GPS L5: The Real Stuff

June 18, 2010 By: Oliver Montenbruck, André Hauschild, Stefan Erker, Michael Meurer, Richard B. Langley, Peter Steigenberger GPS World

L5 Signal on New Block IIF Satellite Now Being Tracked

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.

Yesterday, 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.