- Replaced simple residual ratio test by an F-Test
As soon as level of significance raises above α>85%, RTK solution type will change to the fixed state. I updated the ratio check to reflect a minimum of statistical hypothesis testing, well-knowning that state of the art is a Generalized Integer Aperture Estimation for Partial GNSS Ambiguity Fixing (GIAB) which is far beyond my mathematical and programming skills. -> see [1],[2],[3] for reference. - Updated ecef2pos and pos2ecef functions
The RTKLIB internal functions used to convert Earth-Fixed Earth-Centered Coordinates to Geodetic Coordinates were completely rewritten to reflect the closed form algorithm developed by Mr. Osen, which reduces the worst case transformation error by up to 500 million times. [4] - Improved baseline calculation
RTKLIB internal function used to calculate basline distances was updated to use Vincenty’s formula, which is precise to mm level. [5] - Implemented socket support for chrony daemon in RTKRCV
Add the following line to /etc/chrony/chrony.conf …
refclock SOCK /var/run/chrony.ttyRTK.sock delay 0.0 offset 0.0 poll 2 refid RTK precision 1e-9 prefer
… and restart chrony by service chronyd restart. Besides other minor improvements in the status report of the RTKRCV terminal, an additional line „chrony update delta“ will provide details about socket status and timediff submitted to chrony, e.g. -0.024404792s.
https://github.com/SJuhl75/RTKLIB/tree/precision
Using RTK-GNSS, the position can be determined extremely precisely to within a few centimeters. I wondered if in this context it is not also feasible to use the atomic clocks of the GNSS satellites as a precise source of a network time server. Unfortunately a corresponding setup is not very straightforward: RTKLIB’s streamserver has to feed the gpsd daemon which in turn has to feed the chrony network time client. By analyzing chrony and gpsd open source code I was able to realize a direct socket connection between RTKLIB’s rtkrcv (RTK receiver console app) and the chrony daemon.
Although the results look promising at first look (roughly 75% reduced delay of rtksvr/chrony vs. strsvr/gpsd/chrony „NMEA“ toolchain), the NTP servers connected via 100Mbit fiber optics uplink perform much better in terms of delay and accuracy.
MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== #? NMEA 0 2 377 1 +180ms[ +180ms] +/- 9838us #x RTK 0 2 377 5 +42ms[ +42ms] +/- 2060us ^* ntp1.belwue.de 1 6 377 15 +187us[ +168us] +/- 5008us ^+ ntp2.belwue.de 1 6 377 63 +40us[ +21us] +/- 4930us ^+ ptbtime1.ptb.de 1 6 377 17 +225us[ +206us] +/- 6373us ^+ ptbtime2.ptb.de 1 6 377 76 -149us[ -168us] +/- 6526us ^+ ptbtime3.ptb.de 1 6 377 92 -114us[ -120us] +/- 6401us ^+ ntp1.m-online.net 2 6 377 50 -226us[ -245us] +/- 7253us ^+ ntp2.m-online.net 2 6 377 11 -30us[ -30us] +/- 7587us ^+ ntp3.m-online.net 2 6 377 58 -270us[ -289us] +/- 7748us
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev ============================================================================== NMEA 63 29 257 +6.460 33.254 +187ms 5615us RTK 21 10 81 -317.239 297.191 +32ms 11ms ntp1.belwue.de 18 8 1099 +0.017 0.077 +271us 27us ntp2.belwue.de 64 39 67m -0.000 0.018 +35us 46us ptbtime1.ptb.de 17 9 1032 -0.014 0.055 +218us 18us ptbtime2.ptb.de 47 23 56m +0.011 0.016 +125us 30us ptbtime3.ptb.de 56 26 177m -0.001 0.003 +202us 23us ntp1.m-online.net 10 7 582 +0.081 0.193 -175us 23us ntp2.m-online.net 32 14 33m +0.061 0.052 +74us 49us ntp3.m-online.net 20 14 20m +0.125 0.122 +112us 51us
References
[1] The GNSS integer ambiguities: estimation and validation, Sandra Verhagen
[2] The GNSS Ambiguity Ratio-test Revisited: a Better Way of Using it.
[3] Data-Driven Generalized Integer Aperture Bootstrapping for High-Integrity Positioning
G. Nathan Green and Dr. Todd Humphreys, The University of Texas at Austin
[4] Karl Osen. Accurate Conversion of Earth-Fixed Earth-Centered Coordinates to Geodetic Coordinates. [Research Report] Norwegian University of Science and Technology. 2017. ffhal-01704943v2f
[5] Thaddeus Vincenty. Direct and inverse solutions of geodesics on the ellipsoid with application of nested equations. Survey Review XXII, 176, April 1975
