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Encoder for Azimuth Change Pulses

Figure 1: An old encoder uses in the “ASR-910” ATC-radar

Figure 1: An old encoder used in the “ASR-910” ATC-radar

Encoder for Azimuth Change Pulses

The information in which direction the boresight axis of the antenna points is generated by a mechanically or optically controlled pulse generator, called Encoder. This encoder is attached to the static part of the antenna and its rotor to the rotating part of the antenna. when the antenna is rotating it generates pulses, Azimut Change Pulses” (ACP's). These pulses are counted either in the PPI-display (e.g. ASR–910) or in the radar signal processor (e.g. RRP–117) and are a measure for the angle.

The counter is reset by a North Reference Pulse (NRP) when the antenna passing through the north position so that a unique position can be determined.

Common for the ACP's are 4 096 pulses per revolution for the ASR 910 (corresponds to an accuracy of 0.087° per pulse), up to 16 384 pulses for the RRP 117 (0.02129° per pulse).

The direction of rotation information is usually defined “clockwise”, but with this system, it is always only possible in one direction.

Decoder wiring
Counter
(12…
 14 Bit)
NRP
Reset
ACP
Clock
A0…A11 (…A13)

Figure 2: Principle of decoder wiring

Counter
(12…14 Bit)
NRP
Reset
ACP
Clock
A0…A11 (…A13)
NRP ACP cnt bus

Figure 2: Principle of decoder wiring
(interactive picture)

The decoding circuit consists of counter integrated circuits, which count the arriving ACP's. The ACP's are counted by the decoding circuit. The counter steps are set to zero by the NRP pulse (reset) and thus start counting the angular steps from 0° again. With a total number of 4096 pulses for the full circle (as usual with the ASR 910), the counter has counted 227 pulses at an azimuth of 20°, for example. The absence of a single ACP pulse can be ignored since the counter is reset to zero in the next revolution. The angular error is too small to be detected on the display. However, if the NRP is missing for a longer time, the whole picture on the display turns slowly to the left, because often single ACP pulses cannot be evaluated.

Figure 3: Encoder (dismantled) providing 16384 pulses per rotation of the antenna's turntable

Figure 3: Encoder (dismantled) providing 16384 pulses per rotation of the antenna's turntable

Depending on the electronic version within the encoder, the NRP can occur synchronously to the ACP pulses. This is usually the case if higher accuracy is executed internally (in the encoder) then is required externally (in the radar). In this case, the encoder already contains frequency dividers. However, the north pulse can also be generated completely independently (and thus non-synchronously).