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§ 003. How to check RTCM3 Network RTK (VRS) by WAY?
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KernelSAT
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2023-Sep-01
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Table of contents:
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Network RTK (VRS, MAC or FKP) may provide better RTK performance (accuracy, reliability, TTFF) being compared to
single physical station located on comparable distance.
Sometimes it is needed to check that coming corrections are actually from Network RTK engine, get idea where the nearest physical station is.
WAY service can do such simple analysis. The article described several practical examples of how to check VRS corrections w/ help of WAY service.
Please refer to
Basic RTCM3 analysis w/ WAY article
for general information of how to use and run WAY service.
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Virtual reference station (VRS) is the approach introduced by Trimble, calculates a
set of corrections from a virtual base station in the vicinity of a rover receiver. All baseline-length-dependent
errors, such as troposphere variation, ionospheric disturbances and orbital errors are removed from the
VRS because the virtual base is so close to the rover. The rover sends the position to the network in a
NMEA GGA message and the network software calculates the position of the VRS.
The VRS corrections are sent back to the rover and the rover calculates its position. Most
rovers receive VRS data for a calculated base station that is only a few meters away.
The VRS approach requires bi-directional communication for supplying the rover’s position to the networking software.
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Example: RTCM3 VRS corrections + WAY
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Let's consider some concrete RTCM3 corrections stream logged in Trimble VRS network.
Below is what one may typically see if such file is uploaded into WAY service for analysis.
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General Info
| Parameter |
Value |
| File name |
trimble_vrs.rtcm |
| File Size |
0.542 MB (568202 bytes) |
| Input decoder |
AUTO |
| Used decoder |
RTCM3 |
| Epochs |
| Rate |
1.00 Hz |
| Total |
737 |
| Time issues |
0 (0.000% ) |
| Missed |
0 (0.000% ) |
| Duration |
0 hours 12 min |
| Time start |
09:43:9.00 |
| Time Stop |
09:55:25.00 |
List of detected RTCM3 messages is presented below. [4094] confirms we're dealing w/ Trimble infrastructure. Messages [1030], [1031] & [1032] point on VRS.
Messages: RTCM3
| Message ID |
Good |
Bad |
Description |
| 1007 |
149 |
0 |
Antenna Descriptor |
| 1033 |
149 |
0 |
Receiver and Antenna Descriptors |
| 1005 |
148 |
0 |
Stationary RTK Reference Station ARP |
| 1230 |
148 |
0 |
GLONASS L1 and L2 Code-Phase Biases |
| 4094 |
295 |
1 |
Proprietary Message Trimble Navigation Ltd. |
| 1094 |
738 |
0 |
Galileo MSM4 |
| 1084 |
737 |
0 |
GLONASS MSM4 |
| 1074 |
737 |
0 |
GPS MSM4 |
| 1032 |
74 |
0 |
Physical Reference Station Position Message |
| 1030 |
92 |
0 |
GPS Network RTK Residual Message |
| 1031 |
92 |
0 |
GLONASS Network RTK Residual Message |
"RTCM3 General Statistic" inform us that: it is Network RTK service since Reference-Station Indicator DF141 field from messages [1005] or [1006]
is raised. Virtual station ID is 2689. Usually, VRS station is created just few meters away from rover location.
ID of nearest physical station (got from [1032] message) is 198. Since both positions of virtual and nearest physical stations are available,
WAY computes distance between these stations. Here it is about 8 km.
