Reduction of radar antenna area

• US 5,745,069 A
• Filed: 09/10/1996
• Issued: 04/28/1998
• Est. Priority Date: 09/10/1996
• Status: Expired due to Fees

• Current Assignee: Ball Corporation
• Original Assignee: Ball Corporation
• Inventors: Gail, William B.
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US08/710,092
• Time in Patent Office: 595 Days
• Field of Search: 342/25, 342/162, 342/189, 342/192, 342/194, 342/195, 342/196, 342/197
• US Class Current: 342/25.00F
• CPC Class Codes: G01S 13/30 using more than one pulse per radar period
• CPC Class Codes: G01S 13/904 SAR modes
• CPC Class Codes: G01S 7/2813 Means providing a modification of the radiation pattern for cancelling noise, clutter or interfering signals, e.g. side lobe suppression, side lobe blanking, null-steering arrays specially adapted to secondary radar systems G01S13/762; aerials or aerials systems H01Q21/29, H01Q25/00

Abstract

The traditional minimum antenna area limit for synthetic aperture radar (SAR), imposed by ambiguity considerations, is eliminated by using a transmitter format providing distinguishable sub-pulses. Signal formats which are feasible for implementing such sub-pulses include frequency-division (i.e., a distinct frequency band is used for each sub-pulse), and code-division (i.e., sub-pulses occupying the same frequency band are grouped in distinguishable combinations). The nominal pulse period is divided into N sub-pulses, and the sub-pulse group is transmitted with the nominal pulse repetition frequency (f_p). The range ambiguities are determined by the repetition rate of the sub-pulse group (f_p) and the azimuth ambiguities are determined by the repetition rate of the sub-pulses (N.f_p). The antenna is capable of sampling a Doppler bandwidth that is a factor of N times the traditional value and the antenna area can be reduced by a factor N from the limiting value presently used in SAR antenna design. A reduction in antenna length by a factor of N, for example, can be obtained at the expense of a signal-to-noise decrease of N and an increase in signal bandwidth by the same factor. Alternatively, the same antenna length reduction of N and an increase in signal-to-noise by a factor N can be obtained using the original bandwidth if the resolution is degraded by N.

Claims

1. A method for obtaining data about a body using pulsed signals reflected from the body in a range direction and in an azimuth direction, comprising:

transmitting, toward the body, a predetermined sequence of sub-pulses, said sub-pulses transmitted at a rate that is at least a first predetermined repetition frequency;

detecting, for each said sub-pulse, a corresponding sub-pulse reflection from the body;

storing a plurality of data items, wherein said plurality of data items includes, for each said sub-pulse reflection, a related data item derived from said sub-pulse reflection;

first sampling of said plurality of data items for obtaining a first series of data items;

wherein, there is a first sub-pulse sequence, included in said predetermined sequence of sub-pulses, such that each sub-pulse of said first sub-pulse sequence has a corresponding sub-pulse reflection for which there is a related data item of said first series, and wherein said first sub-pulse sequence has a repetition frequency at least that of the first predetermined repetition frequency;

second sampling of said plurality of data items for obtaining a second series of data items;

wherein, there is a second sub-pulse sequence included in said predetermined sequence of sub-pulses, such that each sub-pulse of said second sub-pulse sequence has a corresponding sub-pulse reflection for which there is a related data item of said second series, and wherein said second sub-pulse sequence has a repetition frequency less than said first predetermined repetition frequency;

providing one or more values related to said plurality of data items, wherein said one or more values have a reduction in sub-pulse reflection ambiguity;

wherein said step of providing includes the following substeps (A1) and (A2);

(A1) reducing, in the azimuth direction, an ambiguity in said one or more of values using said first sampling of said sub-pulse data items;

(A2) reducing, in the range direction, an ambiguity in said one or more of values using said second sampling of said data items;

using said one or more values having a reduced ambiguity for determining a characteristic of the body.

2. A method as claimed in claim 1, wherein said characteristic of the body is a radar cross section used to determine one or more of a position of an object, a soil moisture content, a vegetation type, a mineral type, a vegetation density, an ice/snow cover, a topographical characteristic, an ocean surface feature and an ocean subsurface feature.

3. A method as claimed in claim 1, wherein said first predetermined repetition frequency is substantially the same as the repetition frequency for said first sub-pulse sequence.

4. A method as claimed in claim 1, wherein said step of transmitting uses an antenna on a moving platform above the body for transmitting said predetermined sequence of sub-pulses.

5. A method as claimed in claim 4, wherein said repetition frequency for said second sub-pulse sequence is effective for reducing a range ambiguity.

