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A
SEMINAR REPORT ON
ADVANCE MISSILE GUIDANCE SYSTEM
Submitted by:
Sudhanshu Srivastava
(1205231046)
In partial fulfillment of the requirements for the
Award of the degree of
BACHELOR OF TECHNOLOGY In
ELECTRONICS & COMMUNICATION ENGINEERING
Seminar Guide: Seminar Incharge:
Er. Abhimanyu Dr. S. R. P. Sinha
DEPARTMENT OF ELECTRONICS & COMMUNICATION
ENGINEERING
INSTITUTE OF ENGINEERING AND TECHNOLOGY,
LUCKNOW
(An Autonomous Constituent Institute of Uttar Pradesh Technical University,
Lucknow)
(APRIL- 2015)
[2]
DEPARTMENT OF ELECTRONICS & COMMUNICATION
ENGINEERING
CERTIFICATE
Certified that Seminar Report entitled “ADVANCE MISSILE GUIDANCE SYSTEM” is a
bonafide work carried out in the VI semester by “ SUDHANSHU SRIVASTAVA
(1205231046) ” in partial fulfillment for the award of Bachelor of Technology in
“ELECTRONICS & COMMUNICATION ENGINEERING” from U. P. Technical
University, Lucknow, during the academic year 2014- 2015. Who carried out the seminar
work under the guidance and no part of this work has been submitted earlier for the award of
any degree.
------------------------------------------ -------------------------------------------
ER. ABHIMANYU DR. VINOD KUMAR SINGH
Seminar Guide Head of the Department
Department of Electronics, I.E.T. LKO Department of Electronics, I.E.T. LKO
[3]
ACKNOWLEDGEMENT
I express my gratitude to HOD, Dr. Vinod Kumar Singh, Department of
Electronics Engineering and Er. Abhimanyu, my Seminar Guide for providing me with
adequate facilities, ways and means by which I was able to complete this seminar. I express
my sincere gratitude to him for his constant support and valuable suggestions without which
the successful completion of this seminar would not have been possible.
I am obliged to Staff members of Electronics Department, for the valuable
information provided by them in their respective fields. I am grateful for their cooperation
during the period of my report.
This seminar report has been benefited from the many useful comments provided to
me by the numerous of my colleagous. In addition many other of my friends have checked it
and have offered many suggestions and comments. Besides there are some books and some
online helps.Last but not the least, I thank all others, and especially my classmates and my
family members who in one way or another helped me in the successful completion of this
work.
SUDHANSHU SRIVASTAVA
EC-3rd Year
( 1205231053)
[4]
ABSTRACT
In the modern day theatre of combat, the need to be able to strike at targets that are on the
opposite side of the globe has strongly presented itself. This had led to the development of
various types of guided missiles. These guided missiles are self -guiding weapons intended to
maximize damage to the target while minimizing collateral damage. The buzzword in modern
day combat is fire and forget. GPS guided missiles, using the exceptional navigational and
surveying abilities of GPS, after being launched, could deliver a warhead to any part of the
globe via the interface pof the onboard computer in the missile with the GPS satellite system.
Under this principle many modern day laser weapons were designed.
Laser guided missiles use a laser of a certain frequency bandwidth to acquire their target.
GPS/inertial weapons are oblivious to the effects of weather, allowing a target to be engaged
at the time of the attacker's choosing. GPS allows accurate targeting of various military
weapons including ICBMs, cruise missiles and precision-guided munitions. Artillery
projectiles with embedded GPS receivers able to withstand accelerations of 12,000 G have
been developed for use in 155 mm. GPS signals can also be affected by multipath issues,
where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground ,
etc. These delayed signals can cause inaccuracy.
A variety of techniques, most notably narrow correlator spacing, have been developed to
mitigate multipath errors. Multipath effects are much less severe in moving vehicles. When
the GPS antenna is moving, the false solutions using reflected signals quickly fail to
converge and only the direct signals result in stable solutions. The multiple independently
targeted re-entry vehicles (MIRVs) – ICBMs with many sub-missiles – were developed in the
late 1960s. The cruise missile has wings like an airplane, making it ca pable of flying at low
altitudes. In summary, GPS-INS guided weapons are not affected by harsh weather
conditions or restricted by a wire, nor do they leave the gunner vulnerable for attack. GPS
guided weapons, with their technological advances over previous, are the superior weapon of
choice in modern day warfare.
