OPUS and the Local Weather Pattern
Good post Kent,
I remember a good series of posts some years back in which Bill Strange made some good comments on this subject. What you describe is a very REAL phenomenon and certainly has implications in the real world of GPS positioning.
As I [crudely] understand the operation of OPUS, the QC checks run on each CORS is pretty much limited to running TEQC “qc” mode to check for large cycle slips, and other “obvious” problems in the CORS RINEX file(s). Some of the “problems” associated with Electrical Storms parked over CORS sites, appear to be subtle enough to pass under the TEQC “radar,” BUT be significant ENOUGH to pollute an otherwise good OPUS Solution.
High Energy Electromagnetic Fields (like those associated with Thunder Storms) are NOT conducive to good GPS Data collection. Aside from the tropospheric and ionospheric delay variations, there are other issues involved that are far beyond my meager understanding of such things.
The real trick (as you pointed out), is to IDENTIFY those CORS that MIGHT be problematical, and exclude them from the OPUS solution. The same can be said for “roll-yer-own” (RYO) solutions. There are certainly ways in which one might accomplish this in a RYO solution, but OPUS isn't currently configured in such a way that one could “easily” accomplish this. That “might” change in the reasonably near future, but for now, we are on our own.
Running multiple solutions (2-3 submissions...6-9 CORS) is one way to spot anomalous CORS sites, but I have found that to be more reliable when you have two (or more) static receivers running simultaneously on a given day/project.
Loyal
Some Bum OPUS Solutions
BTW, here are the key elements of a couple of the the bum OPUS solution reports. Note that really the only clues that something was drastically haywire were the peak-to-peak spreads in the Latitude, Longitude, and Ellipsoid Height. While I haven't copied the individual new station positions, it was also true that no pair of the three agreed particularly well.
In case you aren't in the habit of requesting the Extended Output for an OPUS solution, that last bit of information is provided by it.
OPUS Solution #1 for Day 178
NGS OPUS SOLUTION REPORT
========================
EPHEMERIS: igr15900.eph [rapid]
START: 2010/06/27 14:29:00
STOP: 2010/06/27 20:00:30
OBS USED: 10690 / 10960 : 98%
# FIXED AMB: 66 / 73 : 90%
OVERALL RMS: 0.017(m)
REF FRAME: NAD_83(CORS96)(EPOCH:2002.0)
LAT: 31 23 45.46490 0.036(m)
E LON: 260 19 2.35633 0.112(m)
W LON: 99 40 57.64367 0.112(m)
EL HGT: 471.615(m) 0.096(m)
ORTHO HGT: 496.126(m) 0.096(m)
BASE STATIONS USED
PID DESIGNATION DISTANCE(m)
DF7477 TXSA SAN ANGELO RRP CORS ARP 75179.5
DI4783 JCT1 JUNCTION__TX2005 CORS ARP 102252.7
DH3770 TXLL LLANO CORS ARP 120741.9
OPUS Solution #2 for Day 178
NGS OPUS SOLUTION REPORT
========================
EPHEMERIS: igr15900.eph [rapid]
START: 2010/06/27 14:29:00
STOP: 2010/06/27 20:00:30
OBS USED: 10785 / 10978 : 98%
# FIXED AMB: 54 / 61 : 89%
OVERALL RMS: 0.017(m)
REF FRAME: NAD_83(CORS96)(EPOCH:2002.0))
LAT: 31 23 45.46544 0.105(m)
E LON: 260 19 2.35635 0.114(m)
W LON: 99 40 57.64365 0.114(m)
EL HGT: 471.608(m) 0.071(m)
ORTHO HGT: 496.119(m) 0.071(m)
BASE STATIONS USED
PID DESIGNATION DISTANCE(m)
DI4783 JCT1 JUNCTION__TX2005 CORS ARP 102252.7
DG9802 TXBW BROWNWOOD CORS ARP 77808.6
DH3770 TXLL LLANO CORS ARP 120741.9
Some Bum OPUS Solutions
This is pure speculation, of course, but the general pattern of your ellipse suggest that one or more of the CORS mounts may be prone to movement in heavy winds. The bias of your ellipse suggest a southeast movement in one or more of the base stations. If the antenna is mounted high up, such as the top of a tall building, the wind load during an extended storm front could well cause the building to "lean" to the southeast.
There is also the possibility that an antenna mounted on a metal frame would stay cooler on a rainy day, and not be prone to expand and contract in a normal pattern. I know that a tripod set up sturdy on a windy day will stay in level better than on a calm day, due to the cooling effect caused by the wind.
And don't forget that there may not be regular maintenance on the antennas, and any bolted structure will tend to "loosen up" over a period of extreme cooling and heating such as seen in Tejas.
1cm Semi-major Axis
> This is pure speculation, of course, but the general pattern of your ellipse suggest that one or more of the CORS mounts may be prone to movement in heavy winds.
If that were so, I'd think that the OPUS solution would have a high RMS. The RMS value was 0.014m and 0.016m for Day 179 and Day 178, respectively.
The semi-major axis of the 95% confidence error ellipse was 1.0cm and the semi-minor 0.8cm, so there really wasn't anything that unusual about the results.
[Edit: Hmmm, you may be suggesting that the antenna mounts were responsible for the decimeter-level peak-to-peak variations in the bum solutions. The RMS values are still quite low, which I'd think would tend to rule out decimeter-level movements in the antennas.]
