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Re: GPS sat visibility



> From: wchast@utilpart.com
> To: amsat-bb@AMSAT.Org
> Subject: [amsat-bb] GPS sat visibility
> Date: Wed, 20 Jun 2001 10:48:04 -0400
> 
> I have a question, I need to capture data on how many gps sats are
> visible between tall buildings in NY city how long do I need to capture
> data on the sats to find out what the lowest visible count will be ??
> I only need to see one bird but we are looking at the sky between
> tall buildings. The app is for time disciplining.

I am surprised some more knowledgeable folks haven't responded to this,
so...

One approach is to ask your GPS receiver how many satellites it is
receiving.  Depending on the receiver, I believe that there are
several NMEA sentences that tell you how many satellites (even which
satellites) the GPS receiver is seeing.  I don't own an end-user GPS
receiver (one with a display and so forth), but my Garmin GPS-25
(an OEM receiver intended to be embedded in some other device; it
has an RS-232 port, but no display) tells me what satellites it is
seeing every second.

One approach is to get an OEM GPS receiver and connect it to a simple
program running on a laptop (that records how many or what satellites
the GPS receiver is receiving every second).  You can then post-process
the data to display them any way you want.

(There was a program called SAWatch that, among other things, displayed
the number of satellites being viewed by the GPS receiver.  I can't
find the original site for SAWatch, but there are still some copies
on the Internet.)

As long as your program is recording how many satellites are in view at
any particular instant, you probably ought to keep track of how much
of the time your GPS receiver is able to get a valid fix.

(I am still amazed at the amount that goes on in a GPS receiver.  They
get my vote for "the most math per cubic inch".)

An OEM GPS receiver, a laptop, a few hours of Java code, and a bunch of
batteries ought to give you some good initial data.  Better data probably
just required more batteries.

By the way, are you sure you only need to see one satellite to get an
accurate time fix?  Or, do some GPS receivers let you fix the location
so that the distance to (and latency of) the satellite can be determined?

Also, you might get better (or even some) answers if you ask this on
the TAPR APRS list.

-tjs
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