Module 1_ Introduction to GN&C_Lecture Notes.pdf

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1. Module 1
INTRODUCTION

2. Guidance
●determination of the desired path of travel ("trajectory") from the vehicle's
current location to a designated target, as well as desired changes in
velocity, rotation and acceleration for following that path
●Based on mission requirements, specifies desired dynamical states in the
form of waypoints vs time

3. Navigation
●Determination of the vehicle's location and velocity ("state vector") at a
given time and as well as its attitude.

4. Control
●manipulation of the forces by way of steering controls like thrusters, torque
etc.
●needed to execute guidance commands while maintaining vehicle stability.

5. Guidance, Navigation and Control
●branch of engineering dealing with the design of systems to control the
movement of vehicles.
●Especially automobiles, ships, aircraft and spacecraft

6. Measured and Estimated States
●Measured States
○States which are given by the sensors
○e.g position, velocity v/s time
●Estimated States
○States which are estimated by current dynamic states
○Extended/ unscented Kalman filter is used to estimate states

7. State Vector
●State Vector typically contains seven elements
○Three Position Coordinate
○Three Velocity terms
○Time at which these values are valid

8. Attitude in craft
●Orientation of the craft according to earth’s horizon
●Includes craft’s yaw angle, pitch, roll and bank angle.

9. Categories of Navigation
●Land Navigation
●Marine Navigation
●Aeronautic Navigation
●Space Navigation

10. Importance of Navigation System
●Reduces the time lag between measurement and decision
●Increased in no. of aircraft
●Safety requirements

11. Spacecraft GNC
●GNC or AOCS (Attitude orbit control system) is one of the subsystem of
spacecraft

12. GNC systems
●GNC systems are found in essentially all autonomous or semi-autonomous
systems
○Autopilots
○Driverless cars, like Mars rovers
○Guided missiles
○precision-guided airdrop systems
○Reaction control systems for spacecraft
○Spacecraft launch vehicles

13. Importance of Navigation System
●Reduce the time lag between measurements
●Reduce the decision time
●Increase in no. of aircraft
●Improve safety

14. Output of the Navigation System
●Display System
○Gives information to pilot
●Steering Signals
○Give information to Autopilot
●Digital Information
○Give information to central computer

15. Steering Information
●One aircraft should keep out the way of another aircraft
●This includes
○Lateral
○Longitudinal
○Vertical Separation

16. Modern Techniques for Navigation
●Most Modern techniques relies on position determined electronically by
receivers collecting information from satellites.
●Techniques rely on Line of position(L.O.P.)
○Dead Reckoning
○Pilotage
○Celestial Navigation

17. Electronic Navigation
●Electronic Navigation consists of any method of fixing position using
electronic means.
●E.g.
○Radio Navigation
○Radar Navigation
○Satellite Navigation

18. Navigation in Spacecraft
●The function that will locate the spacecraft in term of position, velocity and
also in term in attitude, orientation and angular velocity of the spacecraft

19. Control
●Make sure that the spacecraft applies the required forces and torques to
make sure that it follows as closely as possible the trajectory

20. GNC
●Brain of the spacecraft
●Interface with sensor and hardware
●It will measure the current state of the spacecraft
●Interface with the actuator hardware to make sure that spacecraft applies
required torque and force

21. Basic Principle of GNC

22. GNC Systems
●Attitude GNC
●Orbit GNC

23. Attitude GNC
●Referred as ADCS(Attitude determination and control system)
●It basically consists of orientation or attitude as well as the angular velocity
which is specified through a guidance system and control the attitude using
controller
●Whenever you want to point the specific instrument
●Whenever we try to apply thrust maneuver to accelerate or decelerate the
aircraft, you have to make sure that thrusters are pointing exactly the right
direction.

24. Orbit GNC
●Also known as ODCS(orbit determination control system)
●Responsible to specifying, estimating and controlling the orbit state of the
spacecraft.
○Orbit States: Inertial Position, velocity/Relative velocity
●Specially used in orbit Maintenance
●Example, ISS experiences drag effect at low attitude. Which decelerate the
spacecraft which further decrease the attitude of spacecraft over time.

