Why do radars rotate

Basics of radar technology

Time balance of an impulse radar

Consequences for the time balance (or the time budget) of a pulse radar result from the lighting time and the number of hits, the pulse repetition frequency and the necessary duration of the reception time for a maximum clear measuring distance. As a result, the resolution and various blind speeds also depend on the clock frequencies. With a classic radar (without monopulse technology) which is used as ATC radar, the required data renewal rate is less than 5 seconds. This limits the maximum possible reception time and thus the theoretical range of the radar:

Fig. 1: Time balance of a radar device

Fig. 1: Time balance of a radar device

Fig. 1: Time balance of a radar device

Since the radar signal processing and display still takes place in real time (or with only a small but constant delay), the data renewal rate is approximately equal to the antenna cycle time. The antenna of the radar device must therefore rotate at least 12 revolutions per minute in order to point to the same lateral angle again after 5 seconds and thus measure the target position again.

The lighting time depends on the beam width (opening angle) of the antenna. For example, if this is 1.6 ° with a good parabolic antenna, then the full circle is divided into 360 ° / 1.6 ° = 225 different angles, in which the antenna remains for 5 s / 225 = 22.22 ms.

Radar devices usually require a certain number of hits in order to distinguish the very weak echoes of targets from the interference signals or from thermal noise by means of a so-called pulse application (pulse integration) and to carry out a correct lateral angle measurement. If the number of hits required for signal processing is 20, for example, then only the maximum possible time 1.11 ms remains for one pulse period. Due to the necessary switching measures, for example a constantly changing period duration (staggered PRT) for interference suppression, only 0.8 ms of this can be used for a reception time. During this reception time, the electromagnetic waves cover a maximum distance of 120 km, which corresponds to around 65 nautical miles.

The maximum range therefore depends on the time budget available to the radar for the measurements. To require a longer range or even an additional elevation angle measurement from this radar is simply utopian without revolutionary changes in signal processing (see monopulse method or digital beamforming). Even small changes in the number of hits required as a possible reserve to extend the reception time then have negative effects on the probability of detection by the radar.