The Origins of GPS, Part 2: Fighting to Survive
June 1, 2010 By: Bradford Parkinson, Stephen T. Powers GPS WorldFive Challenges, One Key Technology, the Political Battlefield — and a GPS Mafia
GPS Phase I program approval meant that the real work could begin. The conclusion of a two-part history, told by the people who made it.
With Gaylord Green, Hugo Fruehauf, Brock Strom, Steve Gilbert, Walt Melton, Bill Huston, Ed Martin, James Spilker, Fran Natali, Joe Strada, Burt Glazer, Dick Schwartz, Len Jacobson, AJ Van Dierendonck, and others.
By January 1974, the GPS program at the Joint Program Office (JPO) was well underway. With only about 30 officers, the workload was enormous. Fortunately, the Aerospace cadre of about 25 also made extraordinary contributions. In a flurry of activity, the team developed requests for proposals, made top-level specifications, and published initial interface control documents. The work of converting viewgraphs into real hardware, as many know, is an exacting and sometimes painful process.
Of course there were many challenges, but five of them, principally engineering, stand out as particularly daunting. These were:
- Defining the specific details of the GPS CDMA signal structure;
- Developing space-hardened, long-life, atomic clocks;
- Achieving rapid and accurate satellite orbit prediction;
- Ensuring and demonstrating spacecraft longevity approaching ten years;
- Developing a full family of GPS user equipment.
We discuss each challenge in detail, including the names of those most instrumental in meeting them. The first appearances of their names are highlighted, although if they appeared in Part One of this story (May 2010 issue), their names are not highlighted.
Early GPS manpack worn by JPO Army deputy Lt. Col. Paul Weber.
This photo graced the cover of the first-ever GPS brochure.
Challenge 1. Defining the specific details of the GPS CDMA signal structure (coherence, acquisition, spreading, communication protocol, structure, error correction, message structure, and so on).
The selection of the GPS signal structure was broadly confirmed with the tests that were run by program 621B at the White Sands Missile Range with the help of Joe Clifford, Bill Fees, and Larry Hagerman, all from the Aerospace Corporation.
While the fundamental decision to select CDMA had been made during the Lonely Halls meeting, a vast number of details had yet to be worked out. Fortunately, there were many earlier studies of the signal. Dr. Jim Spilker (then of Philco Ford), who had also written the major reference book on digital communications, authored one of the studies. Dr. Charles Cahn, Nat Natali, Burt Glazer, Ed Martin, and Dr. Robert Gold of Magnavox all made significant contributions. One of the most important details was the decision that the carrier, code, and data of the GPS signal would all be phase-coherent (Figure 1). As discussed later, this decision enabled much of the precision that we now see in advanced GPS receivers.
Figure 1. GPS signals were designed to be all aligned as transmitted, that is, coherent.
(Courtesy Misra and Enge, Global Positioning System).
The exact Gold codes family had to be selected from the original family, since Dr. Gold’s technique did not include the natural Doppler shifts. The data message was integrated into both the civil (C/A ) and military (P/Y) signals through inversion of their codes every 20 milliseconds.
To work out the details of the data message, the JPO had a strong team including Major Mel Birnbaum, Col. Brock Strom, and Capt. Bob Rennard. Outside contractors making major contributions included Dr. Fran Natali, Dr. A. J. Van Dierendonck, and others. Van Dierendonck played a particularly effective role in helping define “GPS time.” This sounds rather mundane, but had some very interesting complexity. Jim Spilker recommended the 1023-bit message length to avoid a correlation problem associated with Doppler shifts (this recommendation was incorrectly attributed in the last issue).
The data stream came down at 50 bits per second. Through this tiny pipe of information, all the precision of GPS had to pass. It included the space-vehicle orbit-position information (ephemerides), system time, space-vehicle clock-prediction data, transmitter status information, and C/A signal handover time to the P/Y code. Also as a part of the message, ionospheric-propagation delay models were incorporated for the single-frequency user. Further, to aid rapid acquisition of new satellites just rising over the horizon, the ephemerides of all other satellites in the full constellation had to be included. Each digital word had to be defined in terms of scaling, bias offset, and precision in terms of the number of bits transmitted.
About 95 percent of the GPS message has endured with no changes needed at all. In a few cases, because the newer user equipment is more accurate, greater precision is desirable. It is a great tribute to the brilliant engineers and scientists who designed the signal structure in 1975 that it has endured for 35 years with so little need for modification.
Some of the JPO Heroes at a "dining-in," a recognition dinner. From left, Major Mel Birnbaum (made many important contributions. He was famous for marathon code reviews that could last 18 hours straight. He hated to miss schedules!); Col. Don Henderson (later Maj. Gen.) second Air Force deputy; Major Ralph Tourino (later Maj. Gen.), Program Control; Lt. Col. Ken Juvette. director of procurement; and LCdr. Joe Strada, a key leader in the extensive test program.
1 2 3 4 5 6 7
RSS
Become a Fan
Join our Network
Follow Us
Please send any technical comments or questions to our webmaster.