This chapter discusses the history of battlefield surveillance radar, foliage penetration (FOPEN) radar, airborne radar, and synthetic aperture dual frequency radar. These early RADARs developed for foliage penetration were in response to military needs to find and locate insurgents in a severe tropical environment.

The development of foliage penetration (FOPEN) radar had a major resurgence in the early 1990s for both military and geoscience applications. A series of data collections was carried out starting in 1990 as a risk reduction for the development of a more reliable FOPEN synthetic aperture radar (SAR) system. Several government laboratories including NASA Jet Propulsion Laboratory (JPL), Sweden's Defence Research Establishment (FOA), US Army Research Laboratory (ARL), US Air Force Research Laboratory (AFRL), and the US Naval Air Development Center (NADC) initiated these collections for both scientific and system technology objectives. A combination of instrumentation collection platforms and airborne brassboard radars were used to obtain data on surface clutter, foliage scattering losses, and the ability to detect objects and terrain features in forested regions. However, it was deemed to be very important that accurate instrumentation and calibration targets be included in the test to characterize the target and clutter phenomena toward the development of operational and commercial radar systems. To this end, the US Defense Advanced Research Projects Agency (DARPA) sponsored MIT Lincoln Laboratory to set up and maintain the scientific and analytic standards. These capabilities would provide the community with a reliable understanding of these measurements' influence on new systems applications.

Section 3.8 presented the problem of the radio frequency interference (RFI) environment in ultra wideband (UWB) radar operation from the standpoint of background clutter. This chapter will cover the significant design factors needed to operate a foliage penetration (FOPEN) radar, in either synthetic aperture radar (SAR) or ground moving target indicator (GMTI) modes, and under most operational conditions. Because of the worldwide regulation of the RF spectrum for telecommunications and active sensing operation, it is necessary to accommodate the limits that are imposed on power spectral density for transmission of the FOPEN UWB waveform. Equally as important is the need to remove the dense, high-power transmissions that will be intercepted by the wide beamwidth SAR antenna and that affect the dynamic range of the signal received by the SAR and the image formation processing.

This chapter will present the major system design of a tactical foliage penetration (FOPEN) synthetic aperture radar (SAR) that could be built with emerging technology. It will go through the steps of the concept of operations (CONOPS) of FOPEN SAR with high area coverage rates (ACRs) and with focused attention for more accurate target detection and characterization. The radar power-aperture product can be chosen, given a standoff range and characteristics of the foliage and targets, to meet the requisite resolution and signal-to-noise ratio (SNR) requirements. Next, guidelines will be summarized for the subsystem specifications, the grazing angle and receiver dynamic range for minimizing the foliage loss and RFI effects, and several modes to meet the CONOPs requirements.

This chapter has illustrated the advantages of a bistatic FOPEN SAR receiver working in concert with a tethered airborne GMTI radar illuminator. The potential for moderate resolution SAR operation is made possible by the shortrange, higher angular rate collection of a bistatic adjunct receiver. Issues of platform motion are evaluated based on a detailed pulse-by-pulse simulation of a large scene, with impact of foliage scattering and loss and target separation. By placing patterns of targets in the scene, the ability to resolve targets in two cardinal planes is provided. Clearly, the collection geometry of a SAR receiver must be fully understood to provide for desired signal to clutter and resolution. In summary, a bistatic adjunct receiver can provide detection of fixed targets by using the GMTI RADAR waveform. The operation closer to the target area on a small UAV will give increase in system sensitivity and resolution over the monostatic platform. More importantly, both the GMTI RADAR and fixed target surveillance can be obtained with a single waveform. Future analysis is required on the synchronization and direct path radiation from the GMTI radar platform to the UAV and the effect of volumetric clutter on the fixed target detection. This is especially true in a dense radiofrequency interference (RFI) environment. Bistatic operation can definitely provide improvement in system operational capability.