[5]
TABLE OF CONTENTS
ABSTRACT iii
S. No. Chapter Page No.
1.0 Introduction 1
1.1 History 1
1.2 Introduction to Missile Guidance 1
1.3 Concept of Missile Guidance 2
2.0 Types of Missile Guidance 3
2.1 Missile Guidance using RADAR signal 3
2.2 Missile Guidance using Wires 4
2.3 Missile Guidance using LASER 4
3.0 Introduction to GPS 6
3.1 Basic concept of GPS 6
3.1.1 Fundamentals 6
3.1.2 More detailed description 6
3.1.3 User-satellite geometry 7
3.1.4 Receiver in continuous operation 7
3.1.5 Non-navigational application 8
4.0 Structure/Segment 9
4.1 Space Segment 9
4.2 Control Segment 10
4.3 User Segment 10
5.0 Functions 12
5.1 How GPS works 12
5.2 GPS Accuracy 13
5.2.1 Factors affecting GPS 14
6.0 DGPS (Differential Global Positioning System) 15
6.1 Operation 15
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6.2 Accuracy 16
6.3 Post Processing 16
7.0 Applications of GPS 17
7.1 Military 17
7.2 Civilian 18
8.0 Missile Guidance and Control 19
9.0 Missile Guidance using GPS 22
9.1 Determining your position 22
9.2 Measuring your distance 22
9.3 Error correction 23
10.0 CONCLUSION 24
REFERENCE 25
[7]
Chapter 1
Introduction
Basically any object thrown at a target with the aim of hitting it is a missile. Thus, a stone
thrown at a bird is a missile. The bird, by using its power of reasoning may evade the missile
(the stone) by moving either to the Left, right, top or bottom with respect to the flight path
(trajectory) of the missile. Thus, the missile in this case has been ineffective in its objective of
hitting the bird (the target) . Now, if the stone too is imparted with some intelligence and
quick response to move with respect to the bird, to overcome aiming errors and the bird's
evasive actions and hit it accurately, the stone now becornes a guided missile. The
incorporation of energy source in a missile to provide the required force for its movement
(propulsion), intelligence to go in the correct direction (guidance) and effective maneuvering
(control) are mainly the technologies of guided missiles. They help in making a missile
specific to a target, that is, they determine the size, range and state of motion of a missile.
1.1 History
Looking back into the history of rockets and guided missiles, we find that rockets were used
in China and India around 1000 AD for fireworks as well as for war purposes. During the
18th century, unguided rocket propelled missiles were used by Hyder Ali and his son Tipu
Sultan against the British. There is a reference that two rockets belonging to Tipu's forces
were captured during the fourth Mysore war in the siege of Seringapatnam in 1799 by
companies of the Bengal and Bombay Artillery of the East India Company. The current phase
in the history of missiles began during the World War I1 with the use of V1 and V2 missiles
by Germany. Since then there has been a tremendous and rapid global advancement in this
field. It spawned the growth and pushed the frontiers of many new technologies in the areas
of materials science, aeronautics, communications, radars and computers. Huge amounts of
prime resources have been channelized into this field resulting in the development of
sophisticated missiles. The readers would no doubt be aware of the importaw role missiles
played in the recently concluded Gulf war.
1.2 Introduction to missile guidance
Guided missile systems have evolved at a tremendous rate over the past four decades, and
recent breakthroughs in technology ensure that smart warheads will have an increasing role in
maintaining our military superiority. On ethical grounds, one prays that each warhead
deployed during a sortie will strike only its intended target, and that innocent civilian will not
be harmed by a misfire. From a tactical standpoint, our military desires weaponry that is
reliable and effective, inflicting maximal damage on valid military targets and ensuring our
capacity for lighting fast strikes with pinpoint accuracy. Guided missiles systems help fulfil
all of these demands.
[8]
1.3 Concept of missile guidance
Missile guidance concerns the method by which the missile receives its commands to move
along a certain path to reach a target. On some missiles, these commands are generated
internally by the missile computer autopilot. On others, the commands are transmitted to the
missile by some external source.
.
Fig 1.1: Concept of missile guidance
The missile sensor or seeker, on the other hand, is a component within a missile that
generates data fed into the missile computer. This data is processed by the computer and used
to generate guidance commands. Sensor types commonly used today include infrared, radar,
and the global positioning system. Based on the relative position between the missile and the
target at any given point in flight, the computer autopilot sends commands to the control
surfaces to adjust the missile's course.
