[Xastir] GPS question, off topic.

[Lance Cotton]

John Ronan wrote:
> Assuming that WAAS signals are quite strong, what would the advantage be
> between a Phoenix 200 Smart Antenna
> http://www.ravenprecision.com/ca/Products/description.jsp?partNum=117-0171-071&Category=1&Type=1
>
> over a garmin GPS-18.
>
> It's for an agricultural guidance solution they are the two GPS options
> available. I suggested to him to borrow my GPS-18 (which I use with my
> OpenTracker 2) to test it before deciding on which to use, as I know
> WAAS signals are quite strong here.

What would the GPS be used for? APRS Tracking? Farming?

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).

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.

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.

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.)

-Lance KJ5O
-- 
J. Lance Cotton, KJ5O
joe at lightningflash.net
http://kj5o.lightningflash.net
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