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A bistatic synthetic aperture radar image of a Pakistani dam reveals features at 50-cm resolution, including elevation changes.
A classified prototype satellite should be in space right now to demonstrate the capability to track moving targets on the ground or at sea.
The details of the prototype developed jointly by the National Reconnaissance Office (NRO) and the U.S. Space Force are not known, but NRO Director Christopher Scolese last year disclosed a launch window between mid-January and mid-April.
- Umbra Space demo funded by AFWerx
- Capella Space demonstrated similar capability last year
What is known is that the technical challenge of providing comprehensive ground moving target indication (GMTI) from a satellite constellation equipped with synthetic aperture radars (SAR) in orbit has foiled previous attempts by the NRO and other agencies.
As the classified effort continues, Santa Barbara, California-based Umbra Space says its nascent constellation of low-Earth-orbit (LEO) microsatellites has proven a new way to achieve space-based GMTI with a relatively high resolution.
The small, nine-year-old company has launched eight satellites—one-fourth of a planned constellation—to a nominal height of 560 km (350 mi.), with a 97.4-deg. orbital inclination. The polar-orbiting satellites operate in clustered pairs, each equipped with an X-band, 102-m (335-ft.) antenna. Each pair is designed to provide SAR imagery at 25-cm (10-in.) resolution, with the bistatic approach adding interferometric capabilities for modeling the elevation of objects within an image. The same bistatic approach also enables “the implementation of moving target indication techniques,” the company said in a March 7 news release.
In a bistatic cluster, there are two satellites—one to transmit a signal and the other to receive it. Receiving a signal from a different angle than a transmitter produces several benefits, including eroding the radar cross-section advantages of stealthy aircraft and ship designs.
Umbra Space received a $1.25 million contract last August from AFWerx, a technology accelerator within the U.S. Air Force Research Laboratory (AFRL), to demonstrate GMTI and maritime moving target indication through its bistatic SAR satellite clusters. DARPA has also selected Umbra Space for the Distributed Radar Image Formation Technology program.
Umbra Space’s announcement follows a similar achievement last year by Capella Space, another small company with a bistatic SAR constellation in orbit.
The goals of the AFWerx and NRO projects are the same. The Air Force retired the last Northrop Grumman E-8C Joint STARS aircraft in November, ending the airborne GMTI mission provided by its AN/APY-7 radar. The Army, meanwhile, plans to fill part of the surveillance gap by fielding a GMTI radar on a fleet of Bombardier Global 6500s by the end of the decade. A prototype system known as the Army Theater-Level High-Altitude Expeditionary Next Airborne (Athena) program flew in March with the GMTI-capable Northrop Grumman Long-Range Radar.
The military has sought to migrate the GMTI capability to the space domain since it canceled the Air Force Joint STARS recapitalization program in 2019. A space-based GMTI capability has a clear appeal: An orbital constellation can provide continuous global coverage well beyond the endurance and advertised 155-km range of the entire E-8C fleet combined.
However, the challenges have overwhelmed previous attempts. The joint Air Force, DARPA and NRO Discoverer II program in the late 1990s, the AFRL’s TechSat 21 in the early 2000s and the Air Force Space Radar program in the late 2000s all sought to demonstrate a GMTI capability from space and failed. A constellation in geosynchronous Earth orbit offers unblinking global coverage but requires an antenna the size of a football field to provide useful resolution. A constellation in LEO theoretically could offer better resolution but at the expense of continuous coverage.
The specifications of the Umbra Space constellation offer a vivid demonstration. The company advertises commercial SAR imagery services but with a seven-day interval between overflights of the same area. While that satisfies the commercial market, it falls short of the military’s goal of continuous custody of critical moving targets such as missile launchers.
Umbra Space also has not revealed how it overcomes other known challenges with GMTI coverage from orbit, such as equipping microsatellites with the processing power required to suppress radio-frequency “noise” produced by ground clutter.
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