The MicroLab-1 spacecraft maximizes solar array illumination by a combination of yaw steering and solar array pitch rotation in order to maintain the solar panels fully exposed to the sun at all times except eclipse. This process interacts with the performance of the GPS-MET payload, which prefers a constant forward-facing attitude as much of the time as possible. [Note: the GPS-MET antenna itself prefers to be facing towards the rear, which entails a forward-facing attitude on the part of the satellite, since it is on the "back" of the satellite -- CB] This attitude both minimizes motion of the GPS-MET antenna relative to the satellite's center-of-gravity motion, and keeps the GPS-MET antenna directly facing the antivelocity direction for optimal reception of GPS satellite set events.
This document is intended to explain how the MicroLab-1 spacecraft performs yaw steering so that its impact on the GPS-MET payload may be best understood. It also lays the groundwork for possible changes to on-orbit operations that may improve GPS-MET performance with minimal impact to the rest of the system and to the OTD payload.
In optimal yaw steering mode, the spacecraft is maintained with its +X (nominal velocity direction) face directed toward the sun at all times, as the spacecraft orbits in its upright attitude. This does not mean that the +X axis points directly toward the sun, only that the sun is kept "in front of" the spacecraft. The solar arrays then rotate so that their faces are kept perpendicular to the sun line at all times. This means that, during the illuminated portion of each orbit, the spacecraft goes from a roughly forward-facing attitude to a rearward-facing attitude. During the eclipse period, the spacecraft continues to track the sun, returning to a forward-facing attitude before returning to sunlight. At all times, the solar arrays are rotated on their drive system in order to accommodate the instantaneous sensed (not commanded) attitude.
This steering mode is called "optimal" because it ideally maximizes solar array output. Assuming perfect attitude control, optimal yaw steering will achieve the maximum possible array illumination at all times. In practice, errors in yaw steering reduce array illumination and therefore array power output.
The amount of steering the spacecraft must do in optimal yaw steering mode depends on the current beta angle of the spacecraft's orbit, that is on the sun's angular proximity to the spacecraft's orbit. For high beta angles (when the sun is near the orbit axis and far from the orbit plane), the spacecraft steers little, to the limit at a 90 degree beta angle when the spacecraft need not steer at all (this corresponds to an orbit lying about the Earth's terminator). For low beta angles, steering becomes more pronounced, to the limit at zero beta angle when the spacecraft must rotate 180 degrees in the instant as the sun passes overhead from "in front of" the spacecraft to "behind" the spacecraft. This same instantaneous rotation would be called for again a half orbit later. The orbit beta angle changes constantly during the mission.
Because extreme yaw steering steering is infeasible, for low beta angles the spacecraft uses "inverse" yaw steering. This means that the spacecraft maintains its +X axis pointing roughly in the velocity direction at all times, while working also to track the sun. This means that the sun passes to the "right" or "left" of the spacecraft once during each illuminated orbit portion. The spacecraft cannot track this motion due to the constraint that it keep facing forward. As the sun passes overhead, therefore, the spacecraft rotates in the opposite direction to again 'pick it up", simultaneously rotating the solar arrays from a forward to a rearward-looking attitude as they continue to track the sun. An analogous distinction is between watching a low-flying aircraft from directly below its flight path, leaning further and further backward as the aircraft goes over (inverse yaw steering), versus watching it from one side and turning in place to see it go by (optimal yaw steering).
For beta angles between about 10 and 30 degrees, inverse yaw steering permits good sun tracking at all times except when the sun is near the zenith. For smaller beta angles, inverse yaw steering is nearly ideal since the sun passes almost directly overhead; solar panel rotation suffices almost entirely to track the sun. Optimal yaw steering is used for beta angles greater then 30 degrees. This transition angle was chosen on the basis of exhaustive power performance tests, taking into account all factors (including attitude control accuracy) that affect power performance.
It is expected that detailed on-orbit mission experience and analysis will reveal whether power will permit commanding yaw maneuvers to improve GPS-MET sensor performance.