25. Attitude Sensors
●Sun Sensor
○Measure the orientation of the spacecraft with respect to sun
●Magnetometer Sensor
○Measure the orientation of the earth’s Magnetic field
●Gyroscope
○Measure the current or actual angular velocity of a satellite
●Digital star tracker
○Measure the current spacecraft attitude in inertial reference frame with the help of
extended kalman filter

26. Spacecraft with GNC





Attitude GNC Software Spacecraft

27. Form (Types) of Navigation System
●Pilotage
○which essentially relies on recognizing landmarks to know where you are.
It is older than human kind.
●Dead reckoning
○which relies on knowing where you started from plus some form of heading
information and some estimate of speed.
●Celestial navigation
○using time and the angles between local vertical and known celestial
objects (e.g., sun, moon, or stars).

28. Conti.
●Radio navigation
○which relies on radio‐frequency sources with known locations (including
GNSS, LORAN‐C, Omega, Tacan,  US Army Position Location and Reporting System…)
●Inertial navigation
○which relies on knowing your initial position, velocity and attitude and
thereafter measuring your attitude rates and accelerations. The operation of inertial
navigation systems (INS)  depends upon Newton’s laws of classical mechanics. It is the only
form of navigation that does not rely on external references.
●These forms of navigation can be used in combination as well.

29. Air Data Information
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Static
Air Temperature (SAT).
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30. Air Data Information
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31. Air Data Information
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32. Air Data Information
● By using the capsule arrangement shown, Total pressure is fed into the capsule while static
pressure is fed into the case surrounding the capsule. The difference between these two
parameters, represented by the deflection of the capsule, represents the aircraft airspeed. This
permits airspeed to be measured.
●In the centre capsule configuration, static pressure is fed into the case of the instrument
while the capsule itself is sealed.
●Here, capsule deflection is proportional to changes in static pressure and therefore aircraft
altitude.
●This allows aircraft barometric altitude to be measured.
●In the arrangement shown in the right of the figure, static pressure is fed into the capsule. It
is also fed via a calibrated orifice into the sealed case surrounding the capsule. In this
situation the capsule deflection is proportional to the rate of change in altitude. This
permits the aircraft rate of ascent or descent to be measured.
●Determination of altitude from pressure measurements is based upon a standard
atmosphere in which pressure, density, and temperature are functions of altitude.

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33. Air Data Information
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34. Air Data Information
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35. Display of Air Data – Contd..
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36. RADAR Systems
●Detection and Ranging by measure of Radio waves
●Radar is an object-detection system that uses radio waves to
determine the range, angle or velocity of objects.
●It can be used to detect aircraft, ships, spacecraft, guided missiles,
motor vehicles, weather formations and terrain.
●A radar system consists of a transmitter producing electromagnetic
waves in the radio or microwaves domain, a transmitting antenna, a
receiving antenna (often the same antenna is used for transmitting and
receiving) and a receiver and processor to determine properties of the
object(s).
●Radio waves (pulsed or continuous) from the transmitter reflect off the
object and return to the receiver, giving information about the object's
location and speed.

37. Principle of operation of Radar
●Radar consists of
○Transmitter (Tx)
○Transmit Antenna (Tx Ant.)
○Receiver (Rx)
○Receiver Antenna (Rx Ant.)

38. Conti..
●Transmitter generates Radio Waves (EM waves)
●Generated EM waves are transmitted by Tx antenna in a particular direction.
Thus a volume of space is illuminated by EM wave. If there exist a target in
this illuminated volume of space, the portion of the EM waves is intercept by
the target and redirected back in various direction
●EM wave is then received by the Rx antenna and is delivered to Rx
●The received EM wave is also known as echo wave.

39. Conti..
● 

40. Parameters of PW
● 

41. Derived Parameters
● 

42. Waveform of RADAR
●Continuous Wave (CW)
○Unmodulated CW
○Modulated CW
●Pulse Wave
○Coherent
○Non-coherent

43. Ambiguous Range
● 

44.  
● 

45. Range Resolution of RADAR
● 

46. RADAR equation
● 

47. Doppler Shift
●change in frequency or wavelength of a wave for an observer who is moving
relative to the wave source
●For waves that propagate in a medium, such as sound waves, the velocity of
the observer and of the source are relative to the medium in which the
waves are transmitted. The total Doppler effect may therefore result from
motion of the source, motion of the observer, or motion of the medium. Each
of these effects are analyzed separately. For waves which do not require a
medium, such as light or gravity in general relativity, only the relative
difference in velocity between the observer and the source needs to be
considered

