Jeff Epler's blog

27 August 2020, 21:10 UTC

GPS and relativity

This is info I've dug up a few times, but it's getting harder to find. Let's put it here, maybe I'll remember its on my own website. The ever-helpful leapsecond.com has a copy of an article from lsu.edu stating:

At the time of launch of the first NTS-2 satellite (June 1977), which contained the first Cesium clock to be placed in orbit, there were some who doubted that relativistic effects were real. A frequency synthesizer was built into the satellite clock system so that after launch, if in fact the rate of the clock in its final orbit was that predicted by GR, then the synthesizer could be turned on bringing the clock to the coordinate rate necessary for operation. The atomic clock was first operated for about 20 days to measure its clock rate before turning on the synthesizer. The frequency measured during that interval was +442.5 parts in 10¹² faster than clocks on the ground; if left uncorrected this would have resulted in timing errors of about 38,000 nanoseconds per day. The difference between predicted and measured values of the frequency shift was only 3.97 parts in 10¹², well within the accuracy capabilities of the orbiting clock. This then gave about a 1% validation of the combined motional and gravitational shifts for a clock at 4.2 earth radii.

It also contains supporting scans of NASA Technical Memorandum 78104, 1977 9th annual PTTI, NTS-2 report showing these statements, although the Technical Memorandum does not state whether the initial pre-tuning value was really due to "doubts" that relativistic effects were real, as compared to other operational reasons.

An earlier document, NTS-2 Cesium Beam Frequency Standard for GPS states (errors mine)

The NTS-2 program office at NRL was tasked by GPS NAVSTAR program office to generate a 10.23 MHz frequency for use with the Pseudo Random Noise System (PRNSA) onboard the NTS-2 Satellite. Frequency requrements for the NTS-2 Orbit Determination and Tracking Systerm (ODATS) was 5MHz with a tunable ΔF offset of approximately +1 × 10^-9 with a resolution of approximately 3 × 10^-12. The ΔF offset was to compensate for relativistic effects and could not be accomplished by offsetting the cesium standard which has a tuning range of ± 1 × 10^-11. The relativistic offset was later added to the 10.23MHz requirement.

We might try to detect a hint of emotion in "The offset was …later added to the requirement".

Citing "Alley, C., “Proper time experiments in gravitational fields with atomic clocks, aircraft, and laser light pulses”, in Meystre, P., and Scully, M.O., eds., Quantum Optics, Experimental Gravitation, and Measurement Theory", we have

There is an interesting story about this frequency offset. At the time of launch of the NTS-2 satellite (23 June 1977), which contained the first Cesium atomic clock to be placed in orbit, it was recognized that orbiting clocks would require a relativistic correction, but there was uncertainty as to its magnitude as well as its sign. Indeed, there were some who doubted that relativistic effects were truths that would need to be incorporated!

Alley: (again, errors mine)

There was considerable uncertainty among the Air Force and contractor personnel designing and building the system whether these effects were being correctly handled, and even, on the part of some, whether the effects were real. The last group was not satisfied until a gravitational frequency shift was measured with a GPS test satellite called NTS-2 by a group at the Naval Research Laboratory in 1977.

Alley cites T. McCaskill, J. White, S. stebbins, and J. Buisson, NTS-2 frequency stability results, "Proceedings, 32nd Annual Symposium, on Frequency Control," U. S. Army Electronics Research and Development Command, Fort Monmouth, N.J. (1978), a different paper with many of the same names as the first Technical Memorandum. However, I don't see that the citation supports the "considerable uncertainty" nor that any doubted "whether the effects were real". It just states that "the Einstein relativistic clock effect was verified to less than one-half percent".

So, left without a citation to follow, I am still in doubt as to whether there were doubters of general relativity on the project. Rather, except for the resigned voice that the offset was "later added" everything seems consistent with: GR (at least once the issue was raised) was assumed correct, but for other reasons (such as synchronizing it with the ground based cesium clocks before launch) the system was built with two modes, and that a story about doubters grew in the retelling.

TODO: Actually read ashby1979.pdf and see if it adds information compared to any of the above.

Files currently attached to this page:

1977ptti.conf..637W.pdf	2.3MB
1978-PTTI-v9-NTS-2.pdf	1.7MB
Ashby2003_Article_RelativityInTheGlobalPositioni.pdf	460.8kB
a088092.pdf	1.1MB
alley1983.pdf	7.8MB
ashby1979.pdf	2.9MB

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