Every ADS-B position message carries two small numbers that describe how much the aircraft's own avionics trust the position it just sent. Most flight trackers hide them. Here is what they mean, why they change, and what to do when they drop.
NIC (Navigation Integrity Category) and NACp (Navigation Accuracy Category for Position) are both small integers, 0 to 11, encoded in the ADS-B airborne position and operational status messages. They are not measurements of the aircraft's speed or altitude. They are the avionics reporting a confidence bound on the position it just broadcast, derived from its own GPS or GNSS receiver's internal error estimate.
Think of NIC as a containment radius with a stated probability: "the true position is within this distance of what I just told you, with at least this much confidence." NACp is a closely related accuracy estimate, tied to the 95% containment bound used for RNP (Required Navigation Performance) applications. In routine operation the two rise and fall together, because they draw on the same underlying GPS solution, but they answer slightly different questions, which is why both exist and why AeroScope tracks both.
Both fields use the same rough idea: a higher number is a tighter, more trustworthy bound. The table below gives the approximate containment radius for common values as used in practice; consult DO-260B/DO-282B for the exact certified definitions.
| Value | NIC containment radius | NACp accuracy bound | Practical read |
|---|---|---|---|
| 0 | Unknown / >20 NM | Unknown / >10 NM | No usable integrity or accuracy claim |
| 5–6 | ~0.5–2 NM | ~0.5–1 NM | Coarse, fine for a general awareness display |
| 7 | ~0.2 NM (370 m) | ~0.1 NM | AeroScope's degraded/normal boundary for NIC |
| 8 | ~0.1 NM (185 m) | ~30 m (0.05 NM) | AeroScope's degraded/normal boundary for NACp |
| 9 | ~75 m | ~30 m | Typical modern GPS, healthy conditions |
| 10–11 | ~7.5–25 m | <10 m | Best-in-class GNSS with augmentation (e.g. SBAS/WAAS) |
A well-equipped airliner cruising normally typically reports NIC 8 or higher and NACp 9 or higher. Values that sit persistently low, or that drop sharply mid-flight without a plausible cause such as a runway turnoff or a known equipment limitation, are the signal worth paying attention to.
NIC and NACp are not independently verified by anything on the ground. They are self-reported by the aircraft's own GPS or GNSS receiver, based on internal figures such as satellite geometry (DOP), the number of satellites being tracked, and receiver autonomous integrity monitoring (RAIM). When a jammer raises the noise floor around a GPS receiver, three things typically happen in sequence: fewer satellites remain trackable, the geometry of the remaining satellites worsens, and the receiver's own error estimate grows. A well-behaved receiver responds to that growing uncertainty by lowering the NIC and NACp it broadcasts, sometimes before a human looking at a raw track would notice anything unusual at all.
This is exactly why the integrity fields matter more than the position itself for detecting interference. A jammed aircraft's reported latitude and longitude can still look smooth and plausible on a map while its NIC and NACp quietly fall, because the receiver is honestly reporting that it no longer trusts its own fix. Watching the position alone would miss this. Watching NIC and NACp catches it early.
Four integrity and accuracy fields travel together in ADS-B messages, and they are easy to conflate:
In short: NIC and NACp tell you how good the position is, NACv tells you how good the velocity is, and SIL tells you how much to trust the whole reporting chain in the first place. All four are worth checking together, which is exactly why AeroScope surfaces them jointly rather than picking one.
AeroScope reads NIC, NACp, NACv and SIL from every ADS-B message it fuses from public feeds and treats an aircraft as degraded on the GPS jamming map when its NIC drops below 7 or its NACp drops below 8. Those thresholds were chosen because they mark the point where the position uncertainty is too large to trust for the proximity and spoofing checks the platform runs, not because of a certification requirement. A single aircraft briefly dipping below those values is usually nothing, terrain shadowing and short GPS dropouts happen. A cluster of aircraft in the same area losing integrity at the same time is the pattern that indicates GNSS interference on the ground below, which is what the jamming map is built to surface.
Degraded integrity fields are also one of several inputs into AeroScope's spoofing checks, alongside cross-source disagreement and physics plausibility tests. See how AeroScope detects ADS-B spoofing for how the pieces fit together, and be aware that known limitations apply: AeroScope has no independent position reference of its own, so it can only reason about what the aircraft itself is reporting.