GPS World, September 2010

LETTERS TO THE EDITOR L5 An Advisor Bids Farewell WWW GPSWORLD COM THE BUSINESS TECHNOLOGY OF GNSS MAY 2010 I wish to second Jim Spilkers 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 117645 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 Th e authors wish to acknowledge the contributions of Dr C R Cahn and Th omas 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 Th e L5 signal design included several innovations which infl uenced subsequent development of modernized GPS signals and of signals for other GNSS My ranking of the most important L5 innovations is Center frequency of 117645 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 Th e list doesnt include the 1023 MHz code clock rates or having two signals in phase quadrature which were included in the fi rst GPS satellite launched in 1978 Th e 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 Th is 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 fi rst 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 Th e 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 signifi cant boost to performance and has been adopted for almost every subsequent GNSS signal Th e problematic C A bit synchronization process has been eliminated by the data symbol overlay code or equivalent in all subsequent signals Th e CNAV message format was principally developed by Karl Kovach with signifi cant 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 signifi cant innovations mentioned above Charlie was key to similar improvements made in the subsequent L2C M code and L1C signal designs Tom Stansell Kauai Hawaii M any thanks for the kind invitation to GPS Worlds Leadership Dinner I have to decline as I wont attend ION GNSS this year I will retire from University College London at the end of September I dont 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 Ive very much enjoyed my association with GPS World and have benefited enormously from it Paul Cross 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 dont know anyone of any significance who doesnt 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 London UK GPS World September 2010 www gpsworld com 10