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required may be deleted so long as the remaining functions are transmitted in the designated time positions.
2.1.4.3 The sequence pair shown in Figure G-3B accommodates the high rate approach azimuth function. Any function
not required may be deleted so long as the remaining functions are transmitted in the designated time positions.
2.1.4.4 Figure G-3C shows the complete time multiplex transmission cycle which may be composed of the sequence
pairs from Figure G-3A or from Figure G-3B. The open time periods between sequences can be used for the transmission of
auxiliary data words as indicated. Basic data words also may be transmitted in any open time period.
2.1.4.5 Sufficient time is available in the cycle shown for the transmission of the basic data and the auxiliary data
defined in words A1-A4, B1-B39, B40-B45 and B55, provided that data are also transmitted during unused time slots or slots
devoted for data words within the sequences.
2.1.4.6 More efficient sequences may be designed by adjusting the timing within the sequences and the inter-sequence
gaps to allow the transmission of additional auxiliary data words. Such sequences must be designed to provide equivalent
freedom from synchronous interference as the sequences shown in Figures G-3A, G-3B and G-3C. Frequency domain
analysis techniques may be utilized to demonstrate that alternative sequences are sufficiently randomized.
2.2 Angle guidance parameters
2.2.1 The angle guidance parameters that define the MLS angle measurement process are specified in Chapter 3,
3.11.4.5. Two additional parameters that are useful in visualizing the operation of the system are the midscan time (Tm) and
the pause time. They may be derived from the Chapter 3 specifications and are shown for reference in the following table.
* All figures are located at the end of the Attachment.
Attachment G Annex 10 — Aeronautical Communications
Signal format midscan and pause times
(see Figure G-2)
Midscan1 Pause
time, Tm time
Function (μs) (μs)
Approach azimuth 7 972 600
High rate approach azimuth 5 972 600
Back azimuth 5 972 600
Approach elevation 2 518 400
Flare elevation 2 368 800
1 Measured from the receiver reference time
(see Appendix A, Table A-1).
2.2.2 Function timing accuracy. Because of the inaccuracy in the determination of the reference time of the Barker
code, and because the transmitter circuits smooth the phase or amplitude during phase transitions of the DPSK modulation, it
is not possible to determine the timing of the signal with an accuracy better than 2 microseconds from the signal-in-space. It
is therefore necessary to measure the timing accuracy specified in Chapter 3, 3.11.4.3.4 on the ground equipment. Suitable
test points should be provided in the ground equipment.
2.3 Azimuth guidance functions
2.3.1 Scanning conventions. Figure G-4 shows the approach azimuth and back azimuth scanning conventions.
2.3.2 Coverage requirements. Figures G-5 and G-6 illustrate the azimuth coverage requirements specified in Chapter 3,
3.11.5.2.2.
2.3.2.1 When the approach or back azimuth antenna sites are necessarily offset from the runway centre line, the
following factors should be considered:
a) coverage requirements throughout the runway region;
b) accuracy requirements at the applicable reference datum;
c) approach azimuth to back azimuth transition; and
d) potential disturbances due to moving vehicles, aircraft or airport structures.
2.3.2.2 An offset azimuth antenna is normally adjusted such that the zero-degree azimuth is either parallel to the
runway centre line or intersects the centre line extended at an operationally preferred point for the intended application. The
alignment of the zero-degree azimuth with respect to the runway centre line is transmitted on the auxiliary data.
2.3.3 High rate approach azimuth. Where the approach proportional guidance sector is plus or minus 40 degrees or
less, it is possible to use a higher scanning rate for the azimuth function. The high rate approach azimuth function is available
to offset the increase in CMN caused by large beamwidth antennas (e.g. 3 degrees). Reducing the CMN provides two benefits:
1) angle guidance signal-in-space power density requirements can be reduced; and 2) dynamic side-lobe level requirements
can be relaxed.
ATT G-3 23/11/06
Annex 10 — Aeronautical Communications Volume I
23/11/06 ATT G-4
2.3.3.1 In general, this function will reduce the CMN caused by wide bandwidth, uncorrelated sources such as diffuse
multipath or receiver thermal noise by a factor of 1/ 3 relative to the basic 13 Hz function rate. However, the full reduction
of power density by 1/ 3 cannot be realized for all ground antenna beamwidths because of the requirement to provide
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