Navigational Aids
Precision Approach Radar
|
Major system characteristics |
|
|
System type |
Scanning beam radar controlled approach system |
|
Frequency band |
3,000 Mc/s |
The text that follows in this section is taken from "The Services Textbook of Radio, Volume 7, Radiolocation Techniques" by Brig. J. D. Haigh, O.B.E., M.A., M.I.E.E., Edited by the staff of "Wireless World", H.M.S.O., London, 1960 pages 202 - 204. This volume was known to the British armed forces as Admiralty B.R.600(7), War Office 10224(7) and Air Ministry A.P.3214(7).
Another type of radar with a similar specification to that of surveillance radar is that used to control the approach of aircraft to an airfield, usually known as a g.c.a. (Ground Controlled Approach) radar. The requirement here is twofold. First, all aircraft flying within a range of thirty or forty miles must be detected; secondly, they must be accurately 'talked down' on to the correct runway. The detection of the aircraft is made easier by the fact that they will be flying in well-defined height zones. The radar will not have to deal with very low flying targets and will not need to take an interest in very high targets. Consequently the detection beam is usually set with its maximum at an angle of about 3o to reduce the likelihood of ground clutter. Nonetheless, mti* techniques are widely used for g.c.a. radars in order that the controllers shall have the clearest possible picture of aerial activity in the vicinity of the airfield. The shape of the beam is 'cosecant squared', designed to give good coverage up to a height of about 5000 feet.
With the ever increasing density of air traffic it is essential that the information on all targets be frequently renewed and scanning rates of as much as 30 r.p.m. are used. Apart from this high scanning rate, which reduces the number of pubes per paint, the other features of the specification make it easier to meet than that of a surveillance radar. The targets are large, and co-operative, the range is not great and no extreme accuracy is required.
To get the required results a peak power of about 100 kW at a wavelength in the 10-cm band is normally used.
To obtain the more accurate data for talking the aircraft down on to the runway a second radar is used which scans a limited angle in azimuth about the required direction of approach, and a limited angle in elevation about the recommended glide path. It is common practice to use one radar which is continuously switched between the azimuth and elevation aerial systems, the output being switched in synchronism between the bearing and elevation displays as shown in diagrammatic form in Fig. l3.4.
The azimuth beam has a width of something less than 1o and commonly scans ±10o about the required direction of approach. The elevation beam has a width (in the vertical plane) of the same order as the azimuth beam and normally scans ±3.5o about the required glide path. The displays used are both of the expanded, partial, p.p.i. type with the required direction and angle of approach, and range markers, superimposed either electronically or by means of a transparent mask suitably engraved. The range scales on the two displays should be identical to facilitate the work of the final approach controller.
Fig. 13.5 shows the appearance of the two displays with an aircraft approaching rather high and to the left of the correct approach.
*moving target indication.
