Autonomous On-Board Orbit Control

With autonomous orbit control provided by Microcosm’s Orbit Control Kit (OCK) software, the traditional process of ground-based satellite tracking and orbit maintenance is replaced by a fully autonomous satellite-based system. OCK currently uses a GPS receiver onboard the satellite, as well as control software to process the GPS measurements, and to compute and execute the orbit maintenance burns at the correct time. It can also function with other autonomous navigation sensors.

Microcosm’s revolutionary capability to control the spacecraft’s orbit autonomously is important for multiple reasons:

  1. All orbit elements are controlled — the satellite’s position is continuously controlled and known in advance, which introduces a whole new set of capabilities whenever coordination between satellites or between the satellite and the ground is required.
  2. Ground operations are simplified — planning time and the number of replanning cycles are reduced, ordinarily one of the most time-consuming parts of mission operations.
  3. Propellant usage is decreased — frequent, small burns are used to maintain the satellite position in its orbit, resulting in lower average drag.
  4. Orbit maintenance costs are reduced — maneuvers are calculated/implemented onboard.
  5. The need for traditional orbit propagation is eliminated — All other spacecraft, ground hardware, and data users can know where the satellite is at all times, without continuous communication of ephemeris updates.
  6. System risk is reduced — the complex tracking and communications chain is eliminated, thus reducing the potential for operator errors and communications failures.
  7. More time is available for coordination and planning to avert problems — RF interference and potential satellite collisions are known further in advance.
  8. The impact on the spacecraft attitude control system (ACS) system is decreased — may allow a reduction in the ACS size, weight, and cost.
  9. Interference with payload operations is eliminated — maneuvers required to achieve orbit control are so small; orbit adjustments can be performed while the payload is operational.
  10. The user is able to know satellite schedules for the life of the mission.
  11. The cost and complexity of constellation maintenance is greatly reduced — the need for “rephasing” the satellites in a constellation is avoided entirely.

The OCK algorithms are designed to provide absolute autonomous orbit control, such that each satellite’s position is controlled in inertial space. Therefore, if each spacecraft within a constellation is maintaining a known orbit, the constellation itself is fully controlled. Nearly all constellations require some type of orbit maintenance or control to prevent collisions between satellites and to maintain the constellation configuration over time. Each spacecraft in the figure below is maintained within a mathematically defined control-box moving with the constellation pattern. All in-track stationkeeping maneuvers are done firing in the direction of motion to put back energy taken out by atmospheric drag.

The current OCK includes a navigation filter that increases the reliability of the system by assuring that a continuous stream of state vectors are provided to the control algorithms. The in-track control algorithms are very robust, and the spacecraft orbit period can be maintained to within ±0.1 seconds (+ 750 m in in-track position), if GPS inputs are acquired on a regular basis. The OCK software can also incorporate more complex filtering techniques for improving on the GPS measurements.

The OCK software, which enables a spacecraft to very precisely maintain its orbit period and phase, was flown on the Surrey Satellite Technology Limited (SSTL) UoSat-12 spacecraft in 1999 (650 km altitude, 65 deg inclination, chemical propulsion) and further validated on TacSat-2 in 2007 (415 km altitude, 40 deg inclination, electric propulsion).

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