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S3 1, 2, 3 (S73-5, S73-6, S74-2)

S3 1 [USAF]

S3 (Small Secondary Satellites) were launched as piggy back payloads on some KH-9 spysat launches. Each one had his own solid fuel motor to perform injection into an higher orbit.

S3 1 carried eigth complementary types of experiments, designed to study atmospheric density and its changes at low altitudes:

  • Mass Spectrometer
  • Electrostatic Analyzer
  • ELF-VLF Receiver
  • Accelerometer Density Observations
  • Ion Density Gauges
  • Solar UV Experiment
  • Retarding Potential Analyzer

The satellite was spin stabilized, with the spin axis perpendicular to its polar orbit. Orbit precession, when the orbit is inclined more than 70 deg, was limited to less than 0.5 deg per day. This limited observations for sampling to less than one-fourth of the possible 25 h of local time over the 7-month satellite lifetime.

S3 2 carried following set of instruments:

  • Velocity Mass Spectrometer
  • Low Energy Proton Spectrometer
  • Energetic Electron (0.1-1.0 Mev) Sensor
  • Magnetometer
  • Triaxial Piezoelectric Accelerometer
  • Spherical Electron Sensor and Planar Aperture Ion Sensors
  • Proton Time-of-Flight and Proton Alpha Counters
  • Neutral Density Experiments (Cold and Hot Cathode Gauges)
  • Neutral Density Experiment (Cold Cathode Gauge)
  • Proton-Alpha Particle Detector
  • Electric Field Observations
  • Electrostatic Analyzer (1-20 keV.
  • Retarding Potential Analyzer (RPA)
  • Electrostatic Analyzer (2-300 eV)

S3 3 was a small observatory in a near-polar orbit with eight different sensors on board. It was designed to observe various magnetospheric parameters and their interrelationships. Sensors, which observed energetic protons and Alpha particles, also provided real-time observations for use by the U.S. Air Force Air Weather Service. It carried following set of instruments:

  • Low-Energy Particle Spectrometer
  • Low-Energy Proton Spectrometers
  • ELF/VLF Receiver
  • Energetic Electron Magnetic Spectrometer
  • Ion-Electron Mass Spectrometer
  • DC Electric Fields
  • Proton Telescope
  • Electric Fields-Ion Drift
Nation: USA
Type / Application: Experimental
Operator: US Air Force (USAF) STP (Space Test Program)
Contractors: Boeing
Equipment: see above
Configuration: ?
Propulsion: Star-17 (#1), Star-13A (#2), Star-17A (#3)
Power: Solar cells, batteries
Mass: 276 kg kg (#1)
Orbit: 148 km × 3483 km, 96.99 (#1); 233 km × 1544 km, 96.28 (#2); 232 km × 6038 km, 97.46 (#3)
Satellite COSPAR Date LS Launch Vehicle Remarks
S3 1 (S73-5, OPS 8452) 1974-085C 29.10.1974 Va SLC-4E Titan-3D with KH-9 9, Raquel 1
S3 2 (S73-6, OPS 5547) 1975-114B 04.12.1975 Va SLC-4E Titan-3D with KH-9 11
S3 3 (S74-2, OPS 3986) 1976-065B 08.07.1976 Va SLC-4E Titan-3D with KH-9 12, Ursala 3


  • NSSDC Master Catalog: S3 1
  • NSSDC Master Catalog: S3 2
  • NSSDC Master Catalog: S3 3
Further STP missions:

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