Optical GNSS Receiver for the ESA's NGGM-MAGIC Mission and for LEO Satellites with the Highest Orbit Accuracy
Authors: Svehla Drazen 
Published: March 2021

&lt;p&gt;Precise orbit determination (POD) of LEO satellites is done with a geodetic grade GPS receiver measuring carrier-phase between a LEO and GPS satellites, and in some cases this is supported with a DORIS instrument measuring Doppler between LEO and ground DORIS stations. Over the last 20 years we have demonstrated 1-2 cm accurate LEO POD and about 1 mm for inter-satellite distance. In order to increase the accuracy of the single satellite POD or satellites in LEO formation we propose an &amp;#8220;optical GNSS receiver&amp;#8221;, a cw-laser on a LEO satellite to measure Doppler between a LEO and GNSS satellite(s) equipped with SLR arrays and to develop it for the next gravity field mission.&amp;#160;&amp;#160;&amp;#160;&amp;#160; &amp;#160;&lt;/p&gt;&lt;p&gt;The objective of the ESA mission NGGM-MAGIC (Next Generation Gravity Mission - Mass-change and Geosciences International Constellation) is the long-term monitoring of the temporal variations of Earth&amp;#8217;s gravity field at high resolution in time (3 days) and space (100 km), complementing the GRACE-FO mission from NASA at 45&amp;#176; orbit inclination. Currently, the GRACE-type mission design is based on optical carrier-phase measurements between two LEO satellites flying in a formation and separated by 200 km.&lt;/p&gt;&lt;p&gt;We propose an extension of the GRACE-type LEO-LEO concept by the &amp;#8220;optical GNSS receiver&amp;#8221; to provide Doppler measurements between a LEO satellite and GNSS satellite(s) equipped with SLR corner cubes by means of a cw-laser onboard a LEO satellite. Such a &amp;#8220;vertical&amp;#8221; LEO-GNSS observable is missing in the classical GRACE-type LEO-LEO concept. If Doppler measurements are carried out from the two GRACE-type satellites in the LEO orbit to the same GNSS satellite and by forming single-differences to that GNSS satellite one can remove any GNSS-orbit related error in the measured LEO-GNSS Doppler. In this way, radial orbit difference can be obtained between the two GRACE-type satellites (free of all GNSS orbit errors) and complement &amp;#8220;horizontal&amp;#8221; LEO-LEO measurements between the two GRACE-type satellites in the LEO orbit.&lt;/p&gt;&lt;p&gt;The non-mechanical laser beam steering has been developed for an angle window of -40&amp;#176; to +40&amp;#176; and it does not require a rotating and a big telescope in LEO (no clouds and atmosphere turbulences in LEO). Therefore, in such a beam-steering window, one could always observe with a fiber cw-laser one GNSS satellite close to the zenith from both GRACE-type satellites. The non-mechanical beam steering concept in zenith direction can be supported by a small 10-cm like (fixed) Ritchey-Chr&amp;#233;tien telescope (COTS), a Cassegrain reflector design widely used for LEO satellites, e.g., for James Webb Space Telescope or for an optical Earth imaging with Cubesats with the 50 cm resolution.&lt;/p&gt;&lt;p&gt;Considering that several GNSS satellites in the field of view could be observed from a LEO satellite with this approach (including LAGEOS-1/2 and Etalon satellites) and the non-mechanical laser beam steering could be extended towards the LEO horizon, an &amp;#8220;optical&amp;#8221; GNSS receiver is a new concept for POD of LEO satellites. Here, we provide simulations of this new concept for LEO POD with GNSS/SLR constellations equipped with SLR arrays and discuss all new applications this new concept could bring.&lt;/p&gt;