The System: GPS L5, the Real Stuff

By Oliver Montenbruck, Andre Hauschild (DLR/GSOC), Stefan Erker, Michael Meurer (DLR/IKN), Richard B. Langley (UNB), and Peter Steigenberger (TUM)

The L5 signal of the new Block IIF satellite shows a very favorable signal strength (Fig. 1), which is somewhere in between the L1 and L2C signal strength for the employed antenna and slightly higher than that of the GIOVE-A/B satellites. While the L5 test signal of the second-last Block IIR-M satellite (PRN1/SVN49) is transmitted through a narrow beam antenna and shows a steep variation with elevation angle, the new satellite exhibits an almost constant flux irrespective of the boresight angle.

Following the successful launch of the first Block-IIF GPS satellite (PRN25/SVN62) on May 28, 2010 (UTC), and the activation of the legacy signals on June 6, users around the world have eagerly awaited the first transmission of PRN25 signals in the L5 band.

In June, at last, the L5 payload was activated for more than five hours transmitting nominal signals with the PRN25 ranging code. This enabled standard tracking receivers to collect the first real L5 measurements from the new satellite.

Scientists of the German Aerospace Center (DLR), the University of New Brunswick (UNB), and the Technische Universität München (TUM) spotted the first L5 data at 15:17:11 UTC from a station in Fredericton, Canada, followed a second later by stations in Japan, Singapore, the Canary Islands, and Germany. The stations are part of the CONGO network, which is the first global network of tri-band (L1/E1, L2, L5/E5a) GNSS receivers monitoring the GPS, GLONASS, GIOVE, and SBAS satellites. For background on the CONGO network, see the September 2009 GPS World article.

Fig.1 Carrier-to-noise-density ratio of GPS (left) and GIOVE-A/B signals measured at the Wettzell station on June 17, 2010. Red curves refer to signals in the L5/E5a band and include data from the PRN1 test satellite and the new PRN25 satellite.

The L5 signal of the new Block IIF satellite shows a very favourable signal strength (Fig. 1), which is somewhere in between the L1 and L2C signal strength for the employed antenna and slightly higher than that of the GIOVE-A/B satellites. While the L5 test signal of the second-last Block IIR-M satellite (PRN1/SVN49) is transmitted through a narrow beam antenna and shows a steep variation with elevation angle, the new satellite exhibits an almost constant flux irrespective of the boresight angle.

Fig. 2 Multipath plots of L1 C/A code, semi-codeless L2 P(Y) code, and L5 code tracking for the Singapore station of the CONGO network (10-second smoothing).

While the new Block IIF satellite has not yet been set healthy and made available for public use, the early measurements collected on June 17 already demonstrate good tracking quality. This is illustrated in Fig. 2, showing the so-called multipath combination for pseudorange measurements from L1 and L2 legacy signals (the upper two panels) as well as the new L5 signal for Singapore, which had continuous visibility of PRN25 during the period of interest. Except for low elevation angles that are affected by strong multipath from structures in the vicinity of the antenna, root-mean-square tracking errors well below 30 centimeters were obtained for all signals.

Fig. 3 L5 spectrum of PRN25 collected on June 17, 2010 with a 30-meter high-gain antenna at Weilheim, Germany.

In addition, the GNSS signal monitoring facility at DLR’s ground station in Weilheim has been used to record high-rate radio-frequency samples and spectra of the new signal, a snapshot of which is shown in Fig. 3. The raw sampling also confirmed that the L5 signal of PRN25 comprises both in-phase and quadrature modulation (in contrast to the PRN1 test signal, which contains a Q-component, only).

To the regret of U.S. scientists, the first publically traced L5 signals were only transmitted when the satellite was over Europe and Asia (see Fig. 4). Nevertheless, the test transmission provided an excellent sneak preview of what we can expect when the regular transmission starts. The satellite is presently expected to be set healthy and to start regular service by the end of August at the latest.

Fig. 4. The ground track of PRN25 during the transmission of L5 signals on June 17, 2010. Also indicated is the footprint of the satellite showing the 0°, 30°, and 60° elevation angle contours at the beginning of the transmission. The ground track is almost centered over Diego Garcia, one of the GPS monitoring stations.

