VHF and UHF spectrum occupancy surveys from orbit
Wideband SDR payloads in LEO measure how densely VHF and UHF spectrum is occupied across entire regions in a single pass, revealing usage patterns no terrestrial monitor network can replicate at comparable scale.
Sensors
- HawkEye 360 RF constellation: Clusters of three formation-flying microsatellites carrying wideband SDR receivers covering roughly 144 MHz to 15 GHz. Time-difference-of-arrival and frequency-difference-of-arrival between cluster members allows geolocation of emitters; revisit over a given point is on the order of hours with the current constellation, improving as additional clusters are launched. Spectral resolution and dwell time per pass are constrained by orbital velocity: a typical LEO pass yields a dwell of roughly 5-10 minutes over a fixed ground footprint.
- Spire Global LEMUR-2 SDR payload: Spire's LEMUR-2 satellites carry software-defined radio payloads capable of receiving in VHF and UHF bands, primarily deployed for AIS and GNSS-RO but configurable for wideband spectrum monitoring tasks. The constellation of over 100 satellites provides frequent revisit, though dwell per pass remains limited by orbital geometry. Spectral snapshots rather than continuous occupancy records are the realistic product.
- NORSAT-1 VHF/AIS payload: Norwegian NORSAT-1, launched 2017, carries a VHF AIS receiver and a UHF payload demonstrating that small satellites can monitor the 156-174 MHz maritime VHF band from orbit. It established the detection sensitivity floor for ship-borne VHF emissions from LEO, informing subsequent wideband survey mission designs. Single-satellite revisit is poor; it is a technology demonstrator rather than a survey constellation.
- ITU-documented terrestrial monitoring networks (reference baseline): The ITU Radio Regulations require member states to operate spectrum monitoring stations, but coverage is uneven: dense in Western Europe and North America, sparse across open ocean, equatorial Africa and Central Asia. Orbital surveys are most valuable precisely where the terrestrial network is thinnest. Ground stations provide calibration truth for orbital measurements where they overlap.
What the orbital vantage point actually measures
A LEO satellite carrying a wideband SDR receiver does not hear what a terrestrial monitor hears. From roughly 500-600 km altitude, the radio horizon extends to a ground footprint of approximately 2,500 km diameter, integrating emissions from thousands of transmitters simultaneously. The received power from any single low-power transmitter is correspondingly weak: free-space path loss at 300 MHz over 500 km is around 153 dB, which demands sensitive receivers and careful noise-figure management. What the orbit gains is geographic breadth; what it sacrifices is the ability to resolve individual low-power emitters in a crowded environment.
The practical result is that orbital VHF/UHF surveys are most reliable for detecting moderately powerful emitters: broadcast transmitters, maritime VHF base stations, military tactical radios operating at elevated power, and regional paging networks. Handheld push-to-talk radios at a few watts are at or below the detection floor for most current payloads unless they are numerous and co-channel, in which case aggregate occupancy is measurable even if individual units are not.
The three-way trade-off that governs every survey design
Spectral resolution, dwell time, and geographic coverage cannot all be maximised simultaneously from a moving platform. A satellite travelling at roughly 7.5 km/s crosses a 2,500 km footprint in about five to six minutes. During that window, a wideband receiver sweeping from 30 MHz to 3 GHz must either step through sub-bands with high spectral resolution (capturing fine channel structure but missing fast-changing occupancy) or capture the full band at coarser resolution in a single snapshot.
HawkEye 360's approach uses multiple satellites in close formation to apply interferometric geolocation across the dwell window, trading some spectral breadth for the ability to locate emitters spatially. Spire's architecture prioritises constellation size to increase revisit frequency, accepting that each individual pass is a brief snapshot. Neither approach produces a continuous occupancy record comparable to a fixed terrestrial monitor; they produce a statistical sample that, aggregated across many passes and many satellites, approximates a time-averaged occupancy map.
Survey planners should be explicit about what they are commissioning. A single-pass snapshot reveals which channels are occupied at the moment of overpass. A multi-week aggregation across dozens of passes begins to reveal duty cycle, diurnal patterns, and geographic boundaries of regional frequency assignments. The two products serve different regulatory and intelligence purposes.
Where the physics creates honest limits
Cloud cover is irrelevant to RF monitoring, which is one of the method's genuine advantages over optical sensing. Ionospheric effects are not irrelevant, however. Below roughly 30 MHz, the ionosphere refracts and reflects signals unpredictably; the VHF band starts at 30 MHz precisely because the ionosphere becomes progressively more transparent above that frequency, though sporadic-E propagation can still carry VHF signals thousands of kilometres beyond their intended service area, creating false geographic attribution in orbital surveys.
At UHF and above, the ionosphere is largely transparent, but the sheer density of emitters in urban environments creates a noise floor that can mask weaker signals. Orbital surveys in the 400-512 MHz land-mobile band over a large city will detect aggregate occupancy reliably; they will not resolve individual channel assignments at 12.5 kHz spacing. Spectral resolution of current wideband SDR payloads in operational constellations is typically in the range of tens of kilohertz to low hundreds of kilohertz, sufficient for band-level occupancy assessment but not for individual narrowband channel auditing.
Geolocation accuracy for a single emitter using TDOA/FDOA across a three-satellite HawkEye 360 cluster is publicly cited as being in the range of kilometres for cooperative geometries, degrading with baseline length and signal duration. Occupancy surveys that do not require precise geolocation are less constrained, but any regulatory enforcement application that needs to locate a specific illegal transmitter to within a city block will require terrestrial follow-up.
