VHF Data Exchange System (VDES) signal monitoring from orbit

VDES is the IMO-standardised successor to AIS, offering higher data rates and two-way satellite links in the 157–158 MHz band. Space-based receivers can already track adoption rates, coverage gaps, and protocol compliance, even though operational deployments remain sparse in the mid-2020s.

Sensors

Kongsberg / NORCE VDES prototype payload (NorSat-3 and related): NorSat-3, launched in 2022, carries a publicly documented VDES receiver payload developed by Kongsberg Seatex and NORCE. It is designed to receive ASM (Application Specific Messages) and VDE-SAT uplink transmissions in the 157–158 MHz band from LEO, demonstrating space-segment feasibility before any commercial constellation exists.
Spire LEMUR-2 multi-mission CubeSat: Spire's LEMUR-2 satellites carry software-defined radio payloads capable of VHF reception alongside AIS and GNSS-RO. The constellation of roughly 100 satellites provides global revisit of under 30 minutes on average, making it the most plausible near-term platform for operational VDES monitoring at scale.
ITU-R M.2092 compliant ground reference receivers: Ground-based VDES base stations conforming to ITU-R M.2092 provide calibration references for space-based signal characterisation. Comparing received signal parameters against the published standard allows assessment of transmitter compliance, Doppler correction quality, and protocol version.
Software-defined radio (SDR) payloads on technology-demonstration smallsats: Several national and commercial smallsat programmes have flown wideband SDR payloads covering the VHF marine band. These can capture raw IQ data across the full VDES channel plan (VDE-SAT channels at 157.1875–157.3375 MHz and 161.7875–161.9375 MHz) for post-pass analysis, though they are not yet operational monitoring systems.

What VDES actually is, and why it is not AIS

AIS works. It is also a 1990s design running at 9.6 kbit/s on two 25 kHz channels, and it was never intended to carry the data volumes that modern maritime domain awareness demands. VDES, standardised by the IMO and ITU under resolution MSC.428(98) and ITU-R M.2092, is the planned replacement. It retains backwards compatibility with AIS but adds three new channel groups: ASM (Application Specific Messages) for short data bursts between ships and shore, VDE-TDM for terrestrial digital exchange, and VDE-SAT for direct two-way links between vessels and satellites.

The satellite component is the significant change. Unlike AIS, where satellites are passive eavesdroppers on a terrestrial protocol, VDE-SAT is designed from the outset with a space segment in mind. Uplink slots are coordinated, data rates reach up to 307.2 kbit/s in the downlink direction, and the system is intended to carry weather routing data, electronic chart updates, and port authority instructions directly to vessels at sea. That two-way capability is what makes space-based monitoring of VDES qualitatively different from AIS monitoring.

The architecture a LEO receiver actually sees

VDES occupies a defined channel plan within the 156–162 MHz VHF maritime mobile band. The VDE-SAT uplink from ships sits in channels around 157.1875 to 157.3375 MHz; the downlink from satellites uses 161.7875 to 161.9375 MHz. A LEO receiver passing over at roughly 500 km altitude has a footprint of approximately 2,500 km diameter and a pass duration of around six to eight minutes over any fixed point. Within that window, it can collect uplink transmissions from every VDES-equipped vessel in the footprint.

The practical challenge is that VDES uses a TDMA-based slot structure coordinated partly by the satellite itself. A monitoring satellite that is not the designated service satellite sees transmissions but cannot participate in slot assignment. It is, in effect, a passive observer of a protocol designed around active coordination. That is not a fatal limitation for monitoring purposes, but it means raw packet capture rates will undercount total traffic unless the receiver is integrated into the network. Honest assessment: a non-participating LEO receiver gives you presence detection and approximate message rates, not a complete traffic picture.

Adoption mapping is the most useful product right now

As of the mid-2020s, VDES-equipped vessels are rare. The NorSat-3 mission has demonstrated reception of ASM transmissions from prototype shore stations and a small number of equipped vessels, but the commercial rollout of shipborne VDES transponders has been slow. IMO carriage requirements have not yet mandated VDES, and the shore-side infrastructure for terrestrial VDE-TDM is patchy outside Scandinavia and a handful of port authorities running trials.

This immaturity is precisely what makes space-based monitoring valuable to regulators and maritime authorities. A LEO constellation can produce a global map of VDES signal activity: which vessels are transmitting, in which ocean regions, and at what message rates. Comparing that against the vessel registry gives an adoption-rate estimate by flag state, vessel class, and trade route. Coverage gap analysis, identifying ocean areas where no VDE-SAT ground segment or orbiting service satellite provides downlink, is equally tractable from passive monitoring data. These are planning inputs for the authorities and industry bodies deciding where to invest in shore infrastructure and satellite capacity.

Signal quality and protocol compliance assessment

Beyond simple detection, a calibrated SDR payload can assess whether a received VDES transmission conforms to the M.2092 specification. Frequency accuracy, symbol timing, modulation index, and packet structure are all measurable from the received signal. Vessels transmitting malformed ASM packets, or using incorrect channel assignments, can be flagged. This is relevant for type-approval enforcement and for identifying firmware versions in early transponder generations that may have known interoperability issues.

