Ground penetrating radar system

• US 6,445,334 B1
• Filed: 12/29/2000
• Issued: 09/03/2002
• Est. Priority Date: 12/29/2000
• Status: Expired due to Fees

• Current Assignee: Foster-Miller Incorporated (Thermo Fisher Scientific Incorporated)
• Original Assignee: Planning Systems Incorporated (Perspecta, Inc.)
• Inventors: Bradley, Marshall R., McCummins, Robert J. Jr., Crowe, Michael P., Duncan, Michael E.
• Primary Examiner(s): Gregory, Bernarr E.
• Application Number:
  US09/752,085
• Publication Number:
  US 20020122000A1
• Time in Patent Office: 613 Days
• Field of Search: 342/21, 342/22, 342/27, 342/28, 342/61, 342/70, 342/71, 342/72, 342/118, 342/175, 342/195, 342/25
• US Class Current: 342/22
• CPC Class Codes: F41H 11/136 Magnetic, electromagnetic, acoustic or radiation systems, e.g. ground penetrating radars or metal-detectors
• CPC Class Codes: G01S 13/88 Radar or analogous systems specially adapted for specific applications electromagnetic prospecting or detecting of objects, e.g. near-field detection, G01V3/00
• CPC Class Codes: G01S 13/885 for ground probing prospecting or detecting using electromagnetic waves G01V3/12
• CPC Class Codes: G01S 13/90 using synthetic aperture techniques , e.g. synthetic aperture radar [SAR] techniques
• CPC Class Codes: G01S 7/20 Stereoscopic displays; Three-dimensional displays; Pseudo-three-dimensional displays
• CPC Class Codes: G01S 7/35 Details of non-pulse systems
• CPC Class Codes: G01S 7/4073 involving an IF signal injection

Abstract

A ground penetrating radar system includes a cart configured to be movable along the ground. A computer is mechanically coupled to the cart. A radar electronics module is mechanically coupled to the cart and electrically coupled to the computer. A first antenna array is mechanically coupled to the cart, electrically coupled to the radar electronics module, and oriented to radiate into the ground and receive radiation from the ground. A second antenna array is mechanically coupled to the cart, electrically coupled to the radar electronics module, and oriented to radiate into the ground and receive radiation from the ground. A movement detector, which is configured to detect movement of the cart, is coupled to the computer. The computer is configured to trigger the radar electronics module when the computer detects that the cart has moved a predefined distance. The system uses nearfield beam forming, which is accomplished through fully coherent signal processing and synthetic aperture reception and processing, to image buried objects in three dimensions. The system displays a plan, or top, view and a side view of the area being scanned to provide a three dimensional perspective on a two dimensional computer screen.

Claims

1. A ground penetrating radar system comprising

a cart that moves over ground;

a computer mechanically coupled to the cart;

a radar electronics module mechanically coupled to the cart and electrically coupled to the computer;

a first antenna array mechanically coupled to the cart, electrically coupled to the radar electronics module, and oriented to radiate into the ground and receive radiation from the ground;

a second antenna array mechanically coupled to the cart, electrically coupled to the radar electronics module, and oriented to radiate into the ground and receive radiation from the ground;

a movement detector coupled to the computer, wherein the movement detector detects movement of the cart over the ground; and

the computer triggers the radar electronics module when the computer detects that the cart has moved a predefined distance over the ground.

2. The ground penetrating radar system of claim 1 wherein

the radar electronics module comprises

a first radar electronics module electrically coupled to the first antenna array; and

a second radar electronics module electrically coupled to the second antenna array.

3. The ground penetrating radar system of claim 1 wherein

the first antenna array is configured to radiate and receive radiation from a first series of points along a first set of curves parallel to the direction of movement of the cart;

the second antenna array is configured to radiate and receive radiation from a second series of points along a second set of curves parallel to the direction of movement of the cart; and

the first set of curves is interleaved with the second set of curves.

4. A ground penetrating radar system comprising

a first bank of receive antennas arranged along a first axis;

a first bank of transmit antennas arranged along a second axis approximately parallel to the first axis and horizontally displaced from the first axis;

a second bank of receive antennas arranged along a third axis approximately parallel to the first axis and horizontally displaced from the first axis;

a second bank of transmit antennas arranged along a fourth axis approximately parallel to the first axis and horizontally displaced from the first axis;

a first radar electronics module coupled to the first bank of transmit antennas and the first bank of receive antennas;

a second radar electronics module coupled to the second bank of transmit antennas and the second bank of receive antennas;

wherein

the transmit antennas in the first bank of transmit antennas are interleaved with the receive antennas in the first bank of receive antennas;

the transmit antennas in the second bank of transmit antennas are interleaved with the receive antennas in the second bank of receive antennas; and

the receive antennas in the first bank of transmit antennas are offset along the first axis from the receive antennas in the second bank of transmit antennas.

