Using Mode S and ADS-B to Filter Primary Radar Targets

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A primary air defence radar does not know what it is looking at. It detects reflections - anything with sufficient radar cross-section within range returns an echo, and the radar reports it as a plot. A busy radar picture over a nation with active civil aviation may contain several hundred simultaneous tracks at any given moment. The air defence crew's task is not to watch all of them. It is to find, with minimum delay, the ones that do not belong.

Mode S transponders and ADS-B receivers provide exactly the data needed to address this problem. Every civil aircraft in controlled airspace carries a Mode S transponder broadcasting a globally unique ICAO 24-bit address on 1090 MHz. ADS-B-equipped aircraft add their own GPS-derived position, speed, and altitude to that broadcast. When this cooperative data stream is fused with the primary radar picture in real time, the result is a structured, classified air picture in which the known civil traffic is systematically separated from the residual - and the residual is where air defence attention belongs.

What Primary Radar Gives You - and What It Does Not

Primary radar measures range and azimuth from the skin return of a target. Processed through a plot extractor, these returns become discrete plot reports: position, scan time, estimated radial velocity if Doppler-capable. A tracker then links successive plots into tracks, producing a heading, speed, and altitude estimate for each sustained contact.

What primary radar cannot provide is identity. A radar track is a physical object in a location moving at a speed - nothing more. Whether that object is a scheduled airliner, a training aircraft, a private jet on a legitimate charter, or something that should not be there is entirely outside the primary radar's capability to determine. Every single track starts its life as unknown.

Secondary surveillance radar (SSR) partially addresses this by interrogating transponders and associating the reply with the radar return. Mode A gives a 4-digit squawk code assigned by ATC. Mode C adds barometric altitude. Mode S goes further, providing the unique ICAO address and enabling selective interrogation of individual aircraft. But SSR requires its own infrastructure - a rotating interrogator antenna, high-power transmitter, and the associated installation and maintenance burden. ADS-B passive receivers achieve cooperative data collection from Mode S Extended Squitter transmissions at a fraction of that cost, with no transmitter required.

The Processing Pipeline

Fusing cooperative Mode S/ADS-B data with primary radar tracks involves a sequence of processing steps, each of which adds to the classification state of every track in the picture:

Steps 1 and 2 are standard radar tracking functions. Steps 3 through 5 are where Mode S/ADS-B integration transforms the picture from raw density into a structured, filtered display.

Step 4 in Detail: The Correlation Gate

The correlation step asks a simple question for every radar track: is there a Mode S or ADS-B return that is consistent with being the same physical aircraft? Consistency is tested across several parameters simultaneously. Each parameter has a gate - a threshold within which the two data sources are considered to represent the same object:

Parameter	Radar-derived value	Cooperative value	Typical gate
Position	Range/azimuth converted to lat/lon	ADS-B GNSS lat/lon	0.5 - 2.0 NM radius depending on radar accuracy and NIC
Altitude	SSR Mode C barometric (if available)	ADS-B barometric	±300 ft
Ground speed	Track-derived from successive plots	ADS-B reported ground speed	±50 kt
Track angle	Track-derived heading	ADS-B track angle	±20 degrees
Time	Scan timestamp	ADS-B message timestamp	Within one radar scan period (typically <12 s)

A correlation is accepted when all parameters fall within their gates simultaneously. A single parameter outside its gate does not necessarily fail the correlation - gate widths are tuned for the expected measurement noise of each sensor - but multiple simultaneous out-of-gate parameters are treated as a failed correlation and the track is flagged for further evaluation.

The position gate deserves particular attention. Primary radar position accuracy degrades with range due to the fixed angular resolution of the antenna beamwidth: a 1-degree beam produces a cross-range position uncertainty of roughly 0.87 NM at 50 NM range, and 1.75 NM at 100 NM. The ADS-B position gate must be sized to accommodate this degradation at range, or legitimate correlations will be missed at long distances. The ADS-B NIC (Navigation Integrity Category) field encodes the GPS position containment radius and can be used to adaptively tighten the gate for high-integrity GNSS reports.

Step 5: Classification - The Three Track States

After the correlation attempt, every track in the picture is assigned one of three classification states:

State	Condition	Operational handling
WHITE	Correlated with a valid Mode S/ADS-B return; ICAO address matches an active, flight-plan-conformant aircraft	Identified cooperative traffic. Displayed in suppressed form or removed from the primary threat display entirely. Logged and monitored passively.
GREY	Correlated with a cooperative return, but one or more parameters fall outside gate, or no flight plan match, or route deviation detected	Cooperative but anomalous. Retained on display with alert flag. Requires operator evaluation - may be a transponder fault, a GNSS error, or a deviation requiring ATC coordination.
BLACK / UNKNOWN	No cooperative return correlated within gate after sustained tracking. Primary radar track exists; cooperative picture is silent.	Non-cooperative contact. Highest priority for operator attention and, if appropriate, further action. This is the residual the system exists to surface.

The display value of this classification is immediate and profound. Instead of presenting the operator with several hundred undifferentiated radar tracks, the fused system presents a small number of grey and black tracks on a suppressed white background. The operator does not need to scan the whole picture - the system has already done it.

