Innovation: The WAAS L5 Signal

May 1, 2009 By: Hyunho Rho, Richard B. Langley GPS World

An Assessment of Its Behavior and Potential End Use

L5 signals have been continuously transmitted by a pair of satellites for the past several years. The geostationary Earth-orbiting (GEO) satellites used by the Wide Area Augmentation System to provide enhanced integrity and accuracy include not only an L1 payload but an L5 payload as well. Although the WAAS L5 signals are not yet intended for end users, can they be used now for positioning and navigation and, if so, are there any caveats?

INNOVATION INSIGHTS with Richard Langley

THE RECENT LAUNCH of the GPS Block IIR-20(M) satellite and the commissioning of its L5 demonstration payload herald the beginning of a bright new era in space-based positioning, navigation, and timing. The new satellite signal is anticipated to provide better-quality range measurements and possibly improve the tracking performance of a GPS receiver compared with current civil L1 and L2 signals through use of improved signal structures. The L5 signal will be standard on the future Block IIF and Block III satellites.

Richard Langley

However, some readers may be surprised to learn that L5 signals have been continuously transmitted by a pair of satellites for the past several years. The geostationary Earth-orbiting (GEO) satellites used by the U.S. Federal Aviation Administration's (FAA's) Wide Area Augmentation System to provide enhanced integrity and accuracy include not only an L1 payload but an L5 payload as well. While the WAAS L5 signals have been broadcast from space for some time, they did not come from a satellite in medium Earth orbit, and so it was necessary to include the demonstration payload on the GPS Block IIR-20(M) satellite to guarantee the L5 frequency filing with the International Telecommunication Union.

There are some differences between the WAAS L5 signals and the future fully fledged GPS L5 signals. The WAAS L5 signals only use a single-channel carrier (there is no quadrature or Q channel) and the data rate is 250 bits per second (bps) rather than 50 bps. The WAAS signals are actually generated on the ground and relayed through the GEOs using a "bent pipe" approach. The FAA uses the L5 signals, in conjunction with the L1 signals, to compute ionospheric delays as part of the closed-loop control of the broadcast signals.

Although the WAAS L5 signals are not yet intended for end users, can they be used now for positioning and navigation and, if so, are there any caveats? In this month's column, I am joined by one of my graduate students, Hyun-ho Rho, who has looked at the WAAS L5 transmissions, examining their signal strengths, multipath characteristics, and instrumental bias issues. Precise positioning performance of WAAS pseudoranges has also been assessed as an independent check on instrumental bias compensation by the WAAS control segment. The favorable results point to a future of the L5 signal, on both the WAAS satellites and the next-generation GPS satellites, which is bright indeed.

"Innovation" is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick.

As part of the GPS modernization effort, a new third civil signal, L5 at 1176.45 MHz, will join the current civil legacy signal on L1 at 1575.42 MHz and the second civil signal on L2 (L2C) at 1227.60 MHz, which is also being deployed during the modernization effort. This new satellite signal is anticipated to provide better quality range measurements and possibly improve the tracking performance of a GPS receiver compared with the current L1 and L2 signals by adopting improved signal structures.

This includes increasing the chipping rate to 10.23 megachips per second (Mcps) compared to 1.023 Mcps for the L1 C/A (C1) code, a longer spreading code than L1 C1, and a higher transmitted power than that of the L1/L2 signals (see TABLES 1 and 2). It will also be beneficial for mitigating the ionospheric error — potentially the largest GPS error source for most civil users — by use of the multiple frequencies. More detailed descriptions of the L5 signal can be found in publications listed in Further Reading.

TABLE 1 Characteristics of GPS L5 signal vs. WAAS L5

Since the Wide Area Augmentation System (WAAS) should be compatible with GPS modernization, both of the current WAAS geostationary Earth-orbiting satellites (GEOs), Intelsat's Galaxy XV (PRN135) and TeleSat's Anik F1R (PRN138), contain L1 and L5 WAAS payloads and broadcast both signals on the air. The WAAS payloads are operated by Lockheed Martin for the Federal Aviation Administration.

TABLE 2 Received minimal signal strength

The WAAS L5 signal structure is similar to the GPS L5 signal except that only a single channel carrier is used, and the data rate is increased to 250 bits per second (bps). The different characteristics of the GPS and WAAS signals are illustrated in Table 1.

A receiver equipped with specialized firmware that allows acquisition of both L1 and L5 signals simultaneously from the WAAS GEOs was used to obtain test data sets at the University of New Brunswick (UNB) in Fredericton, Canada. A data set spanning four continuous days in August 2008 has been used to study the WAAS L5 signals.

This article discusses the overall observation quality of the WAAS L5 signal. Since the carrier-power-to-noise-density ratio (C/N0) indicates the level of signal power versus the level of background noise in the observables, C/N0 was used as a first signal quality indicator, and the C/N0 values of the L1 and L5 signals were compared. The receiver tracking noise and multipath (MP) characteristics of the L1 and L5 signals were also compared. In this comparison, the magnitude of the possible improvement from the enhanced signal structures in the L5 signal is quantified.

In the following sections, the WAAS differential code bias (DCB) between L1 C1 and L5 code (C5) are analyzed. Since the WAAS GEO ranging signals are generated by the ground control segment and uplinked to the GEO satellites for rebroadcast, the ionospheric delays as well as the DCBs should be estimated and compensated for in both the uplink and downlink signals. Since another important role of DCBs in WAAS might be to resolve the clock referencing issue in the observables for single-frequency users (like the group delay term, TGD, for GPS L1/L2), the overall behavior of the estimated DCBs were further analyzed. Finally, the possible benefit of using WAAS GEO ranging measurements in the positioning domain is also discussed.