IMP-J

Telecommunications Description

Convolutionally encoded PCM (pulse code modulated) data were transmitted at VHF frequency (137.98 MHz). The data rate was 1600 bits/s; total transmitted bit rate, including redundancy, was 3200 bits/s. A 400 bits/s mode was available but not used. In the early years of the IMP lifetime, NASA provided a worldwide VHF ground network which yielded a net telemetry coverage in the 85-95% range. (There was no tape recorder on IMP-8.) Coverage fell as NASA descoped its ground station network in the 1980's. By the late 1980's, an ad hoc IMP VHF ground network had been assembled, with the NASA Wallops station as the anchor. Additional stations were located at Belgium (ESA), Hawaii (NASA VLBI Project), U. Tasmania, and U. Chile. This array of stations provided coverage in the 65-80% range from the late 1980's through the mid-1990's, after coverage had fallen to close to 50% in the early 1980's. In the mid 1990's, an IMP VHF antenna was built at McMurdo Sound, Antarctica, with National Science Foundation cooperation. With the McMurdo station operational, IMP coverage reached 90-95% in 1996. Subsequently McMurdo was replaced with a station at Canberra, Australia. In October, 2001, IMP 8 was terminated as an independent mission. Telemetry acquisition resumed after about three months at Canberra only (30-50% coverage), as an adjunct to the Voyager and Ulysses missions.