[9]
Chapter 2
Types of Missile Guidance
Many of the early guidance systems used in missiles where based on gyroscope models.
Many of these models used magnets in their gyroscope to increase the sensitivity of the
navigational array. In modern day warfare, the inertial measurements of the missile are still
controlled by a gyroscope in one form or another, but the method by which the missile
approaches the target bears a technological edge. On the battlefield of today, guided missiles
are guided to or acquire their targets by using:
1. Radar signal
2. Wires
3. Lasers (or)
4. Most recently GPS.
2.1 Missile guidance using radar signal
Many machines used in battle, such as tanks, planes, etc. and targets, such as buildings,
hangers, launch pads, etc. have a specific signature when a radar wave is reflected off of it.
Guided missiles that use radar signatures to acquire their targets are programmed with the
specific signature to home in on. Once the missile is launched, it then uses it’s on board
navigational array to home in on the pre-programmed radar signature. Most radar guided
missiles are very successful in acquiring their targets; however, these missiles need a source
to pump out radar signals so that they can acquire their target. The major problem with these
missiles in today’s battlefield is that the countermeasures used against these missiles work on
the same principles that these missiles operate under. The countermeasures home in on the
radar signal source and destroy the antenna array, which essential shuts down the radar
source, and hence the radar guided missiles cannot acquire their targets.
Figure 2.1: Missile guidance using Radar Signal
[10]
2.2 Missile guidance using wires
Wire guided missiles do not see the target. Once the missile is launched, the missile proceeds
in a linear direction from the launch vehicle. Miles of small, fine wire are wound in the tail
section of the missile and unwind as the missile travels to the target. Along this wire, the
gunner sends navigational signals directing the missile to the target. If for some reason the
wire breaks, the missile will never acquire the target. Wire guided missiles carry no
instrument array that would allow them to acquire a target. One strong downside to wire
guided missiles is the fact that the vehicle from which the missile is fired must stay out in the
open to guide the missile to its target. This leaves the launch vehicle vulnerable to attack,
which on the battlefield one wants to avoid at all costs.
Figure 2.2: Missile guidance using wires
2.3 Missile guidance using lasers
In modern day weaponry the buzzword is fire and forgets. Under this principle many modern
day laser weapons were designed. Laser guided missiles use a laser of a certain frequency
bandwidth to acquire their target. The gunner sights the target using a laser; this is called
painting the target. When the missile is launched it uses it’s on board instrumentation to look
for the heat signature created by the laser on the target. Once the missile locates the heat
signature, the target is acquired, and the missile will home in on the target even if the target is
moving. Despite the much publicized success of laser guided missiles, laser guided weapons
are no good in the rain or in weather conditions where there is sufficient cloud cover. To
overcome the shortcomings of laser guided missiles presented in unsuitable atmospheric
conditions and radar guided missiles entered GPS as a method of navigating the missile to the
target. So, before going to GPS guided missile we will have an introduction to GPS.
[11]
Figure 2.3: Missile guidance using LASER
[12]
CHAPTER 3
Introduction to GPS
The Global Positioning System (GPS) is a space-based satellite navigation system that
provides location and time information in all weather conditions, anywhere on or near the
earth where there is an unobstructed line of sight to four or more GPS satellites. The system
provides critical capabilities to military, civil, and commercial users around the world. The
United States government created the system, maintains it, and makes it freely accessible to
anyone with a GPS receiver.
3.1 Basic concept of GPS
3.1.1 Fundamentals
The GPS system concept is based on time. The satellites carry very stable atomic clocks that
are synchronized to each other and to ground clocks. Any drift from true time maintained on
the ground is corrected daily. Likewise, the satellite locations are monitored precisely. GPS
receivers have clocks as well—however, they are not synchronized with true time, and are
less stable. GPS satellites continuously transmit their current time and position. A GPS
receiver monitors multiple satellites and solves equations to determine the exact position of
the receiver and its deviation from true time. At a minimum, four satellites must be in view of
the receiver for it to compute four unknown quantities (three position coordinates and clock
deviation from satellite time).
3.1.2 More detailed description
Each GPS satellite continually broadcasts a signal (carrier frequency with modulation) that
includes:
A pseudorandom code (sequence of ones and zeros) that is known to the receiver.
