From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 



>From: root{at}mauve.demon.co.uk
>>
>>
>> I'd find that an amateur radio group would be something rare to be working
>> on a GPS locator system.  There are a lot of details which are not public
>
>All the details of the CA (coarse/aquisition) code are published.
>This is what is used in 99.9% of commercial recievers.
>
>> info on the data stream for a start and the frequencies are rather high as
>
>Frequencies are high, but technology has come a hell of a long way since
>the GPS system was designed.
>
>> well as the huge amount of calculations that need to be done on the input of
>> the device.  It took some serious manyears to just write the code for those
>
>Actually, for a very basic reciever, it's pretty simple.
>You take a code generator that generates a 1024 bit code for a satellite
>you hope is in the sky, running at 1Mhz (plus a hair)
>Now, take the incoming signal, and multiply with this code, until you get
>a signal.
>This is a lock, and now you slow down the 1Mhz code generator a fraction.
>Now you have the raw bit-stream from the satellite, from which you can
>obtain navigation messages and ephemiredes(sp??).
>Do this with three other satellites, and you've got a very basic fix, to
>within about 75m, once you've done the coordinate conversion.
>Track the phase accurately, and you can get a meter.
>You don't Need to use the ephemiredes, but it makes aquisition a lot faster.
>
>
>> little things and that sort of work will be a serious undertaking for
>> anybody.  Please note that the code isn't something that you can quickly
>> slapdash together with a windoz C++ program in a few weeks and have it
>> running somewhat by that time.
>
>It's not really complex.
>
>
>

Hi
 A little more work and you can add differential. This
gets close to the ability to determine direction with multiple
receivers/antennas. Still, most commercial machines have about 0.5 man
years in their software. Then again, they have more user interface. You can
buy complete GPS modules as well but they are not a cheap as a regular GPS.
 Some of the units have a one second pulse output that is
very accurate. I have such a receiver made by Trimble. You still need a
computer to display the results. Dwight

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