This radar works on a similar frequency (176 MHz) to the AI Mk4 and on very similar principles. With ASV only two aerials, port and starboard are required as the vertical displacement between the target and aircraft is simply the height of the aircraft. The range of the radar exceeds that of the AI Mk4 and this is assumed to be due to the fact that the sea acts like a mirror and only the sea surface beneath the aircraft results in a strong echo and targets beyond this distance are not affected. The early ASV systems used separate transmit and receive aerials but a later development used an Aerial Coupling unit containing a motor driven switch. Using a combination of quarter wave lines and spark gaps it was possible to stop the receiver being overloaded by the transmitting pulse and therefore the same aerial could be used for both functions, giving improved performance

Another variation used a very large aerial array mounted along the top of the fuselage. This was used broadside on for sweeping the shipping lanes. The system could also be used as a beacon receiver at ranges up to 90 miles

For demonstration purposes I have used the basic system with only the receiver (R3132) and indicator unit (6A), together with a signal simulator similar that is simpler than the AI MK4 version.

The receiver also bears a great similarity to the AI Mk4 version except that only a two pole, two way motor driven switch is required to cater for the twin aerials. In the R3132 the motor is powered from the high frequency A.C. supply rather than the 24v DC. The primary indicator unit used is a 6A although a second simplified version, a 6C could be added and made available to the pilot. Both of these indicators used a single 6" VCR 197 tube with a vertical trace.

Again the signal appears on the trace as a blip either to the left or the right depending whether the signal is port of starboard and the distance from the bottom of the trace is equivalent to the range. Three distance ranges are selectable on the indicator and the appropriate scales are engraved on the cursor.

The lower and larger blip is the simulated sea echo pulse at the height of the aircraft. Often the aircraft would fly low and the noise echo would be lower down the trace. The upper pulse is a simulated target, slightly to port of the aircraft flight line and at a greater distance.

ASV MKII Animation

The left hand screen simulates the radar screen and the right hand rectangle is a simulation of a ship on the horizon. The colour of the echo would normally be the same as the rest of the screen but has been changed from green to yellow to make it easier to identify.

The radar can pick up a target out of visual range, for whatever reason. Note that as the aircraft manoeuvres to approach head on to the target, the radar echo equalises either side of the axis or moves off to the right or left if the aircraft turns away. Note also that the echo get larger as the target is approached and the echo moves down the radar tube. The range of the target is measured from the bottom of the trace by means of a graticule attached in front of the screen. The noise at the bottom of the trace is that due to sea echo immediately below and slightly in front of the aircraft. The sea acts as a mirror to radar echoes and at greater distances little noise echo is received.