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SECOR 13

NSSDCA/COSPAR ID: 1969-037B

Description

SECOR 13 (SEquential COllation of Range 13), or Engineer Geodetic Research Satellite XIII (EGRS XIII), was a joint US Army/Navy satellite developed for the U.S. Army Engineer Topographic Laboratories (USAETL) by the Cubic Corporation. Its primary objective was to provide geodetic coordinates of a ground point located from 160 to 4800 km from known geodetic positions. A series of these Type II SECOR satellites were launched from 1964 to 1970, supported by for transportable ground stations. The goal of the program was to keep at least one operational satellite in orbit at all times. The satellite would transmit signals from orbit that would be received by the four ground stations to conduct range measurements. Three of these stations were placed at known geodetic locations, and the fourth at an unknown location, which could then be calculated using the known satellite and station locations. In this way a network of geodetic points was "filled in" over time.

Spacecraft and Subsystems

The Type II SECOR satellites were rectangular prisms measuring 25.3 x 29.8 x 34.9 cm (9.95 x 11.75 x 13.75 in.). Mass was approximately 17.7 kg (39 lbs.). The satellite construction was primarily aluminum, built around four structural subassemblies. The baseplate subassembly provided mounting surfaces for the transponder and the center support and solar panel support subassemblies. The transponder was a Cubic-built transistorized model TR-30B, which would demodulate the incoming composite signal and phase modulate two carriers of 449 and 224.5 MHz. The signals were demodulated at the ground receiving stations and combined to give effective ranging frequencies.

The center support subassembly provided mounting surfaces for the telemetry and power system components and housing for the batteries, as well as supporting the telemetry antenna. The batteries were hermetically sealed Ni-Cd cells with nominal output of 13.75 volts, charged at 600 milliamperes for 14 hours. Most of the early SECOR satellites used eight commutated telemetry channels for housekeeping data: two for calibration, three for temperatures, and one each for battery voltage, input signal strength, and transponder output power. Later satellites used 16 channels. Telemetry transmission was a 136 MHz carrier phase modulated by a subcarrier voltage controlled oscillator with a center frequency of 730 Hz.

The solar power support subassembly provided mounting surfaces for the solar cells, which covered all the outer surfaces. There were 10 solar panels composed of a total of 1456 N-on-P solar cells. Each cell was protected by a 60 mil (0.15 cm) thick quartz cover and produced 405 millivolts at 60 milliamperes. Each panel was wired (series-parallel) to provide 17 volts. The wraparound subassembly supported the solar panel subassembly and the transponder ranging antennas. There were nine flexible hardened tempered steel tape antennas (1.27 x 0.023 cm), four for 224.5 MHz, four for 449 MHz, and one 136 MHz telemetry antenna. All were simple dipoles, silver plated with a teflon coating, and had numerous 1/8 in. (0.32 cm) holes to minimize the effect of the antenna shadows on the solar cells.

Satellite orientation was achieved by a magnetic stabilization system with a 0.5 x 3.5 inch (1.27 x 8.89 cm) bar magnet and damping rods. Spin-damping (to less than 1 rpm) was accomplished using shorted-coil magnetic despin rods.

Mission Profile

SECOR 13 was launched as a secondary payload with Nimbus 3 on a Thor-Agena rocket from the Western Test Range at Vandenberg Air Force Base on 14 April 1969 at 08:00:00 UT. It was put into an initial 1085 x 1141 km altitude near-polar orbit with an inclination of 99.90 degrees and a period of 107.3 minutes. The spacecraft performed nominally and the final measurements to complete the geodetic belt were established.

Experiments with the SECOR satellites led to Timation, and finally to the GPS Navstar system.

Image from Corliss, 1967

Alternate Names

  • 03891
  • EGRS 13
  • SECOR13

Facts in Brief

Launch Date: 1969-04-14
Launch Vehicle: Thor-Agena
Launch Site: Vandenberg AFB, United States
Mass: 20 kg

Funding Agency

  • Department of Defense-Department of the Army (United States)

Discipline

  • Navigation/Global Positioning

Additional Information

Questions and comments about this spacecraft can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail

Selected References

  • Corliss, W. R., Scientific satellites, NASA, SP-133, Wash., D.C., 1967.
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