RTCM3 General Statistic
Some general statistic and information about RTCM3 stream.
| Parameter |
Value |
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| Base Station Coordinates [1005] / [1006] |
| Base Station ID |
2689 |
| Coordinates, m |
X: 3104792.401
Y: 1010836.216
Z: 5460690.718
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| Reference-Station Indicator DF141 |
1
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0 - Real, Physical Reference Station
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1 - Non-Physical or Computed Reference Station (Network RTK)
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| Receiver Oscillator Indicator DF142 |
0
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0 - All raw data observations may be measured at different instants
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1 - All raw data observations are measured at the same instant
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| Quarter Cycle Indicator DF364 |
0
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0 - Correction status unspecified
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1 - PhaseRanges are corrected
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2 - Phase observations are not corrected
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3 – Reserved
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| Receiver & Antenna [1007] / [1008] / [1033] |
| Receiver Description |
TRIMBLE ALLOY
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| Receiver FW |
Nav 5.43 / Boot 5.43
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| Receiver SN |
52237R38931
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| Antenna Description |
ADVNULLANTENNA
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| Antenna SN |
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| Nearest Physical Reference Station [1032] |
| Physical Station ID |
198 |
| Coordinates, m |
X: 3108110.298
Y: 1003125.866
Z: 5460205.950
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| Distance from Physical Station [1032] to Base Station [1005/1006] |
8.378 km |
WAY also shows these stations (both virtual and nearest physical) on Google Map.
Next, WAY provides analysis of RTCM3 stream and raise warnings in case of obvious issues are
detected (like no base position, base ID change, low number of satellites or bad SNR etc). Please check the article:
§ 002. How WAY service may help in RTCM3 corrections analysis and RTK rover troubleshooting?
Here (file w/ VRS corrections) all is fine, WAY didn't detect any obvious issues in RTCM3 stream. All is GREEN.
WAY RTCM3 Simple analysis
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WAY will try to highlight and rank most valuable and important characteristics of RTCM3 stream which may be useful in RTK troubleshooting. Definitely, WAY can't give 100% reliable answers in 100% cases on questions 'Why RTK doesn't work?' or 'Why RTK performance is not good?'. Both rover measurements and base corrections are needed to be analyzed together. However, WAY may point attention on some obvious issues.
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+3.0
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Good! Base coordinates messages [1005] or [1006] are available
RTK base position is VITAL information required for RTK rover operation. W/o it no precise positioning is possible.
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+1.0
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Good! Antenna Descriptor messages ([1007] or [1008] or [1033]) are available and Antenna type is provided.
RTK rover may have performance issues especially on long base lines if RTK Base antenna type is unknown. Some RTK rovers may even refuse to compose RTK positions at all w/o knowledge of Base Antenna type.
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+1.0
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Good! Receiver Descriptor messages [1033] is available and Receiver type is provided
RTK rover may have performance issues especially on long base lines if RTK Receiver type is unknown.
Information about Base receiver type may be used by RTK rover for so called inter-receiver GLONASS biases table. These pre-calibrated phase biases being used may speed up GLONASS ambiguity resolution if base and rover are of different manufactures.
Some RTK rovers may even refuse to compose RTK positions at all w/o knowledge of Base Receiver type.
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+3.0
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Good! Mean number (24.25) of available SVs is OK
Lack of SVs data coming from RTK base directly affects RTK rover performance (TTFF) especially on long base lines or under challenging environment.
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+1.0
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Good! Max GPS L1CA SNR (54 dbHz) on base is fine.
Low SNR on base may point on issues w/ antenna-feeder tract, wrong system installation, interference etc. All of it may directly affect RTK rover performance resulted in long TTFF, outliers and wrong fixes.
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+1.0
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Good! Mean GPS L1CA SNR (46 dbHz) on base is fine.
Low SNR on base may point on issues w/ antenna-feeder tract, wrong system installation, interference etc. All of it may directly affect RTK rover performance resulted in long TTFF, outliers and wrong fixes.
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+0.3
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Not Bad! There are just few messages w/ bad CRC (0.0298 %). Data link is fine.
Corrupted messages deprive RTK engine of corrections in time what may degrade RTK rover performance.
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+0.0
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INFO: According to Reference-Station Indicator DF141 field, corrections are provided not but single physical reference station but by some Network-RTK service like VRS / MAC / FKP or similar.
