Re: Notions of Time in Computer Architecture

Date	2011-07-04 08:15 +0200
From	Terje Mathisen <"terje.mathisen at tmsw.no">
Newsgroups	comp.arch
Subject	Re: Notions of Time in Computer Architecture
References	(3 earlier) <ko13e8-bm4.ln1@ntp6.tmsw.no> <4e0f0fb6.148514@news.aioe.org> <VuLPp.21570$Md1.571@newsfe19.iad> <e762f889-bf20-45e9-b9fd-4a126c9901f5@e21g2000vbz.googlegroups.com> <iuqi5p$ghb$1@dont-email.me>
Message-ID	<bd28e8-dac.ln1@ntp6.tmsw.no> (permalink)

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Stephen Sprunk wrote:
> GPS is a prime counterexample: any given receiver must be able to track
> at least three satellites, and many can track a dozen or more.  The
> satellites move at speeds where relativity becomes relevant, and AIUI
> receivers have to compensate for it.  I don't know if the satellites

GPS receivers do compensate for one (purely) relativistic effect:

Since the orbits are only nearly circular, the sats experience slightly 
variable gravity fields, depending upon current orbit height.

This means that the onboard clocks run (very slightly) slower during the 
lower parts of each orbit and faster during the higher parts, right?

Each receiver will then adjust the received clock signal from each sat 
(via the calculated pseudorange and the current sat orbit position and 
calculate how much the onboard clock should be early/late due to the 
integrated gravity drag.

> talk to each other directly or if their signals are coordinated solely
> by ground stations, but the same concerns would apply there as well.

GPS sats don't talk between themselves, they are just updated by the 
monitoring base stations ("the control segment") exactly what they 
should transmit from now on, i.e. based on orbit and clock adjustments 
parameters.

The requirement is to keep each sat clock within 10 us of UTC(USNO), but 
they have been doing much better than this for many years.
>
> Keeping time is an embarrassingly parallel problem, but I wouldn't be
> surprised if the military had other satellites in orbit for which
> relativity was an even greater concern.

As in highly elliptical, semi-polar orbits? :-)

Terje

-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

Thread

Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-06-29 15:12 -0700
  Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-06-29 22:08 -0400
  Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-06-30 14:13 +0200
    Re: Notions of Time in Computer Architecture Morten Reistad <first@last.name> - 2011-06-30 18:19 +0200
    Re: Notions of Time in Computer Architecture ChrisQ <meru@devnull.com> - 2011-06-30 21:23 +0100
      Re: Notions of Time in Computer Architecture Rick Jones <rick.jones2@hp.com> - 2011-06-30 19:42 +0000
        Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-01 10:46 +0200
          Re: Notions of Time in Computer Architecture Rick Jones <rick.jones2@hp.com> - 2011-07-01 15:17 +0000
            Re: Notions of Time in Computer Architecture ChrisQ <meru@devnull.com> - 2011-07-04 16:51 +0100
      Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-01 05:54 +0100
    Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-06-30 22:42 -0700
    Re: Notions of Time in Computer Architecture jsavard@excxn.aNOSPAMb.cdn.invalid (John Savard) - 2011-07-01 20:16 +0000
      Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-02 10:33 +0200
        Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-02 09:26 +0100
        Re: Notions of Time in Computer Architecture jsavard@excxn.aNOSPAMb.cdn.invalid (John Savard) - 2011-07-02 12:40 +0000
          Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-02 16:14 +0100
          Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-02 16:48 -0400
            Re: Notions of Time in Computer Architecture Quadibloc <jsavard@ecn.ab.ca> - 2011-07-02 14:27 -0700
              Re: Notions of Time in Computer Architecture Stephen Sprunk <stephen@sprunk.org> - 2011-07-03 15:07 -0500
                Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-04 08:15 +0200
                Re: Notions of Time in Computer Architecture Morten Reistad <first@last.name> - 2011-07-04 17:50 +0200
                Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-04 21:21 +0200
                Re: Notions of Time in Computer Architecture Robert Wessel <robertwessel2@yahoo.com> - 2011-07-04 23:44 -0500
                Re: Notions of Time in Computer Architecture Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-07-05 07:51 +0200
                Re: Notions of Time in Computer Architecture Robert Wessel <robertwessel2@yahoo.com> - 2011-07-05 01:56 -0500
            Re: Notions of Time in Computer Architecture Morten Reistad <first@last.name> - 2011-07-03 19:28 +0200
          Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-03 11:27 -0700
            Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-03 22:13 +0100
              Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-03 18:53 -0700
                Re: Notions of Time in Computer Architecture nmm1@cam.ac.uk - 2011-07-04 08:34 +0100
            Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-03 18:51 -0700
            Re: Notions of Time in Computer Architecture anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2011-07-04 09:20 +0000
              Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-04 12:26 -0700
                Re: Notions of Time in Computer Architecture EricP <ThatWouldBeTelling@thevillage.com> - 2011-07-04 19:35 -0400
                Re: Notions of Time in Computer Architecture Bernd Paysan <bernd.paysan@gmx.de> - 2011-07-08 15:48 +0200
            Re: Notions of Time in Computer Architecture Michael S <already5chosen@yahoo.com> - 2011-07-05 15:24 -0700
              Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-08 09:25 -0700
                Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-08 12:12 -0700
                Re: Notions of Time in Computer Architecture "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-07-08 17:27 -0700
                Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-09 18:07 -0400
                Re: Notions of Time in Computer Architecture EricP <ThatWouldBeTelling@thevillage.com> - 2011-07-10 12:03 -0400
                Re: Notions of Time in Computer Architecture Robert Myers <rbmyersusa@gmail.com> - 2011-07-10 13:02 -0700
    Re: Notions of Time in Computer Architecture Bernd Paysan <bernd.paysan@gmx.de> - 2011-07-08 15:13 +0200
  Re: Notions of Time in Computer Architecture Gubbi <gubsan100@hotmail.com> - 2011-07-01 00:50 -0700

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