This ppt was made as a part of Video Assignment activity for 18AS741 in 7th sem, 2022-23 (BTech, Aerospace, Jain University) by Chaitanya Shukla (19BTRAS051). This is not the best formatted or structured ppt. Should be used for minimalistic applications.

1. Assignment - Video Recording -
18AS741
Shukla Chaitanya Vilas
19BTRAS051
SENSORS
For Attitude Determination in Satellites

2. Attitude Control Electronics
ATTITUDE
SENSORS
Processing
Electronics
Actuator
Drive
electronics
Attitude
actuators
Schematic of Attitude Control System
Power
conditioning &
switching unit
Propulsion
module
interface
To
Propulsion
module
Command
telemetry
power
interface

3. SENSORS
1. Sun Sensor
1. Digital Solar Aspect Detectors
(DSADs)
2. Earth Sensor
1. Earth Horizon sensor
2. Conical Earth sensor
3. Horizon crossing indicators
3. Star sensor Tracker
4. Gyroscope
5. Magnetometer
For Attitude Determination

4. 1. Sun sensor
• A sun sensor is a device that senses the direction to the Sun.
This can be as simple as some solar cells and shades, or as
complex as a steerable telescope, depending on mission
requirements.
• Sun sensors can be of different types like sun presence
sensor, analogue sun sensor and digital sun sensor( output
of DSS can be used directly by attitude determining
electronics)
• Sun sensors have been developed with FOV ranging from
several square arc minutes to 1280x1280 and resolutions of
less than an arc second to several degrees.

5. 1.1 Digital Solar Aspect Detectors
Also known as DSADs, these
devices are
purely digital Sun sensors.
They determine the angles
of the Sun by determining
which of the light-sensitive
cells in the sensor is the
most strongly illuminated.
By knowing the intensity
of light striking
neighboring pixels, the
direction of the centroid of
the sun can be calculated
to within a few
arcseconds.

6. 2. 1. EARTH HORIZON SENSOR
• A scanning horizon sensor is an optical instrument that
detects light from the 'limb' of the Earth's atmosphere,
i.e., at the horizon. Thermal Infrared sensing is often
used, which senses the comparative warmth of the
atmosphere, compared to the much colder cosmic
background. This sensor provides orientation with
respect to the earth about two orthogonal axes. It
tends to be less precise than sensors based on stellar
observation. Sometimes referred to as an Earth Sensor
or Scanning Earth Sensor
• Horizon crossing Indicator ( another type of sensor) is
used in spinning satellite,

7. 2.2 Conical Earth Sensor
Conical Earth Sensor in
Orbit Usage
Scan cone
Satellite
Earth
Pitch axis
Scan Path
Motion of spacecraft
Earth
Roll axis
Conical Earth Sensor
scan pattern

8. Proceedings, ITC/USA '81 (arizona.edu)

9. 2.3.1 Horizon crossing Indicator
• Horizon crossing Indicator ( another type of
sensor) is used in spinning satellite, In each spin
when sensor view passes from space to earth’s
horizon or from the land view to space, such
change is detected and timed. Successive
intervals provide information on orientation of
spin axis. A digital signal processor helps in
accurate horizon location independent of
radiance levels, crossing angles and field of view
effects. Some versions have capability to steer
line of sight of the sensor

10. 2.3.2 Horizon crossing indicator outputs
SCAN
EARTH
Horizon sensor scan pattern and its output

11. 3.1 Star Sensor/Tracker
• A star tracker is an optical device that measures the position(s) of
star(s) using photocell(s) or a camera
• Star field databases are used to determine spacecraft orientation.
A typical star catalog for high-fidelity attitude determination is
originated from a standard base catalog (for example from the
United States Naval Observatory) and then filtered to remove
problematic stars, for example due to apparent magnitude
variability, color index uncertainty, or a location within the
Hertzsprung-Russell diagram implying unreliability. These types of
star catalogs can have thousands of stars stored in memory on
board the spacecraft, or else processed using tools at the ground
station and then uploaded.

12. 3.2 Star tracker
• A star tracker is an optical device that measures the positions of stars
using photocells or a camera.
• As the positions of many stars have been measured by astronomers to
a high degree of accuracy, a star tracker on a satellite may be used to
determine the orientation (or attitude) of the spacecraft with respect
to the stars.
• In order to do this, the star tracker must obtain an image of the stars,
measure their apparent position in the reference frame of the
spacecraft, and identify the stars so their position can be compared
with their known absolute position from a star catalog.
• A star tracker may include a processor to identify stars by comparing
the pattern of observed stars with the known pattern of stars in the
sky.
Design and
implementatio
n of a star-
tracker for
LEO satellite

14. 3.3 Fixed Head Star tracker
A FIXED HEAD STAR TRACKER has electronic searching and tracking capability
over a limited Field of View.
• A star tracker tracks stars within the designated field of view (FOV) and of a
visual spectral magnitude range.
• As an example, LANDSAT satellite had an FOV of 8o x 8o and tracked stars over
a visual magnitude of 2 to 6.
• In the acquisition mode the star tracker is held in position; then the
spacecraft or the gimbal on which it is mounted is rotated until the star
tracker acquires a star of sufficient brightness. Then the star tracker enters
the tracking mode and tracks the star until it leaves the FOV. Once the star
tracker loses a star, it again goes into acquisition mode. Once the star is
acquired, its magnitude is evaluated to match the star table. This process
continues until the desired star is located

