Richard Ferranti, SRI International
Keywords: Space situational awareness, Space object surveillance, Bistatic radar, Non-cooperative radar illumination, Phased array antenna, Machine learning
Abstract:
Ground-Based Bistatic Radar for Space Surveillance using a Non-Cooperative Radar Illuminator
R.L. Ferranti
SRI International
The existing network of US ground-based space, aircraft, and weather radars (FAA, NWS, NSF) can be used as non-cooperative illuminators to detect space objects and debris. Ground-based receivers using multi-beam array antennas can receive bistatic reflections from space objects whenever a non-cooperating radar beam points at the space vehicle or debris. Space-object radar cross section (RCS) can be enhanced where the bistatic angle is large, providing increased detection probability for small space debris. A network of passive sensors with common beam volumes can harvest and combine multiple bistatic reflections, energy otherwise lost in space, and enhance small object detection and target disambiguation. The goal of this research is to provide a means of achieving ground-based, bistatic space radar surveillance with passive receive-only array antennas using existing, non-cooperating radars as illuminators.
An example of a powerful non-cooperating phased-array illuminator is the NSFs Advanced Modular Incoherent Scatter Radar (AMISR) space-weather radar. These radars have unpublished and largely unpredictable scan patterns, making it nearly impossible for a bistatic receiver to establish a common beam volume with an AMISR illuminator. This paper proposes a novel means for determining a non-cooperative radars beam-pointing angle. This method places a wideband software-defined reference receiver with a small antenna within the illuminating radars line-of-sight. The data gathered from this small sensor provides a reference waveform for the bistatic receivers matched filter and a sample of the magnitude and phase of the radars transmissions weighted by its antenna sidelobes and steering phases. Just as a unique set of complex weights are applied to the illuminators phased array to scan a beam in a specific direction, so does the reference receiver obtain complex RF samples unique to those scanned antenna angles. By determining the illuminating radars beam-pointing angles, the bistatic receiver can point its beam to surveil an illuminated target in the common beam volume.
To illustrate this beam-pointing determination method, the left side of Figure 1 shows the 3D pattern of a 192-element phased-array antenna electromagnetic model, arranged as an 8 by 24 element planar grid, and a single dipole reference antenna spaced 500 wavelengths distant, well into the antennas far field. The phased arrays main beam is pointed over a 24-point grid of beam angles covering six 5-degree steps in azimuth and four 10-degree steps in elevation, with none pointed directly at the dipole. The right side of Figure 1 shows the complex current (magnitude and phase) induced in the reference dipole antenna for each of these illuminating beam angles. Because each point is unique, these measurements can be employed to reveal the beam-pointing angle of the radar.
Figure 1: 192-element phased array beam pattern simulation (left) with polar plots (right) showing unique magnitudes and phases of the signal received at a reference antenna in the far field of the array.
Since the reference antenna is located within line-of-sight of the illuminating radar, the signal-to-noise ratio (SNR) of the received reference signal will be very high, even from radar sidelobes 40 dB down from beam peak. This high SNR will ensure accurate measurements of sidelobe emissions. The bistatic radar receiving system is built as a phased array that continuously digitizes and stores signals received at each element. When the illuminating radars beam-pointing angle is determined, the receiver can beamform its stored data to form a common surveillance volume with the transmitted signal, achieving bistatic operation in near real-time.
Figure 2 is a block diagram of the non-cooperative bistatic radar system concept.
Figure 2: Bistatic space surveillance radar using a non-cooperative illuminator. A reference receiver within line-of-sight of the illuminator provides the reference waveform and measurements to determine the illuminating radars beam pointing angle.
This non-cooperative bistatic system can also exploit the scan characteristics of many types of radars. Radars typically perform periodic horizon scans and calibration runs against standard orbiting radar targets. If a reference receiver measurement is corrupted by multipath or other interference, the bistatic receiver can form multiple simultaneous beams in a few likely directions to discover the one with a target. The signal processor can then preferentially associate that reference measurement with a particular beam direction. Every bistatic target detection will reinforce the association of the reference receivers beam-pointing measurement with the actual beam-pointing direction employed by the illuminating radar.
Some radars randomly change their waveforms center frequency, and the wideband reference receiver will record each frequency and waveform. Each of these frequencies will have its own table of complex values to associate with the illuminating radars beam-pointing direction. Since space radars are in constant operation, the bistatic receiving system has continuous opportunities to refine its illuminating radars beam-pointing directions and enhance its target detection capabilities. This evolving augmentation may be instantiated with a machine-learning system, as also shown in Figure 2.
Date of Conference: September 19-22, 2023
Track: SDA Systems & Instrumentation