Do You Know OPUS Can Do This?

March 7, 2014  - By
Image: GPS World

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 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)
PID            DISTANCE(m)
xxxxxx       3125832.0
xxxxxx      3743350.2
xxxxxx      3756756.5
Not bad, considering the monster baselines. Yes, that’s 3+ million meters.
I ran the same data set later with better orbits available, as well as more GPS reference data became available.
SOFTWARE: page5  1209.04 072313      START: 2014/01/30  13:49:00
EPHEMERIS: igr17774.eph [rapid]                    STOP: 2014/01/30  21:13:00
NAV FILE: brdc0300.14n                        OBS USED: 15218 / 16133   :  94%
ANT NAME: NONE            NONE             # FIXED AMB:    74 /    87   :  85%
ARP HEIGHT: 0.000011                         OVERALL RMS: 0.013(m)
REF FRAME: IGS08 (EPOCH:2014.0814)
X:      xxxxxx3.383(m)   0.008(m)
Y:      xxxxxx1.704(m)   0.026(m)
Z:      xxxxxx9.425(m)   0.013(m)
LAT:  xxx xx xx.xxxxx      0.006(m)
E LON:  xxx xx xx.xxxxx      0.015(m)
W LON:  xxx  xx xx.xxxxx      0.015(m)
EL HGT:          386.851(m)   0.025(m)


xxxxxx        271186.8
xxxxxx      1277894.3
xxxxxx        978536.0

Wow, 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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About the Author: Eric Gakstatter

Eric Gakstatter has been involved in the GPS/GNSS industry for more than 20 years. For 10 years, he held several product management positions in the GPS/GNSS industry, managing the development of several medium- and high-precision GNSS products along with associated data-collection and post-processing software. Since 2000, he's been a power user of GPS/GNSS technology as well as consulted with capital management companies; federal, state and local government agencies; and private companies on the application and/or development of GPS technology. Since 2006, he's been a contributor to GPS World magazine, serving as editor of the monthly Survey Scene newsletter until 2015, and as editor of Geospatial Solutions monthly newsletter for GPS World's sister site Geospatial Solutions, which focuses on GIS and geospatial technologies.