SPACE-BORNE ALTIMETRY APPARATUS, ANTENNA SUBSYSTEM FOR SUCH AN APPARATUS AND METHODS FOR CALIBRATING THE SAME

• US 20100073217A1
• Filed: 09/23/2008
• Published: 03/25/2010
• Est. Priority Date: 09/23/2008
• Status: Active Grant

• Current Assignee: Agence Spatiale Europeenne
• Original Assignee: Agence Spatiale Europeenne
• Inventors: MARTIN NEIRA, Manuel, D'ADDIO, Salvatore
• Granted Patent: US 7,808,425 B2
• Time in Patent Office: Days
• US Class Current: 342/120
• CPC Class Codes: G01S 13/882 for altimeters measuring height using barometric means G01C5/06
• CPC Class Codes: G01S 7/4017 of HF systems
• CPC Class Codes: G01S 7/4021 of receivers
• CPC Class Codes: H01Q 3/267 Phased-array testing or checking devices

Abstract

The present invention is directed to a space-borne altimetry apparatus having a first receiving antenna, pointing to outer space, for receiving at least one signal emitted by a remote satellite emitter via a direct path; a second receiving antenna, pointing to the Earth, for receiving said signal via an indirect path including a reflection from the Earth surface; and a signal processing means for computing a distance of the apparatus from a specular reflection point of the signal on the Earth surface by cross-correlating the signals received by said first and second antennas; wherein both the first and second receiving antennas are high-gain steerable antennas; and wherein the apparatus may also include antenna control means for steering at least one receiving lobe of the first antenna toward the remote satellite emitter, and at least one receiving lobe of the second antenna toward a specular reflection point on the Earth surface.

Claims

1. A space-borne altimetry apparatus comprising:

a first receiving antenna, pointing to outer space, for receiving at least one signal emitted by a remote satellite emitter via a direct path;

a second receiving antenna, pointing to the Earth, for receiving said signal via an indirect path including a reflection from the Earth surface; and

signal processing means for cross-correlating the signals received by said first and second antennas;

wherein both said first and second receiving antennas are directive and steerable antennas; and

wherein said apparatus further comprises antenna control means for steering at least one receiving lobe of said first antenna toward said remote satellite emitter, and at least one receiving lobe of said second antenna toward a point on the Earth'"'"'s surface specularly reflecting said signal emitted by a remote satellite emitter toward said second receiving antenna.

2. A space-borne altimetry apparatus according to claim 1, wherein both said first and second receiving antennas are multibeam antennas, said antenna control means being adapted for steering a plurality of receiving lobes thereof.

3. A space-borne altimetry apparatus according to claim 2, wherein both said first and second receiving antennas are phase-array antennas, each comprising a plurality of antenna elements.

4. A space-borne altimetry apparatus according to claim 3, wherein said first and second receiving antennas are identical to each other, except for polarization.

5. A space-borne altimetry apparatus according to claim 3, wherein said first and second receiving antennas are mounted back-to-back on a common supporting structure.

6. A space-borne altimetry apparatus according to claim 5, further comprising front-end electronic circuitry disposed between said first and second receiving antennas.

7. A space-borne altimetry apparatus according to claim 6, wherein said front-end electronic circuitry comprises:

a plurality of first and second front-end amplifiers; and

a plurality of respective switching circuits for connecting each element of said first receiving antenna to an input port of a respective first or second amplifier, and for connecting each element of said second receiving antenna to an input port of a respective second or first amplifier.

8. A space-borne altimetry apparatus according to claim 7, wherein said switching circuits are also for connecting the input ports of said first and second front-end amplifiers to respective temperature-monitored loads in order to perform absolute gain calibration.

10. A method of calibrating a space-borne altimetry apparatus according to claim 8 comprising the steps of:

(a) connecting the first receiving antenna to the input ports of the first front-end amplifiers while directing the receiving lobes thereof toward cold space, and performing first power measurements;

(b) connecting said temperature-monitored loads to the input ports of said first front-end amplifiers and performing second power measurements;

(c) repeating steps (a) and (b) while introducing constant attenuation values after said first or second front-end amplifiers, thus performing third and fourth measurements;

(d) determining at least a first set of end-to-end gain values for signal processing chains including said first front-end amplifiers on the basis of said first to fourth measurements;

(e) turning the apparatus upside-down, for pointing the first receiving antenna toward the Earth and the second receiving surface toward the outer space;

(f) connecting the second receiving antenna to the input ports of the second front-end amplifiers, while directing the receiving lobes thereof toward cold space, and performing fifth power measurements;

(g) connecting said temperature-monitored loads to the input ports of said second front-end amplifiers and performing sixth power measurements;

