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Alarms
The FrSky system comes with some built-in alarms that you can set on the DHT-U or FLD-02 screen. How to set them and what they mean can be confusing. The important thing is that the default values for the RSSI alarms have been determined by FrSky and unless you really know what you are doing you should not change the numbers. You can only change the sounds and values of Alarm 1 and Alarm 2 in any case. Note that FrSky also sometimes call the alarm levels 0, 1, 2, 3 as Green,Yellow, Orange, Red for no useful reason. By default the alarm thresholds are set at:
Green = No alarm = 0 beeps = Signal strength is strong
Yellow = Alarm1 = 1 beep = Value 45 means signal strength is adequate
Orange = Alarm2 = 2 beeps = Value 42 means model is at far, but safe, range
Red = Alarm3 = 3 beeps = Value 40 means maximum range and indicates imminent failsafe (which actually occurs at about 37-38)
However since there are only three distinct alarm sounds in the whole system (1, 2, or 3 beeps) you may get the same sounds for RSSI alarms and any voltage (A1 or A2) alarms you have set.
Beware that there is a catch. Suppose you are at the limit of range and the telemetry receiving system in your transmitter module system loses the signal. Then a "feature" of the FrSky system is that the whole display freezes and displays the last good data. If you were just at the point of a control loss (Receiver RSSI) warning you will not get that warning. If you glance at the screen you might see an RSSI value displayed in the left box that is still safe but in fact it is not the current value but the one that was there when the link back from the model dropped out. So if you get a 3 beep alarm it pays to check which of the two RSSI values set it off. If it was the right hand one - don't trust anything you see on the screen until it starts updating again!