Tropospheric and range biases in Satellite Laser Ranging

Abstract

The Satellite Laser Ranging (SLR) technique provides very accurate distance measurements to artificial Earth satellites. SLR is employed for the realization of the origin and the scale of the terrestrial reference frame. Despite the high precision, SLR observations can be affected by various systematic errors. So far, range biases were used to account for systematic measurement errors and mismodeling effects in SLR. Range biases are constant for all elevation angles and independent of the measured distance to a satellite. Recently, intensity-dependent biases for single-photon SLR detectors and offsets of barometer readings and meteorological devices were reported for some SLR stations. In this paper, we study the possibility of the direct estimation of tropospheric biases from SLR observations to LAGEOS satellites. We discuss the correlations between the station heights, range biases, tropospheric biases, and their impact on the repeatability of station coordinates, geocenter motion, and the global scale of the reference frame. We found that the solution with the estimation of tropospheric biases provides more stable station coordinates than the solution with the estimation of range biases. From the common estimation of range and tropospheric biases, we found that most of the systematic effects at SLR stations are better absorbed by elevation-dependent tropospheric biases than range biases which overestimate the total bias effect. The estimation of tropospheric biases changes the SLR-derived global scale by 0.3 mm and the geocenter coordinates by 1 mm for the Z component, causing thus an offset in the realization of the reference frame origin. Estimation of range biases introduces an offset in some SLR-derived low-degree spherical harmonics of the Earth’s gravity field. Therefore, considering elevation-dependent tropospheric and intensity biases is essential for deriving high-accuracy geodetic parameters.

Satellite laser ranging to GNSS-based Swarm orbits with handling of systematic errors

“…When we estimate additional parameters, i.e., troposphere biases, horizontal gradients (TB, TB + G, RB + TB + G), or coordinate corrections (CRD + RB), the co-estimated range bias values change. Range biases are correlated with troposphere zenith correction and station coordinates, especially the Up component, as shown by Drożdżewski and Sośnica (2021). However, the origin of the systematic effect may be different for each station.…”

“…Different satellites at different heights and/ or 7-day solutions, as in Drożdzewski and Sośnica (2021), should be employed to improve the estimates and separate the SLR troposphere delay parameters and horizontal gradients from those that are satellite-and station-specific. Moreover, we tested a 1-day solution with an estimated daily range and tropospheric biases (RGB + TRP, not shown) in analogy to the 7-day, LAGEOS-based solution from Drożdżewski and Sośnica (2021). The estimated parameters from RGB + TRP better correspond to those from LAGEOS, but the reduction of residual statistics is relatively small and similar to the RB-D solution.…”

“…The source of range bias is related to a variety of factors, including station calibration error, deficiencies in the determination of station coordinates (station height in particular), errors of the measurement system, e.g., delays in system circuits and cables, or detector performance (Pearlman et al 1984;Appleby et al 2016). Tropospheric bias is related to troposphere modeling, errors in the meteorological records on the station, especially the atmospheric pressure, and calibration errors (Drożdżewski and Sośnica 2021). However, the nature of tropospheric biases is similar to elevation-dependent biases, which originate from the limitations of the detectors in the case of different returning signal strength at various elevation angles or other errors, such as the interval counter or event timer bias.…”

“…A time bias is caused by errors in time measurements, clock stability, event timer resolution, and station clock synchronization with respect to the UTC system (Exertier et al 2017;Otsubo et al 2019;Varghese et al 2019). Barometer biases have been discovered at SLR stations, e.g., Wettzell (7827), Graz (7839), and Borowiec (7811), and can reach the level of 1 to even 5 hPa (Wang et al 2020;Celka and Schillak 2003), resulting in elevation-dependent systematic errors (Drożdżewski and Sośnica 2021). Therefore, the ILRS investigates methods accounting for tropospheric errors in SLR and organizes a measurement campaign in 2022 using mobile barometers to compare barometer values and discover possible biases at SLR stations.…”

GENESIS: co-location of geodetic techniques in space

“…Range biases are calculated in SLR processing as additive constants in the modeled range, which should be in essence independent of the epoch of observation and measurement conditions, such as station elevation/azimuth angles, or measured range. However, the range biases not only compensate for ranging machine errors, but also absorb the modeling errors such as satellite center of mass offsets, orbit force model deficiencies, or tropospheric delay (Appleby et al 2016;Luceri et al 2019;Drożdżewski and Sośnica 2021). The presence of ambiguous range biases corrupts the estimation of fundamental geodetic products including site coordinates, the terrestrial reference frame scale and origin (geocenter motion), and geocentric gravitational constant (GM) (Couhert et al 2020).…”