February 5, 2016
Radar cross section (RCS) is the measure of an object’s ability to reflect radar signals in the direction of the radar receiver. Accuracy in measuring RCS can have a direct result on safety and successful aviation operations. In military, stealth technology is used to reduce RCS and make it difficult to detect aircraft. It is desirable for commercial aircraft to have a large RCS so air traffic control radar systems can easily locate aircraft and safely guide them as they approach or depart from the airport.
Commercial Air Traffic Control (ATC) systems may begin to track aircraft from as far as 200 nautical miles from the radar antenna(s), but more typically tracking will begin at 30-50 nautical miles from the radar antenna(s). In either scenario there is potentially multiple radar systems handing off to each other as the aircraft continues its approach or departure.
Engineers have traditionally used Vector Network Analyzers (VNAs) to measure RCS because of their speed and accuracy in making S-parameter measurements. Engineers, however, should consider other capabilities, specifically time domain gating, when selecting a VNA. Currently, there are handheld microwave VNAs that have these features, all of which make it easy to conduct RCS tests on the flight line or in the field.
Although VNAs are most commonly used to provide measurements vs. frequency, the addition of time domain analysis and time gating help simulate pulsed radar functionality by removing reflection distances not associated with the target. A VNA with 12-term error correction, such as the VNA Master, will minimize the systematic errors due to mismatch and leakage to accurately establish a reference plane.
The handheld VNA shown in figure 1 measures the S-parameters in the frequency domain. The frequency range for the measurements should correspond to the radar frequency band; for example, 8.2 – 12.4 GHz for a WR-90 X-band waveguide. The time domain function of the VNA will transform the S-parameter frequency domain measurement (G vs. frequency) to the time domain (G vs. time or distance).
Figure 1: VNA Master with Tx/Rx waveguide antennas.
A typical aircraft RCS measurement configuration using a VNA is shown in figure 2. The transmit antenna (connected to port 1 of the VNA) and receive antenna (connected to port 2 of the VNA) are positioned in the same plane. The measurement target consists of the aircraft either mounted on a low reflection pedestal or a stand-alone on a flight line.
Figure 2: Block diagram for VNA measurement of RCS.
The operation of a S21(f) measurement for the VNA is the equivalent to the radar when configured as shown. The coaxial cable output of port 1 is connected to the coax to rectangular waveguide transition (E field in the vertical direction). The output of port 2 is connected to the output of the receive waveguide antenna. Both antennas are located as close as possible, in either the vertical or horizontal plane. To develop the polarization matrix, the transmit and receive antennas should be capable of 90° rotation. The target should be located at a distance less than AFR/2 but far enough from the antenna to insure that the entire target is within in the beam of the antennas.
A full 12-term calibration can be performed at the output of the coaxial cables to establish the reference plane for the RCS measurements. A S21(f) frequency domain measurement is performed on the target area. The S-parameter data S21(f) is transformed to the distance domain mode S21(D) using band-pass processing. All reflections from the target area or support structure are shown in figure 3. The system can be calibrated in RCS by measuring a target of known RCS and referencing all other targets to the one that is known.
Figure 3: Reflections from target area or support structure.
A S21(f) frequency domain measurement is performed on the standard to be measured. The S-parameter data is transformed to the time domain mode and an appropriate time gate is placed at the standards location. The magnitude of the S21(std) amplitude of the standard reflection is measured, and the value is the reference for the RCS measurement. If the standard were a sphere of having a RCS of 1 m2 then the RCS of the target is given by:
RCStgt (dBsm) = RCSstd (dB) - RCStgt (dB)
The data is expressed in dBsm, or decibels referenced to one square meter. RCS in square meters can be converted to dBsm by the following equation:
dBsm = 10Log(RCSm2) (dB)
The target in this example is a known calibration standard which is positioned in the target area. The calibration standard reflection is identified and a range gate is placed on the calibration standard to remove all other reflections, as shown in figure 4. The amplitude S21tstd of the calibration standard reflection is measured, and the S21 measurement in dB corresponds to the known RCS (in meters2).
Figure 4: Target reflection from a 6” Diameter Calibration Sphere (RCS = 0.018 m2).
You can learn more about conducting RCS measurements by downloading a free application note entitled Measurement of Radar Cross Section Using the VNA Master Handheld VNA.