5. The ground penetrating radar system of claim 4 wherein

the first bank of transmit antennas is offset along the second axis with respect to the second bank of transmit antennas.

6. The ground penetrating radar system of claim 4 wherein

the banks of receive antennas alternate with the banks of transmit antennas.

7. The ground penetrating radar system of claim 4 wherein

each transmit antenna is adjacent to at least one receive antenna;

and

each transmit antenna is oriented to minimize electromagnetic coupling to at least one of its adjacent receive antennas.

8. The ground penetrating radar system of claim 7 wherein

each transmit antenna comprises at least one spiral arm of conductive material;

each receive antenna comprises at least one spiral arm of conductive material; and

a tangent to the inside of the spiral arm at the edge of a transmit antenna is approximately perpendicular to a tangent to the inside of the spiral arm at the edge of a receive antenna adjacent to the transmit antenna.

9. The ground penetrating radar system of claim 8 wherein

each transmit antenna comprises two spiral arms of conductive material;

and

each receive antenna comprises two spiral arms of conductive material.

10. The ground penetrating radar system of claim 4 wherein

the transmit antennas and the receive antennas have faces with centers;

two adjacent first bank receive antennas from the first bank of receive antennas and a first bank transmit antenna from the first bank of transmit antennas interleaved between the two adjacent first bank receive antennas are positioned such that lines between the centers of the faces of the two adjacent first bank receive antennas and the interleaved first bank transmit antenna form a first triangle having sides of approximately the same length;

two adjacent second bank receive antennas from the second bank of receive antennas and a second bank transmit antenna from the second bank of transmit antennas interleaved between the two adjacent second bank receive antennas are positioned such that lines between the centers of the faces of the two adjacent second bank receive antennas and the interleaved second bank transmit antenna form a second triangle having sides of approximately the same length;

a vertex of the first triangle is displaced in the direction of the first axis relative to a corresponding vertex of the second triangle by an amount approximately equal to one-half the distance from the center of one side of the first triangle to the center of another side of the first triangle.

11. The ground penetrating radar system of claim 10, wherein

the third axis is horizontally displaced from the first axis by an amount approximately equal to eight times the distance from the center of one side of the first triangle to the center of another side of the first triangle.

12. The ground penetrating radar system of claim 4 wherein

the transmit antennas are not in contact with the ground when in operation.

13. The ground penetrating radar system of claim 4 wherein

the receive antennas are not in contact with the ground when in operation.

14. A ground penetrating radar system comprising

a digital module comprising:

a direct digital synthesizer that generates a digital IF reference signal;

a digital to analog converter coupled to the direct digital synthesizer that converts the digital IF reference signal to an analog IF transmit signal;

an analog to digital converter that converts an analog IF receive signal to a digital IF receive signal;

a digital down converter that digitally mixes the digital IF receive signal with;

the digital IF reference signal to produce an in-phase product;

the digital IF reference signal shifted in phase by ninety degrees to produce a quadrature product;

an RF module coupled to the digital module comprising

an up-converter that converts the analog IF transmit signal into a transmit signal;

a down-converter that converts a receive signal into an analog IF receive signal;

a transmit antenna array coupled to the up-converter for radiating the transmit signal; and

a receive antenna array coupled to the down-converter for receiving the receive signal.

15. The ground penetrating radar system of claim 14 wherein

the transmit antenna array comprises a plurality of transmit antennas;

the receive antenna array comprises a plurality of receive antennas, and the system further comprises

a digital signal processor;

a transmit switch for applying the transmit signal to one of the plurality of transmit antennas, the transmit switch being controlled by the digital signal processor; and

a receiver switch for receiving the receive signal from one of the plurality of receive antennas, the receiver switch being controlled by the digital signal processor.

16. The ground penetrating radar system of claim 15 wherein

the digital signal processor controls the direct digital synthesizer, the digital down converter, the up-converter and the down-converter.

17. The ground penetrating radar system of claim 14 wherein

the transmit signal comprises a stepped-frequency transmit signal;

and

the receive signal comprises a stepped-frequency receive signal.

