Do You Know OPUS Can Do This?
I recently was involved in a project outside of the United States. Part of the project involved setting up a couple of RTK base stations. Of course, I wanted the antenna surveyed with reasonable accuracy with respect to ITRF. Even though supporting OPUS outside of the U.S. is out of the scope of the NGS mission (I assume), it works the same outside of the U.S. as it does within the U.S. Ok, somewhat the same.
As you imagine, the network of GPS reference stations outside of the U.S. is not nearly as dense as within the U.S., so you can remove OPUS-RS from the discussion immediately. OPUS-RS only requires a minimum of 15 minutes of data, but there must be three GPS reference stations within 250 km that form a polygon around your occupation point. Obviously, in many parts of the world, you aren’t going to be in a location that meets those specifications. Those requirements can be difficult to meet even in the United States. I recall a project on the West Coast where I had plenty of GPS reference stations within 250 km, but because I was near the Pacific Ocean, I wasn’t within the polygon of three GPS reference stations that OPUS-RS could find.
Back to my ex-U.S. project. With OPUS-RS being out of the consideration, OPUS-S was my choice. What you may not know is that OPUS doesn’t just look at CORS inside the U.S. when post-processing GPS data. It also looks at IGS Stations, which are located all over the world. Granted, I knew the distance to the GPS reference stations would be long, perhaps many hundreds of kilometers to each one, so I planned for long occupation times. This was easy because I was setting up high-quality (choke-ring) permanent antennas on building roofs. I set the GPS receiver to log data overnight at 15-second intervals.
I apologize ahead of time for needing to hide some of the data in order to preserve the privacy of my client, but you can try this same exercise on data you collect, or grab data from an IGS station and chop it into smaller pieces to process.
I logged data for about seven hours. Of course, I had ants in my pants, so I didn’t wait for the rapid orbits (used ultra-rapid), but knew I could reprocess at a later date and use rapid and precise orbits. Here’s what I got:
SOFTWARE: page5 1209.04 master51.pl 072313 START: 2014/01/30 13:49:00 EPHEMERIS: igu17774.eph [ultra-rapid] STOP: 2014/01/30 20:59:30 NAV FILE: brdc0300.14n OBS USED: 3219 / 10519 : 31% ANT NAME: NONE NONE # FIXED AMB: 39 / 56 : 70% ARP HEIGHT: 0.0001 OVERALL RMS: 0.015(m) REF FRAME: IGS08 (EPOCH:2014.0814) X: xxxxxxx.203(m) 0.396(m) Y: xxxxxxx.943(m) 0.287(m) Z: xxxxxxx.554(m) 0.173(m) LAT: xx xx xx.xxxxx 0.122(m) E LON: xxx xx xx.xxxxx 0.470(m) W LON: xx xx xx.xxxxx 0.470(m) EL HGT: 387.047(m) 0.212(m) BASE STATIONS USED PID DISTANCE(m) xxxxxx 3125832.0 xxxxxx 3743350.2 xxxxxx 3756756.5BASE STATIONS USED
xxxxxx 271186.8 xxxxxx 1277894.3 xxxxxx 978536.0Wow, the baselines sure improved, and that’s reflected in the solution. That’s because the GPS reference data isn’t immediately accessible from some IGS Stations. In the interest of privacy, I erased the Lat/Lon but kept the elevation. You can see the elevation difference between the two is about 20 cm. I assume it’s an improvement. For confirmation, I decided to run the same dataset through Australia’s AUSPOS online processing service.
X: xxxxxx3.390(m) 0.008(m) Y: xxxxxx1.676(m) 0.006(m) Z: xxxxxx9.405(m) 0022(m) LAT: xxx xx xx.xxxxx E LON: xxx xx xx.xxxxx W LON: xxx xx xx.xxxxx EL HGT: 386.822(m)The results were comparable to the OPUS solution, differing by 0.7cm in X, 0.08cm in Y and 2.9cm in Z.
AUSPOS used substantially more GPS reference stations (14 total) than OPUS:
STATION, Positional uncertainties (95%) for X, Y, Z (in meters)
XXXX , 0.012, 0.008, 0.016
XXXX, 0.008, 0.005, 0.013
XXXX 0.006, 0.005, 0.013
XXXX 0.009, 0.006, 0.021
XXXX 0.007, 0.005, 0.013
XXXX 0.006, 0.005, 0.012
XXXX 0.006, 0.005, 0.013
XXXX 0.006, 0.005, 0.013
XXXX 0.009, 0.005, 0.015
XXXX 0.006, 0.005, 0.013
XXXX 0.007, 0.006, 0.013
XXXX 0.006, 0.006, 0.013
XXXX 0.006, 0.006, 0.012
XXXX 0.008, 0.005, 0.015
Baseline distances ranged from 341 km to 3,700 km.
So, do I believe the OPUS solution or AUSPOS solution? I split the difference at the time. However, I set up the GPS reference stations in such a way that I can access them remotely and log data at any time from my laptop computer, so I’m running a series of eight-hour (or whatever in convenient) occupations and processing them through both services. So yes, OPUS is an international service (shsh, don’t let the bureaucrats and politicians know).
Thanks, and see you next month.
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