Equipment. The CONGO network stations use JAVAD GNSS Triumph Delta-G2T/G3TH receivers. A Leica AR25R3 chokering antenna is used at Wettzell, while the Singapore station is equipped with a Leica AX1203+ GNSS antenna. The L5 spectrum was recorded with an Agilent PSA E4443A vector signal analyzer.

Beidou G3

China launched another Beidou/ Compass satellite, named G3, on June 2. By June 9, its apogee kick motor had placed the satellite in geostationary orbit at 84°38’ east, according to NORAD tracking reports.This is close to the position initially occupied by G2 (83°30’) before it started drifting. By June 9, G2 had drifted to 64°29’. By June 11, G3 had started transmitting signals on three frequencies.

China now has two properly functioning geostationary satellites in its second-generation system, out of a total of five it expects to place by 2012 for a regional operating system; also needed for this concept are four mid-Earth orbit satellites (one currently aloft), and five inclined geosynchronous orbit satellites (zero in orbit now). A planned global system would require 5, 27, and 3 satellites in GEO, MEO, and IGO orbits, respectively, by 2020.

Current regional-system signals on three frequencies use quadrature phase shift keying. Global-system signals will be binary offset carrier waveforms.

Opinions on SVN-49

The public comment period on proposed mitigation options for GPS satellite IIR-20M (SVN-49) ended May 28, and comments are viewable at www.regulations.gov under RITA Docket 2010–0002. Among others, the U.S. GPS Industry Council, NovAtel, Garmin, Septentrio, Raytheon, Boeing Commercial Airplanes division, General Motors OnStar, the European Commission, the MITRE Corporation, STMicroelectronics, the German Space Operations Center, and Cessna Aircraft have all filed comments expressing a preference for one option or another.

Unfortunately for the U.S. Air Force and the GPS Wing, no clear consensus emerges. Indeed, differences of opinion naturally follow the respective orientation of each company or organization toward their customers’ or members’ specialized needs.

Devote It to Science. Perhaps in recognition of this imbroglio, at the Air Force Space Command- Industry Exchange on June 15, Lt. Colonel Todd Parks briefed the PNT Functional Capability Team, explaining that the Air Force now was soliciting from industry “innovative applications” for the SVN-49 signal in space.This echoes a suggestion by Javad Ashjaee at last year’s unprecedented ION/ USAF session on SVN-49, where he proposed that the signal be used for studying multipath.

A website article at www.gpsworld.com/49opinions recaps commentary and preferred options from several companies and organizations.

The potential mitigations are each designed to reduce the impact of the unique nature — that is, errors — of the SVN-49 signal to a portion of the user segment. They are (so far):
1. Set healthy with current 152- meter antenna phase center (APC) and associated clock offsets.
2. Set healthy with factory APC offset.
3. Users switch to multipath-resistant receivers.
4. Modify receiver software to use look-up table corrections.
5. Increase user range accuracy (URA) index to a minimum value of 3.
6. Remove data modulation from L2 P(Y)-code, and
7. Change L2C PRN code to a “unique sequence.” (6 and 7 are considered a pair, to be jointly implemented for desired effect.)
8. Change SVN-49 from PRN-01 to PRN-32.
9. Use spare health code so future users could use SVN-49 despite unhealthy setting. For background on the SVN-49 situation, see Richard Langley’s Expert Advice column from August 2009. Briefly, the pseudorange data broadcast by the satellite contains larger than normal errors that vary according to the elevation of the satellite above the horizon.

The comments filed by the U.S. GPS Industry Council (USGIC), available as a PDF file at both URLs listed in this story, are the most detailed and extensive across all the options. However, the stated preference of the USGIC for Option 9 does not necessarily reflect agreement across all sectors of industry. As the USGIC points out, “Options 1 through 8 propose to designate SVN 49 as healthy using techniques that enable mitigation for some user applications, but that are unable to also mitigate adverse impacts to otherusers.”