18. The ground penetrating radar system of claim 14 further comprising

a computer coupled to a processor through an extensible network, the processor being configured to communicate with the digital signal processor.

19. The ground penetrating radar system of claim 14 wherein the extensible network is a local area network.

20. A ground penetrating radar system comprising

a digital module that generates an analog IF transmit signal and receives an analog IF receive signal;

an RF module comprising

a triple-heterodyne up-converter for converting an analog IF transmit signal into a stepped-frequency transmit signal; and

a triple-heterodyne frequency converter for converting a stepped-frequency receive signal into an analog IF receive signal;

a transmit antenna coupled to the up-converter for radiating the stepped-frequency transmit signal; and

a receive antenna coupled to the down-converter for receiving the stepped-frequency receive signal.

21. The ground penetrating radar system of claim 20 wherein the triple-heterodyne up-converter comprises

a first up-converter that mixes the analog IF transmit signal with a signal from a first local oscillator to produce a first intermediate signal and an aliased first intermediate signal;

a first filter coupled to the first up-converter for attenuating the aliased first intermediate signal;

a second up-converter coupled to the first filter that mixes the first intermediate signal with a signal from a second local oscillator to produce a second intermediate signal and an aliased second intermediate signal;

a second filter coupled to the second up-converter for attenuating the aliased second intermediate signal; and

a down-converter coupled to the second filter that mixes the second intermediate signal with a stepped frequency signal to produce the stepped-frequency transmit signal and an aliased stepped-frequency transmit signal; and

a third filter coupled to the down-converter for attenuating the aliased stepped-frequency transmit signal.

22. The ground penetrating radar system of claim 20 wherein the triple-heterodyne up converter comprises

an up-converter that mixes the stepped-frequency receive signal with a stepped-frequency signal to produce a first intermediate signal and an aliased first intermediate signal;

a first filter coupled to the first up-converter for attenuating the aliased first intermediate signal;

a first down-converter coupled to the first filter that mixes the first intermediate signal with a signal from a first local oscillator to produce a second intermediate signal and an aliased second intermediate signal;

a second filter coupled to the first down-converter for attenuating the aliased second intermediate signal;

a second down-converter coupled to the second filter that mixes the second intermediate signal with a second local oscillator to produce the analog IF receive signal and an aliased analog IF receive signal;

a third filter coupled to the second down-converter for attenuating the aliased analog IF receive signal.

23. A ground penetrating radar system comprising

a transmitter;

a receiver

an array of transmit antennas;

an array of receive antennas interleaved with the array of transmit antennas;

a transmit switch that selectively couples the transmitter to one of the array of transmit antennas;

a receive switch that selectively couples the receiver to one of the array of receive antennas;

wherein

the array of transmit antennas is arranged in one or more rows;

the array of receive antennas is arranged in one or more rows, each row being approximately parallel to, adjacent to and offset from one of the rows of transmit antennas, so that each receive antenna in a row except one is adjacent to two transmit antennas, and each transmit antenna in a row except one is adjacent to two receive antennas; and

the transmit switch and the receive switch couple the transmitter and receiver, respectively, to a first transmit antenna and a first adjacent receive antenna, and subsequently to the first transmit antenna and a second adjacent receive antenna.

24. A method for collecting and displaying data from a ground penetrating radar system, the ground penetrating radar system comprising a plurality of transmit antennas and a plurality of receive antennas, each transmit antenna, except one, having two adjacent receive antennas, the system being mounted on a cart that moves over ground, the method comprising

collecting raw data, comprising

a. selecting a first of the plurality of transmit antennas for transmitting a signal into the ground, wherein a portion of the signal is reflected back from the ground;

b. selecting a first receive antenna that is adjacent to the selected transmit antenna;

c. collecting raw data from the portion of the reflected signal received by the selected receive antenna, the raw data being collected at spatial location (x_m, y_n), the spatial location (x_m, y_n) being denoted by {tilde over (Ψ

)}_mnp where the indices m, n are used to denote position in a grid of spatial locations where the raw data was collected, and p is an index ranging from 1 to P corresponding to a frequency f_p at which the raw data was collected;

d. repeating step c for both receive antennas adjacent to the selected transmit antenna;

e. repeating steps b, c and d for all transmit antennas;

f. repeating steps a, b, c, d, and e each time the cart moves to a new location;

preconditioning the raw data to produce preconditioned data;

analyzing the preconditioned data; and

displaying images of the analyzed data.

