GEO Radar Warning Receivers
I just came across the US Space Force SBIR SF254-D1101, which is asking for a low SWaP Radar Warning Receiver (RWR) payload for small satellites in Geostationary Orbit (GEO). The deadline is September 24, 2025. At first glance, it may look like a straightforward request to shrink an RWR, but in reality, this is a physics-driven problem where every design choice has to work within hard constraints.
https://www.dodsbirsttr.mil/topics-app/?baa=DOD_SBIR_2025_P1_C4
At GEO, a satellite is nearly 36,000 kilometers above Earth. Compared to 500 kilometers in LEO, that distance adds roughly 37 dB of free-space path loss in the radar bands of interest. That is the central obstacle. The mission is still feasible because the radars that matter are high-EIRP systems: fire control, tracking, and certain surveillance or early warning radars. These emitters can produce hundreds of kilowatts through large antennas with gains of 30 dB or more, driving their effective isotropic radiated power high enough that even sidelobes can be detected at GEO. With a low-noise front end, modest Earth-facing gain, and integration across pulse trains, those signals are detectable. GEO’s geometry even helps, since a satellite can dwell on PRIs and stack pulses for about 10 dB of processing gain.
The solicitation is clear that this is not a strategic ELINT collector. The mission is to provide indications and warnings, threat response, and own-ship awareness. That makes the payload a tactical ELINT system focused on warning and survivability. It must detect, classify, and characterize radar emissions sufficiently to cue defensive actions and provide forensic data. For example, once illumination is detected, the spacecraft can take protective steps:
Insert attenuation or blank sensitive receivers to avoid saturation
Safe payloads to prevent corruption or damage
Cut transmissions to deny an adversary refined targeting data
Slew slightly to change geometry and reduce exposure
Log and time stamp pulse descriptors for secure downlink and attribution
Even at the threshold of under 8 kilograms and 35 watts, that kind of payload is protecting a satellite worth hundreds of millions or even billions of $$$. Meeting the more ambitious objective of 5 kilograms and 15 watts would make scaling easier across a fleet, but threshold performance is still valuable.
The design does not have to start from scratch. Other domains have already solved many of these challenges. Aircraft RWRs rely on a sentinel and analyst split: an instantaneous frequency measurement channel runs continuously at very low power, while a software-defined path activates only when cued. Naval EW systems use multiple antennas for coarse direction finding, enough to support defensive actions. However, more advanced approaches, such as interferometry or amplitude monopulse, are options if a finer bearing is ever needed. Ground-based systems package electronics tightly in hot environments, using rugged enclosures, conduction cooling, and duty cycling, which are exactly the methods that small GEO buses will require.
What will separate credible proposals from promises is proof that the numbers add up. A strong response will show a link budget that demonstrates detection of high-EIRP radars at GEO, a power budget that explains how the payload stays under 15 watts on average with duty-cycled SDR paths, a radiation plan that lasts more than five years without wasting mass on brute-force shielding, and a secure communication approach that delivers pulse descriptors without draining the margin.
This SBIR is not about promising magic. It is about showing that you understand why GEO is hard, defining the mission as a tactical tripwire, and adapting proven ideas from air, sea, and land. A payload that does those things will not be exquisite, but it will be survivable, actionable, and worth the cost of putting it into orbit.