TOPO 1, the U.S. Army Topographic Command's first satellite, was launched from the
Western Test Range (WTR) on 8 April 1970, piggyback with the Nimbus 4 meteorological
satellite. Approximately 1 hr and 41 min after launch, the satellite was ejected from the
final stage of the launch vehicle into a near-circular, near-polar orbit.
TOPO I was used primarily for research and development in determining the ultimate accuracy of SECOR when using transponders with ultra-stable oscillators. The objectives of the spacecraft were:
- to study new techniques for accurate real-time determination of positions on the earth's
surface involving a triangulation technique using satellite transponders and ground-based
tracking facilities,
- to test relay and one-way ranging techniques used by the Defense Navigation Satellite
System, and
- to obtain information about ionospheric effects on satellite geodetic surveys.
The 18.14 kg satellite was based on the SECOR Type II satellite and was shaped like a rectangular box (0.36 m by 0.30 m by 0.23 m).
It was equipped with modified SECOR components including:
- a transponder that retransmitted the 136.84 MHz telemetry signals received from three
ground stations whose locations were determined from satellite ranging measurements using
phase comparison techniques and
- a 590-KHz high-resolution ranging channel to obtain an ionospheric correction factor.
Power was supplied by solar cells mounted on the satellite's exterior recharing onboard
batteries. The telemetry transmissions were made by flexible metal tape antennas that were
wound around the satellite's structure and deployed after orbital injection. The satellite
obtained excellent data for a 6-month period (May 1970 to October 1970). Owing to a lack
of funding, however, further tracking was discontinued and, as of 14 January 1972, the
satellite was orbiting in an operational off mode.
