RTPK Elevations vs Base Processed RTK Elevation from 6 Mile Baseline

Tyler

Member
We've recently setup our Javad LS + units to use cellular corrections from our T3 Base. I thought I would share some of the elevation data we've gathered to discuss which should be used, RTPK or base processed RTK and what should be expected from longer Baselines..

**This is not an actual experiment but an analysis of data from a recent project.

Equipment: T3 Base station, Javad LS+ receiver with cellular corrections. Baseline of ~6 miles

Method: Set 3 Control Points using Total Station and Take repeated measurements on each with LS+. Process Base CORS & Base-Rover through DPOS (Process all GNSS turned off). Each observation was set to only RTK fixed, stop after 50 epochs, and at a minimum of 60 seconds with RTPK values stored for all. First and second set of observations were taken ~3 hours apart.\


Point 1Point 2Point 3
MeasurementRTPKRTKMeasurementRTPKRTKMeasurementRTPKRTK
am 1
443.892​
443.732​
am 1
366.463​
366.424​
am 1
458.08​
458.055​
am 2
443.885​
443.612​
am 2
366.487​
366.505​
am 2
458.499​
457.961​
am 3
443.934​
443.708​
am 3
366.543​
366.564​
am 3
458.347​
458.01​
am 4
443.911​
443.725​
am 4
366.607​
366.607​
pm 4
458.346​
458.155​
pm 5
443.832​
443.819​
pm 5
366.666​
366.616​
pm 5
458.303​
458.128​
pm 6
443.812​
443.889​
pm 6
366.697​
366.652​
Avg
458.315​
458.0618​
Avg
443.88​
443.75​
pm 7
366.711​
366.655​
Avg
366.5963​
366.5747​

Holding the RTPK Elevation at Point 1, the following elevations are from our Total Station which was a closed loop.
TS RTPK Avg RTK Avg
Point 1 - 443.88 443.88 443.75
Point 2 - 366.62 366.60 366.57
Point 3 - 458.20 458.32 458.06

Points 1 and 2 were in very open sky, and Point 3 had a decent amount of open sky with some canopy on either side.

I was impressed how close everything matched elevation-wise. But I'm wondering if RTPK should be held when dealing with elevations on longer baselines in the future or if RTK should be held. Based on what I found it seems that there was more redundancy using RTPK elevations. What results would we see on a 25 mile baseline?

Any thoughts?
 

Tyler

Member
What part of the T3 should we be pointing to the North? Is it the back of the unit or is it opposite like the LS?
 

Tyler

Member
RTPK should be more accurate that RTK if both the base and rover are aligned to north.

Would you expect the orientation to North to have that great of an impact on our values? In another one of your posts: post-51178 I saw you had 95% of your measurements from a study within 0.021 ft vertical. In my measurements above I was getting a difference in ~0.10 feet in back to back measurements and ~0.25ft under different satellite geometry.
 

Matt Johnson

Well-Known Member
5PLS
Would you expect the orientation to North to have that great of an impact on our values? In another one of your posts: post-51178 I saw you had 95% of your measurements from a study within 0.021 ft vertical. In my measurements above I was getting a difference in ~0.10 feet in back to back measurements and ~0.25ft under different satellite geometry.

In that test the baseline was less than 100' so the results will be better than with a 6 mile baseline.

The spec sheet shows the following accuracies when in the open:

1648567867628.png
 

Tyler

Member
In that test the baseline was less than 100' so the results will be better than with a 6 mile baseline.

The spec sheet shows the following accuracies when in the open:

View attachment 12780
Since your baseline was only 100' are there other studies that show the RTPK value to be better than the RTK value on long baselines (over 10km)? Does your study show that RTPK is just the better value to use in general, regardless of length of baseline? I saw the other post Alexey pointed me too but it seemed like there was not a clear answer regarding the few tenths of difference that appear on longer baselines.

Perhaps I'll run a static session on a control point next time I'm in the field and see whether the RTPK or RTK is more accurate when compared to a 4 hour static measurement.
 

