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 -- [EMAIL PROTECTED]
Texas Mesonet -- AATLT, Texas A&M University
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843
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