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The PDP-8/E Minicomputer used for LORAN-C

Front Panel of the PDP-8/E S M L XL This machine came from the Canadian Coast Guard in Newfoundland. Notice how it doesn't say "Digital Equipment" anywhere -- the Coast Guard used "CDDP" as the manufacturer code for DEC.

I received it in the summer of 2000 (I think :-)) and just recently (summer 2003) received some information about the machine:

The PDP-8/E in this collection was once part of a suite of equipment used by the US and Canadian Coast Guards as part of the worldwide LORAN-C radionavigation system. To really appreciate the critical role of the PDP-8 in this system, a brief intro on LORAN-C is necessary.

Long-Range Aid to Navigation, type "C" is a system of high-powered radio transmitters and receivers used heavily for precise navigation since the 1950s until the recent advent of lower-cost Global Positioning System (GPS) receivers. The system was installed, operated, and controlled by the United States Coast Guard. The stations located in Canada and Norway were operated by personnel of those nations.

The LORAN-C system used chains of transmitting stations placed at specific geographic locations in a specific geometric relationship. Each chain gave coverage to a specific global region and consisted of three or more transmitting stations - a "Master" station and two or more "Secondary" or "Slave" stations. Beginning with the Master, each station would transmit a group of pulses in a specific order and timing. The timing was very precise, being measured at resolutions of 10-12 seconds. A properly programmed receiver in a user's boat, plane, or whatever, would measure these time differences and calculate its position in a sophisticated variation of triangulation.

To ensure each station in the chain was operating within very tight tolerances for signal pulse shape, power, and timing, a series of receiver sites were placed within the chain's area of operation. This was where the PDP-8 was used. These sites, known as Primary Chain Monitor Sets (PCMS), were often located in remote locations and were unmanned. They were connected by a dedicated data network to a centralized Control Station where technicians maintained a 24-hour watch.

The role of the PDP-8 was central. It was used to perform real-time data acquisition and control of the receiver unit. During my tenure at the LORAN-C engineering facility a project was begun to develop a replacement system for the PDP-8 because of its age. It wasn't long before the engineers on the project realized they couldn't just plug-in an IBM system, do a simple re-write of the code, and move on. The PDP-8 didn't just control the receiver. It WAS the receiver. Using digital I/O interfaces, the processor directly controlled all aspects of the receiver including gain control, timing gate control, analog to digital conversion, waveform envelope calculations, signal-to-noise calculations - ALL in real time! It's pretty amazing to think that this 1960s vintage processor could process data that fast, but in addition to those functions, it also interfaced with the remote data network to format acquired data and send textual reports to the Control Station watchstanders and allow them to manipulate all aspects of the receiver's operation. These formatted text reports included signal strength, time difference measurements with a resolution of 10 nanoseconds, and pulse shape measurements. Additionally, event reports were transmitted when the receiver system detected such things as loss of a station's signal, and deviations in pulse shape and timing that exceeded specified tolerances.

The DEC PDP-8 performed this critical role from the late 1970s until they were phased out around 2000 (I'm not exactly certain of the date as I had been out of that field a few years by then). Millions of airline travelers, maritime users, defense forces, and many others depended on accurate LORAN-C navigational data to keep them off the rocks, in the correct sea lanes and channels, on course, and safe for decades.

And the PDP-8 was central to that mission.

Many thanks to ET1 William F. Leyh III, USCG(Ret.) for the information!

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