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7.3.3.1 Data acquisition. Performance in the airborne acquisition of data provided on either the basic or auxiliary data
function is broken into two items: the time allowed to acquire the data and the probability of an undetected error in the
acquired data.
7.3.3.1.1 At the minimum signal power density, the time to acquire basic data word 2 which is transmitted at a rate of
6.25 Hz does not exceed two seconds on a 95 per cent probability basis. The time to acquire data that are transmitted at a rate
of 1 Hz does not exceed 6 seconds on a 95 per cent probability basis.
7.3.3.1.2 In the acquisition process, the receiver decodes the appropriate data words and applies certain tests to
ensure that the probability of undetected errors does not exceed 1 × 10–6 at the minimum signal power density for those
data requiring this level of integrity. The recommended performance specifications for undetected errors may require
additional airborne processing of the data beyond simple decoding. For example, these may be achieved by processing
multiple samples of the same data words.
7.3.3.1.3 If the receiver does not acquire data required for the intended operation, a suitable warning is to be provided.
7.3.3.1.4 At the minimum signal power density the time to acquire all data words required to support MLS/ RNAV
operations (auxiliary data words B1-B41, A1/B42, A2, A3, A4/B43 and basic data word 6) must not exceed 20 seconds on
a 95 per cent probability basis. The MLS/RNAV equipment has to ensure that the probability of undetected errors for this
block of data does not exceed 0.5 × 10–9. This performance assumes a 2 dB improvement in signal to noise. This may be
achieved through reduced cable loss, margin or improved receiver sensitivity (see the airborne power budget given in
Table G-2). Additionally, with signal levels above this, the acquisition time is intended to be less than 20 seconds.
7.3.3.2 Data validation. After acquisition of data, the receiver repeatedly confirms that the data being received are the
same as the acquired data. The receiver decodes several consecutive and identical data different from that previously acquired
before taking action to accept the new decoded data.
7.3.3.2.1 For data required to support MLS/RNAV operations, the airborne equipment applies the cyclic redundancy
check (CRC) to the data to ensure sufficient integrity has been achieved. Data that continue to be received continue to be
validated. The MLS/RNAV equipment does not accept a new block of data to be used until it is validated with the CRC.
7.3.3.3 Data loss. Within 6 seconds after the loss of basic data or auxiliary data that is transmitted with a maximum
interval of 2 seconds or less, the receiver provides a suitable warning and removes the existing data. Within 30 seconds after
the loss of auxiliary data other than that referred to above, the receiver provides a suitable warning.
7.3.3.3.1 For data required to support MLS/RNAV operations, the airborne equipment does not remove existing data
following validation except under the conditions described in 7.3.3.2.1. An MLS/RNAV data block that has been validated
by the CRC is not removed until a new data block has been received with a different ground equipment identification in basic
data word 6, a new MLS channel is selected, or power is removed. Additionally the data block is not removed when
transitioning to back azimuth coverage.
ATT G-25 23/11/06
Annex 10 — Aeronautical Communications Volume I
7.3.4 Multipath performance
7.3.4.1 Where the radiated signal power density is high enough to cause the airborne equipment thermal noise
contribution to be insignificant, the following specifications should apply for scalloping frequencies between 0.05 Hz and
999 Hz.
7.3.4.1.1 In-beam multipath. Multipath signals coded less than two beamwidths from the direct signal and with
amplitudes of 3 dB or more below the direct signal should not degrade the angle guidance accuracy output by more than plus
or minus 0.5 beamwidth (peak error).
7.3.4.1.2 Out-of-beam multipath. Multipath signals coded 2 beamwidths or more from the direct signal and with
amplitudes of 3 dB or more below the direct signal should not degrade the angle guidance accuracy by more than plus or
minus 0.02 beamwidth.
7.3.5 Clearance
7.3.5.1 The airborne equipment should provide clearance guidance information whenever the antenna is in the presence
of a valid clearance guidance signal.
7.3.5.2 When the decoded angle indication is outside the proportional guidance sector defined in Chapter 3, Table A-7,
the MLS guidance signal should be interpreted as clearance guidance.
7.3.5.3 When clearance pulses are transmitted, the receiver shall be able to process the range of pulse envelope shapes
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