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+0.0
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INFO: Nearest Physical station location message [1032] detected. Means that corrections are very likely are provided by some Network RTK service like VRS or similar.
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Next, we may see statistic of number of tracked GNSS detected in RTCM3 observables messages.
Min / Mean / Max SVs Tracked
Min, mean and max number of tracked SVs per GNSS.
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SVs Tracked |
| GNSS |
Min |
Mean |
Max |
| GPS |
11 |
11.00 |
11 |
| GLONASS |
6 |
6.25 |
7 |
| GALILEO |
7 |
7.00 |
7 |
Maximum and average SNR per each detected GNSS and signal. Also gives
information about actually available signals types.
Max / Mean SNR
Max and mean SNR over all SVs of concrete GNSS and signal.
| GNSS |
Signal |
Max SNR, dbHz |
Mean SNR, dbHz |
| GPS |
L1CA |
54.00 |
46.00 |
| GPS |
L2P |
47.00 |
34.70 |
| GPS |
L2C(L+M) |
53.00 |
44.53 |
| GPS |
L5(I+Q) |
54.00 |
46.90 |
| GLONASS |
L1CA |
52.00 |
45.76 |
| GLONASS |
L1P |
50.00 |
44.38 |
| GLONASS |
L2CA |
50.00 |
44.50 |
| GLONASS |
L2P |
50.00 |
43.99 |
| GALILEO |
E1(B+C) |
52.00 |
47.84 |
| GALILEO |
E5A(I+Q) |
51.00 |
45.83 |
| GALILEO |
E6(B+C) |
56.00 |
49.66 |
| GALILEO |
E5B(I+Q) |
53.00 |
47.07 |
| GALILEO |
E5(A+B)(I+Q) |
54.00 |
49.37 |
SNR (or Code) measurements counters - allow to get idea what concrete satellites are provided by VRS service, what signals and
how many such observables.
Measurements counter (SNR)
Total number of SNR measurements for each GNSS / satellite per Signal.
GPS
Total number of SNR measurements for each satellite per Signal.
| SV |
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| G01 |
737 |
737 |
737 |
737 |
| G03 |
737 |
737 |
737 |
737 |
| G06 |
737 |
737 |
737 |
737 |
| G09 |
737 |
737 |
737 |
737 |
| G14 |
737 |
737 |
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| G17 |
737 |
737 |
737 |
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| G19 |
737 |
737 |
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| G22 |
737 |
737 |
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| G23 |
737 |
737 |
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| G25 |
737 |
737 |
737 |
737 |
| G31 |
737 |
737 |
737 |
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GLONASS
Total number of SNR measurements for each satellite per Signal.
| SV |
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| R03 |
737 |
737 |
737 |
737 |
| R05 |
737 |
737 |
737 |
737 |
| R13 |
737 |
737 |
737 |
737 |
| R14 |
185 |
185 |
185 |
185 |
| R19 |
737 |
737 |
737 |
737 |
| R20 |
737 |
737 |
737 |
737 |
| R21 |
737 |
737 |
737 |
737 |
GALILEO
Total number of SNR measurements for each satellite per Signal.
| SV |
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| E03 |
737 |
737 |
737 |
737 |
737 |
| E05 |
737 |
737 |
737 |
737 |
737 |
| E08 |
737 |
737 |
737 |
737 |
737 |
| E13 |
737 |
737 |
737 |
737 |
737 |
| E15 |
737 |
737 |
737 |
737 |
737 |
| E24 |
737 |
737 |
737 |
737 |
737 |
| E31 |
737 |
737 |
737 |
737 |
737 |
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The articles described example of RTCM3 VRS corrections pre-view and simple analysis done by KernelSat WAY service.
WAY could easily confirm that used corrections are provided by VRS network service.
Also it gives a possibility to check location of virtual station generated by VRS and nearest physical station used by VRS service.
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