The L5 Signal
I wish to second Jim Spilker’s comments in his recent letter to the editor regarding the two wonderful GPS history articles by Brad Parkinson. My endorsement of his comments also includes those about the origins of the L5 signal with reference to the 1999 paper by Spilker and Van Dierendonck, “Proposed New Civil GPS Signal at 1176.45 MHz.” Jim commented in the letter that “. . . . the work I did in designing the GPS L5 signal was performed as a gift to the U.S. Air Force, Federal Aviation Administration, and our country, . . .” It was a generous contribution, and I applaud it.
However, this leads me to comment on other very important but underreported gifts to L5 and subsequent signal developments. A small indication of the L5 contributions is given in the brief acknowledgement at the end of the referenced paper, “The authors wish to acknowledge the contributions of Dr. C.R. Cahn and Thomas Stansell in the selection of this signal.” It also is important to recognize that the L5 signal structure was formulated within an RTCA committee of mostly volunteers. Among other key participants, in addition to A.J., was Dr. Chris Hegarty.
The L5 signal design included several innovations which influenced subsequent development of modernized GPS signals and of signals for other GNSS. My ranking of the most important L5 innovations is:
- Center frequency of 1176.45 MHz in an ARNS band
- Two signal components, one with a data message and the other without (pilot signal)
- Forward error correction (FEC) (first GPS use, borrowed from WAAS)
- Overlay code to frame symbols and eliminate need for bit synchronization
- CNAV message structure for better accuracy and more flexibility
The list doesn’t include the 10.23 MHz code clock rates or having two signals in phase quadrature, which were included in the first GPS satellite launched in 1978. The new center frequency was recommended by Karl Kovach (then with ARINC and now with Aerospace) and adopted before the signal design began, but it was central to the L5 purpose of having a civil signal in an ARNS band. This same center frequency also will be provided by Galileo and Compass, so it was a vital innovation. Although forward error correction had been adopted for WAAS, the first use on GPS was the L5 design. In one form or another, it too will be used on most if not all other GNSS signals.
The second and fourth innovations in the list above were contributed by Dr. Charlie Cahn with help and encouragement from Richard (Rich) Keegan and myself. Having a dataless or pilot signal provides a significant boost to performance and has been adopted for almost every subsequent GNSS signal. The problematic C/A bit synchronization process has been eliminated by the data symbol overlay code (or equivalent) in all subsequent signals. The CNAV message format was principally developed by Karl Kovach with significant help from Art Dorsey of Lockheed Martin.
In summary, Brad Parkinson helped memorialize many of the early “GPS Heroes” who made GPS what it is today. Other heroes have contributed to GPS modernization, and credit should be given where credit is due. Brad mentioned Charlie Cahn, one of my real heroes, who helped shape the 621B and early GPS signals and has continued to contribute in many ways. In addition to the very significant innovations mentioned above, Charlie was key to similar improvements made in the subsequent L2C, M-code, and L1C signal designs.
— Tom Stansell
An Advisor Bids Farewell
Many thanks for the kind invitation to GPS World’s Leadership Dinner. I have to decline as I won’t attend ION-GNSS this year. I will retire from University College London at the end of September. I don’t plan to remain active in the world of GNSS after my retirement so this would be a good time for me to step down from the Editorial Board. I’ve very much enjoyed my association with GPS World and have benefited enormously from it.
I wish you and the magazine continued success. You have come a long way over the past twenty years or so and you are now, and have been for some time, the premier source of news (and very useful gossip!) relating to GNSS worldwide. I don’t know anyone of any significance who doesn’t read GPS World every month. Your highly accessible technical articles have been of enormous help to many cohorts of students here at UCL, and all over the world.
Take care and stay healthy! All the very best,
— Paul Cross