If the allegation is that your aircraft penetrated the lateral boundary of controlled airspace — a CTR, CTA, TMA, ATZ or other zone — the evidence comes from PSR or SSR positional data. The regulatory accuracy requirements for that data are set out in CAP 670, with specific numerical thresholds in the EUROCONTROL ATM Surveillance System Performance Specification.
CAP 670 SUR02.47–49 references the EUROCONTROL ATM Surveillance System Performance Specification (ESASSP) for the specific performance thresholds. The specification sets two levels of accuracy limit:
These are not faults — they are the certified accuracy limits of a fully compliant system. An alleged infringement of 50–100 m is many times smaller than what the radar is certified to detect. [ESASSP Vol 1, Ed 1.3]
| Error Source | Max Permitted Error | Reference | Notes |
|---|---|---|---|
| Average RMS 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 | A single plot can be this far out and the radar is still within specification |
| Average RMS position error per flight — 3 NM terminal | ≤ 330 m | ESASSP 3N_C-R4 | Up to 3 plots can exceed this average |
| Single-plot worst case — 3 NM terminal | 555 m (0.3 NM) | ESASSP 3N_C-R5 | A single plot 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 - that is the airspace boundary could be off by 450m on the controllers screen and still be within specification. |
| 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 CAA alleges you were 50–500 m inside controlled airspace, a single radar plot can be up to 926 m from your true position (en-route) or 555 m (terminal) — many times larger than the alleged penetration. The airspace boundary on the controller's screen adds a further 450 m of potential error. The smaller the alleged infringement, the more powerful this argument becomes.
Positional error is not uniform across radar coverage. PSR azimuth resolution is determined by antenna beamwidth — typically 1–2° at the 3 dB point. At 30 NM, 1° of azimuth error produces approximately 555 m of lateral positional uncertainty. At 40 NM, it exceeds 700 m. Request the documented positional accuracy of the specific installation at the range and bearing of the alleged infringement, not just the headline specification figure. [CAP 670 SUR02.18–21]
The airspace boundary on the controller's screen is a map overlay referenced to a geodetic coordinate system (WGS-84 is recommended). The map must undergo formal verification and validation before entry into service, and the accuracy of features must be documented. [CAP 670 SUR11.119, SUR11.121–126, SUR11.131, SUR11.134]
The displayed boundary map be up to 450m out and still be within specification. If the displayed boundary was itself 300 m displaced from the true boundary, an aircraft shown 50 m inside the displayed boundary could actually have been 250 m outside the real boundary.
CAP 670 requires all failures to meet required performance to be recorded and logged, and mandates that the CAA Regional Inspector is informed if performance is not met. Request the fault log, BITE records, and performance deficiency log for the 30-day period around the incident. Any logged performance failures — interference, equipment degradation, transmitter power issues — are directly relevant to the reliability of the data being used as evidence. [CAP 670 SUR12.98–101]
A single radar plot showing marginal penetration is far weaker evidence than a continuous track well inside the boundary over multiple scans. A single anomalous plot is consistent with a tracking error, a false target, or a momentary position error. CAP 670 requires tracking performance assessment to determine track deviations, which must not exceed the maximum horizontal position error. [CAP 670 SUR12.53–57]
Request the complete plot-by-plot track history. How many scans showed the aircraft inside the boundary? By how much on each? An abrupt position jump between adjacent scans is consistent with a tracking artefact, not aircraft movement.
If your aircraft had a GPS device or EFB application (SkyDemon, Garmin Pilot, ForeFlight, or similar), the track log is likely your most powerful evidence. GPS horizontal accuracy is typically ±5–15 m under normal conditions — up to 60 times more precise than a single en-route radar plot, which can be 926 m out. Preserve the full track log immediately, export it (GPX, KML), and overlay it on a current chart showing the airspace boundary.
Note that CAP 1404 Edition 6 (p.11) explicitly asks whether you used a VFR moving map. A "yes" answer supported by a GPS track showing no infringement directly addresses the ICG's evaluation criteria. [CAP 1404 Ed. 6, p.11]
If the allegation is that you entered airspace vertically — above a ceiling or below a floor — the evidence comes from SSR Mode C altitude data. This has a well-documented error chain governed by CAP 670 and ICAO Annex 10, Volume IV.
The altitude number displayed to the controller is the output of four sequential steps, each introducing potential 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 |
If you are alleged to have been 100 ft above a ceiling or below a floor, the Gillham encoding tolerance alone (±125 ft) may exceed the alleged infringement. Combined with display quantisation (±100 ft) and altimeter error (±75 ft), the total uncertainty is up to 300 ft. 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. Additionally, 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 the allegation, this is procedurally incorrect. [CAP 670 SUR07.29–31]
Obtain the METAR for the relevant aerodrome at the time of the incident and compare the QNH with the value recorded in the ATC system. CAP 670 SUR11.18–20 requires QNH display capability and mandates double-entry validation for any manual QNH change. Any discrepancy shifts the displayed altitude for every aircraft in the system. [CAP 670 SUR11.18–21]
Your transponder and altitude encoder are required to be calibrated within airworthiness requirements. Even a correctly calibrated system operates within a permitted tolerance. If you have your aircraft's most recent transponder calibration and altimetry check records, produce them. They establish that your equipment was operating correctly and quantify the known error tolerance of your specific installation.
Mode C altitude is reported at each radar scan (typically every 4–12 seconds). A single anomalous readout is consistent with a transient Gillham code decode error. Request the complete sequence of altitude readouts over the relevant period. If only one or two scans show an altitude outside the limit while adjacent scans are within it, this pattern is characteristic of a momentary decoding error, not a sustained altitude exceedance. [CAP 670 SUR12.47–48]