[Xastir] GPS question, off topic.

[Gerry Creager]

Couple of  minor points, although Lance's pretty well on-track.

Lance Cotton wrote:
> 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 Phoenix also claims to use a helical antenna.  There's little 
benefit of a quadrifilar helix over a dielectric patch antenna for 
low-angle satellites, and low angle satellites tend to detract from 
accurate positioning because of the interplay of more atmospheric 
traversal than higher-elevation constellation selection.  I think 
they're letting marketing invade technical space here.

> 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.

Aside from the fact that I've got and use dual-frequency receivers and 
have no current connections to the military other than having being 
married to a retired Army Nurse, dual-frequency is usually expensive to 
obtain. Relatively few are purchased compared to Garmin and other OEM 
and consumer units, so there's no economy of scale.  Although I've 
theorized about how to determine iono and tropo delays over the years 
using single-frequency techniques, it's virtually always required two or 
more receivers to make the theory work, over known baselines.

> 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.

Some experiments concerning "local radius" conducted here several years 
ago suggest that said radius was a tool designed by Trimble to sell more 
DGPS base stations... or more precisely, that "local radius" was pretty 
general, and on the order of hundreds to thousands of kilometers.

> 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.)

A lot of precision agriculture applications do need meter or better 
accuracies, especially if the GPS is driving the tractor.  I'm not sure 
the Phoenix is the right answer for this, though, unless I got more 
information and had the opportunity to play with a pair of them for 
evaluation.

gerry
-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
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