Absolute coordinate system of GPS satellites

In 1977 my uncle had one of the first GPS systems available to civilians. He said that every so often the reported position would jump 50 feet or so to one side - deliberately designed that way so that the Russians couldn't use the technology for military purposes. (A 50-foot discrepancy could be confusing to a cruise missile.)

 

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Center of the Earth? Seems totally impractical. Wouldn't the center of the earth's GPS coordinates be ANY GPS coordinates with an altitude of the Earth's radius * -1? (Assuming the Earth was a perfect sphere).

 

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This seems to be an interesting booklet free from the UK Ordnance Survey
https://www.ordnancesurvey.co.uk/business-government/tools-support/os-net/coordinates
The link on that page is '' ''A guide to coordinate systems in Great Britain''
https://www.ordnancesurvey.co.uk/documents/resources/guide-coordinate-systems-great-britain.pdf
Quote from page 20 of booklet:
GPS positioning... My bold below...

March 2020

 

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I always thought that GPS is a result of local triangulation and expressed in latitude and longitude

Triangulation (Trilateration)

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https://www.nationalgeographic.org/photo/triangulation-sized/

How Does GPS Work?
The triangulation method

Exercises:
1. The GPS satellites move in near-circular circumterrestrial orbits with radii near 26,000 km. The period is around 12 hours. What is the speed in the orbit?

2. Assuming that the distance between a GPS satellite and the GPS receiver is 24,000 km. What is the time delay that would be measured? (The speed of light is 300,000 km/sec)

3. The best clocks on-board GPS satellites are accurate to about 1:10-14 (short-term stability). How long (in years) would it take for such a clock to be off by 1 second?

4. What kind of timing accuracy corresponds to a position uncertainty of 15 metres.

5. The circumference of the Earth is 40,000 km and corresponds to 360°. Which angle (in arcseconds) corresponds to 15 metres?

https://www.eso.org/public/outreach/eduoff/seaspace/docs/navigation/navgps/navgps-3.html[/quote][/quote]