Architecture
The 6-DOF Simulator is composed of elements which model a typical spacecraft Attitude Determination and Control (ADAC) System. The SMAD[1] Section 19, provides the best example of such a system and the 6-DOF Simulator models that represent it.
The Figure shows the SMAD ADAC system interpreted as a simulation architecture directed towards execution under GMAT. For instance, since GMAT does not have sensor or actuator hardware, models of sensors and actuators will have to be developed.
Starting from the top left of this figure, the "Control Laws" and "Attitude Determination" functions are assigned to "Flight Software" rather than the "Spacecraft Computer" shown in SMAD figure 19-2. Also the GMAT execution does not replicate real-time behavior, it is an analytical simulation, not a real-time simulation. The "Desired Attitude" is realized as a GMAT Command originating from the scripting language.
The internal interface between the "Attitude Determination" function and the "Control Laws" function is shown as an "Attitude State Vector" which is the data item that will be updated by "Attitude Determination". An additional external interface is added to "Attitude Determination" with respect to the SMAD figure, Kinematics. The actual principal axis quaternion and/or Euler angles must be computed each time the Attitude State Vector is computed to describe the incremental spacecraft model pointing geometry in the body reference frame.
The "Control Laws" function outputs "Actuator Commands to the "Spacecraft Actuator Models" function. Reference the requirements section of this Wiki for the collection of actuator models that need to be developed.
The Spacecraft Actuator Models translate Actuator Commands into "Control Torques" which are provided to the "Spacecraft Dynamics Models". The Spacecraft Actuator Models also interface with the "Internal Disturbance Models" function to trigger and configure disturbance torque computations associated with the actuators. An example of such a model would be the disturbance torques associated with the solar array positioning.
GMAT provides a very detailed force model which incorporates a hi-fidelity 3-body gravitational model, drag models, Solar Radiation Pressure models, and thrust models for a variety of different propulsion types. Thus the "External Disturbance Torques" block of the SMAD Figure 19-2 simplifies to determining the torques due to misalignment of the resultant external force vector provided by GMAT in the 6-DOF Simulator architecture.
The "Spacecraft Dynamics Model" function may be viewed as the core of the 6-DOF Simulator. The Spacecraft Dynamics Model interfaces with the GMAT script language for modeled spacecraft mass properties, including Initial Mass, Moments of Inertia, and Propellant Mass. It also contains the location of each attitude sensor and actuator in the spacecraft geometric coordinate system and can translate and rotate between the geometric coordinate system and the GMAT quasi-inertial body reference frame. The Spacecraft Dynamics Model interfaces with both the Internal Disturbance Models function and the External Disturbance Models function. It ultimately calculates attitude angles and rates which it passes to the Attitude Sensor Models.
Since GMAT does not control hardware, a number of Attitude Sensor Models must be provided, which are enumerated in the requirements section of this Wiki. Each of these sensor models must simulate both the modes and substates of the sensors and most importantly, the measurement error model associated with the sensor. A hardware Inertial Reference Unit exhibits bias, drift, and noise in its measurements, and a very important feature of the Attitude Determination flight software is the filter used for processing each sensor measurement stream. The sensors also output different measurement types, for instance a star-tracker will provide an estimate of the pointing angle of its aperture, whereas an accelerometer assembly will provide integrated angular rate.
The Attitude Sensor Model must have knowledge of the location of the spacecraft relative to the Sun, the Earth (or other primary body) and the Moon (or other natural satellites). Sun sensor models need to know the ephemeris of the sun. Earth sensors models need to know the ephemeris of the Earth. For star trackers, the epoch is important such that it can use the proper star map. GMAT provides this orbital location and epoch information, which is known as an Ephemeris.
Lastly, the SMAD Figure shows a Definitive Attitude Determination function which is not incorporated in the 6-DOF Simulation Architecture. This function is a ground processing function, which essentially becomes an analysis of the GMAT Report File output in the the context of the Simulation.
The following paragraphs provide a summary of the information provided in the Figure. Each topic will be expanded within the below sections of this Architecture specification.
- Control Laws (CL)
- Attitude Determination (AD)
- Attitude Sensor Model (ASM)
- Spacecraft Actuator Model (SAM)
- Internal Disturbance Model (IDM)
- External Disturbance Model (EDM)
- Spacecraft Dynamics Model (SDM)
- GMAT Command
- Kinematics
- Ephemeris
- External Force
- Mass Properties
- Actuator Commands
- Actuator Modes
- Control Torques
- Disturbance Torques
- Attitude Angles
- Attitude Rates
- Attitude Measurements
- Attitude State Vector