Doppler shift is substantial at VHF from LEO. A satellite at 500 km altitude and 7.6 km/s orbital velocity produces a Doppler rate of roughly ±3.5 kHz across a pass at 157 MHz. VDES receivers are specified to handle this, but monitoring payloads must apply accurate Doppler correction before demodulation. The correction itself, once applied, gives a useful cross-check on vessel velocity when combined with the vessel's reported position.

Limits that matter for anyone planning a programme

The fundamental limit is the scarcity of VDES transmitters. A monitoring system is only as informative as the signals it receives, and with VDES penetration still in low single-digit percentages of the global fleet, the dataset is thin. Revisit rates from a small constellation are adequate for daily adoption mapping but insufficient for real-time traffic management. A single LEO satellite revisits any ocean point roughly every 90 minutes; a constellation of 20 to 30 satellites would bring that below 15 minutes, which is the threshold where VDES monitoring starts to complement AIS operationally rather than simply auditing it.

VHF propagation at 157 MHz is largely line-of-sight from LEO, which is an advantage over HF but means the horizon geometry matters. At 500 km altitude, the radio horizon is around 2,500 km, but atmospheric ducting can occasionally extend this. Interference from terrestrial VHF services is a real concern in coastal and port areas, where the marine band shares spectrum with adjacent allocations. In open ocean, the noise floor is lower and detection of even low-power ASM transmissions is reliable. Near busy ports, discrimination between VDES and adjacent-channel interference requires careful filtering.

Satellize can integrate VDES monitoring data with AIS and optical vessel detection layers.

From raw captures to actionable intelligence

The analytic pipeline for VDES monitoring runs from raw IQ capture through demodulation, packet decode, and message parsing to a structured database of vessel identifiers, message types, timestamps, and signal parameters. That database supports several distinct products. Adoption dashboards show VDES penetration by flag state and vessel class over time. Coverage assessments show which ocean regions have no VDE-SAT service satellite in view for more than a defined interval. Compliance reports flag transmitters whose signal parameters fall outside M.2092 tolerances.

None of these products require the monitoring satellite to be part of the VDES service network. Passive reception is sufficient. The constraint is that without active participation, you cannot measure downlink performance or latency, only uplink characteristics. For the planning and regulatory use cases that dominate the current market, that is an acceptable trade-off.

Typical figures

Frequency band monitored	VDE-SAT uplink: 157.1875–157.3375 MHz; ASM channels within 156–162 MHz VHF maritime mobile band (per ITU-R M.2092)
LEO footprint diameter (500 km altitude)	Approximately 2,500 km radio horizon radius; practical decodable range depends on transmit power and antenna gain
Pass duration over fixed point	6–8 minutes per pass for a single LEO satellite at 500 km
Revisit (single satellite)	Approximately every 90 minutes at mid-latitudes; polar regions see more frequent coverage
Revisit (small constellation, 20–30 satellites)	Under 15 minutes average globally, based on published Spire LEMUR-2 constellation geometry
Doppler shift range at 157 MHz from LEO	Approximately ±3.5 kHz across a full pass; must be corrected prior to demodulation
VDES uplink data rate (VDE-SAT)	Up to 307.2 kbit/s downlink; uplink rates defined in M.2092 depend on channel bandwidth (25 kHz or 100 kHz modes)
Minimum detectable transmitter power	Class-B VDES transponders specified at 1 W minimum; Class-A at higher power; open-ocean detection at 500 km is feasible for both classes with a sensitive SDR payload
Archive depth (NorSat-3 reference)	NorSat-3 operational from 2022; limited public archive; commercial SDR constellation archives vary by operator agreement

Analytics Satellize can run

VDES adoption rate by flag state and vessel class	Packet decode and vessel identifier matching against IMO registry; time-series aggregation	Monthly adoption dashboard (PDF or interactive web layer) with penetration percentages by flag, class, and trade route
Ocean coverage gap map for VDE-SAT service	Orbital geometry modelling combined with observed transmission absence; comparison against ITU filing data for licensed VDES satellites	GIS layer (GeoJSON or shapefile) showing ocean regions with greater than defined interval without VDE-SAT coverage
Transmitter compliance assessment	Signal parameter extraction from raw IQ: frequency offset, modulation index, symbol timing error; comparison against ITU-R M.2092 tolerances	Flag list of non-compliant transmitters with MMSI, signal parameter deviations, and confidence score
ASM message type distribution analysis	Packet parsing and message-type field extraction; statistical aggregation by region and vessel class	Quarterly report on which ASM application categories are in active use, informing shore-side infrastructure investment decisions
VDES versus AIS coverage comparison	Co-registration of VDES detection events with AIS track database; gap identification where vessels transmit AIS but not VDES	Vessel-level comparison table and regional heatmap showing AIS-only, VDES-only, and dual-mode populations
Shore-station and port VDES activity monitoring	Detection and characterisation of terrestrial VDE-TDM base station transmissions; signal strength mapping by pass geometry	Geo-located inventory of active VDES shore infrastructure with estimated coverage radius and operational status

Sources

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.