This report summarizes the current progress of the Attitude Determination and Control (ADC) Division of the Acme Aerospace Company for February 11-17, 1998. This report will describe the different options of ADC, as well as make preliminary suggestions as to which are more applicable to the Odysseus missions.

The attitude of a spacecraft, or its orientation, is determined by position, velocity, attitude and attitude motion. The initial two parameters are translational parameters, and are not dealt with under the topic of Attitude Determination and Control. The latter two are rotational parameters which must be determined and controlled by the ADC instrumentation. Instrumentation for ADC usually involves major subsystems of the vehicle. They are often massive, power consuming and have stringent orientation demands.

A spacecraft is in a relatively low torque environment, however the absence of other disturbances, slight torques from different sources can greatly affect the vehicles attitude. The main sources of torque are :

• Aerodynamic torque : arises in vehicles in low earth orbit from the satellite drag, producing a torque due to the offset of the center of gravity and the aerodynamic center of pressure. The magnitude of this effect varies widely in the upper atmosphere as the density can vary greatly from standard models. Left uncorrected the atmospheric drag grows quadratically with time. Fortunately, this drag is only a factor at altitudes less than 500 km.
• Magnetic field of the Earth (or other large body) : arises in low orbits around planets with substantial magnetic fields such as the Earth and Jupiter. It is not yet certain whether Mars has a strong enough magnetic field to exert this torque on the orbiting craft. The torque arises from the interaction of the magnetic fields of the spacecraft and the planet. It is a factor at altitudes between 500 and 35,000 km.
• Gravity Gradient : arises from the small difference in the gravitational force between its "lower" and "upper" sides. This causes the body to rotate until its longest axis, the axis with the minimum inertia, points toward the planet. This torque is also a factor between 550- 35,00 km.
• Solar Radiation : arises from both electro-magnetic radiation and particles radiating from the Sun. This is the only torque which is present during interplanetary flight. It is negligible when in close proximity to a planet or large body.

Since torques exist throughout the entire environment of the mission, some method of attitude control is required. This can come in several forms.

• The spacecrafts own angular momentum
• In this situation, the vehicle is "spin stabilized" by spinning the spacecraft. In inertial space, the angular momentum of a spinning object will remain fixed for extended periods of time with little or no external torques. If disturbed, a spinning body will return to its stable position.
• This method is very simple and reliable. Long-term stability, such as is needed for an interplanetary mission, requires pure spin around the satellites major axis, that with the most inertia. Therefore, for spin stabilized spacecraft, a wheel shape is desired over a pencil shape.
• A problem with spin stabilization is that it is difficult to keep tele-communications and other directional instrumentation pointed in a specific direction (i.e. at the Earth.)
• Spin stabilization may be a good choice for the Odysseus I, providing that the instrumentation aboard is compatible with a rotating vehicle.
• The spacecrafts response to environmental torques
• In this case, the body aligns itself in a stable manner according to the environmental torque. For example, in the presence of gravity-gradient torque, if the minor axis is pointed toward the planet.
• These environmental torques (except solar radiation) are only present near planets and might be useful in orbits around Earth and Mars.
• Active control (for spacecraft which are "Three-Axes Stabilized," having all axis fixed in inertial space)
• Reaction Wheels : consist of a flywheel rigidly mounted to the vehicle. Using a motor to rotate the wheel will generate an opposite rotation in the spacecraft in order to conserve angular momentum. Since the inertia of the spacecraft is much greater than that of the wheel, the wheel will spin much faster than the vehicle. This make the wheel able to react to very sensitive variations in the vehicles attitude. Three such wheels are required in order to control all the possible directions of torque, with at least one more provided for redundancy.
• Reaction wheels are weighty, expensive and have fast moving parts subject to wear and malfunctions.
• They respond quickly relative to other systems.
• The reaction wheel can only store, not remove momentum. Therefore, in order to compensate for a continual torque, the wheel must spin faster and faster. When the wheel reaches the fastest speed of the motor, it becomes saturated. At this point, some secondary system, such as jets, must be utilized to hold the vehicle steady while the wheels wind down, called "momentum dumping."
• Control Moment Gyros : use another configuration of reaction wheels. These are mounted with gimbals and is constantly spinning. Adjusting the direction of the gyro will alter the direction of the spacecraft.
• Gyros are very heavy, but provide greater accuracy than flywheels.
• Gyros also involve momentum dumping.
• Each can control the motion around 2 perpendicular axes, so three gyros can exert complete redundancy over all three axes.
• The greater power of each gyroscope makes them a good possibility for use in the Odysseus II. Reaction time is on the order of that of reaction jets.
• Reaction Jets : common and effective ways of inducing a torque on the vehicle. Two opposing jets, centered around the CG can create a torque with out creating a translational disturbance.
• Standard on manned spacecraft because they can quickly exert large forces.
• They use consumable fuel and need redundant systems which become heavy and complex.
• Can perturb orbit if the two opposing jets are not precisely centered around the CG.
• Will be necessary for momentum dumping if reaction wheels or gyroscopic control is used in the Odysseus II, although they probably use too much fuel to be the primary source of attitude control on an interplanetary mission.

In order to detect the deviation of the attitude from the expected value, measurement of the divergence must be made. There are several different ways of doing so.

• Spin Stabilized Spacecraft
• Sun Sensors : uses a narrow slit and a photosensitive sensor to measure the angle between the sun and the spin axis of the vehicle, the Sun angle, b .
• Earth horizon telescope : has a narrow field of view which sweeps out a cone, overlapping the horizon of the earth. Sensing the reflected light from the planet, the sensor produces a pulse. Another pulse is generated when the telescope crosses back off the earth and into dark space. The time between these pulses, the spin frequency of the spacecraft and the known size of the earth allow the attitude of the vehicle to be calculated.
• This is only available in orbit around the Earth.
• Relies on the rotation of the vehicle.
• Three-Axis Stabilized Spacecraft
• Two Axis Sun Sensor : use two sun sensors to locate the direction of the sun. Needs another reference point to fix the rotation of the vehicle around the suns vector. The other vector can be a measurement of the magnetic field of the Earth. This is only available in Earth orbits.
• Star Sensors : Very accurately measure the direction of stars with respect to body frame of reference.
• Independent of orbit.
• Complex and heavy. Usually use different attitude determination method for an initial estimate and only refine measurements.
• Gyroscopes : measure incremental changes in attitude, but require another system (i.e. star sensors) to provide absolute measurements.

Therefore, the initial research points to a more wheel-like, spin stabilized structure for the Odysseus I. Due to the further complications of the manned nature of the Odysseus II, the structure will probably be three-axis stabilized with gyroscopes and reaction jets for attitude control. Attitude determination will likely be a combination of gyroscopic response and star sensors or sun sensors.

The next step in design of the missions ADC is to further refine the instrumentation which is to be used for both Odysseus I and II. After a flight trajectory is mapped out, including the parking orbit altitudes at both Earth and Mars, the influence of different environmental torques will be calculated. Also, the refinement of the type of attitude determination devices to