Randie Ohtsji [4555] wrote:
> Hello,
> I am interested in more information on GPS.  Does anyone have any good links?

Try:
http://www.inmet.com/~pwt/gps_gen.htm

This page gives a huge number of links. Look out for the pages that
describe "NMEA sentences" which explain the format of the RS232 data
that comes out of a GPS device. NMEA is "National Maritime Engineering
Association" and they have laid down the standards for decades; not just
GPS, but Loran, Decca, etc.

Newsgroup: sci.geo.satellite-nav gives extremely up-to-date info about
commercially available GPS devices and software (but not tonight, it
seems, because my newsfeed has gone down again). You can ask really
stupid questions and nobody minds. In fact they don't always notice.
Example (I read this recently): "Why don't marine GPS receivers give out
the altitude information sentences like the aircraft GPS receivers do?"
Answer: "Boats generally operate at sea level...."

Robin Lovelock's Home Page:
http://www.gpss.co.uk
Here, you can download huge amounts of his GPSS software, especially
maps of just about anywhere in the world. The system is meant for use in
vehicles - with the complete setup you talk to it and it answers
(obviously, you can't operate a keyboard and drive as well). For example
you say to it, "McDonalds", and it replies, "2 miles ahead, first
right".

Robin is a very kind man and you can ask him stupid questions, as I did.
His GPSS software suddenly stopped working for me. I had written a small
program, in Basic, on an old PC, that emulated a GPS receiver. It "flew"
from London to Edinburgh at any speed I set, and sent the data out the
RS232 at 4800 baud in NMEA sentence format, and into the RS232 on the PC
running the GPSS software. I can recommend this as a way of getting to
grips with NMEA data. You can keep clear of difficult maths by making
gross assumptions about the Latitude and Longitude of the source and
destination; find out how miles there are in a degree (Lat and Long) in
your part of the world, and then become a flat-earthist (for this
project only), and it's simple geometry.

The problem: it turned out Robin had at last correctly implemented the
checksum that follows an NMEA sentence (a simple little 1-byte thing, no
problem for a PIC). Some older GPS devices didn't bother with the
checksum, but GPSS ignores these sentences now. And quite right too,
since they might contain serious errors.

John Blackburn,
South London UK.