By time-aligning a receiver-generated version and the receiver-measured version of
the code, the time of arrival (TOA) of a defined point in the code sequence, called
an epoch, can be found in the receiver clock time scale.
A message that includes the time of transmission (TOT) of the code epoch (in GPS
system time scale) and the satellite position at that time.
Conceptually, the receiver measures the TOAs (according to its own clock) of four satellite
signals. From the TOAs and the TOTs, the receiver forms four time of flight (TOF) values,
which are (given the speed of light) approximately equivalent to receiver-satellite range
differences. The receiver then computes its three-dimensional position and clock deviation
from the four TOFs.
[13]
In practice the receiver position (in three dimensional Cartesian coordinates with origin at the
earth's centre) and the offset of the receiver clock relative to GPS system time are computed
simultaneously, using the navigation equations to process the TOFs.
The receiver's earth-centred solution location is usually converted to latitude, longitude and
height relative to an ellipsoidal earth model. The height may then be further converted to
height relative the geoids (e.g., EGM96) (essentially, mean sea level). These coordinates may
be displayed, e.g. on a moving map display and/or recorded and/or used by other system
(e.g., vehicle guidance).
3.1.3 User-satellite geometry
Although usually not formed explicitly in the receiver processing, the conceptual time
differences of arrival (TDOAs) define the measurement geometry. Each TDOA corresponds
to a hyperboloid of revolution (see Multilateration). The line connecting the two satellites
involved (and its extensions) forms the axis of the hyperboloid. The receiver is located at the
point where three hyperboloids intersect.
It is sometimes incorrectly said that the user location is at the intersection of three spheres.
While simpler to visualize, this is only the case if the receiver has a clock synchronized with
the satellite clocks (i.e., the receiver measures true ranges to the satellites rather than range
differences). There is significant performance benefits to the user carrying a clock
synchronized with the satellites. Foremost is that only three satellites are needed to compute a
position solution. If this were part of the GPS system concept so that all users needed to carry
a synchronized clock, then a smaller number of satellites could be deployed. However, the
cost and complexity of the user equipment would increase significantly.
3.1.4 Receiver in continuous operation
The description above is representative of a receiver start-up situation. Most receivers have a
track algorithm, sometimes called a tracker, which combines sets of satellite measurements
collected at different times—in effect, taking advantage of the fact that successive receiver
positions are usually close to each other. After a set of measurements are processed, the
tracker predicts the receiver location corresponding to the next set of satellite measurements.
When the new measurements are collected, the receiver uses a weighting scheme to combine
the new measurements with the tracker prediction. In general, a tracker can (a) improve
receiver position and time accuracy; (b) reject bad measurements, and (c) estimate receiver
speed and direction.
The disadvantage of a tracker is that changes in speed or direction can only be computed with
a delay, and that derived direction becomes inaccurate when the distance travelled between
two position measurements drops below or near the random error of position measurement.
GPS units can use measurements of the Doppler shift of the signals received to compute
velocity accurately. More advanced navigation systems use additional sensors like a compass
or an inertial navigation system to complement GPS.
[14]
3.1.5 Non-navigation applications
In typical GPS operation as a navigator, four or more satellites must be visible to obtain an
accurate result. The solution of the navigation equations gives the position of the receiver
along with the difference between the time kept by the receiver's on-board clock and the true
time-of-day, thereby eliminating the need for a more precise and possibly impractical
receiver based clock. Applications for GPS such as time transfer, traffic signal timing, and
synchronization of cell phone base stations, make use of this cheap and highly accurate
timing. Some GPS applications use this time for display, or, other than for the basic position
calculations, do not use it at all.
Although four satellites are required for normal operation, fewer apply in special cases. If one
variable is already known, a receiver can determine its position using only three satellites.
For example, a ship or aircraft may have known elevation. Some GPS receivers may use
additional clues or assumptions such as reusing the last known altitude, dead reckoning,
inertial navigation, or including information from the vehicle computer, to give a (possibly
degraded) position when fewer than four satellites are visible.
[15]
Chapter 4
Structure/Segment
The current GPS consists of three major segments. These are the space segment (SS), a
control segment (CS), and a user segment (US). The U.S. Air Force develops, maintains, and
operates the space and control segments. GPS satellites broadcast signals from space, and
each GPS receiver uses these signals to calculate its three-dimensional location (latitude,
longitude, and altitude) and the current time.
