Attitude Determination and
Attitude Control
• Placing the
telescope in orbit
is not the end of
the story.
• It is necessary to
point the telescope
towards the
selected targets,
or to scan the
selected sky area
with the telescope.
• So the satellite
attitude needs to
be measured and
controlled.
Attitude Control
• Is control of the angular position and rotation
of the spacecraft, either relative to the object
that it is orbiting (Earth, Moon ..), or relative to
the celestial sphere.
• The attitude control system (ACS) is composed
of : Attitude Sensors, Controller, Actuators.
Desired
Attitude
Estimated
Attitude
determines
the desired
torques, and
send electrical
commands
applies the
desired
torques
take the
measurements
Estimates the
current attitude
D e s c r i p t i o n:
ACS: Gravity Gradient
Actual
Attitude
The entire spacecraft spins, which provides inertial
ACS: Single Spin
orientation. The spinning provides gyroscopic
D e s c r i p t i o n:
stiffness, and stability.
U s e s t h e c h a n g e i n g r a v i t y w i t h a l t i t u d e t o c r e a t e a t o r q u e w h e n the principal axes are not
The nutation damper damps out the nutation, but
aligned with the orbital reference frame. Long booms are usually extended to create the
precession still exists. Spin is about the axis of
torque. The gravity gradient provides the restoring or stabilizin g t o r q u e b u t d o e s n o t d a m p
maximum moment of inertia, that is usually, but
the librational motion. Viscous dampers are added to dissipate t h e e n e r g y a n d d a m p o u t t h e
not always, the orbit normal.
librations. The gravity gradient torque can be a control torque but it can also be a disturbance
Solar arrays are fixed to the body.
torque if there are products of inertia. For example, LANDSAT had large products of inertia
A dvant age s :
and the gravity gradient was the dominant disturbance torque.
Simple, reliable, long lifetime.
Disadvantages:
Advantages:
Sensor collection is limited to scanning obtained
Simple, reliable, cheap, long lifetime.
by spin motion. High angular momentum results in
poor maneuverability, i.e., reorientation of spin
Disadvantages:
axis. Poor power efficiency, only half the solar
Poor pointing accuracy (5 deg), poor yaw control, thermal bendin g o f b o o m c a u s e s
cells are illuminated at any one time. Must spin
oscillations, tendency to flip upside down.
about the axis of maximum moment of inertia.
(from D. Mortari)
(from D. Mortari)
Spinning Satellites Examples
• Spinning is used in
satellites for astronomy
when sky scanning is
required.
• COBE, WMAP, Planck all
spin at 1-10 rpm, so the
telescopes (antennas) can
scan the sky at a rate of
deg/s.
• Complex scan patterns can
be obtained by combining
satellite spin and slow reorientation manouvers (see
COBE, WMAP, Planck)
Flat spin: in the absence of perturbations
the spin axis orientation remains constant
with respect to the inertial frame of distant
stars – Not useful for astronomy.
Using the ACS and perturbations, the spin
axis orientation can be continuously
reoriented away from the earth center.
This is useful for full-sky surveys (COBE)
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Dual-Spin Satellite (2/6)
A vehicle consisting of two primary components free to perform relative rotations
about a common bearing axis is called a "dual - s p i n " v e h i c l e , a n d t h e i d e a o f
stabilizing such a vehicle by spinning one part (the "rotor") wh i l e t h e o t h e r p a r t
( t h e " d e s p u n p l a t f o r m " ) r o t a t e s m o r e s l o w l y - or not at all - is called the dual -spin
attitude stabilization concept.
ACS: Dual Spin
D e s c r i p t i o n:
The limitations of single spin satellites are partially overcome with dual spin satellites.
With a dual spin satellite the payload, e.g. the antenna, is desp u n w h i l e t h e o t h e r p o r t i o n o f
spacecraft spins to provide gyroscopic stability. Nutation is dam p e d b y n u t a t i o n d a m p e r ,
located on either the spun or despun portion. Many of the geosynchronous communication
This dual -spin configuration thus serves to create a spin stabilized satellite.
satellites have been dual spin.
