The EtherScan RADAR (“ESR” as it is known for short) is a device that alerts you to proximity of things. It has two modes, described below.
To toggle monster detection on/off, press R. This acts as a kind of RADAR that lets you know where enemies are. The faster the beeps, the nearer the monster. The sound is actually two close-together beeps; the pitch difference between which tells you where (vertically) the monster is with respect to you. For example, if the monster is lower or higher than you. If the monster is roughly on the same level as you, the beeps will both have the same pitch.
Just as in regular Quake, you're not given the ability to detect enemies through walls with the ESR. In AccessibleQuake, this was the case and it proved to be both unnecessary and potentially confusing. This way is much more fun :-).
If there is a hazard in front of you – some kind of ledge/drop – you'll be told about it by the navigation helper. If you want to try and jump over the pit, you can use this alternative ESR mode to help you do so. Pressing the R key again when you have a drop in front of you will make the ESR lock onto it instead of enemies. It automatically falls back to detecting enemies when you lose the lock on the pit (by jumping over it successfully, falling in or just turning and facing another way).
If it can't find a pit, it will just turn the ESR off.
It is a good idea to check that you can make the jump (with the K key) first. Also, if you are not terribly confident of bing able to make it, you should check, with the D key, that the pit doesn't contain something harmful.
The ESR is capable of detecting the nearest monster, enemy (player or bot not on your team) and friend (player or bot team member) to the player. It uses a different sound to indicate which is which, so that you know not to fire at team mates (vertical level information, as described for monsters above, is provided for all three types of detection).
By default, the T key toggles if monsters are detected, Y toggles enemy detection and U enables/disables the announcement of friends. All three features are enabled by default.