6. NIC / NAC

NIC, NAC, NUC, and SIL, those acronyms do sound confusing. They are measurement for the integrity, accuracy, or uncertainties of the position measurement from GPS unit.

  • NIC - Navigation Integrity Category
  • NUC - Navigation Uncertainty Category
  • NAC - Navigation Accuracy Category
  • SIL - Surveillance/Source Integrity Level

Two of those values are more interesting for us, NICp and NACv, represent the position integrity and velocity accuracy respectively.

Before dive into decoding and interpolation, let’s introduce two parameters:

  • Rc: Horizontal Containment Radius Limit, interpolated from NICp number
  • HFOM: Horizontal Figure of Merit, interpolated from NACv number

6.1. NIC and Rc

Bring back the message from position decoding previously:

MSG: 8D40621D58C382D690C8AC2863A7

|    | ICAO24 |      DATA      |        |
|----|--------|----------------|--------|
| 8D | 40621D | 58C382D690C8AC | 2863A7 |

Convert both messages to binary strings:

| DF    | CA  | ICAO24 ADDRESS           | DATA                                   ->
|                                        | TC    | SS | SBnic | Altitude     | T  ->
|-------|-----|--------------------------|-------|----|-------|--------------|--- ->
| 10001 | 101 | 010000000110001000011101 | 01011 | 00 | 0     | 110000111000 | 0  ->

  ->  Data (cont.)                                   | CRC                      |
  -> | CPR-F | CPR Latitude      | CPR Longitude     |                          |
  -> |-------|-------------------|-------------------|--------------------------|
  -> | 0     | 10110101101001000 | 01100100010101100 | 001010000110001110100111 |

Not the *2 field (bit-40), where we have the NIC Supplement-B (S[B]) in combination with TC number, we are able to determine the NIC value.

The relation of TC, NIC, and Rc are as follow:

TC SBnic NIC Rc
9 0 11 < 7.5 m
10 0 10 < 25 m
11 1 9 < 74 m
0 8 < 0.1 NM (185 m)
12 0 7 < 0.2 NM (370 m)
13 1 * 6 < 0.3 NM (556 m)
0 < 0.5 NM (925 m)
1 ** < 0.6 NM (1111 m)
14 0 5 < 1.0 NM (1852 m)
15 0 4 < 2 NM (3704 m)
16 1 3 < 4 NM (7408 m)
0 2 < 8 NM (14.8 km)
17 0 1 < 20 NM (37.0 km)
18 0 0 > 20 NM or Unknown
  • * NIC Supplement-A = 0
  • ** NIC Supplement-A = 1

In our example:

TC -> 11
SBnic -> 0

We have:
  NIC -> 8

So, what happened to the NIC Supplement-A and C? Those two bits are broadcast in Aircraft Operational Status Message (TC=31, see Introduction page). For Surface Position Message, you will need the combination of A and C to determine the NIC number (note: Rc values are different from Airborne Position Messages). However, with Supplement-B bit we are already able to decode the NIC and Rc for airborne positions.

6.2. NAC and HFOM

NAC is reported in the Airborne Velocity Message.