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Space domain awareness (SDA) is shifting from knowing where satellites are to understanding what they are doing. Radio frequency (RF) sensing is how you see that behavior, and no single vantage point sees all of it.
This week HawkEye 360 published results from an AFRL-funded demonstration showing its LEO satellites could detect and characterize RF emissions from spacecraft in LEO, MEO, and GEO. It's a good moment to step back and look at how RF sensing fits into SDA as a whole: space-based and ground-based RF sensors each answer a different part of the SDA question.
This post walks through what each approach contributes, where each one is limited, and why a complete picture needs both.
What RF adds to space domain awareness
Radar and optical sensors tell you where an object is and how it moves. RF tells you whether it is alive and what it is doing.
A satellite's emissions reveal things its orbit cannot:
- Activity: whether it is transmitting at all, and when.
- Communication links: which ground stations or other spacecraft it talks to.
- Capability: frequencies, bandwidths, and signal types it uses.
- Behavior change: new emissions, a shifted carrier, or silence where there used to be traffic.
- Interference: who or what is degrading a link, and from where.
The DoW framed its HawkEye 360 study around exactly these questions: is the satellite transmitting, is it talking to a particular ground station, and is it changing its usual behavior. Answering them well depends on two things: the geometry of where you listen from, and how continuously you listen.
Space-based RF sensing: geometry the ground can't reach
Satellites carrying RF receivers can listen from orbit, which opens angles that ground sensors never get. As HawkEye 360 COO Todd Probert put it to Payload, some satellite emissions are hard or impossible to observe from the ground because of antenna directionality or atmospheric effects.
Where it shines:
- Hearing emissions pointed away from Earth, such as crosslinks between spacecraft.
- Global reach, including over oceans and denied areas where no ground site can be placed.
- Characterizing signals from many orbital regimes with one fleet.
Tradeoffs:
- Collection is pass-based. In HawkEye's demo, LEO targets required new concepts of operations because of high relative speed and short collection windows.
- GEO and MEO targets required adapted pointing techniques, since the sensors were built to look down at Earth.
- Each collection is a tasked snapshot, so watching one target continuously means many satellites or long gaps between looks.
- Spacecraft are expensive to build and launch, and hard to upgrade once in orbit.
Ground-based RF sensing: persistence and baselines
Ground sensors listen from fixed sites on Earth. Pointed at the geostationary belt, they can stare at the same satellites around the clock, because GEO spacecraft never leave the sky above a given site.
Where it shines:
- Continuous collection on GEO targets, with no revisit gaps.
- Long baselines of normal behavior, which is what makes a change stand out.
- Geolocating uplink interference, since uplinks originate on the ground and arrive at the satellite from Earth.
- Lower cost per site, faster to deploy, and hardware that can be upgraded anytime.
- A network of sites in different locations adds geometric diversity and redundancy.
Tradeoffs:
- Each site only sees the part of the sky above it, so coverage depends on where sites are.
- Emissions aimed away from Earth, like crosslinks, are out of view.
- LEO satellites pass quickly over any single site, so ground collection on LEO is also time-limited.
- Weather and local RF noise can affect some bands.
Space-based and ground-based RF side by side
Why a complete picture needs every layer
Each approach covers the other's blind spots. Space-based sensors catch geometries the ground can't reach. Ground-based networks watch continuously what orbit only sees on a pass.
Consider a GEO communications satellite whose service suddenly degrades:
- Service on the satellite degrades, and the cause isn't on the operator's own equipment.
- A ground-based network that has been watching that slot for months spots a new carrier that wasn't in the baseline, and geolocates its uplink.
- A space-based collection adds what the ground can't see, such as a nearby spacecraft emitting in an unexpected direction.
No one layer gets you from symptom to source to intent on its own. Together, they turn an outage report into an explanation.
As HawkEye's Probert told Payload, the question is no longer whether commercial RF can contribute to SDA. The work now is fusing these layers so defense, civil, and commercial users can see the whole RF environment, not just a slice of it.
At Rebel, our ground-based RF sensor network provides the persistent, ground-based layer of that picture.
We're also extending that sensing into orbit. Through our partnership with Portal Space Systems, first reported by SpaceNews, Rebel is working to bring spectrum monitoring and electronic threat detection aboard Portal's highly maneuverable spacecraft. Because Portal vehicles can change orbits quickly, onboard sensing can reach geometries that fixed constellations can't, and pair that space-based view with what our ground network sees every day.
Frequently asked questions
What is RF space domain awareness? It is the use of radio frequency sensing to understand what satellites are doing: whether they transmit, who they communicate with, and how their behavior changes over time. It complements radar and optical tracking, which focus on position and motion.
Is space-based or ground-based RF sensing better for SDA? Neither is better on its own. Space-based sensors offer unique viewing geometries and global reach. Ground-based networks offer persistent, continuous coverage of GEO and strong uplink geolocation. Most complete SDA architectures use both.
Why does persistent RF monitoring matter? Detecting a change in behavior requires knowing what normal looks like. Continuous collection builds that baseline, so new emissions, drift, or interference stand out quickly instead of being missed between snapshots.
Can RF sensing identify the source of satellite interference? Yes. Networks of RF sensors can detect interference and geolocate where it originates, which helps operators and regulators attribute it and act on it.
Sources
- Advancing Space Domain Awareness with Commercial RF Intelligence, HawkEye 360
- HawkEye 360 Turns its RF-Sensors to Orbital Targets, Payload
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