48. MTI & Pulse Doppler RADAR
Doppler frequency shift is sometimes used to measure relative velocity of a
target using a pulse radar, its most interesting and widespread use has been in
identifying small moving targets in the presence of large clutter. Such pulse
radars which use the doppler frequency shift to distinguish (or discriminate)
between moving and fixed targets are called MTI (Moving Target Indicators)
and Pulse Doppler Radars. The physical principle of both these radars are the
same but they differ in their mode of operation.
MTI are high-quality air surveillance radars that operate in the presence of
clutter

49. Conti..

50. Conti..

51. Conti.
●For instance the MTI radar operates on low pulse repetition frequencies
thus causing ambiguous Doppler measurements (blind speeds) but
unambiguous range measurements (no second-time-around echoes). On the
other hand the pulse doppler radar operates on high pulse repetition.
frequency thus causing unambiguous doppler measurements (no blind
speeds) but ambiguous range measurements (second-time-around echoes).
The meaning of these terms will become clear later when we describe the
actual operational principles of these radars.

52. Principle operation
●CW radar can be converted to a pulse radar by providing a pulse modulator
which turns on and off the amplifier to generate pulses.
● We need to note that there is no local oscillator here since the reference
signal is supplied directly from the CW oscillator. Apart from this function
the CW oscillator also supplies a coherent reference needed to detect the
doppler frequency shift. By coherent we mean that the phase of the
transmitted signal is preserved in the reference
signal.
●This kind of reference signal is the distinguishing feature of a coherent MTI
radar.

53. CW oscillating voltage

56. ●The reference signal and the target echo signal are heterodyned in the
mixer stage. The difference frequency component is
●
●For stationary targets the doppler frequency shift fd will be zero; hence
Vdiff will not vary with time and may take on any constant value from
+A
4

to -A
4
, including zero. But when the target is in motion relative to the
radar, fd has a value other than zero and the voltage corresponding to
the difference frequency from the mixer will vary with time. Note that
all these
frequencies are with reference to the carrier waveform and has
nothing to do with the pulse repetition frequency.

57. Doppler Shift frequency and blind speed
●Doppler Shift frequency
●n-th blind speed
●Whenever the target relative velocity with respect to the radar along the
line of sight matches with these speeds, an MTI radar fails to detect the
moving target. Thus to avoid doppler ambiguities (due to blind speeds) the
first blind speed must be larger than the maximum expected relative velocity
of the target.

58. Conti..
●can be achieved by either making f
p
large or by making λ large. So MTI radars
should operate at long wavelengths (low carrier frequencies) or high pulse
repetition frequencies, or both. But, unfortunately other constraint prevent
this kind of choice. Too low radar frequencies make the beam-width wider
and cause deterioration in angular resolution. Too high pulse repetition
frequencies cause ambiguous range measurements.

59. Conti..
●MTI radars operate on low pulse repetition frequencies and thus are prone
to blind speeds, but they do not have the problems of range ambiguities. On
the other hand, pulse Doppler radars operate at high pulse repetition
frequencies and thus are affected by ambiguous range measurements. But
they do not have the problem of blind speeds. MTI radars are usually used as
high-resolution surveillance radars in airports. Pulse doppler radars are used
for detection of high-speed extraterrestrial objects like satellites and
astronomical bodies.

60. Numerical
●In a MTI radar the pulse repetition frequency is 200 Hz and the carrier
transmission frequency is 100 MHz. Find its first, second and third blind
speeds?

62. Moving target indicator
●Delay line circuit saves previous phase evaluation.
●Cancellation circuit subtracts previous phase from current phase.
●Return from Stationary targets will have same phase comparison and be
cancelled out.
●Return from Moving targets will have different phase comparison and will be
retained / displayed.

65. Delay Line Canceller
●It is a time domain filter
●It operates at all ranges and does not require separate filter for each range
resolution cell.

66. Limitations of MTI
●Blind speeds are the limitations of the MTI.
●The target will not be visible to the radar though it is advancing towards it.
This is very dangerous in most of the cases.

67. How to overcome
●If the blind speed is to be greater than maximum radial velocity expected,
then λf
p
is to be large.
●Long wavelength and high PRF are preferable.
●Then with more than one PRF, operate more than one λ.

