LEO-to-GNSS Laser Interferometer for Space Geodesy with Laser DORIS and Laser SAR

In order to increase the accuracy of precise orbit determination for a single satellite or satellites in LEO formation, we propose using a LEO-to-GNSS laser interferometer, what we call a “laser GNSS receiver”, to measure the Doppler shift with a continuous-wave (CW) laser between LEO and GNSS satellites equipped with SLR arrays (Galileo, GPS, GLONASS, Beidou). LEO orbit is above atmosphere (no  atmospheric attenuation and turbulence in laser signal) and this makes the “laser GNSS receiver” very attractive for future LEO missions. At the EGU and AGU conferences, over the last several years, we have presented the link budget, design and feasibility of such a new instrument in space geodesy and discussed applications in: reference frame missions; gravity field missions; laser atmospheric sounding (above the clouds) and combination with microwave GNSS-RO; time/frequency transfer for ground optical clocks at 10^-18 frequency uncertainty (TAI, UTC); and Earth-to-Moon laser interferometry using an ILRS telescope. Here we extend this new instrument in space geodesy to laser DORIS and laser SAR.

Laser altimetry is an established technique that uses a pulsed laser to measure a range from LEO orbit to the ground in the nadir direction. In a similar way, interferometric laser tracking could be established on the continuous-wave laser signal transmitted from the LEO orbit in the nadir direction and reflected from the ground. This could be done, e.g., by modulating a microwave-like signal on a CW laser, providing a microwave phase on a laser carrier. The main advantage of the laser SAR/inSAR is that microwave modulation on a laser carrier is not going to be affected very much by the wet delay of the atmosphere and in this way does not require radiometers in LEO to correct atmospheric propagation effects, and instead, they can be corrected a priori using models like those used for the SLR measurements.

Therefore, compared to the microwave SAR/inSAR, laser SAR/inSAR opens up the possibility of using the SAR/inSAR technique along with space geodesy techniques if permanent geodetic stations are equipped with the well-defined laser retro-reflectors on the ground. Compared to the pulsed lasers used by ILRS, a continuous-wave laser is more appropriate for higher laser powers since the lower laser peak power avoids damage to the transmitting optics and allows simplified optics with non-mechanical laser beam steering. We present link budget of such a laser DORIS technique to observe Doppler shift from LEO orbit to the ground laser retro-reflectors and laser SAR/inSAR based on laser signal reflected from the ground surface. The IceSAT-2 mission from NASA indirectly confirmed the link budget with the onboard pulsed laser used for laser altimetry, opening up the possibility of a laser SAR/inSAR technique from LEO.