Methods and apparatus for orbit determination and time synchronization based on two-way user measurements

A satellite comprising:

• a transmitter and receiver unit for, in each of a plurality of time periods, communicating with a respective pair of ground stations comprising a first ground station and a second ground station, to exchange a first downlink message and a first uplink message with the first ground station and to exchange a second downlink message and a second uplink message with the second ground station, wherein the first and second downlink messages and the first and second uplink messages are time-stamped messages;
• and
• a processing unit for determining one or more orbit parameters indicative of an orbit of the satellite based on known positions of respective first and second ground stations relating to one or more of the plurality of time periods and based on first timing information relating to times of reception and transmission of respective first uplink and downlink messages and second timing information relating to times of reception and transmission of respective second uplink and downlink messages in the one or more of the plurality of time periods.