Which is better, Omnistar or JD Starfire?
Study and report by Lance Lefebure 3-14-2009
... and OmniStar wins ... by a lot.
I've heard numerous discussions about how SF2 and OmniStar XP compare to each
other, but the discussion has been largely based on what each company's
marketing department publishes. I decided to find out for myself how they
compare via some static accuracy testing.
Keep in mind that most companies publish position drift specs as pass-to-pass,
meaning how much movement you can expect to see over 15 minutes. In my testing,
I do 24 hour tests to eliminate variability caused by the satellite visibility
changing through out the day. All of my results are showing how much position
drift was seen over 24 hours. The pass-to-pass drift will be considerably less.
My test results are the average of 7 different 24 hours tests on each system. In
the event of a receiver reboot prior to the test, the receiver was allowed to
converge for a minimum of 4 hours prior to capturing any data. Over the course
of my testing, there were a couple storms. Results from those periods were
discarded as they showed even more drift than the 7 tests used in the
comparison.
To test SF2, I used a JD iTC receiver, tilt compensation was disabled, running
firmware version 3.05D.
To test OmniStar XP, I used a Trimble 252 running firmware version 3.57.
The OmniStar XP test was done last summer in July/August. The SF2 test was done
over the past 3 weeks and finished this morning. Due to the tests not being
conducted simultaneously, different solar conditions may explain some of the
extra position drift seen on SF2. A truly fair comparison would involve testing
both systems simultaneously.
In my testing, for the 3 Sigma results, SF2 had an average of 98% more position
drift than OmniStar XP.
OmniStar XP had a position drift of 4.19 inches or less, 99.7% of the time.
StarFire 2 had a position drift of 8.30 inches or less, 99.7% of the time.
Not a single one of the SF2 test results even approached the accuracy level of
the worst results I got on OmniStar XP, for any of the sigma levels.
INTERPRETATION: OmniStar XP exhibited in a mostly circular drift pattern, which
is what I expected. In contrast, SF2 seemed to have an affinity to drift to the
West and to the South East. I have no idea why, but that pattern is consistent
across all of the SF2 tests that I did. If you were using SF2 in a guidance
application, it appears you will see increased position accuracy by driving your
A-B lines in an generally East-West direction compared to lines going
North-South.
I really expected SF2 and XP to be similar. Some time ago, John Pointon (from
OmniStar) commented on this site that their XP signal and JD SF2 both come from
the Jet Propulsion Labs. Since they're using the same data stream, the accuracy
can probably be attributed to a difference in how that correction data is used
within the receiver.
OTHER NOTES: The iTC (at least on this firmware version) would randomly nag
about "reset tilt and turn logic for positioning. dt = 7.0000" approximately
every 10 seconds. This data showed up in the NMEA data stream. While this isn't
inherently bad, the nag message wasn't followed my a carriage return like other
NMEA data sentences are. Effectively, this would be on the same line as a GGA
string, causing that line to fail the checksum verification. This always showed
up on GGA sentences, and since my software uses the RMC sentences for position
data, I wasn't overly concerned with it. Strangely, this issue seems to
completely go away after the receiver has been running for a day or two, and
returns upon reboot of the receiver. In the future, I might want to look at
splitting the NMEA data at the dollar sign marking the beginning of the line and
discard any data before that.
Full test results (1, 2, and 3 Sigma distances):
http://spreadsheets.google.com/pub?key=pQEFqfMI4UEND1BBDtyJsag
-Lance
Edit: Special thanks to collegeboy for supplying me with the cable needed to
connect an iTC receiver to a computer.
Used by permission.
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