Low-power satellite-based geopositioning system
First Claim
1. A method for determining a position of a transceiver on the surface of the earth comprising the steps of:
- a) transmitting a signal from the transceiver in response to a query from a signal from a satellite;
b) transmitting the response after a precisely controlled time interval after the transceiver receives the query;
c) estimating a length of a propagation path from the satellite to the transceiver from a time delay in the response;
d) measuring a Doppler shift in the response from the transceiver;
e) estimating a first derivative of a path length from the satellite to the transceiver from the measured Doppler shift;
f) estimating the satellite position and velocity from satellite telemetry data;
g) determining an angle between the direction of satellite motion and a line of bearing to the transceiver from the first derivative and the satellite position and velocity; and
h) determining a position of the transceiver on the surface of the earth as being one of two points where the surface of the earth intersects with a base of a cone defined by the angle in step g) and the estimated path length.
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Abstract
A Low Earth Orbiting satellite system provides location and data communications services to mobile users equipped with a receiver/transmitter. The receiver/transmitter acts as a transponder that responds to a query transmitted over the satellite network. The response is sent after a precisely controlled time interval after the transponder receives the query so that the ground station can estimate the length of the propagation path from the satellite to the transponder. The transponder also transmits the response at a frequency that is proportional to the frequency of the received query so that the ground station can estimate the first and second derivatives of the length of the propagation path according to the measured Doppler shift. The ground station also estimates the satellite positioning using telemetry from the satellite obtained from the on-board GPS receiver. The position of the user terminal relative to the satellite position is then determined from the path length measurements. Given the satellite position and velocity, the measured path length and first and second derivatives determine the angle between the direction of satellite motion and the line of bearing to the user terminal. This angle defines a cone with the satellite at the origin. The user terminal position is somewhere on the circle defined by the cone and the estimated path length. The intersection of this circle with the surface of the Earth yields two possible user positions, which ambiguity can be resolved by three techniques: (1) use of knowledge of which beam the signal was received in; (2) use of earlier position data; or (3) using nearby satellites to receive the signal. The user terminal uses a single frequency reference to provide timing for the receive and transmit frequency synthesizers and for the analog-to digital and digital-to-analog converters. The frequency tracking algorithm shifts the baseband frequency of the response by a factor k so that the output frequency is related to the frequency of the received signal by the same factor k. This eliminates the absolute frequency of the reference as a source of error and allows the use of less expensive oscillators in the user terminal. In addition to the above, the present invention is able to rapidly acquire the signal at very low signal levels in the presence of large frequency uncertainties.
85 Citations
10 Claims
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1. A method for determining a position of a transceiver on the surface of the earth comprising the steps of:
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a) transmitting a signal from the transceiver in response to a query from a signal from a satellite;
b) transmitting the response after a precisely controlled time interval after the transceiver receives the query;
c) estimating a length of a propagation path from the satellite to the transceiver from a time delay in the response;
d) measuring a Doppler shift in the response from the transceiver;
e) estimating a first derivative of a path length from the satellite to the transceiver from the measured Doppler shift;
f) estimating the satellite position and velocity from satellite telemetry data;
g) determining an angle between the direction of satellite motion and a line of bearing to the transceiver from the first derivative and the satellite position and velocity; and
h) determining a position of the transceiver on the surface of the earth as being one of two points where the surface of the earth intersects with a base of a cone defined by the angle in step g) and the estimated path length. - View Dependent Claims (2, 3, 4, 5)
where v represents the satellite velocity and d represents the first derivative of the path length.
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3. The method according to claim 1, further comprising the step of:
i) determining on which of the two points that the surface of the earth intersects with the base of the cone from step h) the transceiver is located from which satellite beam on the satellite received the response from the transceiver.
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4. The method according to claim 1, further comprising the step of:
i) determining on which of the two points that the surface of the earth intersects with the base of the cone from step h) the transceiver is located by comparing earlier positions of the transceiver.
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5. The method according to claim 1, further comprising the step of:
i) transmitting the response from the transceiver to the satellite using a frequency that is proportional to the incoming frequency.
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6. A method for determining a position of a transceiver on the surface of the earth comprising the steps of:
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a) transmitting a signal from the transceiver in response to a query from a signal from a satellite;
b) transmitting the response after a precisely controlled time interval after the transceiver receives the query;
c) estimating a length of a propagation path from the satellite to the transceiver from a time delay in the response;
d) measuring a Doppler shift in the response from the transceiver;
e) estimating a second derivative of the path length from the satellite to the transceiver from the measured Doppler shift;
f) estimating the satellite position and velocity from satellite telemetry data;
g) determining an angle between the direction of satellite motion and a line of bearing to the transceiver from the second derivative and the satellite position and velocity; and
h) determining a position of the transceiver on the surface of the earth as being one of two points where the surface of the earth intersects with a base of a cone defined by the angle in step g) and the estimated path length. - View Dependent Claims (7, 8, 9, 10)
where v represents the satellite velocity, d represents the path length, and d represents the second derivative of the path length.
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8. The method according to claim 6, further comprising the step of:
i) determining on which of the two points that the surface of the earth intersects with the base of the cone from step h) the transceiver is located from which satellite beam on the satellite received the response from the transceiver.
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9. The method according to claim 6, further comprising the step of:
i) determining on which of the two points that the surface of the earth intersects with the base of the cone from step h) the transceiver is located by comparing earlier positions of the transceiver.
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10. The method according to claim 6, further comprising the step of:
i) transmitting the response from the transceiver to the satellite using a frequency that is proportional to the incoming frequency.
Specification