Passive altimeter employing GPS signals

• US 6,256,559 B1
• Filed: 03/13/2000
• Issued: 07/03/2001
• Est. Priority Date: 03/13/2000
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Tsui, James B. Y.
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): MARC-COLEMAN, MARTHE Y
• Application Number:
  US09/524,372
• Time in Patent Office: 477 Days
• Field of Search: 701/4, 701/5, 701/14, 701/207, 701/208, 701/213, 340/970, 340/977, 342/357.09, 342/357.13
• US Class Current: 701/4
• CPC Class Codes: G01S 13/003 Bistatic radar systems; Multistatic radar systems
• CPC Class Codes: G01S 13/882 for altimeters measuring height using barometric means G01C5/06
• CPC Class Codes: G01S 19/36 relating to the receiver frond end
• CPC Class Codes: G01S 19/39 the satellite radio beacon positioning system transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
• CPC Class Codes: G01S 3/16 using amplitude comparison of signals derived sequentially from receiving antennas or antenna systems having differently-oriented directivity characteristics or from an antenna system having periodically-varied orientation of directivity characteristic

Abstract

A passive radio frequency signal-enabled aircraft altimeter employing signals of one or more global position system satellites as a source of terrain illumination. The altimeter determines altitude of the host aircraft with respect to specific terrain features beneath the aircraft rather than an altitude above a mean or nominal level of the earth'"'"'s surface—as is already provided in a global position system signal. The altimeter employs two signal paths between the global position system satellite and the host aircraft, one direct signal path and one earth-reflected signal path, together with elementary geometric/trigonometric relationships, involving length difference in these paths and signal angle of arrival, in determining aircraft altitude. Equal angles of satellite signal incidence and reflection at the point of satellite signal reflection from the earth is an enabling principle in the altimeter.

Claims

1. In combination:

a host aircraft having first and second global position system signal reception antennas disposed on opposite extremities thereof, said first and second antennas having direct signal and earth-reflected signal reception capabilities respectively with respect to radio frequency signals originating in a global position system satellite;

radio frequency signal receiver apparatus connected with said first and second global position system signal reception antennas and generating output signals representing first and second samples of global position system data received from said global position system satellite via said first and second antennas respectively;

signal processing apparatus connected with said radio frequency signal receiver apparatus and generating, from said first and second samples of global position system data, first distance signals representative of direct physical distance between said aircraft first global position system signal reception antenna and said satellite and second distance signals representative of greater ground reflection-inclusive indirect physical distance between said aircraft second global position system signal reception antenna and said satellite;

said second distance signals representative of greater ground reflection-inclusive indirect physical distance between said aircraft second global position system signal reception antenna and said satellite including equal-angled incident and reflected signal paths to and from an earth point of satellite signal illumination attending said aircraft;

geometric algorithm signal processing apparatus generating from said first and second distance signals and angular relationships attending said equal-angled incident and reflected signal paths an aircraft altitude signal representative of orthogonal distance between said aircraft and a ground point below said aircraft.

2. The combination apparatus of claim 1 wherein said first and second antennas are disposed on upper and lower fuselage portions of said aircraft.

3. The combination apparatus of claim 2 wherein said first and second antennas are patch antennas of differing physical configuration and of opposite received radio frequency signal circular polarization characteristics.

4. The combination apparatus of claim 2 wherein said radio frequency signals are of 1.57542 gigahertz frequency and said first and second antennas are of less than three centimeters overall physical dimension.

5. The combination apparatus of claim 1 wherein said apparatus includes satellite selection apparatus capable of selecting between concurrently receivable signals from a plurality of said global position system satellites.

6. The combination apparatus of claim 1 wherein said geometric algorithm signal processing apparatus includes trigonometric relationships definitive of a first right triangle having an angle formed by said equal-angled incident and reflected signal paths at said earth point as one angle therein.

7. The combination apparatus of claim 1 wherein said geometric algorithm signal processing apparatus includes trigonometric relationships definitive of a second right triangle having a coincident hypotenuse with said first right triangle and one angle equal to an angle of arrival of said radio frequency signals from said global position system satellite at said aircraft.

8. The combination apparatus of claim 7 wherein said geometric algorithm signal processing apparatus trigonometric relationships definitive of a second right triangle further include one second right triangle leg, a leg adjacent said angle equal to an angle of arrival of said radio frequency signals from said global position system satellite, a leg representing said aircraft altitude and another leg coincident with an earth surface portion below said aircraft.

9. The combination apparatus of claim 1 wherein said first and second antennas are disposed on upper and lower fuselage portions of said aircraft and said geometric algorithm signal processing apparatus includes trigonometric relationships definitive of first and second right triangles having a common side.