15. 3.3 Fixed Head Star tracker
Fixed-head star tracker attitude updates on the
Hubble SpaceTelescope - NASATechnical Reports
Server (NTRS)
The Hubble Space Telescope (HST) was
launched in April 1990 to begin
observing celestial space to the edge of
the universe. National Aeronautics and
Space Administration (NASA) standard
fixed-head star trackers (FHST's) are used
operationally onboard the HST to
regularly adjust ('update') the spacecraft
attitude before the acquisition of guide
stars for science observations. During the
first 3 months of the mission, the FHST's
updated the spacecraft attitude
successfully only 85 percent of the time.
During the other periods, the trackers
were unable to find the selected stars --
either they failed to find any star, or
worse, they selected incorrect stars and
produced erroneous attitude updates. In
July 1990, the HST project office at
Goddard Space Flight Center (GSFC)
requested that Computer Sciences
Corporation (CSC) form an investigative
'tiger' team to examine these FHST
update failures.
This paper discusses the work of the FHST tiger team, describes the investigations that led
the team to identify the sources of the errors, and defines the solutions that were
subsequently developed, which ultimately increased the success rate of FHST updates to

16. 4. Gyroscopes
• Gyroscopes are devices that sense rotation in
three-dimensional space without relying on the
observation of external objects.
• Rate gyros and rate integrating gyros (RIGs)
are attitude sensors used to measure changes
in spacecraft orientation.
 Rate gyros measure angular rates and
 RIGs the spacecraft angular displacements directly.
• Classically, a gyroscope consists of a spinning
mass, but there are also "Laser Gyros" utilizing
coherent light reflected around a closed path.

17. GYROSCOPE
An apparatus consisting
of a rotating wheel so
mounted that its axis can
turn freely in certain or
all directions, and
capable of maintaining
the same absolute
direction in space in spite
of movements of the
mountings and
surrounding parts: used
to maintain equilibrium,
determine direction, etc. Gyroscope

18. GYROSCOPE
One typical type of gyroscope is made by suspending a relatively
massive rotor inside three rings called gimbals. Mounting each of
these rotors on high quality bearing surfaces insures that very
little torque can be exerted on the inside rotor.

19. GYROSCOPE
• Input is a “rate of turn” = Angular rate
= Input torque
Output of a gyro is a change in gimbal angle
(gimbal turns) in response to change in input
turn due to gyroscopic precession.
The input is a turn with respect to Input axis – IA
The output is a turn with respect to output axis-
OA
• Ref: Rotational Motion Transducers:Alan S.
Morris, Reza Langari, in Measurement and
Instrumentation (Second Edition), 2016

20. RATE GYROSCOPE (RG)
• The rate gyro, illustrated in has an almost
identical construction to the rate-integrating
gyro, and differs only by including a spring
system that acts as an additional restraint on
the rotational motion of the frame

21. RATE GYROSCOPE (RG)
The instrument measures the absolute angular velocity of
a body, and is widely used for generating stabilizing
signals within vehicle navigation systems.
The typical measurement resolution given by the instrument is
0.01°/s and rotation rates up to 50°/s can be measured.
The angular velocity, α, of the body is related to the
angular deflection of the gyroscope, θ, by the equation:
[θ/α](D)=H/[MD2+βD+K]
M is the moment of inertia of the system,
β is the viscous damping coefficient,
K is the spring constant, and
D is the D-operator α = angular velocity

22. Rate Integrating Gyro (RIG)
FRAME
OUTPUT
INPUT
VISCOUS DAMPER

23. Rate indicating gyroscope (RIG)
• In a rate indicating gyroscope, the gyroscope is turned
at a steady rate about its input axis and a torque is
applied to the spin axis. This causes the gyroscope to
precess about the output axis. The rate indicating
gyroscope consists of a damping fluid between the
float assembly can and the outer casing. This viscous
fluid resists the motion of the gimbal precession. This
causes the gimbal to accelerate initially in the fluid,
until the damping effect is equal to the precessing
force. The rate of precession, will hence be directly
proportional to the rate of turn of the gyroscope about
its input axis and the total angle of movement about
the output axis will be proportional to the speed and
length of time the input axis is turning.

24. 5.1 Magnetometer
• A magnetometer is a device that senses magnetic field
strength and, when used in a three-axis triad, magnetic
field direction. As a spacecraft navigational aid, sensed
field strength and direction is compared to a map of the
Earth magnetic field stored in the memory of an on-
board or ground-based guidance computer. If spacecraft
position is known then attitude can be inferred.
• Magnetometers are not accurate inertial attitude sensors
as earth’s field is not completely known, field strength
varies inversely with cube of distance and residual field in
the spacecraft also influences its performance.

25. 5.2 Magnetometer
• Accuracy possible: Milli Gauss
• It can measure both magnitude and
direction of magnetic field.
• Since the Earth’s magnetic field not
exactly known, the magnetometer is
not very accurate; models are also
inaccurate.
• The magnetic field varies as 1/r3,
where r is the distance from Earth ;
thus the magnetometers are useful
only for LEO orbits of about 1000 km,
where the Earth’s magnetic field is
about 0.5 Gauss

26. 5.3 Magnetometer
Magnetometer has a magnetic sensor and an
electronics unit which processes the signal into a
useful output. There are two types of sensors:
a.One type uses atomic properties such as
Zeeman splitting or nuclear magnetic resonance.
b.The second type uses magnetic inductance.
This is based on Faraday’s law, according which
an electromagnetic force (EMF), E, is induced in a
conducting coil placed in a time-varying magnetic
flux.

28. Thankyou
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