Environmental Engineering Reference
In-Depth Information
A
Attitude and Orbit Determination and Control
In order to perform its science mission, a spacecraft must, in general, know its
orientation in space and its position relative to the targets it plans to observe.
The term
orbit
refers to the spacecraft's position and velocity with respect
to an inertial reference frame. The term
attitude
refers to the spacecraft's
orientation with respect to some inertial reference frame (for Earth-orbiting
spacecraft, this ordinarily is the geocentric inertial (GCI) frame).
Traditionally, the determination of a spacecraft's orbit was exclusively a
ground system function. While in communication with the spacecraft, the
ground system would collect tracking data, and then the Flight Dynamics
computational facility (in the context of NASA/Goddard missions) processed
the data to calculate the definitive (i.e., actual) spacecraft orbit associated
with the time(s) corresponding to the collected tracking data. Flight Dynamics
then would calculate a
predicted
spacecraft orbit by combining the definitive
orbit data with mathematical models describing the gravitational interactions
and orbital perturbations experienced by the spacecraft. If the spacecraft's
flight software (FSW) did not include these models, the ground system would
uplink a set of “fit parameters” (or a position/velocity-vector file) tailored
to the FSW's orbit propagator (or orbit interpolator), which the spacecraft
would then use to compute its position and velocity. If the spacecraft FSW did
include at least a subset (or simplified version) of these models, the ground
system would uplink “seed” vectors that the FSW would utilize as starting
input when integrating the spacecraft equations of motion to calculate its
position and velocity at some arbitrary time. In either case, the FSW output
was a predicted orbital position and velocity as opposed to a measured orbital
position and velocity. More recently, for low earth orbit (LEO) spacecraft,
orbital position can be directly measured to high accuracy using the onboard
global positioning system (GPS), which also can be used to synchronize the
spacecraft clock to GPS time.
By contrast, measuring spacecraft attitude has been a standard on-
board function practically since flight computers were introduced. What has
