Positioning determination using one low-Earth orbit satellite

A position determining system for a satellite communications system, comprising:

• a user terminal;
• a satellite with a known position and a known velocity;
• a gateway for communicating with said user terminal through said satellite;
• range parameter determining means for determining a range parameter;
• range-rate parameter determining means for determining a range-rate parameter;
• and
• position determining means in said gateway for determining a position of said user terminal on the Earth's surface based on said known position and velocity of said satellite, said range parameter, and said range-rate parameter, said position determining means comprising
• means for generating a M.times.1 parameter vector z comprising said parameters, wherein M is the number of parameters determined;
• means for generating a position vector x representing an initial reference point;
• means for generating a partial derivative matrix H containing information regarding said known positions and velocities of said satellite and an Earth model describing the shape of the Earth wherein the relationship between x and H is given by ##EQU7## means for generating a M.times.M weight matrix W to emphasize the influence of specific parameters;
• and
• means for executing the iterative equation
• x.sub.I+1 =x.sub.I +(H.sup.T WH).sup.-1 H.sup.T W(z-z)
• wherein x.sub.1 and x.sub.I+1 are the current and next position estimates, respectively, and I represents the iteration number, until the difference between x.sub.I and x.sub.I-1 falls below a first predetermined threshold.

In a communications system comprising a user terminal, a satellite with a known position and a known velocity, and a gateway for communicating with the user terminal through the satellite, a method for determining the position of the user terminal, comprising the steps of:

• (a) determining a range parameter with respect to the satellite;
• (b) determining a range-rate parameter with respect to the satellite;
• and
• (c) determining the position of the user terminal on the Earth's surface based on the known position and known velocity of the satellite, said range parameter, and said range-rate parameter, wherein step (b) further comprises the steps of:
• (i) measuring, at the user terminal, a frequency of a first signal received from the gateway through the satellite;
• (ii) sending said frequency measurement of said first signal to the gateway;
• (iii) transmitting a second signal from the user terminal to the gateway through the satellite;
• and
• (iv) measuring, at the gateway, a frequency of a second signal received from the user terminal through the satellite, wherein said range-rate parameter represents said frequency measurements of said first and second signals.

In a communications system comprising at least one satellite with a known position and a known velocity and a gateway for communicating with user terminals through the satellite, a user terminal comprising:

• means for retransmitting a first signal received from the gateway through the satellite;
• and
• means for measuring a frequency of a second signal transmitted by the gateway through the satellite;
• means for sending said frequency measurement to said gateway;
• and
• means for transmitting a third signal to said gateway through the satellite;
• wherein the position of the user terminal on the Earth's surface can be determined based on said retransmitted first signal, said frequency measurement, said third signal, and the known position and known velocity of the satellite.