# Radio coherent transponders are the foundation of deep space navigation, have optical analogues been used? If so, the furthest distance measured?

Coherent radio transponders receive a signal from Earth containing specially encoded modulation, amplify it and retransmit it back to Earth. Correlating the received signal with the transmitted signal allows very precise measurement of the spacecraft's range and its rate of change. (delay-doppler)

Optical links provide potential to improve on the signal to noise for these measurements at large distances because with the far shorter wavelength (micron vs centimeter) a 30 cm telescope on a spacecraft or a 3 meter telescope can have transmit and receive gains $$(d/\lambda)^2$$ 60 dB larger than a 3 or 30 meter radio dish, respectively. (caveat1)

An optical analog to a coherent transponder would not need the returning laser light to be coherent with the incoming light, but only an encoded, modulated signal in the light to be coherent in some way. It could be the traditional way this could be just transmitting a pulse or burst of light whenever a pulse or burst is received.

Question: Radio coherent transponders are the foundation of deep space navigation, have optical analogues been used? If so, the furthest distance measured?

1This is an over-predictor of overall S/N improvement because of the physics of detection; current optical detectors converting the photon's energy to carrier pair production whereas radio detection is based on the amplitude incoming electric field only.