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second obstructed approach
Ranging source ID 8 1 to 255 1 12 0000 1100
Source availability sense 1 — — Will cease 0
Source availability duration 7 0 to 1 270 s 10 s 220 s 0010 110
Message Block CRC 32 — — — 1101 1011 0010 1111 0001 0010 0000 1001
APPLICATION FEC 48 — — — 0011 1110 1011 1010 0001 1110 0101 0110 1100 1011 0101 1011
Input to the bit scrambling
(Note 2)
1 82 20 18 55 05 4B 30 A0 38 17 C0 40 20 50 C0 94 40 A8 40 30 4C 70 13 70 80 30 34 90 48 F4 DB DA D3 6A 78 5D 7C
Output from the bit scrambling 1 A4 17 90 1F 1A 53 1B 7F A2 C2 19 72 FC 16 10 62 81 E1 43 2C 48 5F E3 1A 3F 56 60 18 86 EA 33 F3 B3 09 07 26 28
Fill bits 0 to 2 — — 0
Power ramp-down 9 000 000 000
D8PSK Symbols
(Note 3)
0000003511204546316504322056660551067602416124477363463220700103224006601332124166231163643777110173115
74302323445146644444
Notes.—
1. The rightmost bit is the LSB of the binary parameter value and is the first bit transmitted or sent to the bit scrambler. All data fields are sent in the order specified in
the table.
2. This field is coded in hexadecimal with the first bit to be sent to the bit scrambler as its MSB. The first character represents a single bit.
3. Symbols are represented by their differential phase with respect to the first symbol of the message, in units of π/4 (e.g. a value of 5 represents a phase of
5π/4 radians) relative to the first symbol.
23/11/06 ATT D-42
Attachment D Annex 10 — Aeronautical Communications
7.18 Type 101 message
Type 101 message is an alternative to Type 1 message developed to fit the specific needs of GRAS systems. The primary
difference in the contents and application of these two message types is two-fold: (a) Type 101 message has a larger available
range for σpr_gnd values and (b) ground subsystem time-to-alert is larger for a system broadcasting Type 101 messages. The
first condition would typically occur in a system where a broadcast station covers a large area, such that decorrelation errors
increase the upper limit of the pseudo-range correction errors. The second condition may be typical for systems where a
central master station processes data from multiple receivers dispersed over a large area.
8. Signal quality monitor (SQM) design
8.1 The objective of the signal quality monitor (SQM) is to detect satellite signal anomalies in order to prevent aircraft
receivers from using misleading information (MI). MI is an undetected aircraft pseudo-range differential error greater than
the maximum error (MERR) that can be tolerated. These large pseudo-range errors are due to C/A code correlation peak
distortion caused by satellite payload failures. If the reference receiver used to create the differential corrections and the
aircraft receiver have different measurement mechanizations (i.e. receiver bandwidth and tracking loop correlator spacing),
the signal distortion affects them differently. The SQM must protect the aircraft receiver in cases when mechanizations are
not similar. SQM performance is further defined by the probability of detecting a satellite failure and the probability of
incorrectly annunciating a satellite failure.
8.2 The signal effects that might cause a GBAS or SBAS to output MI can be categorized into three different effects on
the correlation function as follows:
a) Dead zones: If the correlation function loses its peak, the receiver’s discriminator function will include a flat spot or
dead zone. If the reference receiver and aircraft receiver settle in different portions of this dead zone, MI can result.
b) False peaks: If the reference receiver and aircraft receiver lock to different peaks, MI could exist.
c) Distortions: If the correlation peak is misshapen, an aircraft that uses a correlator spacing other than the one used by
the reference receivers may experience MI.
8.3 The threat model proposed for use in assessment of SQM has three parts that can create the three correlation peak
pathologies listed above.
8.4 Threat Model A consists of the normal C/A code signal except that all the positive chips have a falling edge that
leads or lags relative to the correct end-time for that chip. This threat model is associated with a failure in the navigation data
unit (NDU), the digital partition of a GPS or GLONASS satellite.
8.4.1 Threat Model A for GPS has a single parameter Δ, which is the lead (Δ < 0) or lag (Δ > 0) expressed in fractions
of a chip. The range for this parameter is –0.12 ≤ Δ ≤ 0.12. Threat Model A for GLONASS has a single parameter Δ, which
is the lead (Δ < 0) or lag (Δ > 0) expressed in fractions of a chip. The range for this parameter is –0.11 ≤ Δ≤ 0.11.
　
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