RTPK - Post Processed GNSS - Long Range, Short Occupation Test
Saturday evening I took a few minutes to scratch an itch I've had for some time now. I wanted to test RTK and RTPK precision at long distances. Several years ago I tested RTK at distances of 10+ miles and found some very interesting results using GPS and Glonass which convinced me that our stated precisions at the time were overly pessimistic. With multiple constellations it seemed that we could probably achieve even better results today. With the old GPS and Glonass only v6 engines, the furthest I ever used RTK was about 25 miles. Precision started to open up and time to fix also opened up. The fixes were correct, but precision was so loose as to not really have a use for most of my surveying work.
I setup a Javad GNSS Triumph-3 at my office, using a Javad issued Verizon SIM card with static IP, transmitting RTCM3 MSM4 Short, GPS+Glonass+Galileo+Beidou corrections to my Javad GNSS Triumph-LS Plus with J-Field 184.108.40.2061 and GNSS 220.127.116.11918 firmware. I used Javad DPOS derived coordinates for the base (at present I'm not sure what the statistics are on the coordinates I used). I drove 30+ miles to a HARN monument at a county airport DATASHEETS (noaa.gov)
I occupied the monument for 30 seconds. Due to default settings, I did not immediately have an RTK fix, but I logged the data and processed using RTPK, the onboard post-processing application in J-Field. The baseline distance was 170255 feet (or 32.2 miles or 52km). I was shocked that RTPK reported a fixed solution. Moreover I was blown away by the repeatability of the following points collected and the low residuals compared to the published coordinate of the mark.
N 6898032.82 sFT
E 3259192.75 sFt
U 348.9 sFt
Texas North Central Zone
NAD83, 2011 Adjustment, Epoch 2010, Orthometric Height Geoid18
Point 1 - 30 seconds
N +0.084 E -0.009 U +0.3
Point 2 - 30 seconds
N +0.076 E -0.042 U +0.2
Point 3 - 180 seconds
N +0.023 E -0.038 U +0.25
Point 4 - 60 seconds
N +0.067 E -0.038 U +0.25
Point 5 - 60 seconds
N +0.062 E -0.034 U +0.1
Point 6 - 30 seconds
N +0.031 E -0.016 U +0.25
I would not consider this a thorough test. While I do trust the coordinates of the base and of the NGS monument, I cannot say that their precision is better, or worse, than the precision RTPK appears to produce. Also, I cannot say that through a 24 hour cycle that the precision would continue to be as good as what I saw here. The sample size is just two small and the test site too unknown to make definitive assertions. Having said all of that, I've been using GPS for more than 20 years now. There was a time when L1 only GPS was ubiquitous. The general useful range was limited to about 20km (12 miles) and precision was noticeably poorer at those ranges than at 10km (6 miles) or less. My dad and a few colleagues across the country experimented with long range L1 solutions with great success, but the required procedures to achieve those results was beyond practicality for most surveying applications. Later as we invested in dual frequency GPS equipment I found that I could acquire fixed positions, post processed, with only five minutes of data within about 10 miles of the base. I never really pushed the limits much further, except to say that even with OPUS and then DPOS, I would see how short an occupation I could use to get a reliable fix with the CORS spacing I had (about 30 miles). I found that 15 minutes was the bare minimum.
So you can imagine my surprise when I consistently acquired tight fixes using RTPK at 32 miles with only 30 second observations. Eventually I made some changes to a few settings in the advanced settings of the RTK engines which allowed me to get reliable RTK fixes at that range as well.
I do not know how important the addition of Galileo and Beidou were to getting a fix at this distance with these short observations, but I suspect it was critical. This definitely makes an office base station more practical for many surveyors.