6. A method as claimed in claim 4, wherein said repetition frequency for said second sub-pulse sequence is a function of at least one of:

(a) a carrier frequency of said sub-pulse reflections, (b) an extent of the antenna substantially traverse to a direction of movement of the platform, (c) an altitude of the antenna above the body, and (d) an angle of incidence between a propagation direction of said predetermined sequence of sub-pulses and a tangent to a surface of the body at a point where the propagation direction and the tangent intersect.

7. A method as claimed in claim 4, wherein the body is a celestial body with the platform orbiting the celestial body.

8. A method as claimed in claim 4, wherein the antenna is a synthetic aperture radar antenna.

9. A method as claimed in claim 4, wherein said step of transmitting includes transmitting said sub-pulses from the antenna.

10. A method as claimed in claim 4, wherein said repetition frequency for said first sub-pulse is a function of:

(a) a speed of the platform and (b) an extent of the antenna substantially parallel to a direction of movement of the platform.

11. A method as claimed in claim 1, wherein said repetition frequency for said first sub-pulse sequence is effective for reducing an azimuth ambiguity.

12. A method for obtaining data about a body from a moving platform using pulsed signals reflected from the body in a range direction and in an azimuth direction, comprising:

transmitting, toward the body, a sequence of sub-pulses from the platform wherein a consecutive plurality of said sub-pulses is transmitted within each pulse interval of a series of pulse intervals, so that the following conditions (A1), (A2) and (A3) are satisfied;

(A1) said sub-pulses are transmitted at a rate that is at least a first predetermined repetition frequency;

(A2) said pulse intervals repeat at a rate that is less than or equal to a second predetermined repetition frequency, said second predetermined repetition frequency being less than said first predetermined repetition frequency; and

(A3) for each of said pulse intervals, a reflection from the body of at least one sub-pulse transmitted in the interval, wherein the reflection is distinguishable from every other body reflection of a sub-pulse from the pulse interval;

detecting, for each transmitted said sub-pulse, an associated reflection of the sub-pulse from the body;

storing a plurality of data items, wherein said plurality of data items includes, for each said associated reflection, a related data item derived from said associated reflection;

determining one or more values for a given characteristic of the body by;

(B1) using the related data item for each associated reflection of each sub-pulse of said sequence of sub-pulses to reduce azimuth ambiguity in said one or more values, and

(B2) using said distinguishable reflections for reducing range ambiguity in said one or more values.

13. A method as claimed in claim 12, wherein each said distinguishable reflection includes one of a waveform and a signal frequency that uniquely identifies the distinguishable reflection from every other body reflection associated with a sub-pulse transmitted in a same interval as the sub-pulse with which the distinguishable reflection is associated.

14. A method as claimed in claim 12, wherein said range ambiguity is a function of a length of time during which an associated reflection for one of said sub-pulses is detected.

15. A method a claimed in claim 12, wherein said azimuth ambiguity is a function of a Doppler bandwidth shift in an associated reflection for one of said sub-pulses.

16. A method as claimed in claim 12, wherein at least one of the steps of transmitting and detecting utilizes an antenna on the platform.

17. A method as claimed in claim 12, wherein said step of transmitting includes transmitting said consecutive plurality of sub-pulses for a pulse interval as a chirp signal.

18. A method as claimed in claim 12, wherein said step of transmitting includes transmitting said consecutive plurality, N, of sub-pulses for a first pulse interval, such that one of the following holds:

(a) there is no time gap between said sub-pulses of said consecutive plurality of sub-pulses for said first pulse interval;

(b) there are time gaps between said sub-pulses of said consecutive plurality of sub-pulses for said first pulse interval;

(c) said consecutive plurality of sub-pulses for said first pulse interval are uniformly distributed in said first pulse interval.

19. A method for dimensioning an antenna for a moving platform, comprising:

establishing a length at least as long as each pulse interval of a sequence of pulse intervals, said sequence having an interval repetition frequency effective for reducing, along a direction substantially transverse to a movement of the platform, a likelihood of correlating a reflection from a body of a signal pulsed in one of said pulse intervals with a reflection from the body of a signal pulsed in a different one of said pulse intervals;

first determining a sequence of sub-pulse signals such that;

(a) said sequence of said sub-pulse signals can be transmitted within said length,

(b) for each said sub-pulse signal of said sequence of sub-pulse signals having a corresponding reflection from the body, the corresponding reflection is distinguishable from every other reflection from the body of said sub-pulse signals in said sequence of sub-pulse signals; and

(c) the sub-pulse signals of said sequence of sub-pulse signals have a sub-pulse repetition frequency effective for reducing an ambiguity in detecting said corresponding reflections, along a direction substantially parallel to a movement of the platform, wherein the ambiguity relates to a bandwidth aliasing of said corresponding reflections;

second determining an area for said antenna, said area being greater than or equal to a predetermined value, said predetermined value being inversely related to a predetermined number of sub-pulse signals within the sequence of sub-pulse signals.

