At the aircraft receiver, a pulse is detected each
time the respective beams sweep past the aircraft.
Consider an aircraft on the approach as illustrated
in Figure 13.3. The (azimuth) time referenced
scanning beam sweeps from left to right (‘TO’),
and then returns from right to left (‘FRO’). If the
aircraft is in position A, it is to the left of the
centreline and will receive a pulse at time interval
t1 as the beam sweeps ‘TO’, and then at time t2
when the beam sweeps ‘FRO’. The two pulses
are therefore close together with the aircraft to
the left of centreline. If the aircraft were in
position B, i.e. to the right of the centreline, it
would receive pulses at t3 and t4 due to the
relative position of the aircraft.