Matt Johnson

Well-Known Member
5PLS
RTPK should always be better than RTK since it processes and filters all data forward and reverse and uses all signals where as RTK is looking at data epoch by epoch and is using a limited number of signals. RTPK is also using absolute antenna calibrations by collecting many points at the same location.

The LS can easily be configured to run tests with Auto Accept and Auto Repeat so anyone is free to run there own tests. Comparing a few points from a survey you are working on is not the optimal way to answer these type of questions.
 

nusouthsc

Active Member
We have recently noticed a seemingly scaled difference in RTK/RTPK from our office base. It appears to be showing up in just a mile or 2. By the time we reach 5 or 6 miles we see 0.10’ or more N/E separation on every shot. To test we have switched over to the network and get great agreement. Not sure whats going on.

I am going run a test soon at 1, 2, 3,… miles out to 10 or 12 miles to see if the error is consistently growing. Anyone else seeing that? Or anyone have any suggestions of things I can check?
 

nusouthsc

Active Member
I ran my little test today and pretty much got the same results. Within 100' of the base RTPK agreed perfect. At 5 miles it was about 0.12-0.14' difference. At 10 miles it had grown to 0.21' . I have sent the project to support "NuSouth Office Base Range RTPK" Serial# 00783.
I took most of the shots on our office base. However, I did take a few on the VRS network for comparison.

As another side observation, there were times during my shots that the LS showed 4 engine fixes and would not count any epochs. At one point it was almost 2 solid minutes of 4 engines fix and it didn't collect a single epoch. Is anyone else experiencing this? I reset engines several times and even rebooted the LS and no change. This does happen from time to time and its generally when I am in the wide open.
 
Last edited:

ken larson

Member
I have had 4 engine fixes latley in the wide open that were collecting epochs on one engine only, but did not pay too close att. as it worked itself out after min or two. I have not moved to the pre-release yet at have a ls+ and t3.
 

Alexey Razumovsky

Well-Known Member
JAVAD GNSS
5PLS
I ran my little test today and pretty much got the same results. Within 100' of the base RTPK agreed perfect. At 5 miles it was about 0.12-0.14' difference. At 10 miles it had grown to 0.21' . I have sent the project to support "NuSouth Office Base Range RTPK" Serial# 00783.
I took most of the shots on our office base. However, I did take a few on the VRS network for comparison.

As another side observation, there were times during my shots that the LS showed 4 engine fixes and would not count any epochs. At one point it was almost 2 solid minutes of 4 engines fix and it didn't collect a single epoch. Is anyone else experiencing this? I reset engines several times and even rebooted the LS and no change. This does happen from time to time and its generally when I am in the wide open.
Thank you for pointing this. We will investigate the issue.
 

nusouthsc

Active Member
Alexey,
I also saved a static file from the base while I was taking the shots. Let me know if you think you need it.
Thank you for your help.
 

Wes Hand

Member
I sent a project called RTPK TEST to support. I have been seeing the same thing that nusouthsc is reporting. Its so large of a difference that I have been reluctant to go more than 5 miles from my base. Please notice that the first point 1.1 has agreement with rtk & rtpk at within thousandths of a foot. The next 32 shots the easting differs 0.10'-0.18' in the east. Its like the vector for the rtpk is longer than the vector for the rtk result. My rover was roughly 9 degrees off of due west from the base and that is the direction. I have seen the difference at 5-7 miles and it grows with distance from base. I was 10.8 miles from the base.
SN 00162
 

Matt Johnson

Well-Known Member
5PLS
What is really needed is data with many points collected at the same location to see if the precision is good for both RTK and RTPK and if there is a percistant bias between the two.
 

nusouthsc

Active Member
Certainly, I will be glad to do this as this is becoming a crippling issue for us. We have a permanent office base that we are now not sure of its integrity. We use it daily and need to get this resolved asap. How many points do you suggest in one location? Also, would sets of points at different ranges be needed? We currently are using 1 min RTPK processing, is that sufficient for the testing?

There is for sure a bias for us. I worked for several hours this week at 4 miles and did not get one shot to agree less than N0.06/E0.06. And that was in the wide open as well.
Any suggestions would be greatly appreciated.
 
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