By broadcasting the L5 signal before August 26, 2009, the United States has fulfilled requirements associated with International Telecommunication Union-Radiocommunication Sector filings and avoided a potential claim of the frequency band by another country.

In contrast to the GPS specification IS-GPS-705, the L5 signal transmitted by SVN49 contains only a dataless Q5 component modulated with the pseudorandom noise ranging code PRN63 and therefore acts mainly as a placeholder to preserve the L5 frequency for oncoming Block IIF and Block III satellites, which are supposed to transmit signals in full compliance with IS-GPS-705. SVN49’s L5 signal is not authorized for navigation purposes but may be used for a proof of concept and for receiver tests.

Intended for safety-of-life applications (SoL) such as civil aviation, the GPS L5 signal is located in an Aeronautical Radionavigation Service (ARNS) band and will bring significant benefits to high-precision users and those interested in increased integrity. L5 is anticipated to provide better quality range measurements and possibly improve the tracking performance of a GPS receiver compared with current L1 and L2 civil signals by adopting improved signal structures. This includes an increased chipping rate of 10.23 megachips per second and a longer spreading code than L1 C/A. In combination with L1, it will end worries about the ionospheric refraction error for SoL applications and potentially offers an almost perfect mitigation of ionospheric error through dual-frequency techniques.

To independently verify L5’s potential, a team of experts from the German Aerospace Centre (DLR) and the University of New Brunswick (UNB) captured and analyzed the new L5 signal from the beginning. A detailed look at the signal structure was undertaken using DLR’s signal verification and analysis facility (SVAF), a highly optimized and accurately calibrated facility for GNSS verification that also supports the European Galileo program.

The core of this facility is the 30-meter parabolic deep-space antenna at DLR groundstation Weilheim, Germany. Using the SVAF, a characterization of the L5 signal in the time and frequency domains has been carried out and the power levels and time variability studied. Figure 1 shows an example spectrum and a 10-microsecond snapshot of the Q-channel of the L5 signal. The figure indicates that the L5 signal is significantly band-limited by the output filters of the satellite. Furthermore, it appears to have a slight spectral asymmetry, requiring further study.

FIGURE 1. The new GPS L5 signal — example spectrum and snapshot (Q-channel)

For assessing the ranging performance, the L5 signal has also been tracked using NovAtel EuroPak-15a and JAVAD Delta-G2T receivers at UNB and DLR that were equipped with specially adapted firmware (Figure 2). Though the signal is not yet intended for precision measurements, preliminary results confirm favorable noise and multipath characteristics in comparison with the legacy L1 C/A code.

FIGURE 2. C/N0 of SVN49 L1 (red) and L5 (green) signals captured at
the University of New Brunswick

In the June issue of GPS World, we will publish a detailed article on the GPS L5 demonstration signal and its verification. We will discuss in depth the power behavior of the signal with respect to elevation angle and spectrum characteristics, present an analysis of the PRN code, and assess the ranging performance of the L5 signal in more detail.

— M. Meurer, S. Erker, S. Thölert, J. Furthner, A. Hauschild, DLR; R.B. Langley, S. Carcanague, University of New Brunswick

Witness to History

OVER THE COURSE of a 30-year military career, you get used to being up for some event or other at the break of dawn, or oh-dark-thirty as we fliers like to say. Many of those events have been special, as was the turn on of the L5 GPS next-generation civilian signal at 04:58:18 on April 10, 2009.

I was proud to be an invited guest of the GPS Wing at the 150-foot parabolic radio reflector antenna or “Big Dish” built and run by the Stanford Research Institute (SRI) for the Naval Research Laboratory.

The dish sits atop a serene setting in the Stanford Hills. This large, beautiful plot of land was dedicated to Stanford University by Leland Stanford, Sr., for perpetuity and will remain rural for as long as someone pays the lease. It sits in the middle of Menlo Park and its value must be astronomical, no pun intended, but on this beautiful morning as we waited for the sun, the signal, and history, it made the perfect setting. No one thought about land prices; nothing disturbed us.

It was dark, with birds calling quietly and cows lowing. The huge 300,000-pound dish and the building below it, all on a huge platform, moved silently along a prescribed 140-foot track at 1 degree per second to intercept the signal from Navstar 63, or GPS IIR(M)-20.

When the L5 signal came through, just a few seconds early, it was so perfect and so clear that initially we all thought it was a simulation. Then we were assured that it was indeed the actual signal from space. All told, there were only 12 of us on hand for the event, but all of us — and uncounted more — had been working hard and waiting years for it to take place.

