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Publication Number: FHWA-HRT-10-037
Date: October 2010

Step Frequency Ground Penetrating Radar Characterization and Federal Evaluation Tests

Test Design

SF GPR emissions characterization testing was conducted using the configuration shown in figure 2 and figure 3 and pictured in figure 4 and figure 5. The side view in figure 2 illustrates the maximum 9.84-ft (3-m) distance that was maintained between the SF GPR array under test and the emission measurement antennas. A biconic antenna was used to measure frequencies in the 50–299-MHz range, a log periodic antenna was used to measure frequencies in the 300–2,000-MHz range, and a horn antenna was used to measure frequencies in the 2,001–3,000-MHz range. Detailed make and model information for the test equipment is provided below. Figure 2 illustrates the seven angles that the SF GPR antenna array was aligned with for each test configuration to determine the orientation corresponding to maximum emissions. Due to antenna array symmetry, angles from 0 to 180 degrees were representative. As the test plan required, the antenna array was always directed at the ground and was suspended 10 inches (254 mm) above a standard sand pit.

Emission measurement equipment included the following:

  • Agilent Technologies E4440 power spectrum analyzer.

  • A.H. Systems, Inc. biconic antenna, model SAS-542, S/N 769 (9.84 ft (3 m)).

  • A.H. Systems, Inc. log periodic antenna, model SAS-510-2, S/N 1053 (9.84 ft (3 m)).

  • A.H. Systems, Inc. double ridge guide horn antenna, model SAS-571, S/N 549 (9.84 ft (3 m)).

  • Electronic Cable Specialists cable P/N 310801 (two each at 6 ft (1.83 m) and one at 12 ft (3.66 m)).

  • 3D Radar/Curtiss-Wright SF GPR system, model Geoscope GS3F.

The SF GPR system consists of a laptop computer, a rack mountable radar control unit, a 26.24-ft (8-m)-long cable bundle, and an antenna array that has 23 antenna elements (see figure 6).

The SF GPR emission characterization test procedure follows:

  1. Measure ambient emissions to establish a baseline (50–6,000 MHz).
  2. Position the SF GPR antenna array in each of the seven orientation angles (see figure 2).
  3. Measure emissions from the SF GPR array using each of the three emission measurement antennas (biconic, log periodic, and horn) using the unnotched SF GPR emission spectrum for peak and root mean squared (RMS) measurements (50–6,000 MHz).
  4. Determine SF GPR antenna array orientation corresponding to maximum emissions for the unnotched SF GPR emission spectrum configuration.
  5. Measure the peak and RMS for the notched SF GPR emission spectrum with the SF GPR antenna array orientation corresponding to maximum emissions using each of the three emission measurement antennas (biconic, log periodic, and horn).
  6. Collect the complete general measurement set by collecting data corresponding to the test matrix (see figure 15 and figure 16 in appendix B).
  7. Make measurements to characterize SF GPR emissions in frequency bands corresponding to critical Federal systems. Make measurements for maximum emissions orientations of the SF GPR antenna array according to the test matrix (see figure 17 and figure 18 in appendix B).

Maximum SF GPR emissions corresponding to the worst case orientation of the SF GPR antenna array relative to the emissions measurement antenna were determined by comparing emission measurement plots for each unnotched measurement configuration in the frequency range of interest and selecting the configuration corresponding to the maximum emissions.

Another set of measurements was made using the same procedure previously defined, but for these measurements, the number of active antenna array elements in the SF GPR array varied. These measurements were made in the event that Federal agencies would analyze emission characteristics when a subset of antenna array elements is active. The original test scenario, using all antenna array elements, represents worst case emissions. Therefore, the supplemental measurements are only expected to be used for a deeper analysis of SF GPR emission characteristics if needed.

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1ft = 0.305 m

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Figure 2. Diagram. SF GPR emissions characterization test configuration.

 

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1ft = 0.305 m

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Figure 3. Diagram. Orthogonal position of antenna array relative to figure 2.

 

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Figure 4. Photo. SF GPR testing at the FAA Technical Center.

 

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Figure 5. Photo. SF GPR test site at the FAA Technical Center.

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Figure 6. Diagram. Antenna array with a bank of 23 transmitter elements and 23 receiver elements.

Additional follow-up emission measurements were made using procedures described in the test report in appendix E. The minimum distance between the SF GPR antenna array and the measurement antenna was 6.88 ft (2.1 m), as illustrated in figure 3. This distance often corresponded to the orientation where maximum emissions were measured.

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