Regulatory and strategic applications that justify the cost
National telecommunications regulators in countries with limited monitoring infrastructure have the clearest use case. An orbital survey can produce a baseline occupancy map of the entire national territory in weeks, identifying bands where de-facto usage diverges from the assigned frequency plan, flagging regions where interference sources are active, and prioritising where terrestrial inspectors should be deployed. This is a fundamentally different workflow from dispatching inspectors to locations chosen by complaint.
Defence spectrum managers use occupancy surveys to characterise the electromagnetic environment in regions of interest before operations. Knowing which VHF and UHF bands are heavily used in a given theatre, and which are relatively clear, informs frequency planning for tactical communications. The orbital survey provides a picture that no single ground-based intercept station can replicate without extensive geographic distribution.
Spectrum auction preparation is a third application. Regulators releasing new spectrum in the 600 MHz or 700 MHz digital dividend bands need to understand existing incumbents before clearing timelines can be set. An orbital survey identifies geographic pockets where analogue television or legacy land-mobile users remain active, allowing targeted clearance rather than blanket assumptions.
Aggregating passes into a usable occupancy product
Raw data from a wideband SDR payload is a time-frequency power matrix: received power in dBm as a function of frequency and time, tagged with satellite position and attitude. Converting that into a geographic occupancy map requires associating each spectral measurement with a ground footprint, which depends on antenna beam pattern, satellite altitude, and attitude knowledge. For a nadir-pointing antenna with a broad beam, the footprint is the entire radio horizon; for a directional antenna, it can be narrowed to a few hundred kilometres.
Standard occupancy metrics from the terrestrial monitoring literature, including time-occupancy (fraction of time a channel exceeds a threshold), bandwidth occupancy, and duty cycle, can be estimated from orbital data if revisit is sufficient. The ITU-R SM.2256 report on spectrum occupancy measurement provides the methodological baseline that orbital survey analysis should reference, even though it was written for terrestrial stations. Satellize applies this class of analysis to aggregated pass data, producing band-level occupancy heat maps and anomaly flags for clients commissioning spectrum environment assessments.
Archive depth for commercial RF constellation data varies by operator. HawkEye 360 has been collecting data since 2018, providing a multi-year baseline for trend analysis. Comparing occupancy across years can reveal spectrum migration patterns, growth in unlicensed device populations, and the effect of regulatory interventions.
Typical figures
| Frequency coverage (typical operational SDR payloads) | VHF from ~144 MHz; full UHF to ~3 GHz (HawkEye 360 extends to ~15 GHz) |
| Spectral resolution (wideband survey mode) | Tens of kHz to low hundreds of kHz; insufficient for 12.5 kHz narrowband channel auditing |
| Ground footprint per pass | Up to ~2,500 km diameter radio horizon at 500-600 km altitude; directional antennas narrow this |
| Dwell time over a fixed point | Approximately 5-10 minutes per pass at LEO altitudes |
| Revisit (multi-satellite constellation) | Hours to sub-daily with HawkEye 360 cluster architecture; improving with constellation growth |
| Emitter geolocation accuracy (TDOA/FDOA, three-satellite cluster) | Kilometres-class for cooperative geometries; degrades with short signal duration or poor baseline |
| Minimum detectable signal (approximate) | Moderately powerful emitters (tens of watts ERP) reliably detected; handheld units at a few watts at or below detection floor |
| Archive depth (HawkEye 360) | Collection from 2018; multi-year baseline available for trend comparison |
| Ionospheric transparency | Reliable above ~50 MHz; sporadic-E propagation can cause false attribution below ~100 MHz |
| Delivery formats | Time-frequency power matrices, band-level occupancy heat maps, anomaly flag reports, GIS-compatible layers |
Analytics Satellize can run
| National band occupancy baseline map | Aggregated pass power matrices converted to time-occupancy metrics per ITU-R SM.2256 methodology, gridded to geographic cells | GIS layer showing per-band occupancy percentage by region, delivered as GeoTIFF and shapefile with summary PDF report |
| Incumbent user identification ahead of spectrum auction | Multi-pass occupancy aggregation to identify geographic pockets of persistent emission in target band; compared against licensed frequency assignment database | Tabular report of geographic areas with confirmed active incumbents, with estimated duty cycle and recommended clearance priority |
| Anomalous emitter flagging | Statistical thresholding against baseline occupancy model; passes with power exceedances in nominally unassigned channels flagged for review | Alert feed with frequency, approximate location (kilometres-class), timestamp, and estimated signal strength; updated per constellation pass cycle |
| Diurnal and seasonal occupancy trend analysis | Time-series stacking of multi-week pass data to compute occupancy as a function of time-of-day and season per band and region | Time-series charts and statistical summary report; suitable for input to spectrum refarming planning |
| Electromagnetic environment characterisation for operational planning | Band-by-band occupancy profiling of a defined geographic area using aggregated constellation data; cross-referenced against ITU frequency assignment records | Spectrum environment assessment report covering VHF and UHF bands, identifying congested and relatively clear sub-bands within the area of interest |
| Multi-year occupancy trend comparison | Year-on-year comparison of aggregated occupancy metrics using archived constellation data to detect spectrum migration, growth in device populations, or regulatory intervention effects | Comparative GIS layers and trend report with annotated change events |
Who does the work
We can get this done for you. Satellize runs its own analyst desk and a strong science team. You do not buy a data feed and work out what it means; our people source the imagery, run the analysis described on this page, and hand you the answer with its confidence limits stated. Discuss this requirement.