Per Enge from Stanford and Tom Nagle from the U.S. Department of Transportation (DOT) were on hand, as well as personnel from the GPS Wing and SRI. The signal was also received by a 10-foot dish on campus in Menlo Park, by the Jet Propulsion Laboratory on its eight-foot dish, and by a specially configured Trimble GPS receiver. Captain Anil Hariharan from the GPS Wing initiated the required International Telecommunication Union paperwork immediately.

L5 is much more than just another signal from space. It is the culmination of a long, hard road for the GPS to broadcast a critical safety-of-life signal in the ARNS band, not only for the DOT, Federal Aviation Administration, and aviation — although that is a critical part of the plan — but for all civilians around the globe that need and want a protected PNT signal.
I do not have time or space to recount the struggle here, but it started in 1998 at a meeting in the Pentagon between key members of the government. The Secretary of the Air Force was present, and to say it was a contentious meeting would put it mildly.

From it came, among other decisions, the way ahead for the L5 signal broadcast today. From the halls of that five-sided building, the seat of military power amid the hustle and bustle of our nation’s capital, to the bucolic Stanford Hills 11 years later, the L5 signal managed to survive, and I was proud to be on hand to witness its birth.

Second Compass Satellite Launched

ON APRIL 15, China launched a second Beidou/Compass global navigation satellite, destined for geostationary orbit along the equator at an altitude of about 22,300 miles.

The Compass-G2 satellite has been characterized as of a “second generation,” representing a transition from a regional satellite navigation system, for which four test satellites were previously launched, to a global concept. China launched the first second-generation Compass satellite into medium-Earth orbit in April 2007.

China claims it may add as many as 10 more spacecraft to the global constellation by the end of 2010, with a goal of filling out a fleet of 30, in both geostationary and medium-Earth orbits, by 2015. Officials say the system will provide global coverage, “supplanting the U.S. GPS in Chinese cars, cell phones, and other commercial applications.” Among such, they cite transportation, meteorology, petroleum prospecting, forest-fire monitoring, disaster response, telecommunications, and public security.

China plans its space infrastructure “independent from foreign technology,” to provide critical navigation and positioning services, and bring social and economic benefits. The system will also offer “safer” positioning, velocity, and timing communications for authorized users, among which the Chinese military is sure to be paramount.

Conflicts. Interoperability of Compass with other GNSS remains an open question, although negotiations proceed along several levels. Particularly sticky is an impasse with Europe’s Galileo over use of a specified spectrum band. In March, Yin Jun, the director of European affairs in China’s Ministry of Space and Technology, stated that “we have made no concrete progress” in resolving differences, but held out “hope for results at our next meeting in June.”

New Space Center. China earlier announced founding of Shenzhen Aerospace Spacesat Co. Ltd., which may design and build some or all of the future Compass satellites. Located just outside Hong Kong, it is expected to develop six to eight types of satellites and produce four to five satellites every year for global navigation, telecommunications, remote sensing, and space exploration.

Septentrio Offers Live L5 Tracking

Septentrio Satellite Navigation, based in Leuven, Belgium, states that its commercial off-the-shelf GNSS receivers are now successfully tracking the new L5 signal transmitted from GPS satellite SVN49 since turn-on April 10. Researchers and other specialists interested in evaluating and monitoring GPS and Galileo signals on two common frequencies can now perform tests with live signals from both constellations on L1 and L5/E5a.

Septentrio receivers are now tracking a total of five signals broadcast by SVN49: L1-CA, L1-P(Y), L2-P(Y), L2C, and L5. Septentrio offers its customers a special firmware adapted to track the L5 test signal on the standard commercial platform PolaRx3G.

The company states that these receivers are also capable of tracking Galileo signals in the L1 and L5 bands, enabling dual-frequency GPS/Galileo trials using signals in the same frequency bands.

Hans van der Marel from Delft University of Technology, The Netherlands, commented, “It is very useful for researchers like ourselves to be able to simultaneously collect L1/L5 GPS and L1BC/E5a GIOVE measurements. During the previous night, we tracked a complete pass of GPS SVN49, together with GIOVE-A and GIOVE-B, and are excited to analyze one of the first datasets including both GPS L5 and GIOVE-A/B E5a measurements. One of our first observations is that the power of the demonstration GPS L5 signal is even higher than expected when the satellite is in the zenithal direction, while the opposite is true at lower elevations.”

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