20. A method as claimed in claim 19, further including a step of fabricating said antenna having dimensions that provide the area for said antenna.

21. A method as claimed in claim 19, wherein said step of first determining said plurality of sub-pulse signals includes establishing a uniquely identifying one of a frequency and waveform for each said sub-pulse signal in said sequence of sub-pulse signals.

22. A method as claimed in claim 19, wherein said step of second determining includes determining a dimension of said antenna in a direction substantially coincident with a direction of movement of the platform, wherein said dimension is inversely related to said sub-pulse repetition frequency.

23. A method as claimed in claim 19, wherein said step of establishing includes constraining said interval repetition frequency to be less than a function having a value determined substantially by the term:

space="preserve" listing-type="equation">fW cos.sup.2 θ

/2hβ

.sub.w sin θ

where f is a carrier frequency for at least one said sub-pulse signal, W is said width of the antenna, h is an altitude of the antenna above the body, β

_w is a factor accounting for an angular broadening of said sub-pulse signals transmitted and θ

is an incidence angle between the platform and the body.

24. A method as claimed in claim 19, wherein said step of first determining includes determining said sub-pulse repetition frequency to be greater than a function having a value determined substantially by the term:

space="preserve" listing-type="equation">2vβ

.sub.L /L

where v is a speed of the platform, β

_L is a factor accounting for an angular broadening of a signal corresponding to one of said sub-pulse intervals, and L is a length of the antenna.

25. A method for reducing ambiguity in reflections of signals transmitted from a moving platform, comprising:

receiving a reflected signal in each interval of a predetermined series of intervals;

obtaining a data item related to each of said reflected signals;

providing interval and sub-pulse sampling frequencies, wherein each of said interval and sub-pulse sampling frequencies sample said data items at different frequencies;

first using said data items sampled at said interval sampling frequency for reducing reflected signal ambiguity in a direction traverse to a direction of movement of the platform;

second using said data items sampled at said sub-pulse sampling frequency for reducing ambiguity in said reflected signals along a direction coincident with the direction of movement of the platform.

26. A method as claimed in claim 25, wherein said platform includes a synthetic aperture radar antenna for receiving said reflected signals.

27. A method as claimed in claim 26, further including a step of transmitting signals from said synthetic aperture radar antenna, said signals providing said reflected signals when said signals are reflected from a body spaced apart from the platform.

28. An apparatus for imaging an area from a moving platform, comprising:

transmitter means, on the platform, for transmitting a plurality of pulsed signals toward the area;

first signal pulse controller means for determining a first sampling frequency for reducing range ambiguity of reflections of said pulsed signals wherein said range ambiguity is a function of an angle corresponding to signal propagation between the platform and the area in a direction traverse to a direction of movement of the platform;

second signal pulse controller means for determining a second sampling frequency for reducing a reflected signal azimuth ambiguity in a direction coincident with the direction of movement of the platform;

receiver means for receiving a reflection of one of said plurality of pulsed signals from the area in each interval of a predetermined series of intervals, said predetermined series having a repetition frequency of at least said second sampling frequency;

storage means for storing a data item corresponding to each of a majority of said reflections;

imaging means for determining a value for a location in the area, said value being a function of said data items;

wherein said imaging means samples said data items at a rate no greater than said first sampling frequency for reducing a distortion in said value resulting from data items with corresponding reflected signals having signal range ambiguity; and

wherein said imaging means samples said data items at a rate of at least said second sampling frequency for reducing a distortion in said resulting from data items with corresponding reflective signals having azimuth ambiguity.

29. An apparatus as claimed in claim 28, wherein said second signal sampling frequency is at least twice said first signal sampling frequency.

30. An apparatus as claimed in claim 28, wherein said value is a function of said data items corresponding to said reflections that are reflected from the location.

31. An apparatus as claimed in claim 28, wherein said receiver means is on the platform.

32. An apparatus as claimed in claim 28, wherein said area is a swath on a surface of a celestial body orbited by the platform.

33. An apparatus as claimed in claim 28, further including a digitizing means for digitizing said reflections received by said receiver means for obtaining said corresponding data items.

34. An apparatus as claimed in claim 28, wherein said imaging means includes:

range correlation means for transforming said data items from a substantially greater than unity time-bandwidth product format to a format substantially being unity; and

sampling segregation means for iteratively activating said range correlation means on said data items at said first sampling frequency.

35. An apparatus as claimed in claim 28, wherein said transmitter means and said antenna means are each included in a synthetic aperture radar system.

36. An apparatus as claimed in claim 28, wherein said value is included in a radar cross section used to determine one or more of a position of an object in the area, a soil moisture content of the area, a vegetation type of the area, a mineral type of the area, a vegetation density of the area, an ice/snow cover of the area, a topography of the area, an ocean surface feature of the area, and an ocean subsurface feature of the area.