UK air traffic control relies on two fundamentally different radar technologies to detect aircraft. Both have well-documented accuracy limits set out in CAP 670 and the EUROCONTROL ATM Surveillance System Performance Specification. Understanding which type of radar was used — and what its certified error tolerances are — is essential to evaluating any infringement allegation.
PSR detects aircraft by transmitting pulses of radio energy and listening for reflections. It requires no cooperation from the aircraft — no transponder, no electrical power on board. It measures range (distance) and azimuth (bearing), both with inherent errors that grow with distance. PSR provides no altitude information.
SSR works on a completely different principle. The ground station transmits an interrogation signal on 1030 MHz; the aircraft's transponder actively replies on 1090 MHz. This provides identification (Mode A squawk), altitude (Mode C), and in modern systems enhanced data (Mode S). SSR range measurement is subject to a hard accuracy floor of 150 m imposed by the ICAO-permitted transponder delay tolerance of ±0.5 µs if if the radar was perfect - which it isn't.(ICAO Annex 10, Vol. IV, §3.1.1.7.5.1).
CAP 670 SUR02.47–49 references the EUROCONTROL ATM Surveillance System Performance Specification (ESASSP) for the performance thresholds that UK ATC radar must meet:
| Separation standard | Average error per flight | Single-plot worst case | Reference |
|---|---|---|---|
| 5 NM (en-route) | ≤ 550 m | 926 m (0.5 NM) | ESASSP 5N_C-R4, R5 |
| 3 NM (terminal) | ≤ 330 m | 555 m (0.3 NM) | ESASSP 3N_C-R4, R5 |
The average error figures are just that — averages. Individual plots regularly exceed them, and a three consecutive plots can be up to 926 m (en-route) or 555 m (terminal) from the aircraft's true position. In addition, the airspace boundary displayed on the controller's screen may itself be up to 450 m (0.25 NM) from its true surveyed position (CAP 670 SUR11.131). Radar error and map error can act in the same direction — producing a combined worst-case of approximately 1.4 km of permitted error, all within certified specification.
The following table brings together every documented error source from CAP 670 and the EUROCONTROL specification. Every value shown is within normal, compliant operation — these are not faults.
| Error Source | Max Permitted Error | Reference | Notes |
|---|---|---|---|
| Average position error per flight — 5 NM en-route | ≤ 550 m | ESASSP 5N_C-R4 | Individual plots regularly exceed this average |
| Single-plot worst case — 5 NM en-route | 926 m (0.5 NM) | ESASSP 5N_C-R5 | Up to three consecutive plots can be this far out and the radar is still within specification |
| Average position error per flight — 3 NM terminal | ≤ 330 m | ESASSP 3N_C-R4 | Individual plots regularly exceed this average |
| Single-plot worst case — 3 NM terminal | 555 m (0.3 NM) | ESASSP 3N_C-R5 | Up to three consecutive plots can be this far out and the radar is still within specification |
| Display map feature accuracy (en-route / approach) | ≤ 450 m (0.25 NM) | CAP 670 SUR11.131 | Permitted error of displayed boundary vs surveyed position on the controller's screen |
| Display map acceptance threshold at commissioning | ≤ 900 m (0.5 NM) | CAP 670 SUR12.85 | New-system flight trial acceptance criterion |
| PSR north alignment (recommended tolerance) | ≤ 0.1° | CAP 670 SUR04.9 | Recommendation only — not an absolute requirement |
| Combined worst case (radar error + map error, same direction) | ~1.4 km | ESASSP + SUR11 combined | Single-plot error (926 m) + map error (450 m) acting in the same direction |
If the alleged penetration into controlled airspace is 29 m, 50 m, or even 200 m, a single radar plot can be up to 926 m from the aircraft's true position (en-route) or 555 m (terminal) — many times larger than the allegation. The radar simply cannot distinguish between an aircraft just inside and just outside the boundary at these margins. CAP 1404's first evaluation question — "Can the ICG confirm an infringement actually occurred?" — should be answered "No" whenever the alleged penetration falls within the radar's error margin.
