The Scientific Atlanta GPD-111 Tracking Receiver is a receiver covering the frequency range of 200Hz to 100kHz. It is capable of automatically-or manually-scanning for, detecting, and tracking signals within its frequency range. As such, it can receive ELF, VLF, and LF signals with detection of the following modes: AM, USB, LSB, and variable pitch BFO (e.g. CW) signals.

It displays the frequency of the signal, and has wideband equalization of gain vs frequency, along with a noise-responding AGC, to maintain optimal signal detection thresholds.

Detection signal shape and bandwidth are selectable. Adjacent Sideband suppression is at least 40dB below bargraph full scale deflection. All other signals are suppressed at least 55dB below the specified noise-floor-to-prevent-detection.

Automatic Frequency Control (AFC) to 10 Hz, and True Restored Frequency (TRF) options round out its capabilities.

It has three selectable audio bandwidths: 1 kHz, 2 kHz, & 8 kHz; and a Low Pass Filter prior to the AGC and detector A/D processing, to provide anti-alias filtering (at greater than 40 dB below full scale), to prevent out-of-band signal foldback into the audio processing spectrum.

The unit has both Balanced (dual transformer-coupled, summed inputs) and Unbalanced (>10,000 ohms) at an expected Level of 10mV to 3Vrms.

These receivers were specially designed for operation on US Navy Trident Missile Submarines. How they got into the surplus market, I'll never know. I suppose something better came along. They were in service in 1990-and may still be. Mine came with a pallet-load of other stuff, bought by a friend in the surplus business, who then called me. I know of one other in civilian hands-although there may well be plenty of others. Also, I suspect there is one 'loose' in Japan, since there is a website there which mentions them. I have received no reply to my inquiries to that person.