What Generates a Black Track

Understanding what can produce a non-cooperative black track is important for calibrating operator response. Not every black track is a threat - but every black track requires an explanation:

• Deliberate transponder deactivation. An aircraft that switches off its Mode S transponder will lose its white classification and transition to black after the cooperative data ages out. In civil controlled airspace this is a serious regulatory violation; in contested or border airspace it is the primary indicator of non-cooperative intent.
• Below-horizon ADS-B gap. If the aircraft is within primary radar range but below the ADS-B receiver's radio horizon - flying low along a valley or below ridge lines - the primary track will exist without a cooperative return. A distributed ADS-B network with low-sited stations can close many of these gaps.
• ADS-B receiver coverage gap. An area covered by primary radar but not by any ADS-B receive station will produce black tracks for all aircraft in that zone, regardless of transponder status. Coverage mapping before deployment identifies these blind spots.
• Transponder malfunction. A genuine equipment failure can cause a cooperative aircraft to lose its white classification. ATC will typically report this via voice; the track is then manually reclassified by the operator pending confirmation.
• Non-equipped aircraft. Light general aviation operating VFR outside controlled airspace may legitimately carry no Mode S transponder in some jurisdictions. These produce black tracks. Airspace rules, time of day, and track behaviour all contribute to the operator's assessment.

Grey Track Behaviour: The Anomaly Indicators

Grey tracks - those where cooperative data exists but does not fully corroborate the radar return - often carry more intelligence than simple black tracks. Several characteristic grey patterns are operationally significant:

• Position offset without velocity anomaly. The ADS-B position and radar position diverge beyond gate, but heading and speed corroborate. This is the classic signature of GNSS jamming affecting the aircraft's navigation - the ADS-B position is drifting away from the radar truth. The aircraft may not be aware of its own navigation error.
• Velocity anomaly without position offset. Position gates pass but the ADS-B-reported ground speed or track angle diverges from the radar-derived values. Possible causes include a wind model error in the aircraft's FMS, a heading sensor fault, or - in adversarial scenarios - a spoofed ADS-B velocity broadcast intended to falsely represent the aircraft's manoeuvre state.
• Valid ICAO address, wrong location. The correlating ADS-B return carries an ICAO address that matches a known aircraft, but the claimed position is far from the radar-derived position. This is inconsistent with any legitimate equipment fault and is a strong indicator of identity spoofing - a transmitter broadcasting a stolen ICAO address from a different location.
• Route deviation. A correlated, identified track that departs significantly from its filed route. Not necessarily a threat - diversions and emergencies are routine - but warrants ATC coordination to confirm status.

Track History and Temporal Filtering

Cooperative data is not static - it ages. An ADS-B message received 30 seconds ago from an aircraft now at a different position should not be used to corroborate a current radar plot without applying a temporal propagation. The cooperative position must be extrapolated forward using the last known velocity vector to predict where the aircraft should be at the current scan time. If the predicted cooperative position falls within gate of the current radar plot, the correlation is maintained. If the cooperative data ages beyond a configured threshold (typically two to four scan periods) without refresh, the track's classification downgrades - from white toward grey - until new cooperative data arrives or the track times out.

This temporal degradation is important operationally. A track that was white 30 seconds ago and has just gone silent - cooperative data dropped off with no radar track loss - represents a transponder shutdown event in flight. The track transitions from white to grey to black in real time, and the transition itself is an alert. When and where a track went black is as operationally significant as the fact that it is black.

Coverage Geometry: Aligning Radar and ADS-B Footprints

The filtering architecture works only where both sensors have coverage. A primary radar with 300 km range and an ADS-B network covering only 100 km from the coast will produce a large annular zone - 100 to 300 km out - where primary radar tracks are present but cooperative data is absent. Every track in that zone will be classified black by default, not because the aircraft is non-cooperative, but because the ADS-B receiver cannot hear it.

Matching ADS-B coverage to primary radar coverage is therefore a fundamental system design requirement. This typically involves a combination of:

• Elevated ADS-B receive sites on high ground or towers to extend the radio horizon.
• Multiple distributed receive stations to eliminate low-altitude gaps.
• Satellite AIS/ADS-B data feeds for ranges beyond any practical ground station horizon, accepted at the cost of higher data latency.

Where a coverage gap is unavoidable and known, it should be documented and the operator display should indicate that tracks in that region are black by default due to receiver geometry, not necessarily due to non-cooperative behaviour. Treating a coverage-gap black track the same as an in-coverage black track produces false alert workload and erodes operator confidence in the system.

Conclusion

The conjunction of Mode S/ADS-B with primary air defence radar is not a marginal enhancement - it is the mechanism by which a dense, undifferentiated primary picture becomes an operationally usable one. The primary radar provides non-cooperative detection and tracking. Mode S and ADS-B provide the cooperative identity layer. Together, through a correlation and classification process operating across every scan, they produce a picture from which known civil traffic has been systematically removed - leaving the operator with a focused, manageable residual of grey and black contacts that represent the genuine air defence workload.

The quality of this residual - its false alert rate, its classification latency, its grey-track fidelity - is determined by three factors: the accuracy of the primary radar, the coverage and sensitivity of the ADS-B receive network, and the rigour of the correlation and gate logic. All three are engineering problems with known, tractable solutions.

Sparrow Global designs and supplies ADS-B receive systems and supports their integration into air surveillance architectures. Visit our AIS & ADS-B Solutions page or contact us to discuss your air picture requirements.

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