[time-nuts] Satelles PNT from Iridium satellites

[jimlux]

On 8/10/20 7:40 AM, Bob kb8tq wrote:
> Hi
> 
>  From what I saw as the system was developed, the people doing it realized that timing
> was at the core of the design. If there was a timing problem, nothing was going to work.
> There were GPS (and before that other sat-nav) presentations at the Frequency Control
> Symposium for many years. The “big boys” in timing all were involved in GPS one way
> or the other.
> 
> NIST was doing time transfer work on GPS before the system was fully up and running.
> Their results are (to a great degree) what got everybody believing  that GPS *could*
> be a good source of time. Those papers started early and kept on flowing …. Until they
> put their “stamp of approval” on the technique, I don’t think anybody was ready to call it
> a super time source.
> 
> This is by no means to imply that NIST was the only outfit involved or that the others
> who also evaluated GPS somehow did not contribute. That’s far from the case. The only
> point is that NIST got out there early.
> 
> Bob
> 


The original Woodford/Nakamura report from 1966

http://www.xyht.com/wp-content/uploads/2016/07/3-5-redacted-TOR-10012525-17-1-Briefing-navigation-satellite-study_Redacted.pdf


Very much describes the "requirements" as being able to target something 
with an uncertainty of <0.01 nautical Mile (about 20 meters) (the most 
stringent one I see) - there's a whole list of accuracy/cadence 
requirements on slide 27

They also describe the aspect that it's not necessarily absolute 
position that's important, but relative to the target, airport you're 
returning to, etc.  (implying that cartography and map models also 
needed to improve)

They don't really ever say they would be providing "time" per-se 
(comparing Loran C,D, Omega, Transit, and future system), but they do 
mention (page 47) that Cesium clocks are available.

They go through the trades and show that you need some sort of 
"simultaneous" scheme (unlike transit) to accommodate the "high fix 
rate" users - and compare angles, range rate, range differencing as the 
observables.
Page 57 (attached) is the one everyone's seen with the taxonomy of 
various schemes.

Nowhere in the report do they mention time, although they do say that 
one could put a system up without cesium clocks and improve it later.



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