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 like a rifle sight that is slightly off-centre: every shot lands in the wrong place, but consistently in the same direction. It can be corrected by calibration — but only if the calibration is actually performed and verified.
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 the alleged infringement discussed in the case study. 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 case documented in the whistleblowing disclosure, 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. The aircraft's altimeter reading is converted to a Gillham grey code by the altitude encoder, transmitted as part of the transponder reply, decoded by the ground system, and displayed in 100 ft increments. ICAO Annex 10 permits an encoding tolerance of ±100 ft (up to ±200 ft at code transition boundaries). Combined with altimeter instrument error and QNH discrepancies, a total altitude error of 200–400 ft is within normal, compliant operation.
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.
GPS positioning systems used in general aviation — including EFB applications like SkyDemon and Garmin Pilot — typically provide horizontal accuracy of 3–5 metres (civilian GPS). This is an order of magnitude more precise than the radar systems relied upon by the CAA.
| System | Typical Horizontal Accuracy | Notes |
|---|---|---|
| Civilian GPS (modern receiver) | 3–5 m | 95th percentile; no augmentation required |
| SSR range (transponder delay limit) | 150 m floor | Hard limit from ±0.5 µs transponder delay tolerance |
| Terma Scanter 4002 PSR at 30 NM | ~573 m (3σ + bias) | Conservative bound at the range of the alleged infringement |
| CAP 670 en-route radar limit | 926 m (0.5 NM) | 95th percentile; 5% of plots may exceed this |
| CAP 670 terminal radar limit | 463 m (0.25 NM) | 95th percentile |
GPS is so much more accurate than ATC radar that it is routinely used to calibrate radar heads. An aircraft with a known GPS position overflies the radar coverage area, and the difference between the GPS position and the radar-reported position is used to measure and correct the radar's alignment errors. The fact that GPS is used as the calibration reference underscores that it is the more accurate system — you do not calibrate a precise instrument using a less precise one.
The whistleblowing disclosure filed with the CAA documents a systematic practice of: automatically preferring radar data over GPS evidence in all cases; declining to give any proper weight to GPS data provided by pilots in their defence; and failing to disclose to pilots the error margins associated with the radar systems. This practice is the evidential equivalent of preferring a ruler marked in metres over one marked in millimetres — and refusing to explain why.