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SECOR 3 (SEquential COllation of Range 3), or Engineer Geodetic Research Satellite III (EGRS III), 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 was 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 model TR-27, 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 3 was launched as part of a payload of eight satellites on a Thor Augmented Delta-Agena D booster from the Western Test range at Vandenberg Air Force Base on 9 March 1965 at 18:28 UT. Due to schedule slippages, SECOR 3 ended up launching 2 days before SECOR 2. It was put into an initial 906 x 942 km altitude orbit with an inclination of 70.09 degrees and a period of 103.5 minutes. The orbital altitude was too low to complete the SECOR equatorial belt, although the mission did complete the tie between Hawaii and Japan. By exercising the satellite batteries, their lifetime was extended to almost 3 years.

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

Image from Corliss, 1967

Alternate Names

  • 01208
  • EGRS 3
  • SECOR3

Facts in Brief

Launch Date: 1965-03-09
Launch Vehicle: Thor Augmented Delta-Agena D
Launch Site: Vandenberg AFB, United States
Mass: 18 kg

Funding Agencies

  • Department of Defense-Department of the Army (United States)
  • Department of Defense-Department of the Navy (United States)
  • Department of Defense-Department of the Air Force (United States)


  • Navigation/Global Positioning

Additional Information

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



NameRoleOriginal AffiliationE-mail

Selected References

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