Primary Surveillance Radar detects aircraft by transmitting pulses of radio energy and listening for reflections. It requires no cooperation from the aircraft — no transponder, no electrical power on board. The radar measures two things: the range (distance) and the azimuth (bearing) of the target. Both measurements have inherent errors.
The radar transmits a pulse and times how long the echo takes to return. Since radio waves travel at the speed of light (c ≈ 299,792,458 m/s), the range is calculated as:
where t is the round-trip time. The division by 2 accounts for the pulse travelling out and back. Errors in range measurement arise from:
Bias is the residual systematic error that remains after calibration — it cannot be removed. Think of a rifle sight that drifts slightly with temperature: every shot in a session lands in the same wrong place, but the offset changes between sessions. During any given flight, the bias is constant — so every radar plot of your aircraft is shifted in the same direction by the same unknown amount, up to the specified limit.
Sigma (σ) is the random scatter around the biased point. Even with perfect calibration (zero bias), individual measurements will scatter around the true position. At 1σ, approximately 68% of measurements fall within the stated tolerance. At 3σ (99.7% confidence), the scatter is three times larger. For safety-critical evidence, the 3σ bound is the appropriate measure — because the question is not where the aircraft probably was, but whether the radar can reliably prove it was inside the boundary.
The radar antenna rotates continuously. The azimuth (bearing) of a target is determined by the antenna's pointing direction at the moment the echo is received. The antenna beam has a finite width — typically around 1° at the half-power points — so the system must estimate which part of the beam the target lies in. Errors in azimuth arise from:
Critically, azimuth error translates to a cross-range positional error that grows linearly with distance. A small angular error at long range produces a large positional displacement:
At 55.66 km range, an azimuth sigma of 0.15° translates to a 1σ cross-range error of approximately 146 m. At 3σ (99.7% confidence), this becomes 437 m. This is the fundamental reason why radar positional accuracy degrades dramatically with distance.
Because range and azimuth errors are independent and have different magnitudes, the true position uncertainty is not a circle — it is an ellipse, elongated in the cross-range (azimuth) direction. The semi-axes of the 3σ error ellipse are:
The total position error combines the bias offset with the 3σ noise ellipse. The conservative scalar bound is:
The Terma Scanter 4002 is a Primary Surveillance Radar used by NATS at multiple UK sites. Following a Subject Access Request and Information Commissioner appeal, NATS confirmed this was the radar used to detect an alleged infringement. Its published specifications allow us to calculate the actual positional error at any given range.
| Parameter | Specification |
|---|---|
| Frequency band | 9000–9200 MHz (X-band) |
| Transmitter | 6 kW, fault tolerant solid state (GaN), 8 modules |
| Instrumented range | Up to 60 NM |
| Minimum detection range | 0.15 NM |
| Antenna | 18-foot, Cosec² elevation pattern, circular polarisation |
| Rotation rate | 12–20 RPM (default 15 RPM) |
| Accuracy — range | < 25 m bias (< 60 m sigma) |
| Accuracy — azimuth | < 0.1° bias (< 0.15° sigma) |
| Resolution — range | < 36 m (measured at 10 dB SNR) |
| Resolution — azimuth | < 1° |
| Interface | Ethernet UDP/TCP IP, ASTERIX format |
Source: Terma Scanter 4002 product brochure
In the victim's case, the aircraft was 55.66 km from the radar head. The alleged infringement was 29 m inside the Solent CTA boundary. Applying the Scanter 4002's published specifications:
Bias offset (systematic error):
3σ noise (99.7% confidence random scatter):
Conservative bound (bias + 3σ):
The alleged infringement was 29 m. The radar's conservative error bound at this range is 573 m — nearly 20 times larger. A Monte Carlo simulation confirms that the majority of radar returns for an aircraft 30 m outside the boundary would appear to be inside controlled airspace. The radar simply cannot distinguish between "30 m inside" and "30 m outside" at this distance.
