adcs Flashcards
what are the key roles of the ADCS?
orientate the spacecraft in the required directions, stabilise it, and manage its angular momentum
what are the key components of the ADC system?
- attitude sensors to determine attitude
- control actuators to control attitude
list the different spacecraft operating modes and define them
- orbit insertion = during and after boost before spacecraft gets to final orbit
- acquisition = determine attitude and make spacecraft stable, also after emergencies
- normal, on-station operations = majority of mission, mission requirements will drive design
- slew manoeuvres = re-orientating the spacecraft if required
- contingency/safe mode = may use less power or sacrifice operation to meet constraints
- special = during special time-periods such as eclipse
define nadir pointing
earth pointing
list and define ADCS requirements
- item to be pointed = whole payload or specific item?
- pointing direction = relative to what?
- pointing range = all the possible pointing directions
- pointing accuracy = absolute angular control requirement
- pointing knowledge = either real-time or after the fact
- pointing stability = maximum rate of change
- slew rate = time taken to re-orientate from one direction to another
- exclusion zones = e.g. not within 10º of the sun
- sun pointing = for power generation or thermal control
- pointing during thrusting = may need guidance corrections
- communications = antenna pointing towards ground station or relay
define the angles used to define spacecraft attitude
- yaw = about spacecraft Z-axis (nadir pointing)
- pitch = about spacecraft Y-axis
- roll = about spacecraft X-axis (usually pointing in direction of velocity vector)
define cyclic torques
varying over one orbit
define secular torques
building up over time
true or false: sum of all the torques on a spacecraft can exhibit both cyclic and secular behaviour
true
define aerodynamic torques
in LEO, there is sufficient residual atmosphere present to exert a force on the spacecraft due to the momentum exchange and energy accomodation between the residual gas atoms impacting spacecraft surfaces
why do aerodynamic torques arise?
arise when the spacecraft aerodynamic centre is offset from the spacecraft centre of mass
when are aerodynamic torques constant?
constant if nadir pointing
why do solar radiation pressure torques arise?
momentum exchange between photons and spacecraft surfaces
when are solar radiation pressure torques constant?
if spacecraft pointing towards the sun
when are solar radiation pressure torques cyclic?
if the spacecraft is nadir pointing
why do magnetic field torques arise?
due to the interaction between the weak magnetic field generated by the electric currents and residual magnetisation of the spacecraft and the local magnetic field of the Earth
fill in the missing words: local magnetic flux varies with _ and _ and _
altitude, latitude and orbit inclination
are magnetic field torques cyclic or secular?
cyclic
why do gravity torques arise?
difference in gravitational force acting on different parts of a spacecraft due to different moments of inertia
at which angle between the Z-axis and the vertical is the gravity torque maximum?
45º
are gravity torques constant or cyclic?
- constant if nadir pointing
- cyclic if spacecraft inertially fixed
define gyroscopic rigidity
as angular momentum becomes large, the effect of a torque impulse reduces
how do spin stabilised spacecraft overcome disturbance torques?
- use the effect of gyroscopic rigidity to achieve stability by giving the spacecraft a momentum bias
- the direction of the momentum bias (spin axis) should be in an axis that will not need to change during the mission (axis normal to the orbit)
- the angular momentum axis is not necessarily the same as the spacecraft orbit axis
define the principal axis
preferred axis, about which the spin of the axis is the most simple
what is a 3-axis stabilised spacecraft?
a spacecraft that can manoeuvre and keep stable in 3-axes
what do spin stabilised and 3-axis stabilised spacecraft both need?
external torquers to dump momentum built up on reaction or momentum wheels
how do 3-axis stabilised spacecraft overcome disturbance torques?
- momentum bias through momentum wheels on a pitch axis (similar to spinner)
- zero momentum that has a reaction wheel on each axis to respond to disturbances
what can happen when a spacecraft separates from its launch vehicle, and how can this be counteracted?
- spacecraft separates from launch vehicle resulting in angular velocity called ‘tip-off rate’
- ADCS must stabilise the spacecraft with moment of inertia before the attitude reaches a certain amount
what can be said about the torque requirement involving an attitude slew manoeuvre?
- a driven attitude slew manoeuvre resulting in a change in attitude in a specified time
- the accelerating torque is usually applied for half the duration
- decelerating torque applied for remainder
describe the main modes for spin-stabilised spacecraft
- pure rotation = where the rotation axis, principal axis, spacecraft geometry axis, and the angular momentum vector are all parallel
- coning = only the spacecraft axis is not parallel with the others, the physical motion is the same as in pure rotation however, the z axis rotates
- nutation = angular momentum vector, which is fixed in space, is not aligned with the rotation axis or principal axis, therefore rotation occurs about the momentum axis, this movement must be damped
define ‘attitude determination references’
known reference points that can help determine the attitude of a spacecraft
list the different attitude determination references
- sun
- earth (or other central body)
- magnetic field
- stars (including distant planets)
provide the advantages and disadvantages of the sun as an attitude determination reference
+
* bright, low power and weight
* usually must be known for solar cells & to protect sensitive equipment
-.
