Future Space Based Radar Technology Needs for Surveillance

Space Based Radars are gaining significant acceptance for world-wide, all-weather, on-demand surveillance. The main impediment has been affordability, followed closely by the limitations by satellite communications for providing near real time target detection data to the users. However, recent advances in active electronic scanned arrays and on-board signal processing are enabling SBR development for ground moving target indication. The real challenge will be for surveillance of air targets and for targets obscured by camouflage and concealment. This presentation will outline some critical SBR design architectures, and a roadmap for developing the enabling technologies to meet future surveillance needs. SPACE BASED RADAR SIZING Several studies 1,2,3 have addressed the power aperture design of space based radar for both ground moving target indication (GMTI) and air moving target indication (AMTI). For a surveillance radar, the design is primarily determined by considerations of orbit altitude and number of satellites required to maintain a region within the satellite’s instantaneous field of regard. For a single satellite, the power aperture (PA) tradeoff depends on the factors of target cross section σ, range to target from the transmitter and receiver (RT and RR ), and area coverage rate Ω/ts (square radians per second) as shown in (1). The other terms in (1) are familiar receiver, noise and system loss characteristics, along with the minimum signal to noise requirements for target detection and false alarm. 2 2 min ) / ( 4 R T s n o R T R R t N S L F kT A P Ω = σ π . (1) For monostatic operation, it is a simple matter of determining the Power*Aperture requirements, given the desired area coverage rate and target cross section. Fig. 1 provides an indication of the SBR power aperture, as a function of orbit altitude, required for detecting a 10 square meter target, at various area coverage rates. The figure also shows the significant impact of grazing angle on area collection rate; where low grazing angles cover a significantly larger area. The impact of grazing angle on power aperture is reduced for mid Earth orbit (MEO) satellites (altitudes greater than 2000 Km); since the Earth encompasses a smaller angle with respect to the radar, and the range difference to surveillance regions is smaller. It is important to note, however, that the MEO solutions will require much larger apertures, with associated higher power, weight and production cost. The tradeoff then is whether the overall MEO constellation can be made more cost effectively than the low Earth orbit (LEO) constellation. Figure 1 Power Aperture Sizing of Monostatic SBR (target cross section 10 square meters) The constellation of satellites to achieve world wide surveillance operations is a principal design parameter in any system design. A LEO constellation at approximately 1000 Km provides reasonable detection of targets with a modest satellite size, but requires more than 20 satellites to provide the near continuous coverage. Fig. 2 shows a coverage map of a typical LEO constellation including the grazing angle limit to coverage (outer limit), and the nadir hole where reasonable target detection ceases. In Fig. 3, a MEO constellation at 10,000 Km is shown, providing more robust coverage with fewer satellites. However, at MEO orbits, a much larger power aperture antenna design is required; and the radiation belt is significantly stronger, requiring more stringent design of electronics. It is instrumental to compare the surveillance objectives for the GMTI versus the AMTI missions. The target characteristics and mobility are different in the two missions. AIAA/ICAS International Air and Space Symposium and Exposition: The Next 100 Y 14-17 July 2003, Dayton, Ohio AIAA 2003-2825 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. 2 American Institute of Aeronautics and Astronautics This leads to a general comparison of two SBR power aperture design requirements, as summarized in Table 1. Several points are apparent from this comparison: • Airborne targets are typically smaller than ground targets by a factor of 10 to 100 • Because of mobility of the air targets, the AMTI surveillance radar must revisit the area at least 3 times as often than for GMTI • The volume coverage for airborne targets is typically 3 times more than that of ground targets Figure 2 Low Earth Orbit Satellite Coverage (21 satellites at 1000 Km orbit altitude) Figure 3 Mid Earth Orbit Satellite Coverage (4 satellites at 10,000 Km orbit altitude) Table 1 Comparison of GMTI and AMTI Radar Scaling These factors point out that the AMTI requirements are at least 1000 times more difficult, for similar radars, than a GMTI mission. As a result, alternative architectures are needed to obtain surveillance against these targets. Because of the significant demand on radar PA, the architectures will consider wide area surveillance of for nominally large targets, and focused search for small targets. The architecture changes from GMTI are: • Lower frequencies (such as L-Band or 1250 MHz); where the target cross section can be larger and the clutter smaller than at GMTI frequencies. • Monostatic Wide Area Surveillance against moderately sized tactical targets to maintain high area coverage rates. • Bistatic geometries from the monostatic SBR transmitting to a bistatic adjunct receiver. Here the receiver is closer to the target, obtaining an advantage in signal to noise ratio for small targets, at the expense of lower effective area coverage rates. RADAR APERTURE DESIGN Significant progress has been made in developing Active Electronically Scanned Antenna (AESA) technology to achieve the power density, spectral purity and sidelobe control required of a surveillance radar. Fig. 4 depicts a typical AESA design concept with a deployable planar array, large solar cell array for generation of prime power, and spacecraft bus to house the onboard electronics, satellite attitude measurement and control system, and power management and distribution system. The major issue for affordability has been the ability to manufacture the AESA with deployable design techniques, and provide sufficient number of receiver channels for adaptive processing, to compensate for both clutter and environmental effects on the radar performance. Figure 4 Simple concept of Space Based Radar Power Aperture is the most important design parameter for system performance and affordability. Once the available satellite power has been determined, the size of the aperture Ground Based Surveillance Airborne Surveillance Difference (dB) Target RCS (dBsm) Revisit Rate (sec) Volume Coverage 10 30 X 0 to -10 10 3X 10 to 20 5 5 Total 20 to 30