During the investigation of this case, the CAA official handling the infringement reportedly claimed the radar was accurate "to a few centimetres." The Scanter 4002's own manufacturer specifies range accuracy of 25 m bias with 60 m sigma — and that is range only, before azimuth error is added. The claim of centimetre-level accuracy is off by a factor of approximately 1,000 to 10,000. The CAA official was in possession of the radar make and model at the time of making this claim.
Secondary Surveillance Radar works on a fundamentally different principle to primary radar. Instead of detecting reflected energy, SSR relies on the aircraft's transponder to actively reply to an interrogation signal. This provides additional information — identification (Mode A) and altitude (Mode C/S) — but introduces its own error sources.
The ground-based SSR interrogator transmits a coded pulse on 1030 MHz. The aircraft's transponder receives this interrogation, processes it, and after a fixed delay transmits a reply on 1090 MHz. The ground station measures the total elapsed time from interrogation to reply, subtracts the nominal transponder delay, and calculates range from the remainder.
ICAO Annex 10, Volume IV, Section 3.1.1.7.5.1 specifies that the aircraft transponder must reply to a valid interrogation with a delay of 3.0 µs ± 0.5 µs. This tolerance is a fundamental property of every Mode A/C/S transponder in service worldwide.
The ground station assumes a nominal delay of exactly 3.0 µs and subtracts it from the measured round-trip time. But if the transponder's actual delay is anywhere within the permitted ±0.5 µs tolerance, the calculated range will be wrong by up to:
c × 1.0 µs / 2 = 299,792,458 × 0.000001 / 2 ≈ 150 mThis is a hard floor on SSR range accuracy that cannot be overcome by any improvement to the ground radar. No matter how precise the interrogator's timing circuits, no matter how advanced the signal processing, the transponder delay uncertainty of ±0.5 µs translates to a 150 m range uncertainty window. A perfectly engineered, perfectly calibrated SSR ground station — with zero error of its own — would still be unable to determine an aircraft's range to better than 150 m.
ICAO Annex 10, Volume IV — Surveillance and Collision Avoidance Systems, Section 3.1.1.7.5.1
SSR determines azimuth by the same principle as PSR — the antenna's pointing direction when the reply is received. The same azimuth errors apply: bias from antenna misalignment and sigma from beam interpolation. At long range, the cross-range error from azimuth uncertainty dominates, just as with PSR. The combined position uncertainty from transponder delay tolerance plus azimuth error at range makes SSR no more accurate than PSR for lateral position.
Mode C altitude data passes through multiple stages, each adding error:
| Error Source | Magnitude | Reference |
|---|---|---|
| Gillham encoding tolerance | ±125 ft (±38.1 m) | ICAO Annex 10, Vol IV |
| Mode C display quantisation (100 ft steps) | Up to ±100 ft | SSR system design |
| Altimeter instrument error (certified aircraft) | ±30–75 ft typical | EASA Part 21 / airworthiness standards |
| QNH setting / atmospheric variation | Variable | METAR / AIP meteorological data |
| ADS-B ground domain altitude resolution (minimum) | Must not degrade below 100 ft | CAP 670 SUR07.38 |
| Combined plausible error | up to 300 ft | All sources combined |
The controller sees a derived figure, not the aircraft's actual altitude. A displayed value of FL055 (5,500 ft) could represent any actual altitude between approximately 5,200 ft and 5,800 ft under normal, compliant operation. CAP 670 SUR07.31 states that only barometric altitude shall be displayed to controllers for separation purposes — if GPS-derived geometric altitude has been used in an allegation, this is procedurally incorrect. [CAP 670 SUR07.29–31]
For PSR, the error ellipse at typical ranges runs to hundreds of metres. For SSR, even a perfect ground station faces a 150 m hard limit from transponder delay tolerance alone. When the CAA alleges an infringement of 29 m, 50 m, or even 200 m, neither surveillance technology is capable of confirming the allegation to the required standard. The CAA's first question under CAP 1404 — "Can the ICG confirm an infringement actually occurred?" — should be answered "No" whenever the alleged penetration falls within the radar's error margin.