Re: [LEAPSECS] RAS hits the news

From: Poul-Henning Kamp <>
Date: Mon, 26 Sep 2005 17:31:01 +0200

In message <>, "M. Warner Losh" writes:
>In message: <>
> Rob Seaman <seaman_at_NOAO.EDU> writes:
>: The question is whether "at least 20 minutes" (presuming this to be
>: accurate) is intrinsic to the system design or is rather a result of
>: poor implementation by some receiver manufacturers.
>A cold GPS receiver takes about 20 minutes to get the almanac data
>from the GPS constellation. It is intrinsic to GPS that this is the
>case. You cannot get around this.

Actually I think the number is around 15 minutes from the time you
get code data from the first satellite.

(I have always wondered why the almanac isn't staggered between
the satellites, that way you get get it at least four times faster
on average).

>Others are
>cold only if they have been off so long that they have no way of
>knowing the current correct leap count. Given that BIPM only
>publishes 6 months in advance, this means that the longest a receiver
>can be off is about 6 months.

I think you risk confusing two things here.

At least the Oncore receives will happily use a 2 year old alamanc
to aid in getting first fix.

But they will not use anything but the current almanac to report
leap seconds.

>Also, a GPS receiver that has cached the almanac can acquire
>satellites much more quickly than one who has to wait for the almanac
>to be downloaded.

This is btw, the original design rationale for the almanac data

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk_at_FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Received on Mon Sep 26 2005 - 08:41:48 PDT

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