A dvant age s :
A
C
A′
C′
ω3
A
A′
D e s p u n p a y l o a d a l l o w s E a r t h p o i n t i n g p a y l o a d . C a n h a v e s p i n a b o ut m i n i m u m m o m e n t o f
inertia axis. Reliable, long lifetime.
ω3 +ωS
Disadvantages:
Poor power efficiency because solar cells are on spinning portio n. High angular momentum
results in poor maneuverability, i.e., reorientation of spin axis. Sensitive to mass
Rotor
imbalances.
Platform
(from D. Mortari)
ACS: Momentum Bias
D e s c r i p t i o n: T h e m o m e n t u m b i a s s a t e l l i t e i s a v a r i a t i o n o f t h e d u a l s p i n c o n c e p t t h a t o v e r c o m e s
s o m e o f t h e p r o b l e m s o f t h e d u a l s p i n . W i t h t h e m o m e n t u m b i a s s p acecraft the gyroscopic stability
is provided by a rapidly spinning momentum wheel r a t h e r t h a n t h e b u s . T h e p a y l o a d a n d b u s a r e
despun or rotating at orbital rate. The momentum wheel provides control about the spin (pitch) axis
a n d s o m e c o m b i n a t i o n o f n u t a t i o n d a m p e r , m a g n e t i c t o r q u i n g , o r p ropellant provide control about
the yaw and roll axes. Use of propellant is avoided whenever possible. Magnetic or propellant are
used for momentum dumping.
A d v a n t a g e s: B e t t e r p o i n t i n g a c c u r a c y , b e t t e r p o w e r e f f i c i e n c y d u e t o d e s p u n solar arrays. Still
r e l a t i v e l y c h e a p a n d n o t c o m p l e x w h i c h m e a n s l e s s w e i g h t t h a n t hr e e a x i s s y s t e m s . W h e n t h e
pointing requirements are not stringent, i.e., > 0.5 deg, the mo m e n t u m b i a s A C S i s u s u a l l y t h e
desired approach. The momentum bias approach with magnetic torquers is the dominant type of
ACS in LEO.
Disadvantages: U s u a l l y p o o r y a w c o n t r o l . G y r o s c o p i c s t i f f n e s s r e s u l t s i n p o or maneuverability.
Poorer reliability and lifetime. Due to its lower cost and compl exity and relatively accurate
pointing capability this has been the most popular type of ACS.
(from D. Mortari)
• Typically a spacecraft will have several momentum
wheels oriented along orthogonal axes.
• To change its rotation along those axes it will
increase or decrease the spin of the momentum
wheels in the opposite direction. When the
spacecraft achieves its desired orientation, it can
then halt its rotation by braking the momentum
wheels by the same amount.
• Momentum wheels are usually spun by electric
motors. Both spin-up and braking are controlled
electronically by computer controls.
• The strength of the materials of a momentum wheel
establishes a speed at which the wheel would come
apart, and therefore how much angular momentum
it can store.
• Since the momentum wheel is a small fraction of
the spacecraft's total inertia, easily-measurable
changes in its speed provide very precise changes
in angle.
Reaction
(momentum)
wheels
2
ACS: Three Axis
Example:
D e s c r i p t i o n:
• In the CNES ACS:
• Three magnetic-bearing
reaction wheels apply a torque
on the satellite to rotate it about
one of its three axes.
• Two magnetic torquers interact
with the Earth's magnetic field
to generate torques and thus
control the speed of rotation of
the reaction wheels.
• Thrusters complete the system.
All three axes are independently controlled by mass expulsion, r eaction wheels (RWs) or
c o n t r o l m o m e n t g y r o s ( C M G s ) . U s u a l l y R W s o r C M G s a r e u s e d f o r c ontrol and propellant
(sometimes magnetic torquing) is used for momentum dumping.
A dvant age s :
Pointing accuracy, maneuverability and adaptability to changing m i s s i o n r e q u i r e m e n t s .
Disadvantages:
Cost, weight, complexity and lifetime.