10. The passive method of generating altimeter data in an airborne aircraft comprising the steps of:

collecting a first radio frequency signal from a direct signal path connecting said aircraft with a global position system satellite in-earth-orbit;

receiving a second radio frequency signal from an indirect signal path connecting said aircraft with said global position system satellite in-earth-orbit by way of an intervening signal reflecting ground point;

determining a path length difference between said direct and indirect signal paths;

ascertaining from one of said radio frequency signals an angular orientation angle of said signal paths with respect to a coordinate axis at said aircraft;

apportioning said path length difference among a pair of triangle sides formed by equal-angled incident and reflected signal paths located at said signal reflecting ground point;

said triangle sides being components of a right triangle, a right triangle inclusive of one angle having a fixed size relationship with said signal path angular orientation angle;

computing from geometric relationships attending said right triangle a length of a ground-perpendicular element connecting said aircraft with an underlying ground point, said computed length being an altitude determination of said aircraft.

11. The passive method of generating altimeter data in an airborne aircraft of claim 10 wherein said step of determining a path length difference between said direct and indirect signal paths includes computation of said path length difference from data encoded in said first and second radio frequency signals.

12. The passive method of generating altimeter data in an airborne aircraft of claim 10 wherein said intervening signal reflecting ground point is an earth disposed moving point below said aircraft and is displaced from said ground-perpendicular element aircraft-underlying ground point.

13. The passive method of generating altimeter data in an airborne aircraft of claim 10 wherein said step of ascertaining an angular orientation angle of said signal paths with respect to a coordinate axis at said aircraft comprises ascertaining said angular orientation with respect to said direct path first radio frequency signal and an axis normal of said aircraft in level flight.

14. The passive method of generating altimeter data in an airborne aircraft of claim 10 wherein said step of apportioning said path length difference among a pair of triangle sides includes:

forming said right triangle as a first right triangle having one apex at said aircraft; and

with a right angle between said second signal path and a long first right triangle leg connecting with said apex; and

with said apportioned path lengths being a short leg and hypotenuse portions of said first right triangle; and

with an angle of said first right triangle opposite said long triangle leg being said one angle having a fixed size relationship with said signal path angular orientation angle; and

with said fixed size relationship being a two times relationship with said signal path angular orientation angle.

15. The passive method of generating altimeter data in an airborne aircraft of claim 14 wherein said step of computing from geometric relationships attending said right triangle, said first right triangle, a length of a ground-perpendicular element connecting said aircraft with an underlying ground point includes:

computing a long leg length in a second right triangle having a common hypotenuse with said first right triangle;

said second right triangle including a right angle between a long leg representing said aircraft altitude and a short, ground-coincident, leg intermediate said perpendicular element ground point and said signal reflecting ground point.

16. The passive method of generating altimeter data in an airborne aircraft of claim 10 wherein said second radio frequency signal is a signal of 1.57542 gigahertz frequency and first circular polarization along a path of incidence with said signal reflecting ground point and second circular polarization along a path of departure from said signal reflecting ground point.

17. Aircraft altitude determination apparatus comprising the combination of:

aircraft-contained radio receiver means for receiving first and second radio frequency signals from an earth orbit-disposed global position system satellite, said second radio frequency signals being ground-reflected versions of said first signals;

geometric algorithm computation means for determining from an angle of arrival of said first and second radio frequency signals at said aircraft, from a length difference between signal paths traversed by said first and second radio frequency signals in reaching said aircraft and from an equal angled reflection of said ground reflected second signal at a ground point below said aircraft, an aircraft altitude-defining triangle leg length.

18. The aircraft altitude determination apparatus of claim 17 wherein said radio frequency receiver means includes:

one of first and second aircraft-mounted receiver antenna members and a two channel radio receiver and first and second aircraft-mounted receiver antenna members and a pair of single channel radio receivers; and

wherein said first and second aircraft-mounted receiver antenna members are disposed on lower and upper parts of said aircraft.

19. The aircraft altitude determination apparatus of claim 17 wherein signal paths traversed by said first and second radio frequency signals in reaching said aircraft are radial but substantially parallel paths extending from a global position system satellite in orbit through earth atmosphere to upper and lower aircraft fuselage-mounted radio frequency antennas.

20. The aircraft altitude determination apparatus of claim 17 wherein said equal angled reflection of said ground reflected second signal includes a reflection angle, centered on an axis orthogonal of said reflection point, of twice said angle of arrival in size and wherein said ground-reflected second radio frequency signals are of opposite circular polarization with respect to said first radio frequency signals.