RSat (Relay Satellite), later renamed Okina, is a subsatellite to the Japanese SELENE
(Kaguya) lunar orbiter. It is to be used to provide four-way Doppler measurements of
Main Orbiter by using Relay Satellite for far-side gravity field for the first time. he
uplink radio wave from Usuda is relayed to the Main Orbiter via the relay satellite, which
is returned to Usuda via relay satellite again. Then the Doppler frequency is measured at
Usuda. Orbits of the sub-satellites are precisely determined by tracking the radio sources
onboard sub-satellites with differential VLBI, which contributes the accuracy of the
gravity field, especially on the lunar limb areas.
