K-band Ranging Instrument (KBR)

K-band Ranging Instrument (KBR) is the microwave inter-satellite ranging payload used by the GRACE and GRACE-FO twin-satellite gravity missions. The instrument measures changes in separation between paired spacecraft, nominally about 220 km apart, so orbit dynamics can be inverted into monthly gravity-field change ^[1][2]. GRACE-FO retained K/Ka-band microwave ranging as its science ranging system and added the Laser Ranging Interferometer as a technology demonstration ^[2]. On GRACE, K-band range-rate measurements were combined with GPS orbit data and accelerometer measurements to produce Level-1B inputs for gravity-field processing ^[1].