The scientific tourist #245 — IDCSP

Another interesting little piece of technology history from the Air Force Space and Missile Museum at Cape Canaveral Air Force Station, Florida:


The larger of the two models is a life-size model (likely an engineering model or prototype) of an IDCSP satellite.  Starting in 1962, IDCSP (Initial Defense Communications Satellite Program) was an experimental military communications satellite system.  As it turns out, it was the first successful such system, and paved the way for later, more sophisticated systems.

The IDCSP system was made up of 26 small satellites scattered in sub-geosynchronous orbit, launched in four groups (the smaller model on the right shows their launch configuration) between 1966 and 1968.  Small / simple / cheap satellites have come back into favor again (it seems to be a cyclical thing).  But these little gems were built and launched back when small and simple was the only way to build them — each satellite weighed just 45 kg (99 lbs), was spin stabilized, 86 cm (34 inches) in diameter, and had no battery.  Each had a single repeater with a capacity of 5 high-quality voice circuits, 11 tactical voice circuits, or 1 megabit per second of data.  This was enough that in 1967 an operational link was established via IDCSP to transmit digital data from Vietnam to Hawaii through one satellite, then on to Washington, D.C. via another.  In 1968, the system was declared operational and renamed DSCS I (Defense Satellite Communications System I).  Designed to last 18 months, 5 of them were still operational after 8 years, and 2 of them lasted 10 years.

Given that the technology was so new at the time, the satellites were intentionally launched into sub-geosynchronous orbit so that they would “drift” in their orbit — from west to east, at about 30 degrees (equivalent to 2 hours) each day.  This way, a backup satellite would always be visible to an earth station even if one satellite failed.  This was the one and only such system ever flown using this approach — subsequent communications satellites have been launched to either geosynchronous orbit, or well below it (to reduce launch costs).

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