Both the GRR-24 and GRT-22 were single-channel (set to one frequency among 3500 channels in the 225-400 MHz frequency range) voice radio sets installed at GATR Sites under Project RIVET SWITCH, circa 1975. The AN/GRT-22 was the transmitter, and the AN/GRR-24 was the receiver. There was also a multi-channel transceiver set known as the AN/GRC-171 in that same project; it was tuneable (tuneable among 3500 channels in the 225-400 MHz frequency range).

The AN/GRT-22 replaced the AN/GRT-3, and the AN/GRR-24 replaced the AN/GRR-7. Those earlier radio sets were single-channel, and also operated in the 225-400 MHz frequency range.

Reference: http://hereford.ampr.org/millist/m6.html