Four-way Doppler tracking for lunar gravity measurements executed by Kaguya and its relay satellite: Okina

SELENE (Selenological and Engineering Explorer) is a Japan's lunar probe which was launched and injected into the lunar polar orbit in 2007. The Main Orbiter of SELENE named Kaguya has separated the Relay Satellite: Rstar (Okina). We have executed four-way Doppler measurements which determined the orbit of Kaguya aviating above the lunar far side. The ground station up-links ranging signals, and the relay satellite transponder on Rstar (RSAT-1) relays the carrier waves to Kaguya. Then the transponder on Kaguya (RSAT-2) receives the signals and returns to Rstar, and down-linked to UDSC. Three of four receivers through four-way links acquires signals and tracks Doppler frequency shift with phased lock loops. For that purpose, the receivers should be locked sequentially under the condition of large Doppler shift due to mutual velocities of three moving bodies. The receivers were designed in consideration of the above condition and system operability. Eventually, our system has realized the first case to track two fully moving links between the lunar orbiters and carried out Doppler measurements. Results of the orbit determinations for Kaguya above the lunar far side have shown the anomaly distributions of the gravity fields which could be scarcely found by the conventional two-way RARR methods.