68. Possible Solution
●Long range MTI radars operate in L, S or higher bands.
●Operate with blind speeds and ambiguous doppler speed for the sake of
accurate range estimation.
●The practical answer is: Keep the first blind speed out of the expected range
of doppler frequency.
●The standard technique is to operate at multiple frequencies.
○This is known as ‘Staggered PRF – MTI’.

69. Double Cancellation
●It will have better clutter rejection null.

70. Pulse Doppler RADAR
●Attributes of pulse radar / technology of CW radar.
●“Mixer” added to Pulse Radar.
●Sample of transmitted and received signal
are compared at mixer.
○Mixer output is Doppler shift (velocity).
○Doppler sorted into velocity categories.
○Categories identified by color in display.
■Standard Weather Radar.
○More rain / higher wind – higher Doppler.

71. Conti..
●Also used for weapon Fire Control systems
●Pulse Radar – ONE Antenna ONLY
●High PRF
○Many pulses / high frequency / large BW
○Large volume of range and range rate info
○High degree of accuracy
○Duty Cycle > 10%
●A radar that increases its PRF high enough to avoid problems of blind
speeds is called a ‘pulse doppler radar’.
●But it may be acceptable to operate at a slightly lower PRF and accept
both range and doppler ambiguities. Then it is ‘medium PRF – PDR’.

72. Block diagram of PDR

73. Disadvantage
●The large sidelobe clutter viewed by a pulse Doppler radar is a reason why it
requires a high improvement factor than an AMTI (Airborne Moving Target
Indicator) radar of equivalent performance.
●To detect aircraft targets within the sidelobe clutter region, a bank of
narrowband doppler filters with adaptive thresholds can be used. So
antenna must have exceptionally low sidelobes.

74. Eclipsing Loss
●Since the pulse Doppler radar can not receive when it is transmitting
the high duty cycle can result in a loss if the echo signal arrives when a
pulse is being radiated and the receiver is turned off. This is called
eclipsing loss.
●The degree of eclipsing varies as the target range changes with time so
eclipsing can cause periodic holes in the coverage.
●A rapidly approaching target will not remain eclipsed for long so that
detections will occur at a slightly shorter range when eclipsing is
present.
●A reduction of the duty cycle and an increase in the no. of range gates
will
reduce the effect of eclipsing.

75. Medium PRF-PDR
●It has both range and Doppler ambiguities.
●It results in less clutter being seen by the antenna sidelobes than the high
PRF radar since there are fewer pulse viewing ambiguous range cells.
●There is no clutter free region.

76. High PRF-PDR
●No ambiguities in Doppler frequency, no blind speeds but many range
ambiguities.
●Range ambiguities can be resolved by transmitting three redundant
waveforms each at a different PRF.
●Transmitter leakage and altitude return are removed by filtering.
●Main beam clutter is removed by a tunable filter.
●High closing speed aircrafts are detected at long range in the clutter
free region.
●There is poor detection of low radial speed targets that are masked in
the frequency domain by short range sidelobe clutter folded over in
range.

77. Conti..
●Often only a single range gate is employed but with a large doppler filter
bank.
●For comparable performance, a much larger improvement factor is required
than lower PRF systems since the high PRF results in more clutter being
viewed by the antenna sidelobes.
●The antenna sidelobes must be quite low in order to minimize the sidelobe
clutter.
●Range accuracy and the ability to resolve multiple targets in range are
poorer than other radars.

78. Low PRF-AMTI
●No range ambiguities but many doppler ambiguities ( blind speeds).
●Requires TACCAR and DPCA to remove effects of platform motion.
●TACCAR – Time Averaged Compensation for Clutter Doppler Shift.
●DPCA – Displaced Phase Center Antenna.
●It operates clutter free at long range where no clutter is seen due to
curvature of earth.
●Sidelobe clutter is usually not as important as it is in pulse doppler
systems.
●Best employed at UHF or perhaps L band.
●Increase blind speeds and the lower effectiveness of platform motion
compensation prevent its use at high microwave frequencies.

79. Conti..
●The lower RF of AMTI radar results in wider antenna beam-width than
a higher frequency ( S band) pulse doppler radar whose mission is wide
area air surveillance.
●Because there are no range ambiguities to be resolved, redundant
waveforms with multiple PRFs are not needed.
●For comparable performance the required product of average power
and antenna aperture is less than that for pulse doppler radars.
●Usually simper than pulse doppler radar.
●Cost is generally much less than pulse doppler radar of comparable
performance.
●AMTI can not be used in fighter/interceptor X band radars for look
down detection of targets in clutter.

