[Nisgeis]

There are two main groups of radar, search and fire control. Fire control radar is prefixed with an F and search radar is prefixed with an S. The second letter is usually the model of search radar, so the SJ radar is search radar, model J (model 10). Fire control radar used to be denoted by letters, but was standardised to be numbered instead, so what used to be called an FC or FD would subsequently be called just a Mark III or Mark IV.

Any number after the S<letter> system is a modification. So, SJ-1 is the first modification to the SJ radar set, which increased range.