AP 3302 Pt. 3
Section 1
CHAPTER 4
Some examples of the uses of pulsed radar
enable the aircraft to reach the point where it is in visual contact with the runway-from which point the aircraft can land safely under its own control. This special ground-controlled approach (GCA) procedure requires the assistance of a precision radar. One system uses a transmitter with the following parameters:
|
Wavelength |
3 cm (X band) |
|
Frequency |
9 GHz |
|
Peak power |
60 kW |
|
Pulse duration |
0.l8 us |
|
PRF |
3,279 p.p.s. |
|
Slant range |
up to 13 nm. |
The transmitter feeds two aerials alternately:
- Azimuth aerial. This scans 20o in azimuth-normally 10o each side of the runway centre line approach path; the half-power beamwidth is 4o vertically and 0.5o horizontally (in azimuth).
- Elevation aerial. This scans 7o in elevation-normally from -1o to + 6o relative to ground level. The half-power beamwidth is 0.5o vertically (in elevation) and 4o horizontally.
The two beams and their coverage are illustrated in Fig 12, which also shows typical displays. The information on the displays is interpreted by the ground controller and suitable instructions are then passed by radio telephone to the aircraft to enable it to reach the point where the pilot can make a visual touchdown.
Airborne Aids
In addition to the assistance that can be given to an aircraft by primary ground radar installations, aircraft have themselves a number of airborne navigation and landing aids available. Among such aids are:
- Doppler radar-which provides information on drift and groundspeed of the aircraft. This is essential information for dead-reckoning navigation. This input can also be applied to the computer of the automatic pilot where it combines with other inputs (e.g. aircraft heading) to provide positional information.
- Radar altimeter-which indicates the height of the aircraft above the ground over which it is flying at any instant.