80. Comparison Between MTI and PDR
PDR
●F
r
is estimated accurately
●High PRF - not to have blind speeds
●But time around echoes do exist
●Power amplifier with high duty
cycle
●Range gates are used
MTI
●Range is estimated accurately
●Blind speeds do exist
●Low PRF – no time around echoes.
●Delay line cancellers are used
●Frequency gates are used

81. Limitation to MTI Performance
●The improvement in signal-to-clutter ratio of an MTI is affected by factors
other than the design of the doppler signal processor. Instabilities of the
transmitter and receiver, physical motions of the clutter, the finite time on
target (or scanning modulation), and limiting in the receiver can all detract
from the performance of an MTI radar.

82. Conti..
●MTI improvement factor: The signal-to-clutter ratio at the output of the MTI
system divided by the signal-to-clutter ratio at the input, averaged uniformly
over all target radial velocities of interest.
●Sub-clutter visibility : The ratio by which the target echo power may be
weaker than the coincident clutter echo power and still be detected with
specified detection and false alarm probabilities.
●Clutter visibility factor : The signal-to-clutter ratio, after cancellation or
doppler filtering, that provides stated probabilities of detection and false
alarm.
●Clutter attenuation: The ratio of clutter power at the canceller input to the
clutter residue at the output, normalized to the attenuation of a single pulse
passing through the unprocessed channel of the canceller.
●Cancellation ratio: The ratio of canceller voltage amplification for the
fixed-target echoes received with a fixed antenna, to the gain for a single
pulse passing through the unprocessed channel of the canceller.

83. Conti..
●Equipment instabilities : Pulse-to-pulse changes in the amplitude,
frequency, or phase of the transmitter signal, changes in the stalo or
coho oscillators in the receiver, jitter in the timing of the pulse
transmission, variations in the time delay through the delay lines, and
changes in the pulse width can cause the apparent frequency spectrum
from perfectly stationary clutter to broaden and thereby lower the
improvement factor of an MTI radar.
●Internal fluctuation of clutter : Although clutter targets such as
buildings, water towers, bare hills. Or mountains produce echo signals
that are constant in both phase and amplitude as a function of time,
there are many types of clutter that cannot be considered as
absolutely stationary. Echoes from trees, vegetation, sea, rain, and
chaff fluctuate with time, and these fluctuations can limit the
performance of MTI radar.

84. Conti..
●Antenna scanning modulation: The received pulse train of finite duration to
has a frequency spectrum (which can be found by taking the Fourier
transform of the waveform) whose width is inversely proportional to no. of
hits. Therefore, even if the clutter were perfectly stationary, there will still
be a finite width to the clutter spectrum because of the finite time on target.
If the clutter spectrum is too wide because the observation time is too short,
it will affect the improvement factor. This limitation has sometimes been
called scanning fluctuations or scanning modulation.

85. CRT Display
●The cathode-ray tube's principal shortcoming is that it cannot present a true
three-dimensional picture. The fundamental geometrical quantities involved
in radar displays are the RANGE, AZIMUTH ANGLE (or BEARING), and
ELEVATION ANGLE. These displays relate the position of a radar target to
the origin at the antenna. Most radar displays include one or two of these
quantities as coordinate of the crt face. The actual range of a target from the
radar, whether on the ground, in the water, or in the air is known as SLANT
RANGE. The majority of displays use as one coordinate the value of slant
range, its horizontal projection (GROUND RANGE), or its vertical projection
(ALTITUDE).

86. Common Display
●A-scope (the RANGE-HEIGHT INDICATOR (RHI) SCOPE)
●PLAN POSITION INDICATOR (PPI) SCOPE

87. A-Scope Display
●Presents only the range to the target and the relative strength of the echo.
Such a display is normally used in weapons control radar systems. The
bearing and elevation angles are presented as dial or digital readouts that
correspond to the actual physical position of the antenna. The A-scope
normally uses an electrostatic-deflection crt.