25. The method of claim 24 wherein preconditioning the raw data to produce preconditioned data comprises

g. removing a constant frequency component and a system travel time delay;

h. removing a transmit-antenna to receive-antenna coupling effect;

i. prewhitening; and

j. repeating steps g, h and i for each spatial location of the raw data.

26. The method of claim 25 wherein

removing a constant frequency component and a system travel time delay comprises applying the following equation:

${\widehat{\mathrm{\Psi <br><br>}}}_{\mathrm{mnp}}=\left({\widetilde{\mathrm{\Psi <br><br>}}}_{\mathrm{mnp}}-\frac{1}{P}<br><br>\underset{p=1}{\overset{P}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\widetilde{\mathrm{\Psi <br><br>}}}_{\mathrm{mnp}}\right)<br><br>\exp <br><br>\left(\mathrm{<br><br>}·<br><br>2<br><br>\mathrm{\pi <br><br>}·<br><br>f_p·<br><br>\mathrm{\tau <br><br>}\right).$

27. The method of claim 25 wherein

removing the transmit-antenna to receive-antenna coupling effect comprises applying the following equation:

28. The method of claim 25 wherein

removing the transmit-antenna to receive-antenna coupling effect comprises applying the following equation:

${\mathrm{\Psi <br><br>}}_{\mathrm{mnp}}={\widehat{\mathrm{\Psi <br><br>}}}_{\mathrm{mnp}}-\frac{1}{N_2-N_1+1}·<br><br>\underset{n=N_1}{\overset{N_2}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\widehat{\mathrm{\Psi <br><br>}}}_{\mathrm{mnp}}$

where N₁ and N₂ define a region to be imaged.

29. The method of claim 25 wherein

removing the transmit-antenna to receive-antenna coupling effect comprises applying the following equation:

${\mathrm{\Psi <br><br>}}_{\mathrm{mnp}}=\underset{q=-Q}{\overset{Q}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>a_q·<br><br>{\widehat{\mathrm{\Psi <br><br>}}}_{m,n+q,p}$

where a_q are digital filter coefficients chosen to reject low frequency spatial energy.

30. The method of claim 25 wherein

removing the transmit-antenna to receive-antenna coupling effect comprises applying the following equation:

31. The method of claim 25 wherein

prewhitening comprises applying the following equation:

32. The method of claim 24 wherein

analyzing the preconditioned data comprises applying the following equation:

$I_{\mathrm{mnp}}=\frac{1}{\left(2<br><br>U+1\right)<br><br>\left(2<br><br>V+1\right)<br><br>P}·<br><br>\underset{u=-U}{\overset{U}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\underset{v=-V}{\overset{V}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\underset{p=P_1}{\overset{P_2}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\gamma <br><br>}}_{m+u,n+v,p}<br><br>\exp <br><br>\left(\mathrm{<br><br>}·<br><br>2<br><br>\mathrm{\pi <br><br>}·<br><br>f_p·<br><br>{\mathrm{\tau <br><br>}}_{\mathrm{uvw}}\right)$

 

where

I_mnw is the complex image value at spatial location (x_F,m, y_F,n, z_w);

U is the SAR array size in the cross-track direction;

V is SAR array size in the along track direction;

(f_P1, f_P2) is the frequency processing band;

τ

_uvw is the travel time from source (u, v) in the SAR array down to a focal point at depth z_w and back up to receiver (u, v) in the SAR array;

$x_{F,m}=\frac{3<br><br>d}{4}+\frac{\left(m-1\right)<br><br>d}{4};<br><br>$

y_F,m=0.933013d+(n−

1)dy;

d=5.52 inches; and

dy=scan spacing.

33. The method of claim 32 wherein the transmit antennas and the receive antennas are in contact with the ground and wherein ${\tau }_{}$

uvw=1cg

[(xs,u-xr,u)2+(ys,v-yr,v)2+zw2]

where (x_s,u, y_s,v) and (x_r,u, y_r,v) are the location of the transmit and receive antennas and c_g is the speed of light in the ground.

34. The method of claim 24 wherein displaying images of the analyzed data

computing a plan view image of the analyzed data;

computing a side view image of the analyzed data; and

displaying the plan view image and the side view image.

35. The method of claim 34 wherein computing a plan view image of the analyzed data comprises applying the following equation:

36. The method of claim 34 wherein computing a side view image of the analyzed data comprises applying the following equation: