I am struggling with the implementation of a Satellite Laser Ranging (SLR) validation tool.
The situation is as follows: I am provided with a framework which gives me the position of a SLR observation station and the position of a satellite at a specified GPS time. Those positions can be assumed to be sufficiently correct for the given task. From those positions I calculate a reference distance which I will use for comparison with given SLR observations. The observation files I use as input are in CRD format and provided by data centers like http://edc.dgfi.tum.de/en/. From those files I extract, for each observation record, the GPS time stamp and time of flight in seconds. According to the simplest observation equation for SLR given by $$d = c * \frac{\Delta t}{2}$$ I calculate the distance between station and satellite and compare it to the distance provided by my framework.
The problem is the distances deviate seemingly randomly with deviations ranging from 1 to 300 meters in both directions. I am aware that the simple observation equation is missing all correction terms but even without those an accuracy of up to 10 meters should be feasible.
So far I thought I was dealing with a station clock or observation clock bias but adding offsets in positive as well as negative direction to the observation times always resulted in greater deviations of the two distances.
The only thing I am constantly experiencing in all observations is (surprisingly) a greater deviation at steeper elevation angles, which is in contrast to my expectations, since this results in far shorter absolute distances.
One example of my results is the following observation from Graz Lustbühel of the 22 June 2017 monitoring the GRACE A satellite.
| standard Deviation 156.32185
| Time | Ref Distance| Obs Distance| Deviation | Elevation Angle
| ------------------------------------------------------------------------------
| 2017-06-22_13-57-47 | 573630.53 | 573510.16 | -124.37518 | 34.1093
| 2017-06-22_13-57-51 | 550922.4 | 550796.16 | -130.05672 | 35.989845
| 2017-06-22_13-57-56 | 528348.25 | 528215.69 | -136.19948 | 38.060675
| 2017-06-22_13-58-00 | 507464.43 | 507325.52 | -142.39058 | 40.19135
| 2017-06-22_13-58-06 | 484607.77 | 484461.27 | -149.80929 | 42.81356
| 2017-06-22_13-58-11 | 465610.45 | 465457.04 | -156.57817 | 45.279134
| 2017-06-22_13-58-16 | 449075.77 | 448915.81 | -162.99763 | 47.689646
| 2017-06-22_13-58-20 | 435233 | 435067.11 | -168.8336 | 49.94278
| 2017-06-22_13-58-25 | 424230.88 | 424059.89 | -173.84746 | 51.921769
| 2017-06-22_13-58-30 | 415972.62 | 415797.47 | -177.94548 | 53.540971
| 2017-06-22_13-58-35 | 411432.82 | 411255.13 | -180.45426 | 54.491375
| 2017-06-22_13-58-40 | 409363.89 | 409184.61 | -182.0285 | 54.951053
| 2017-06-22_13-58-46 | 411421.31 | 411242.31 | -181.76171 | 54.532429
| 2017-06-22_13-58-50 | 416021.81 | 415844.36 | -180.23949 | 53.598194
| 2017-06-22_13-58-56 | 427050 | 426876.64 | -176.23183 | 51.499173
| 2017-06-22_13-58-59 | 433701.19 | 433530.31 | -173.80953 | 50.323281
| 2017-06-22_13-59-06 | 452168.09 | 452003.85 | -167.28999 | 47.357755
| 2017-06-22_13-59-11 | 468417.38 | 468258.68 | -161.8747 | 45.049964
| 2017-06-22_13-59-15 | 487200.93 | 487048.25 | -155.9925 | 42.663639
| 2017-06-22_13-59-21 | 509791.27 | 509645.32 | -149.43676 | 40.116813
| 2017-06-22_13-59-26 | 532186.88 | 532047.1 | -143.43855 | 37.873845
| 2017-06-22_13-59-30 | 555171.16 | 555037.25 | -137.7555 | 35.809979
| 2017-06-22_13-59-35 | 580787.28 | 580659.4 | -131.91771 | 33.744171
| 2017-06-22_13-59-40 | 607376.04 | 607253.94 | -126.36003 | 31.816812
| 2017-06-22_14-00-11 | 792767.05 | 792675.37 | -97.569278 | 22.258777
Is there any phenomenon explaining this varying offsets?