The low range radio altimeter (LRRA) is a self-
contained vertically directed primary radar
system operating in the 4.2 to 4.4 GHz band.
Airborne equipment comprises a transmitting/
receiving antenna, LRRA transmitter/receiver and
a flight deck indicator. Most aircraft are fitted
with two independent systems. Radar energy is
directed via a transmitting antenna to the ground;
some of this energy is reflected back from the
ground and is collected in the receiving antenna,
see Figure 12.9.