Displacements of the Earth’s crust caused by tidal and non-tidal loading forces are essential in space geodesy. Whereas tidal corrections are commonly accepted by the international scientific community and applied at the observation level, it is not recommended to apply the non-tidal displacement corrections.
The combination of optical (SLR) and microwave (GNSS, VLBI, DORIS) observation techniques requires special attention to be paid to atmospheric pressure loading corrections because of the so-called Blue-Sky effect. The Blue-Sky effect is directly related to the weather-dependency of the SLR observations. SLR measurements can be performed only under almost cloudless sky conditions, typically during high air pressure conditions which deform the Earth crust downwards, and thus, introduce a systematic bias in all SLR-derived products. On the other hand, the microwave techniques are weather-independent. Thus, applying the loading corrections reduces systematic effects from the SLR-derived parameters and enhances the consistency between SLR-based and microwave-based results by removing the impact of the Blue-Sky effect.
The impact of the Blue-Sky effect can be assessed as the difference between the mean atmospheric loading correction applied to SLR stations when an SLR station observes one of the two LAGEOS satellites, and the mean correction to SLR stations for the entire time series. The effect is in the order of 2.5 mm for many inland stations (note that there are no SLR stations in the region with the biggest atmospheric pressure loading deformation). The Blue-Sky effect reaches a maximum value of 4.4 mm for the SLR station located near Kiev, Ukraine. The largest magnitude the Blue Sky effect yields for inland stations in central Asia and Eastern Europe, where the largest atmospheric pressure loading deformation is expected.
Regarding the fact that some SLR stations continuously improve their tracking capabilities, the impact of the Blue-Sky effect becomes smaller for a few stations. The Blue-Sky effect was reduced, for instance, for Zimmerwald from 1.8 mm in 1999 to 0.5 mm in 2010, for Greenbelt from 0.9 mm in 1999 to 0.3 mm in 2010, and for Katzively from 3.1 mm in 1999 to 1.4 mm in 2010. The reduction of the Blue-Sky effect is especially visible for SLR stations that updated and automatized their laser systems or enabled day-time tracking capabilities. For the stations without significant tracking capability improvements, the Blue-Sky effect remains at the same level or even slightly increases.
Although the Blue-Sky effect is at the mm-level it should be considered in SLR analyses because all sources of errors leading to bigger discrepancies than 1 mm between space geodetic techniques should be taken into account in order to reach the goal of the Global Geodetic Observing System (GGOS) for the precision of station positions of 1 mm. The Blue-Sky effect exceeds the goal of GGOS for about 57% of all SLR stations.
The Blue-Sky effect on SLR stations (units: mm). Size of the circles is proportional to the number of normal points to LAGEOS satellites collected by the stations.
The Blue-Sky effect is about 1.1 mm on average over all SLR stations. The systematic shift of the SLR station height due to the Blue-Sky effect has a non-negligible impact on the scale derived from the SLR technique. The shift of 1.1 mm corresponds to a scale discrepancy of about 0.2 ppb w.r.t. the radius of the Earth. Therefore, the disagreement between the scale derived from SLR and VLBI, amounting 8 mm in ITRF2008, can be partly diminished when applying the atmospheric pressure loading corrections.
Applying atmospheric pressure loading corrections slightly improves the inner stability of SLR solutions and reduces the discrepancies between GNSS and SLR solutions. As a result, the estimated GNSS-SLR coordinate differences fit better by about 10% to the local ties at the co-located stations. The impact of the atmospheric pressure loading is clearly detectable in the current high-quality SLR products. Neglecting the loading corrections applied at the observation level, on the other hand, introduces systematic effects in all SLR-derived parameters, i.e., in station coordinates, geocenter coordinates, satellites orbits, Earth rotation parameters, global scale, and gravity field parameters. The total impact of loading deformations cannot be fully accounted for when the station coordinates are corrected in the a posteriori analysis because all estimated parameters are affected. The impact of the atmospheric pressure loading on, e.g., the polar motion is systematic with a dominating annual signal of the amplitude of 45 µas and 42 µas for the X and Y pole coordinates, respectively (see Figure below). Thus, the application of the loading corrections at the observation level is recommended for all space-geodetic solutions.
Differences of the pole coordinates derived from SLR solutions with and without applying atmospheric pressure loading (APL) corrections.
Related references providing further details and results.
Sośnica, K., Thaller, D., Dach, R., Jäggi, A., Beutler G. (2013). Impact of loading displacements on SLR-derived parameters and on the consistency between GNSS and SLR results. J Geod, 87(8): 751-769, doi: 10.1007/s00190-013-0644-1.
Sośnica, K. (2014). Determination of Precise Satellite Orbits and Geodetic Parameters using Satellite Laser Ranging. PhD thesis of the Faculty of Science of the University of Bern.
