# A New Online Service for the Validation of Multi-GNSS Orbits Using SLR
## 1 Introduction
### Ranging the GNSS Constellation  
Satellite Laser Ranging (SLR) is a technique of space geodesy for a direct monitoring of the objects in the Earth's orbit. Since 1964, the increase of both the precision of observations [3; 8; 9; 10] and the development of advanced mathematical models reducing the impact of the systematic errors, e.g., troposphere delay modeling for optical wavelengths [11; 12] could be observed. Nowadays, the SLR technique has assumed a leading position in space and satellite observation techniques among Very Long Baseline Interferometry (VLBI), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) and Global Navigation Satellite Systems (GNSS). The SLR measurements performed to the geodetic satellites (LAGEOS 1 and 2, LARES, Etalon, Stella, etc.) provide geodetic parameters for the International Terrestrial Reference Systems (ITRS) realizations: (1) the origin of the geocentric reference frame; and (2) the global scale [13; 14]. Moreover, SLR delivers values of the standard gravitational parameter, GM, and low-degree spherical harmonics of the Earth's gravity field [15; 16; 17]. On the other hand, nearly all GNSS satellites are equipped with the onboard Laser Retroreflector Arrays (LRAs). The scheduled modernization of GPS spacecraft to the Block-III will begin in 2018 and provide satellites with full SLR tracking support [18]. In the near future, there will be more than 100 GNSS satellites with the capability of SLR tracking [19]. Ranging the GNSS constellation using SLR could strengthen the combined precise orbit determination [20; 21] or even allow the orbit determination using solely SLR data [22; 23], validate new radiation pressure models [24; 25; 26; 27; 28] and enhance the co-location of techniques employing the co-location in space onboard GNSS spacecraft [29; 30]. SLR is also essential in providing information about clock behavior in space [31; 32]. However, the co-location in space requires an advanced analysis of all systematic errors which may limit the consistency level between GNSS and SLR techniques [30; 33]. These errors include: the satellite signature effect, i.e., the effect related to laser reflections from multiple corner cubes; range and time biases in SLR; differences between nighttime and daytime SLR observations due to using optical filters; variable position of satellite center-of-mass and errors in the laser retroreflector array optical centers; microwave antenna phase center offsets and their variations; and errors in modeling satellite attitude and orbits. Moreover, errors in the modeling of optical and microwave signal propagation in the atmosphere, such as higher order ionosphere delay corrections with signal bending for GNSS and currently neglected horizontal gradients of troposphere delay in SLR processing, may also limit the consistency level between SLR and GNSS [34]. The aim of GOVUS is to identify possible issues, such as range biases, orbit errors, signature effects, and wrong antenna offsets, all of which limit the consistency between SLR and GNSS, and introduce discrepancies between both space geodetic techniques. All of these issues must be first identified and then modeled or eliminated to provide a co-location in space free of systematic errors. The main merit of the SLR technique is the very high potential of the measurement accuracy as the result of beneficial propagation of a light pulse in the Earth's atmosphere [3; 11; 29]. The visible and infrared electromagnetic waves are almost insensitive to the water vapor, which is one of the main components of the Earth's troposphere and the source of the microwave delay in GNSS, VLBI or DORIS techniques. Consequently, the tropospheric delay is well modeled for SLR observations and additionally the ionospheric impact on laser beam is practically negligible [35]. These result in an accuracy of the range measurement at the millimeter level.