(from D. Mortari)
Attitude Control Methods and their Capabilities
ACS methods:
Passive
Semi - p a s s i v e
Gravity gradient
Active
Momentum bias with
Propellant
magnetics
Spinner with nutation
Reaction wheels with
Reaction wheels with
damper
magnetics for momentum
propellant for momentum
dumping
dumping
Dual spinner with nutation
CMGs with magnetics for
CMGs with propellant for
damper
momentum dumping
momentum dumping
(from D. Mortari)
Sensors
Actuators
Actuators are usually broken into four classes: mass expulsion, m o m e n t u m e x c h a n g e ,
e n v i r o n m e n t a l a n d d i s s i p a t i v e . A n A C S m a y h a v e a c t u a t o r s f r o m a n y or all the classes.
Mass Expulsion
The types of sensors used for attitude determination are:
1.
horizon sensors (or conical Earth scanners),
Momentum Exc.
Environmental
Dissipative
2.
sun sensors,
Reaction wheel
Gravity gradient
Nutation damper
3.
star sensors,
Momentum wheel
Magnetic
GG viscous damper
4.
magnetometers,
CMG
Aerodynamic
5.
inertial reference units (IMU or attitude reference units ARU), and
6.
GPS receivers.
There are two types of magnetic torquers. Those used for momentum dumping or control in momentum
bias systems, and the eddy current dampers used in gravity gradie n t s y s t e m s .
Range of torques available from some of these actuators (From Ch o b o t o v )
Actuator Type
Reaction Control (RCS)
T o r q u e R a n g e ( N- m )
1 0-2
- 10
Magnetic Torquer
1 0-2
Gravity Gradient
1 0-6 - 1 0-3
Aerodynamic
1 0-5
Reaction Wheel
Control Moment Gyro
- 1 0-1
-
1 0-3
1 0-1 - 1
1 0-2
-
From this table we see that RWs and
H o r i z o n s e n s o r s m e a s u r e p i t c h a n d r o l l t o a n a c c u r a c y o f a b o u t 0. 1 - 0.5 deg. An accuracy less than 0.05
d e g c a n b e a c h i e v e d b y e x t e n s i v e c a l i b r a t i o n a n d a c c o u n t i n g f o r the equatorial bulge. Sun and star
sensors provide directions. An horizon sensor does not provide ya w i n f o r m a t i o n ( f o r m o m e n t u m b i a s
CMGs are used when precision
systems it is not necessary to measure yaw). One wide- F O V o r t w o n a r r o w- F O V s t a r s e n s o r s a r e n e e d e d
pointing and/or high torque is required.
to provide attitude. Since the star sensors cannot provide contin u o u s a t t i t u d e m e a s u r e m e n t s a n
Satellites that perform rapid attitude
I M U / A R U i s n e e d e d t o p r o v i d e t h e a t t i t u d e b e t w e e n m e a s u r e m e n t s . IMUs suffer from drift and biases
maneuvers or have articulating
and need frequent updates which are provided by the star sensors. M a g n e t o m e t e r s m e a s u r e E a r t h ' s
payloads which create large
disturbance torques require CMGs
magnetic. Accuracies no better than 1 deg can be obtained. The GPS receivers are used in an
i n t e r f e r o m e t e r m o d e t o d e t e r m i n e a t t i t u d e . A c c u r a c i e s a s g o o d a s 0.01 deg are expected using GPS.
1 03
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Presence Sun Sensors
Sun Sensors
Most c o m m o n b e c a u s e t h e y are light weight , not expensive , limited p o w e r required a n d
b e c a u s e t h e y p r o v i d e a n a c c u r a c y w h i c h i s a c c e p t a b l e f o r m o s t o f t h e m i s s i o n s.
Photocell
Sun rays
Sun is t h e principal source o f e n e r g y ( exemption: interplanetary spacecraft ). S o l a r f l u x g o e s
2
as 1/ r .
Shadow
Sun ( i n t h e I R F ) is evaluated by using interpolating functions ( armonic series expansion )
rod
which least square fit t h e Sun positions recorded i n t h e s t a r a t l a s.
P r e s e n c e s u n s e n s o r s p r o v i d e t h e sun
Shadow area
crossing t i m e o n l y, o r t h e s u n p r e s e n c e
within the sensor F O V
Entrance slit
T h e y a r e u s e d t o s y n c h r o n i z e c o m m a n d w i t h s p i n p e r i o d, a n d t h e y a r e used to protect s t a r
s e n s o r s.
