[Xastir] GPS question, off topic.

[John Ronan]

>
Evening,
>
> What would the GPS be used for? APRS Tracking? Farming?
>
Farming actually.
> The Phoenix GPS claims sub-meter accuracy. Barring marketing lies,  
> this is not possible with only WAAS. WAAS would likely get you to 3  
> meter accuracy at best (Gerry N5JXS certainly knows more about this).
>
Ok thats what I was reckoning.
> The eDif feature mentioned for th Phoenix appears to attempt to  
> model ionosphere delays by analyzing multiple satellite signals  
> over time. Normally ionospheric delay (signal bending) correction  
> is done using a 2nd GPS frequency, on which the actual data is  
> encrypted, but the clock information can be extracted. This 2nd  
> frequency is what is normally referred to as "Military grade" GPS.  
> The eDif claims to be able to get the same type of correction data  
> out of a single frequency receiver. It also sounds like a costly  
> add-on.
>
ok
> The other differential positioning technologies mentioned are  
> likely DGPS,  where you have a separate GPS receiver at a precisely  
> known position which radios out the offsets (within some "local"  
> radius) between the GPS-derived position and its precisely known  
> position. DGPS receivers recieve and incorporate these offsets into  
> their calculations.
>
ok
> The question back to you,  I guess, would be: do your friend need  
> sub-meter accuracy and 10Hz position updates? (Most NMEA-out GPSs  
> output positions only at 1Hz or less, depending on the NMEA  
> sentences enabled.)
>
Ok.

Short answer is I'm not really sure

I'll try and address Curt and Gerry's questions as well.

Firstly.. WAAS signals aren't a problem, the only mountains are  
North, and going south, next stop would be Spain. Most of the land is  
pretty flat.

The system is being marketed to do several things (apologies, I'm not  
at my desk, so the piece of paper I wrote the URL on is not available  
to me.. scratch that.. found it. http://www.farmworks.co.uk/gps.php  
click on GPS Swath Guidance ), one of which is to allow him to spread  
fertiliser with as little overlap as possible.  This is because over  
the course of a season, the cost of the overlap is quite high  
apparently (in monetary terms).  It must be appreciable as he is  
looking for a method to reduce that cost, but is sceptical of the  
solutions presented.

Purely as an exercise, I was going to try the GPS-18 that I have  
already, put GPSman running (or something) and drive one or two of  
his fields.. just to see what he thinks...all the locals looking at  
their APRS displays will think I've gone barmy...

Regards
John



--
John Ronan <jronan at tssg.org>, +353-51-302938
Telecommunications Software &  Systems Group,  http://www.tssg.org