88. PPI Scope
●The PPI, also called the P-Scope, is by far the most commonly used
radar display. It is an intensity modulated circular display on which
echo signals are shown in plan position with range and azimuth angle
displayed in polar coordinates. It is a polar coordinate display of the
area surrounding the radar platform. Own ship is represented as the
origin of the sweep, which is normally located in the center of the
scope, but may be offset from the center on some sets. The PPI uses a
radial sweep pivoting about the center of the presentation. This results
in a map-like picture of the area covered by the radar beam. A
long-persistence screen is used so that the display remains visible until
the sweep passes again.The origin of the polar coordinates is at the
location of the radar, and is normally located at the center of the
display. The PPI uses a radial sweep pivoting around the center. The
result is a map-like display of the area covered by the radar beam.

90. Thank You

91. Additional

92. Basic Principle of Inertial Navigation
●Given the ability to measure the acceleration of vehicle it  would be
possible to calculate the change in velocity and  position by performing
successive mathematical 
integrations of the acceleration with respect to time.
●In order to navigate with respect to our inertial reference  frame, it is
necessary to keep track of the direction in which 
the accelerometers are pointing.
●Rotational motion of the body with respect to inertial  reference frame
may be sensed using gyroscopic sensors 
that are used to determine the orientation of the 
accelerometers at all times. Given this information it is
possible to resolve the accelerations into the reference 
frame before the integration process takes place.

93. INS Types
●There are many different designs of INS with different performance
characteristics, but they fall generally into two categories: –
○gimbaled or stabilized platform techniques
○Strapdown
●The original applications of INS technology used stable platform 
techniques. In such systems, the inertial sensors are mounted on a 
stable platform and mechanically isolated from the rotational 
motion of the vehicle. Platform systems are still in use, particularly 
for those applications requiring very accurate estimates of 
navigation data, such as ships and submarines.
●Modern systems have removed most of the mechanical complexity 
of platform systems by having the sensors attached rigidly, or 
“strapped down”, to the body of the host vehicle. The potential 
benefits of this approach are lower cost, reduced size, and greater 
reliability compared with equivalent platform systems. The major 
disadvantage is a substantial increase in computing complexity. 

94. INS Consists
●An inertial navigation uses gyroscopes and accelerometers to maintain an
estimate of the position, velocity, and attitude rates of 
the vehicle in or on which the INS is carried, which could be a land 
vehicle, aircraft, spacecraft, missile, surface ship, or submarine.
● An INS consists of the following: –
○An IMU
○Instrument support electronics
○Navigation computers (one or more) calculate the gravitational  acceleration (not
measured by accelerometers) and doubly integrate  the net acceleration to
maintain an estimate of the position of the  host vehicle.

95. Gimbal

96. Gimbal
●A gimbal is a rigid with rotation bearings for isolating the inside of the frame
from external rotations about the bearing axes. At least three
gimbals are required to isolate a subsystem from host vehicle rotations
about three axes, typically labeled roll, pitch, and yaw axes.
●The gimbals in an INS are mounted inside one another. Gimbals and torque
servos are used to null out the rotation of stable platform on which the
inertial sensors are mounted.

97. Working of Gimbal INS
●The gyros of a type known as “integrating gyros” give an output
proportional to the angle through which they have been rotated
●Output of each gyro connected to a servo‐motor driving the
appropriate gimbal, thus keeping the gimbal in a constant orientation
in inertial space
●The gyros also contain electrical torque generators which can be used
to create a fictitious input rate to the gyros
●Applications of electrical input to the gyro torque generators cause the
gimbal torque motors/servos to null the difference between the true
gyro input rate and the electrically applied bias rate. This forms a
convenient means of cancelling out any drift errors in the gyro.

98. Strapdown
●Accelerometers mounted directly to airframe  (strapdown) and measure
“body” acceleration
●Horizontal/vertical accelerations computed  analytically using direction
cosine matrix  (DCM) relating body coordinated and local
level navigation coordinates
● DCM computed using strapdown body  mounted gyro outputs

99. Block diagram of Strapdown

100. Advantages of INS
●It is autonomous and does not rely on any external aids or visibility
conditions. It can operate in tunnels or underwater as well as anywhere else.
●It is inherently well suited for integrated navigation, guidance, and control of
the host vehicle. Its IMU measures the derivatives of the variables to be
controlled (e.g., position, velocity, and attitude).
●It is immune to jamming and inherently stealthy. It neither receivers nor
emits detectable radiation and requires no external antenna that might be
detectable by radar.

