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Composite Structures | SmallSats | Mission Engineering

Software

Software developed by Microcosm includes technologies of orbit and attitude determination and control. Microcosm’s focus here has been primarily on the appropriate level of autonomy to reduce the cost of operating space systems. Microcosm develops the following orbit and attitude flight systems:

  • Autonomous on-board orbit control (demonstrated on orbit on UoSat-12 and TacSat-2)
  • Autonomous navigation
  • Autonomous orbit transfer (large maneuvers)
  • Autonomous guidance, navigation and control for rendezvous and docking (far field and close in maneuvers)
  • Attitude determination and control systems
  • Formation flying
  • Plug-and-Play Orbit and Attitude Determination and Control

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).

MicroGLOBE

MicroGLOBE is designed to aid the engineer or scientist in creating plots representing the celestial sphere and drawing figures on it. It is an invaluable tool in performing conceptual analysis for mission planning, systems engineering, spacecraft design, and sensor placement, to name just a few applications. By accurately representing the celestial sphere or the Earth’s surface, it greatly facilitates the utilization of global geometry techniques which allow the user to obtain quick, rigorous, and often conceptually elegant solutions to engineering problems with greatly enhanced physical insight.

Some measure of the versatility of MicroGLOBE can be seen in the nature of the spheres it is commonly used to represent and their typical applications:

  • Spacecraft-centered celestial sphere in inertial coordinates
    • attitude analysis for spinning spacecraft
    • eclipse analysis
    • measurement loci
    • singularity conditions
    • measurement bias observability
    • sensor coverage in elliptical orbits
    • launch window constraints
  • Spacecraft-centered celestial sphere in spacecraft fixed coordinates
    • attitude analysis for 3-axis stabilized spacecraft
    • sensor placement and coverage
    • Sun, Moon, and appendage interference conditions
    • Earth viewing conditions (as seen from spacecraft)
    • conditions for viewing other spacecraft
  • The Earth’s surface
    • Earth coverage
    • ground station coverage
    • interference potential
    • system efficiency assessments
  • Earth-centered celestial sphere
    • apparent satellite motion
    • Sun or other spacecraft interference
  • Earth-centered sphere containing the spacecraft
    • satellite constellation analysis
    • relative spacecraft motion
    • spacecraft-to-spacecraft viewing conditions

In many cases, global geometry analysis using MicroGLOBE can replace a system simulation that is 1 to 2 orders of magnitude more expensive. In addition, global geometry can provide greater insight and more flexibility than system simulations.

High Precision Orbit Propagator

The High-Precision Orbit Propagator (HPOP), developed by Microcosm, Inc. of El Segundo, CA, is a state-of-the-art orbit generator which can generate orbits for a wide variety of Earth satellites with accuracies on the order of 12 meters per orbit or better. It can handle circular, elliptical, parabolic, and hyperbolic orbits at distances ranging from the surface of the Earth to the orbit of the Moon and beyond, although orbits around the Moon itself are not currently supported.

The HPOP includes modern, ultra-high-fidelity models for all of the major perturbations affecting an Earth satellite:

  • Goddard Earth Model (GEM) 10B, an advanced 21 x 21 spherical harmonic expansion
  • Lunar-solar point-mass gravitational effects using the U.S. Naval Observatory Compressed Ephemeris to predict the positions of the Sun and Moon. This ephemeris is accurate to within one thousandth of an arc-second.
  • Atmospheric drag using the Harris-Priester atmosphere model, modified to take into account the diurnal bulge, to compute the atmospheric density. The drag model assumes single-collision specular reflection, which is appropriate for most satellites. Departures from this can be modeled by changing the area-to-mass ratio of the satellite.
  • Solar radiation pressure. This model assumes that the satellite is either a mirror sphere or a black body, which is appropriate for most satellites.

The HPOP also takes into account all of the major predictable motions of the Earth which affect the apparent position of the satellite:

  • Precession of the equinoxes
  • Nutation
  • Diurnal rotation
  • Barycentric displacement

Unpredictable Earth motions cannot be modeled at the present time, but fortunately these are small. They include polar motion, irregular variations in the Earth’s rotation rate, and continental drift. Polar motion causes the poles to wander in irregular circles in a region about 30m square, taking many years to complete each circle. Irregular variations in the Earth’s rotation rate can change the length of the day by up to 1/4 millisecond per year, but such large changes tend to cancel out over time, leaving a residual secular increase of 1.5 milliseconds per century. Continental drift occurs at rates of up to 5 centimeters per year.

The HPOP also accounts for the differences among the three major astronomical time systems:

  • Universal Time Coordinated (UTC), also known as Greenwich mean time
  • International Atomic Time (TAI)
  • Terrestrial Dynamic Time (TDT), formerly known as Ephemeris Time (ET)

For ultra-high precision, the HPOP uses the Runge-Kutta-Fehlberg method of order 7-8 to integrate the equations of motion.

The HPOP requires the following input:

  • Initial satellite position and velocity
  • Satellite area-to-mass ratio
  • Output schedule

It produces as output a set of satellite positions and velocities at equally-spaced times. Both input and output positions and velocities must be given in mean-of-J2000.0 coordinates.