Why each GPS satellite holds four atomic clocks?

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Discussion Overview

The discussion centers around the rationale for each GPS satellite being equipped with four atomic clocks, exploring the reasons behind this design choice, including redundancy and reliability considerations. The conversation touches on historical context and comparisons to other systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that each GPS satellite holds four atomic clocks and questions the necessity of having four instead of fewer.
  • Another participant suggests that redundancy is a key reason for having multiple clocks, implying that this enhances reliability.
  • A participant queries why the number is specifically four, proposing that the choice could be influenced by the need for a voting mechanism similar to that used in space shuttle computers.
  • It is mentioned that earlier GPS satellites used three rubidium clocks for redundancy, and the addition of a fourth clock was a response to the development of cesium clocks, which later became the standard.
  • One participant emphasizes that the term "not cheap" is relative, noting that the overall costs associated with launching and maintaining equipment in orbit can overshadow the costs of individual components.

Areas of Agreement / Disagreement

Participants generally agree on the importance of redundancy in the design of GPS satellites, but there is no consensus on the specific reasoning behind the choice of four clocks versus other numbers. The discussion remains open-ended regarding the optimal number of clocks.

Contextual Notes

Some participants express uncertainty about the historical changes in clock technology and configurations across different satellite blocks, indicating a lack of clarity on the specifics of these developments.

Who May Find This Useful

This discussion may be of interest to individuals studying satellite technology, aerospace engineering, or those curious about the operational principles of GPS systems.

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Each of the 24 GPS satellites holds four atomic clocks! Why four and not one or two? They are not cheap!
Thanks
 
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Redundancy.

And compared to the cost of launching them into space, they probably are pretty cheap.
 
Ok, redundancy, but ... why 4 and not 5 or 3 or ... XY?

Thanks.
 
Perhaps for the same reason as the redundancy of space shuttle computers; they essentially 'vote' upon decisions. Three might not produce enough of a majority to be fail-safe, and five is overkill.
 
The block 1 satelites used rubidium clocks and had three of them as the normal backup - 3 can vote and still have a backup if one fails.
Prototype ceasium clocks where developed and one clock was added alongside the existing 3 ( so the rubidium ones could be used if it didn't work).
In later model satelites 2 rubidium and 2 ceasium were used, so each had a backup.
Then when ceasium proved best they were switched to all ceasium, but since the design hardware and software was for four clocks it was as easy and safer to fit four of them.

(Not sure of exactly what changes where when and on what block numbers)
 
Thanks for a great explanation, Mgb. :approve:
 
It's also worth pointing out that "not cheap" is relative statement. Nothing in orbit is ever cheap. So the differential cost isn't always driven by the cost of the instrument.
 

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