AP 3302 Pt. 3
Section 1
CHAPTER 4
Some examples of the uses of pulsed radar
To find the elevation angle (or height) of a target on a given bearing at a given range, the height-finding aerial is turned to the indicated bearing and is then 'nodded' up and down between certain limits (typically between - 1o and + 29o from horizontal). The nodding cycle takes about 3 seconds. When the target has been located and identified, the nodding cycle may be reduced. The required reduction depends upon the range; at long ranges, the variation in elevation angle with range is small, and the nod cycle can be reduced to about 3o The height accuracy of this radar is stated to be better than 1,500 feet at a range of 150 nm.
The mode of operation described above is known as 'automatic searchlight'. Other modes are:
- Single shot. This gives a one-nod cycle, on the target bearing, with the amplitude of the nod automatically controlled by the target range.
- Burn through. This gives a continuous nod motion with a fixed nod amplitude of 3o, centred on an elevation angle that can be varied over 27o. The nodding cycle takes 1 second.
- Volumetric scan. This gives continuous azimuth scan, with a stepped elevation scan super-imposed. This mode enables the height-finder to be used also for surveillance if required.
- Sector volumetric scan. This is similar to the volumetric scan mode but with the azimuth scan reduced to a known sector. The sector angle scanned can be varied between 10o and 90o.
Other Radars
The air defence radar system described so far needs at least two radars: one operating as a surveillance radar to give range and bearing on a p.p.i.; and the other operating as a height- finding radar to give range and elevation (or height) of the target. In practice, it is likely that more than two radars will be employed. Others may be operating to 'track' the target and to calculate its speed by means of Doppler (see Sect 7). Also, since the early-warning radars will have provided information on the range and bearing of the target, some radars may be employed in a sector scanning role, concentrating on the given volume of space in which the target will be found (Fig 5).
