Doppler Generation for Moving Target Testing
Flight testing without the flight costs.
Radar systems development, qualification, and testing is an important challenge for test system designers. Testing can include simulators, modeling software, and field-testing. Each of these techniques must deal with and present a variety of signals found in the real-world environment including, main and secondary target returns, sea and ground clutter, multipath, jamming, interference, electromagnetic noise, and stationary and moving targets.
Each testing approach has advantages, and while field-testing is the most compelling, it has some serious drawbacks. It is expensive, time consuming, requires regulatory approval, and does not always provide an opportunity for troubleshooting problems and performing system optimization.
Field-testing can require the coordination of a variety of costly assets and platforms including aircraft, ships, and vehicles — the logistics of which can delay projects for long periods of time. This coordination presents safety hazards (both physical and radiation hazards), along with environmental and financial costs due to fuel expenditures. Simulations and modeling software are excellent tools, but they cannot provide accurate results in all cases, particularly for the development of new radar systems. This is true because the simulator design must be developed to operate with the radar system under test. This requires some understanding of the new system operation that is neither practical nor desirable.
An alternate approach to the testing problem is to re-create the actual environment in which the radar system must operate. This re-creation, or channel environment replicator (CER), must include all of the above features and accept any radar system characteristic variation, including operating frequency, frequency agility, pulsed or continuous wave (CW), encryption, and power level.
CER systems should include the propagation delay and loss associated with a desired target along with multiple target reflections, as well as include the radar target cross-section and target velocity, if any. For a moving target, the CER must vary the target position (distance from the radar to the target) and produce the frequency shift (Doppler frequency) associated with the target velocity (Figure 1).
The Doppler frequency (Fd) of a target moving at velocity (v) is given by Fd ≈ 2 v/λ, where λ is the radar operating wavelength.
The target velocity relative to the radar system source depends on the speed and direction of the target. The three possible target velocity vectors are shown in Figures 2 – 4.
Doppler Frequency Generation
An effective method for reproducing the target propagation delay and loss is an optical fiber CER. This CER system can vary both the target distance and the signal level for a given radar cross-section. It can also create multiple signal paths to replicate clutter and multi-path. The final component the Doppler frequency generation associated with the target velocity. A signal modulator is a broadband approach to Doppler generation. While these systems normally operate over octave bandwidths, they may be extended with only minor performance degradation.
Figure 1 shows a version of a moving target generator that covers radar frequencies from 1 to 18 GHz. The compact systems provide target generation with Doppler frequencies from 0 to ±12 kHz. They allow the user to set target and radar target speeds and directions, including opening, closing, and lateral motion. Systems may be operated remotely using wireless controls with scenario generator features, such as flight path replication, moving source and targets, and multiple target options.
Data may be entered in multiple formats including target and radar speed and direction, start-stop LONG-LAT and speeds, and directly with pre-programmed map placements. The OptX Series 5000 Doppler generators can be used with the OptX Series 3000 CER delay lines to create both Doppler and step-wise target and source movements along with propagation loss associated with a given radar cross-section. This combination creates complete moving target replication.
Doppler generators are an important component in the CER system that, together with target distance and cross-section, can create real-world radar testing in the laboratory. The CER represents a considerable reduction in cost, schedule time, and environmental problems associated with radar field-testing. It also eliminates the need for regulatory approval to radiate the risk of radiation safety hazards and security concerns associated with the transmission of sensitive signal profiles.
This article originally appeared in the November/December print issue. Click here to read the full article.