Used to synchronize pulsed command
(spin -u p , s p i n- d o w n ) to maneuvre , a n d
A t 1 A U t h e sun may be considered as a point ( 0 . 2 6 7 deg .).
to t u r n o n / o f f o n b o a r d e x p e r i m e n t s a n d
instrumentation .
For G E O t h e r e i s t h e parallax error (0.03 d e g a s m a x v a l u e s).
Photocell a t t h e b a s e
Grid slit
(b e l o w t h e slit )
Presence Sun Sensors
B a s e d o n t h e Snell refraction law.
n sin ϑ = sin ϑ'
Aperture
n sin γ = 1
Lens
Image plane
Photocells
mask
Sun image
Photo
Photocell 1
cell 2
Mirror
Analogic Sun Sensors
Reference
axis
Normal
Sun rays
Normal
Sensor # 1
Photocell
Sensor 1
T
I ( ϑ) = I (0) d n = I (0)cos ϑ
Normal
Output current
Sensor 2
Normal
Sensor # 2
Mask
Photocells
Sun rays
Photocell #2
Photocell #1
Angle (d e g)
4
Measurement
Digital
Sun Sensors
Command
C o m m a n d: s u n p r e s e n c e
M e a s u r e m e n t (4 parts ):
1)
ATA,
2)
Sign bit,
3)
C o d e bits ,
4)
Interpolating bits.
Measurement
Sun angle
Command
Grid
Digital
Sun
sensor
slits
Photocells
Binary c o d e
Binary and Gray code
Gray code
Double Digital Sun Sensor
T h e g a i n o f t h e G r a y c o d e consists o f t h e fact that the bit s t r i n g, which
represents t h e a n g l e measure , c h a n g e s one bit only at each digital step .
Decimal
Binary
0
Gray
0
Decimal
Binary
Gray
0
11
1011
1110
1
1
1
12
1100
1010
2
10
11
13
1101
1011
3
11
10
14
1110
4
100
110
15
1111
1000
5
101
111
16
10000
11000
6
110
101
17
10001
11001
1001
7
111
100
18
10010
11011
8
1000
1100
19
10011
11010
9
1001
1101
20
10100
11110
10
1010
1111
21
10101
11111
Photocells
5
Earth
horizon
sensor
Best range: 14µ-16µ of CO 2
(less high altitude clouds contr.)
Relative radiance
400 (IR) and 30,000 (visible)
Hot Moon
Earth horizon sensor
Measure two crossing times t1 and t 2
Moon horizon
sensor
Two measured angles, ϕ1 and ϕ2
allow to evaluate the Nadir direction.
Relative radiance
Hard horizon
With angular speed ω, the measured angle is ϕ = ω(t2 − t1 )
Moon t e m p e r a t u r e r a n g e:
–240 d e g t o +30 deg
H o t a r e a for cold Moon
H o t a r e a for hot Moon
Cold Moon
Moon center d i s t a n c e
Cold horizon position
Earth/Sun sensors for spinning S/C
H o t horizon position
Star sensor for spinning S/C
Meridian slit
Displacement angle (deg)
Inclined slit
Optical axis
Satellite
equator
Elevation slit
Azimuth slit
6
The Archeops Star Sensor
Star Sensor
• Archeops was a telescope on a spinning balloon
payload, designed to map a significant fraction of
the CMB sky with a microwave telescope.
7
Jupiter scans
Focal Plane
Archeops – Kiruna 2002
Star
Trackers
Photomultiplier
Electronics
O p t i c a l system
Star
Grid
Baffle
8
dΦ B
dt
V = ∫ E dl =
Magnetometers
Typical GN&C Sensors (Wertz)
T h e y m a y b e u s e d at a l t i t u d e s l e s s t h a n 1 , 0 0 0 K m , o n l y .
( Earth magnetic field decreases with radius as 1/ r3 )
Small precision is associated with t h e fact
that t h e E M F Models are not so accurate.
Biases not negligeable
Sensors
(residuals o f magnetic fields
caused by onboard circuits )
Electronic unit
Signal processing
Telemetry
and
Link
Analog to digital
converter
Attitude Determination Systems
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