Technical Provisions for Mode S Services and Extended Squitter

DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris
C-26 Technical Provisions for Mode S Services and Extended Squitter
3. The use of Gilham type altimeters is strongly discouraged because of the potential for undetected
altitude errors.
C.2.3.9.11 Source Integrity Level (SIL)
This 2-bit (“ME” bits 45 – 46, Message bits 77 – 78) subfield shall be used to define the probability of the
reported horizontal position exceeding the radius of containment defined by the NIC, without alerting,
assuming no avionics faults. Although the SIL assumes there are no unannunciated faults in the avionics
system, the SIL must consider the effects of a faulted Signal-in-Space, if a Signal-in-Space is used by the
position source. The probability of an avionics fault causing the reported horizontal position to exceed the
radius of containment defined by the NIC, without alerting, is covered by the System Design Assurance
(SDA) parameter (§C.2.3.10.14).
The SIL probability can be defined as either “per sample” or “per hour” as defined in the SIL Supplement
(SILSUPP) in §C.2.3.9.2.
Notes. —
1. For GNSS position sources the HIL or HPL is provided with a probability of 1x10 -7 per hour, which
should be used to set the SIL to 3.
2. The GPS defined HPL probability rate of 10 -7 per hour is based on the GPS constellation fault rate
of 10 -4 per hour and a 10 -3 probability of missed detection, given that the fault occurs. Different
containment radii indicated by the HPL are all defined at the missed detection probability of 10 -3 .
3. Fault detection is an essential consideration in determining the SIL parameter. Fault detection
assures, at a specified probability of missed detection, that the error is no greater than a specified
limit without an alert.
4. For alternate ADS-B position sources to report integrity, they will need to be certified for their fault
detection characteristics.
The “SIL” subfield is encoded in accordance with Table C-15. For installations where the SIL value is being
dynamically updated, if an update has not been received from an on-board data source for SIL within the
past 2 seconds, then the SIL subfield shall be encoded as a value of ZERO (0), indicating “Unknown.”
Table C-15. Source Integrity Level (SIL) Encoding
SIL Coding
(Binary) (Decimal)
00 0
01 1
Probability of Exceeding the NIC
Containment Radius (RC)
Unknown or > 1 x 10 -3
per flight hour or per sample
≤ 1 × 10 -3
per flight hour or per sample
≤ 1 × 10 -5
per flight hour or per sample
≤ 1 × 10 -7
per flight hour or per sample
Draft
10 2
11 3
C.2.3.9.12 Status of MCP/FCU Mode Bits
The “Status of MCP / FCU Mode Bits” subfield is a 1-bit (“ME” bit 47, Message bit 79) field that shall be used
to indicate whether the mode bits (“ME” bits 48, 49, 50, and 52 and 54, Message bits 80, 81, 82, and 84, and
86) are actively being populated (e.g., set) in the Target State and Status Message in accordance with Table
C-16.
If information is provided to the ADS-B Transmitting Subsystem to set either “ME” bit 48, 49, 50, or 52 or 54
(Message bit 80, 81, 82, or 84 or 86) to either “0” or “1,” then bit 47 shall be set to ONE (1). Otherwise, bit
47 shall be set to ZERO (0).
DRAFT - Working Paper ASP TSGWP11-01 for review by the TSG during the meeting in June 2011 in Paris