Laser GNSS Receiver for LEO POD, Laser Occultation and Time & Frequency Transfer of Optical Clocks in the Timing Labs
Authors: Svehla Drazen 
Published: March 2022

&lt;p&gt;At the last EGU and AGU conferences, we have proposed and demonstrated the feasibility of a laser GNSS receiver in the LEO orbit in order to provide carrier-phase measurements on a CW laser between a LEO satellite and GNSS satellites equipped with SLR arrays. This is a novel approach in space geodesy for precise orbit determination (POD) of LEO satellites and the gravity field mapping from space. Considering that the wet delay in signal propagation is typically 67x smaller for optical than for microwaves, we have extended this laser GNSS receiver to laser occultation for atmosphere sounding where use of a modulation on a CW laser could be applied to combine this method with the GNSS radio-occultation (GNSS-RO). In that case, one could compare in LEO orbit microwave GNSS measurements and CW laser measurements between a LEO and GNSS satellites from the top of the atmosphere down to the clouds and the lower troposphere.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Here we propose to further extend the laser GNSS approach in space geodesy, and to demonstrate the combination of a CW laser and GNSS measurements with a ground parabolic antenna of about 60 cm diameter. The CW laser and the receiving photodiode is to be placed in the optical center and collocated with the phase center of the parabolic GNSS antenna. If the same parabolic mirror is used as an antenna to track laser and microwave GNSS measurements to a single GNSS satellite in the zenith direction, all geometry effects can be removed (geometry-free), ending up with the Galileo satellite clock and GNSS receiver clock parameter being the only parameters of such a geometry-free ground-to-space optical/microwave metrology link for Galileo. Considering that optical frequency of a CW laser, stabilized by an internal cavity, can be provided with the frequency stability of &lt;7&amp;#215;10&lt;sup&gt;-16&lt;/sup&gt;, it can be transformed into a microwave band (with frequency comb) and with the same level of stability used as a reference frequency of the GNSS receiver. Therefore, one can use optical frequency of a CW laser via microwave Galileo signal to compare frequency of Galileo satellite clocks or optical clocks in the timing labs. Atmosphere effects for optical band (CW laser) can be applied a priori, whereas for microwave GNSS, troposphere zenith delays (TZDs) need to be estimated with the noise level of about a few millimeters in the zenith direction. Therefore, by selecting one Galileo satellite, close to zenith from two optical clocks on the ground, all Galileo satellite-related errors will be removed including Galileo satellite clock parameter, and time and frequency of optical clocks could be compared at the 10&lt;sup&gt;-17&lt;/sup&gt; - 10&lt;sup&gt;-18&lt;/sup&gt; frequency uncertainty level. This opens up the possibility of using Galileo by the timing labs for the generation of the official time (TAI, UTC) and for metrology in space, along with the laser GNSS applications in LEO orbit for POD and atmosphere sounding that very nicely complement the microwave GNSS.&lt;/p&gt;