101. Disadvantages of INS
●Mean‐squared navigation errors increase with time.
●Cost, including:
○Acquisition cost, which can be an order of magnitude (or more) higher than GPS
receivers.
○Operations cost, including the crew actions and time required for initializing position
and attitude. Time required for initializing INS attitude by gyrocompass alignment is
measured in minutes. TTFF (Time To First Fix) for GPS receivers is measured in
seconds.
○Maintenance cost. Electromechanical avionics systems (e.g., INS) tend to have higher
failure rates and repair cost than purely electronic avionics systems (e.g., GPS).
●Size and weight, which have been shrinking
●Power requirements, which have been shrinking along with size and
weight but are still higher than those for GPS receivers.
●Heat dissipation, which is proportional to and shrinking with power
requirements.

102. Air Data Information

103. Inertial Measurement Unit
●The IMUs consist of an all-attitude, four-gimbal, inertially stabilized platform. They provide inertial attitude and
velocity data to the GN&C software functions. Navigation software uses the processed IMU velocity and attitude
data to propagate the orbiter state vector. Guidance uses the attitude data, along with state vector from the
navigation software, to develop steering commands for flight control.
●Flight control uses the IMU attitude data to convert the steering commands into control surface, engine gimbal
(thrust vector control) and reaction control system thruster fire commands. Although flight could be accomplished
with only one, three IMUs are installed on the orbiter for redundancy. The IMUs are mounted on the navigation
base, which is located inside the crew compartment flight deck forward of the flight deck control and display
panels. The navigation base mounting platform is pitched down 10.6 degrees from the orbiter's plus X body axis.
The navigation base provides a platform for the IMUs that can be repeatedly mounted with great accuracy,
enabling the definition of transformations that relate IMU reference frame measurements to any other reference
frame.
●The IMU consists of a platform isolated from vehicle rotations by four gimbals. Since the platform does not rotate
with the vehicle, its orientation remains fixed, or inertial, in space. The gimbal order from outermost to innermost is
outer roll, pitch, inner roll and azimuth. The platform is attached to the azimuth gimbal. The inner roll gimbal is a
redundant gimbal used to provide an all-attitude IMU while preventing the possibility of gimbal-lock (a condition
that can occur with a three-gimbal system and cause the inertial platform to lose its reference). The outer roll
gimbal is driven from error signals generated from disturbances to the inner roll gimbal. Thus, the inner roll gimbal
will remain at its null position, orthogonal to the pitch gimbal.

104. ●The inertial sensors consist of two gyros, each with two degrees of freedom that
provide platform stabilization. The gyros are used to maintain the platform's inertial
orientation by sensing rotations of the platform caused by vehicle-rotation-induced
friction at the gimbal pivot points. The gyros output a signal that is proportional to
the motion and is used by the gimbal electronics to drive the appropriate gimbals to
null the gyro outputs.
●The spin axis of a gyro is its axis of rotation. The inertial stability of the spin axis is a
basic property of gyroscopes and is used in stabilization loops, which consist of the
gyro pick-off, gimbals and gimbal torquers. When the vehicle is rotated, the platform
also tends to rotate due to friction at the gimbal pivot points. Since the gyro casing is
rigidly mounted to the platform, it will also rotate. The gyro resists this rotation
tendency to remain inertial, but the resistance is overcome by friction. This rotation
is detected by the pick-offs as a deflection of the rotating gyro wheel. A signal
proportional to this deflection is sent to the gimbal electronics, which routes the
signals to the appropriate torquers, which in turn rotate their gimbals to null the
pick-off point. When the output is nulled, the loop is closed.

105. Star Trackers
●The star tracker system is part of the orbiter's navigation system. Its two units are located just
forward and to the left of the commander's plus X window in a well outside the pressurized
crew compartment-an extension of the navigation base on which the IMUs are mounted.
●The star trackers are slightly inclined off the vehicle's negative Y and negative Z axes, for
which they are named. The star trackers are used to align the IMUs on board the orbiter as
well as to track targets and provide line-of-sight vectors for rendezvous calculations.
IMU alignment is accomplished by using the star trackers to measure the line-of-sight vector
to at least two stars. With this information, the GPC calculates the orientation between these
stars and the orbiter to define the orbiter's attitude. A comparison of this attitude with the
attitude measured by the IMU provides the correction factor necessary to null the IMU error.
●The GPC memory contains inertial information for 50 stars chosen for their brightness and
their ability to provide complete sky coverage.
●The star trackers are oriented so that the optical axis of the negative Z star tracker is pointed
approximately along the negative Z axis of the orbiter and the optical axis of the negative Y
star tracker is pointed approximately along the negative Y axis of the orbiter. Since the
navigation base provides the mount for the IMUs and star trackers, the star tracker line of
sight is referenced to the navigation base and the orbiter coordinate system; thus,
the GPC knows where the star tracker is pointed and its orientation with respect to the IMUs.