(h) repeating steps (f) and (g) while introducing constant attenuation values after said second front-end amplifiers thus performing seventh and eighth measurements; and

(e) determining at least a second set of end-to-end gain values for signal processing chains including said second front-end amplifiers on the basis of said fifth to eighth measurements;

11. A method of self-calibrating a space-borne altimetry apparatus according to claim 7 comprising the steps of:

(a) alternatingly connecting the first receiving antenna to the input ports of said first and second amplifiers, and the second receiving antenna to the input ports of the second or first amplifiers;

(b) averaging the values of delay observables computed when the first receiving antenna is connected to the input ports of the first amplifiers and the second receiving antenna to the input ports of the second amplifiers, and when the first receiving antenna is connected to the input ports of the second amplifiers, and the second receiving antenna to the input port of the first amplifiers.

12. A method of calibrating a space-borne altimetry apparatus according to claim 7 comprising the steps of:

(a) alternatingly connecting the first receiving antenna to the input ports of said first and second amplifiers, and the second receiving antenna to the input ports of the second and first amplifiers;

(b) computing a difference between values of delay observables measured when the first receiving antenna is connected to the input ports of the first amplifiers and the second receiving antenna to the input ports of the second amplifiers, and when the first receiving antenna is connected to the input ports of the second amplifiers, and the second receiving antenna to the input port of the first amplifiers; and

(c) determining, on the basis of said difference, a delay calibration parameter for the apparatus.

13. An antenna subsystem for a space-borne altimetry apparatus according to claim 1, comprising a first and second phase-array receiving antennas mounted back-to-back on a common supporting structure.

14. An antenna subsystem according to claim 13, wherein said first and second phase-array receiving antennas are identical to each other, except for polarization.

15. An antenna subsystem according to claim 13 further comprising, disposed between said first and second antenna, a plurality of front-end modules associated to respective antenna elements of said first and second receiving antennas, each front-end module comprising:

a first and a second amplifier; and

a switching system adapted for connecting an antenna element of said first receiving antenna to an input port of the first or second amplifier, and for connecting an antenna element of said second receiving antenna to an input port of the second or first amplifier.

16. An antenna subsystem according to claim 15 wherein the switching systems of said front-end modules are also for connecting the input ports of said first and second front-end amplifiers to respective temperature-monitored loads in order to perform absolute gain calibration.

17. A space-borne altimetry apparatus according to claim 4, wherein said first and second receiving antennas are mounted back-to-back on a common supporting structure.

18. A space-borne altimetry apparatus according to claim 4, wherein said signal processing means comprise delay lines for applying symmetric delays to the signals received by said first and second receiving antennas.

19. A space-borne altimetry apparatus according to claim 4, wherein said signal processing means comprise frequency converters for applying symmetric Doppler compensation frequency shifts to the signals received by said first and second receiving antennas.

9. A method of calibrating a space-borne altimetry apparatus comprising

a first receiving antenna, pointing to outer space, for receiving at least one signal emitted by a remote satellite emitter via a direct path;

a second receiving antenna, pointing to the Earth, for receiving said signal via an indirect path including a reflection from the Earth'"'"'s surface, said first and second receiving antennas being mounted back-to-back on a common supporting structure;

front-end electronic circuitry sandwiched between said first and second receiving antennas; and

signal processing means cross-correlating the signals received by said first and second antennas;

wherein said front-end electronic circuitry comprises;

a plurality of first and second front-end amplifiers; and

a plurality of respective switching circuits for connecting each element of said first receiving antenna to an input port of a respective first or second amplifier, and for connecting each element of said second receiving antenna to an input port of a respective second or first amplifier, said switching circuits also being for connecting the input ports of said first and second front-end amplifiers to respective temperature-monitored loads in order to perform absolute gain calibration;

and wherein both said first and second receiving antennas are directive and steerable antennas; and

wherein said apparatus further comprises antenna control means for steering at least one receiving lobe of said first antenna toward said remote satellite emitter, and at least one receiving lobe of said second antenna toward a point on the Earth'"'"'s surface specularly reflecting said signal emitted by a remote satellite emitter toward said second receiving antenna;

the method comprising the steps of;

(a) connecting the first receiving antenna to the input ports of the first front-end amplifiers while directing the receiving lobes thereof toward cold space, and performing first power measurements;

(b) connecting said temperature-monitored loads to the input ports of said front-end amplifiers and performing second power measurements;

(c) repeating steps (a) and (b) while introducing a constant attenuation value after said first front-end amplifiers, thus performing third and fourth measurements;

(d) determining at least a set of end-to-end gain values for signal processing chains including said first front-end amplifiers on the basis of said first to fourth measurements;

(e) repeating steps (a) to (d) for the second front-end amplifiers.