Space Based Radar (SBR) Electronically Scanned Arrays [ESA]

When it comes to detecting faint objects with any form of electromagnetic radiation embedded in strong background clutter and noise, there is no substitute for aperture. Whether using radio frequencies, light, or infrared, the more photons that can be gathered from a target of interest, the better.

Another inescapable fact of physics is that the further away a sensor is from an object-and space can be pretty far away-the larger the collection aperture needs to be to maintain a prescribed detection and resolution performance. The narrower beamwidths enabled by large apertures also play a significant role in limiting the amount of interference emanating from other locations. The Arecibo antenna in Puerto Rico and the Hubble telescope dramatically illustrate the "bigger is better"

To meet the demands of rapid area coverage and high revisit rates, scanning must be performed electronically. ESAs, or electronically scanned arrays, eliminate the need for mechanical slewing, often a necessity for very large antennas.

For narrowband systems (i.e., when the signals of interest are reasonably approximated as a single frequency sine wave), electronic scanning is accomplished by introducing phase shifts between different parts or subarrays of the antenna. This is because time delay equates to a phase shift when a narrowband signal propagates across the antenna from an angle not aligned with the array boresight. Unfortunately, simple phase shifts are inadequate when wider bandwidth signals are required.

For example, synthetic aperture radar (or SAR) bandwidths can easily approach half a Gigahertz or more at X-band (3 cm wavelength). This limits phase-only electronic scanning antennas to approximately a meter or less resolution depending on how much scanning off boresight is required.

For larger antennas, the array can be divided into separate sections, each with a separate receiver and true time delay introduced between the subarrays. If the outputs of each subarray are digital, time delay is easily introduced with software. The practical consequence of a true time delay requirement for a large spaceborne antenna is increased complexity, weight, volume, and power consumption.

A significant case in point is space-based radar, or SBR. Ideally, SBR will provide continuous surveillance of surface mobile targets via a GMTI (or ground moving target indicator) mode of operation in much the same manner as existing and future planned JSTARS and Global Hawk systems. Consider the impact on antenna size requirements of attempting to replicate JSTARS performance from space. A key antenna metric is resolution and accuracy. The JSTARS antenna is approximately 7 m in horizontal length. At a range of 150 km, this translates into a cross-range resolution of approximately 1000 m. The actual accuracy of the measurement is a function of signal-to-noise ratio (or SNR) and can be a factor of 10 to 20 times better than the fundamental resolution.

In a low earth orbit (or LEO) side-looking configuration, operating ranges of 1500 km are possible - a factor of 10 increase in range. To maintain the same resolution, the antenna would need to be 57 m long. At MEO altitudes, say 10,000 km, the length increases to approximately 320 m.

The primary reason for considering MEO, despite the demands on antenna size, are the significantly reduced number of satellites required to achieve persistent 24/7 coverage and steeper grazing angles, alleviating terrain obscuration.

Another extremely important metric for GMTI radar is the minimum detectable velocity, or MDV, defined as the slowest radial velocity that can be separated from mainbeam ground clutter. Assuming typical cruising speeds for a Boeing 707 and 7 m length antenna, the MDV due to mainbeam clutter spread is approximately 2 m/s. To maintain this MDV at LEO requires an antenna length of approximately 100 m. The greater length is due to the increased ground speed, which translates into a greater clutter Doppler spread. Interestingly, the slower ground speeds at MEO result in a smaller antenna requirement than LEO to maintain the requisite MDV. For example at 10,000 km, an antenna of 80 m has the same MDV as the 100 m LEO.

The two primary functions of an SBR are search and track. The amount of surface area searched per unit time is set by the product of radiated power and antenna aperture area. While search capability is set by the power-aperture (PA) product, for a tracking radar PA2 is the appropriate metric to determine how many individual tracking beams can be supported. This, in turn, implies that there is an extra benefit to increasing antenna size rather than power-doubling the antenna size quadruples tracking capability. For example, the improved resolution, accuracy, and clutter suppression of a larger antenna yield synergistic improvements in all tracking performance metrics; indeed, bigger is better.

Once deployed and functional, these large antennas need to be carefully calibrated. It is no small feat to point such a narrow beam at such great distances. Fractions of an arc-second error can translate into large miss distances on the surface. Here again, some innovative concepts have been identified that take advantage of the dynamic nature of the satellite orbits. Ground beacons placed in friendly territories can provide the essential "truth signals" required to precisely calibrate the antenna array manifold. In MEO orbits, one can expect a ground beacon to always be in the radar's field of regard.