106. CREWMAN OPTICAL ALIGNMENT SIGHT
●The crewman optical alignment sight is used if inertial measurement unit alignment is in error
by more than 1.4 degrees, rendering the star tracker unable to acquire and track stars. The
COAS must be used to realign the IMUs to within 1.4 degrees. The star trackers can then be
used to realign the IMUs more precisely. The COAS is mounted at the commander's station so
the crew can check for proper attitude orientation during ascent and deorbit thrusting
periods. For on-orbit operations, the COAS at the commander's station is removed and
installed next to the aft flight deck overhead right minus Z window.
●By knowing the star being sighted and the COAS's location and mounting relationship in
the orbiter, software can determine a line-of-sight vector from the COAS to the star in an
inertial coordinate system. Line-of-sight vectors to two stars define the attitude of
the orbiter in inertial space. This attitude can be compared to the attitude defined by the
IMUs and can be realigned to the more correct orientation by the COAS sightings if the IMUs
are in error.
●The COAS's mounting relative to the navigation base on which the IMUs are mounted is
calibrated before launch. The constants are stored in software, and COAS line-of-sight
vectors are based on known relationships between the COAS line of sight and the navigation
base.

107. TACAN
●The onboard tactical air navigation units determine slant range and magnetic bearing
of the orbiter to a TACAN or VHF omnirange TACAN ground station. The
ground-based TACAN and VHF omnirange TACAN stations constitute a global
navigation system for military and civilian aircraft operating at L-band frequencies (1
gigahertz).
●The orbiter is equipped with three TACAN sets that operate redundantly.
Each TACAN has two antennas:
○one on the orbiter's lower forward fuselage
○other on the orbiter's upper forward fuselage. The antennas are covered with
reusable thermal protection system tiles.
●The onboard TACAN sets are used for external navigation and for the orbiter during
the entry phase and return-to-launch-site abort. Normally, several ground stations
will be used after leaving L- band communications blackout and during the terminal
area energy management phases. TACAN's maximum range is 400 nautical miles.
●In the GPC mode, 10 TACAN ground stations are programmed into the software and
are divided into three geometric regions: the acquisition region (three stations), the
navigation region (six stations), and the landing site region (one station).

108. MICROWAVE SCAN BEAM LANDING SYSTEM
●The three onboard microwave scan beam landing systems are airborne Ku-band
receiver/transmitter navigation and landing aids with decoding and computational
capabilities. The MSBLS units determine slant range, azimuth and elevation to the ground
stations alongside the landing runway. MSBLS is used during terminal area energy
management, the approach and landing flight phases and return-to-launch-site aborts. When
the channel (specific frequency) associated with the target runway approach is selected, the
orbiter's MSBLS units receive elevation from the glide slope ground portion and azimuth and
slant range from the azimuth/distance-measuring equipment ground station. The orbiter is
equipped with three independent MSBLS sets, each consisting of a Ku-band
receiver/transmitter and decoder. Data computation capabilities determine elevation angle,
azimuth angle and orbiter range with respect to the MSBLS ground station. The MSBLS
provides highly accurate three-dimensional navigation position information to the orbiter to
compute state vector components for steering commands that maintain the orbiter on its
proper flight trajectory. The three orbiter Ku-band antennas are located on the upper forward
fuselage nose. The three MSBLS and decoder assemblies are located in the crew
compartment middeck avionics bays and are convection cooled.
●The ground portion of the MSBLS consists of two shelters: an elevation shelter and an
azimuth/distance-measuring equipment shelter. The elevation shelter is located near the
projected touchdown point, with the azimuth/DME shelter located near the far end of the
runway. Both ends of the runway are instrumented to enable landing in either direction.