IISLS
The IISLS method ( I mproved I nterrogator S ide L obe S uppression) is a technical possibility of preventing responses from reflections at a secondary radar .
The queries and responses caused by sidelobes are a big problem for the secondary radar. The technical requirements are the same as with the ISLS procedure : Two antenna diagrams are used, a strongly bundled directional diagram for the main lobe, and a control pulse (P2) is emitted via an omnidirectional antenna .
With the IISLS method, the P2 and P1 impulses are sent through the suppression channel (remember: with the simple ISLS, only the P2 impulse is sent here). In this case the transponder receives the pulse pair P1-P2 in the direction of reflection in a direct way. A response is usually suppressed because P1 = P2. The transponder is therefore blocked for a short time (approx. 35 µs). As a result, the query arriving later due to the detour of the reflection by Δt cannot be responded to, so no incorrect answer is sent.
The IISLS procedure can really very effectively prevent incorrect answers. However, the disadvantage of the method is that the answers are prevented not only in one side lobe of the query, but in the entire area of the diagram of the suppression channel (up to about 40 nautical miles). This is very fatal in areas with a high number of queries, as it also prevents a lot of correct answers. This is why this procedure has not been able to prevail.