* may be eclipsed during part of orbit
* angular diameter limits accuracy to ~ 1 arc minute (from earth)
provide the advantages and disadvantages of the earth or another central body as an attitude determination reference
+
* always available if s/c nearby
* bright, mostly unambiguous
* needed for many payloads, easy analysis
-.
* requires scan motion for horizon
* must protect sensors from sun
* horizon definition limits to ~ 0.1º
provide the advantages and disadvantages of magnetic field as an attitude determination reference
+
* cheap
* low power
* always there for low altitude s/c
-.
* poor resolution (~ 0.5º), only near earth
* limited by modelling accuracy
* spacecraft must be magnetically clean
provide the advantages and disadvantages of stars as an attitude determination reference
+
* high accuracy ~ 0.001º
* available anywhere
* mostly orbit independent
-.
* heavy, complex, expensive
* id-ing stars is time consuming
* need to protect from the sun
* multiple stars cause problems
describe attitude determination systems for spinner spacecraft
- fan shaped sun sensors = used to detect sun angle
- earth horizon telescopes = used to detect earth horizon
- this results in two intersecting cones, one around sun vector and one around earth vector
- this gives two possible pointing directions, solved using prior knowledge of attitude
describe attitude determination systems for 3-axis stabilised spacecraft
- uses two perpendicular sun sensors, as the spacecraft can rotate about the sun vector
- system needs an additional reference vector such as earth or stars
- suitable source chosen depending on mission pointing accuracy requirements
list and define the two types of attitude hardware
- sensors = determine attitude
- actuators = provide torques that can control attitude
describe how sun sensors work
- angular radius of the sun is nearly orbit independent and small enough to be considered as a point source
- brightness permits simple, reliable and low power equipment
- many missions have solar experiments therefore the sun vector is of interest anyway
list the different types of sun sensors and how they work
- analogue sensors = output signal that is a continuous function of the sun angle
- sun presence sensors = constant output whenever the sun is in the sensor FOV
- digital sensors = encoded, discrete output as a function of the sun angle
describe how earth horizon sensors work
- used in earth orbiting spacecraft where the earth can cover large areas of view, up to 40% at an altitude of 500km
- sensors detect earth horizon, either in visible or IR spectrum, however can be poorly defined due to atmospheric aberration
- need scanning mechanism as they have small FOV
list the different types of Earth horizon sensors, and how they work
- photo-diode = near IR
- bolometer = type of thermistor which changes resistance to perform sun rejection
- thermopile = string of thermocouple junctions in series, simple but slow, cannot be used in scanning systems
- pyroelectric sensors = thin crystal slab, between electrodes, radiation increase crystal temp, char polarization measured across electrodes
list the types of star sensors and how they work
- star scanners = used on spinning spacecraft, star passes into FOV of slits, after several passes, system determines star vector
- star trackers/mappers = 3-axis spacecraft, selects, locates, and tracks one or more stars to get 2-axis or 3-axis attitude information
- systems highly accurate
- CCD sensors can be blinded by sun, moon or planets entering FOV
describe how gyroscopes work
- can be used to measure speed or angle of rotation from initial reference
- often grouped to provide 3-axis information
- cannot be used alone for attitude determination
describe reaction and momentum wheels
type of internal torquer that translates momentum from or to a wheel rotating about a fixed axis
define attitude control
controlling attitude involves exerting external torque on spacecraft or transferring momentum using internal torquers
describe reaction wheels
- spin rate = 0
- spin in either direction to give single axis of control
- spins with constant rate about axis
describe momentum wheels
- non zero spin rate to give nearly constant momentum
- provides gyroscopic stiffness in other two axes
- can spin up or down by few %
describe control moment gyros
rotor spins about axis with constant rate and are on commanded gimbal to enable changes in direction of angular momentum vector
how should wheel systems be sized?
based off angular momentum capacity, must also be able to store the cyclically built momentum without frequent momentum dumping
how do magnetorquers work?
- generate magnetic field which interacts with earth’s magnetic field to produce external couple on spacecraft
- rod-like magnets mounted in three perpendicular directions, can be used for pointing or offloading momentum
- use of magnetorquers limited to LEO or low MEO
- cannot produce torque about local field direction, fine for polar orbits, for equitorial or low inclination they cannot offset North-South torques
- magnetorquers can only rotate perpendicular to the planetary field lines it is immersed in
define gravity gradient torques in attitude control
- passive technique that uses spacecraft interial properties to keep it pointed towards earth
- uses the fact that an elongated object is aligned by gravity, therefore longitudinal axis points towards earths centre
for gravity gradient torques in attitude control, how will a spacecraft align depending on axis and moment of inertia?
- axis with max moment of inertia = align normal to orbit plane
- axis with min moment of intertia = align with local vertical
what can be an issue of using gravity gradients?
can result in pendulum like motion called libration which must be damped
describe thrusters and how they work
- used for re-pointing or momentum dump
- can be mounted in clusters on spacecraft surfaces, pointing in different directions
- magnitude often unalterable, therefore controlled using pulse duration
- on-off control can lead to limit cycle with short period, can result in excessive propellant usage
how should thrusters be sized
sizing done to counter act disturbance torques, meet slew requirements, or dump stored momentum
when does the maximum disturbance occur for a gravity gradient torque?
after about a 1/4 of the orbital period
