While in safe-mode, preservation of a spacecraft is the number one priority. Essential AOCS input needs to be provided by high reliable sensors. For this purpose, Spacetech offers a thermo-optical Coarse Earth and Sun Sensor (CESS), available in two versions:

Heritage CESS (HCESS): used in numerous successful LEO space missions e.g. CHAMP, GRACE, TerraSAR-X, TanDEM-X, Paz, Cryosat, SWARM

Advanced CESS (ACESS): Design improvements result in superior sensor performance and extend the range of application (orbit height, satellite accomodations).

HCESS Alone // ACESS Alone

Sensor at a glance:
  • Dimensions: < 100 mm x 50 mm x 40 mm (width x depth x height)
  • Mass of approx 80 (120) g for HCESS (ACESS)
  • Two OSR surfaces (black and mirror) with known solar absorbance α and emittance ε
  • 6 PT1000 thermistors on the OSR backside
  • Low power consumption of 0.1 W for temperature read-out
  • Simple yet robust design

For more detailed information, find our product datasheet.

Working Principle

 A set of six orthogonal accomodated CESS can be used to determine the relative earth and sun vector in the following way:

  1. Perform a continous measurement of the temperatures of all OSRs by probing the Pt1000 thermistors
  2. Extrapolate temperatures to steady-state
  3. Estimate the irradiated infra-red and solar flux, both for black and mirror OSRs via their respective values for α and ε
  4. Calculate Earth and Sun Vectors by combining the six respective vectors and apply appropriate filters.

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Thermal vacuum test setup

STI CESS Template

ACESS development has been performed in STI thermal vacuum facilities and represenative environmental conditions:

  • Heritage and advanced CESS tested simultaneously in thermal vacuum test chamber
  • (p < 10-5 mbar)
  • 2 kW light source used for sensor illumination and heatable spacecraft I/F (+60 K)
  • Thermal modelling for numerous test cases, such as irradation with variable angle of incidence

 

Flight data correlation

Simulation

  • Thermal testing allows to state a mathematical thermal model (TM) for sensors
  • Models are used to correlate CESS to satellite (GRACE) housekeeping data in order to check consistency
  • Excellent agreement of simulation and data
  •  Modeling allows to simulate and analyse the sensor performance in arbitrary orbits

 

 

Sensor Performance
  • ACESS outperforms the HCESS by approx. 50 % in peak and mean error for different orbits and sun positions
  • Using CERES data (earth albedo infrared map) with GRACE housekeeping data results in the following earth vector errors

irmap

 Erdvektor

 

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