The space segment is composed of 24 to 32 satellites in medium earth orbit and also includes
the payload adapters to the boosters required to launch them into orbit. The control segment
is composed of a master control station (MCS), an alternate master control station, and a host
of dedicated and shared ground antennas and monitor stations. The user segment is composed
of hundreds of thousands of U.S. and allied military users of the secure GPS Precise
Positioning Service, and tens of millions of civil, commercial, and scientific users of the
Standard Positioning Service.
4.1 Space Segment
The space segment (SS) is composed of the orbiting GPS satellites or Space Vehicles (SV) in
GPS parlance. The GPS design originally called for 24 SVs, eight each in three
approximately circular orbits, but this was modified to six orbital planes with four satellites
each. The six orbit planes have approximately 55° inclination (tilt relative to the earth's
equator) and are separated by 60° right ascension of the ascending node (angle along the
equator from a reference point to the orbit's intersection). The orbital period is one-half a
sidereal day, i.e., 11 hours and 58 minutes so that the satellites pass over the same locations
or almost the same locations every day. The orbits are arranged so that at least six satellites
are always within line of sight from almost everywhere on the earth's surface. The result of
this objective is that the four satellites are not evenly spaced (90 degrees) apart within each
orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120,
and 105 degrees apart, which sum to 360 degrees.
Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of
approximately 26,600 km (16,500 mi), each SV makes two complete orbits each sidereal day,
repeating the same ground track each day This was very helpful during development because
even with only four satellites, correct alignment means all four are visible from one spot for a
few hours each day. For military operations, the ground track repeat can be used to ensure
good coverage in combat zones. As of December 2012, there are 32 satellites in the GPS
constellation. The additional satellites improve the precision of GPS receiver calculations by
providing redundant measurements.
4.2 Control Segment
The control segment is composed of:
[16]
a master control station (MCS),
an alternate master control station,
four dedicated ground antennas, and
six dedicated monitor stations.
The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground
antennas (for additional command and control capability) and NGA (National Geospatial-
Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by
dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein Atoll, Ascension
Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared
NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and
Washington DC. The tracking information is sent to the Air Force Space Command MCS
at Schriever Air Force Base 25 km (16 mi) ESE of Colorado Springs, which is operated by
the 2nd Space Operations Squadron (2 SOPS) of the U.S. Air Force. Then 2 SOPS contacts
each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN)
ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension
Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on
board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of
each satellite's internal orbital model. The updates are created by a Kalman filter that uses
inputs from the ground monitoring stations, space weather information, and various other
inputs.
4.3 User Segment
The user segment is composed of hundreds of thousands of U.S. and allied military users of
the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and
scientific users of the Standard Positioning Service. In general, GPS receivers are composed
of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and
a highly stable clock (often a crystal oscillator). They may also include a display for
providing location and speed information to the user. A receiver is often described by its
number of channels: this signifies how many satellites it can monitor simultaneously.
Originally limited to four or five, this has progressively increased over the years so that, as of
2007, receivers typically have between 12 and 20 channels.
GPS receivers may include an input for differential corrections, using the RTCM SC-104
format. This is typically in the form of an RS-232 port at 4,800 bit/s speed. Data is actually
sent at a much lower rate, which limits the accuracy of the signal sent using RTCM.
Receivers with internal DGPS receivers can outperform those using external RTCM data. As
of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS)
receivers.
[17]
Figure 4.1: Three segments of GPS
[18]
Chapter 5
Functions
5.1 How GPS works
GPS signals do not contain positional data. The position reported by the receiver on the
ground is a calculated position based on range-finding triangulation. GPS positioning is
achieved by measuring the time taken for a signal to reach a receiver. Almost one million
times a second the satellite transmits a one or a zero in a complex string of digits that appears
random. In actuality this code is not random and repeats every 266 days. The receiver knows
that the portion of the signal received from the satellite matches exactly with a portion it
generated a set number of seconds ago. When the receiver has determined this time, the
distance to the satellite can be calculated using simple trigonometry where: Distance to the satellite = speed x (tr - tto) (where speed is c, the speed of light, in a vacuum
(299792.5 x 10³ ms-1
). tto is the time at the origin and tr is the time at the receiver).
The DoD maintains very accurate telemetry data on the satellites and their positions are
known to a high level of precision. When the distance to three satellites is known then there
is only one point at which the user can be standing. This principle is demonstrated in the
diagrams on the following pages. From one measurement we know the receiver can be
anywhere at a uniform distance from the satellite with a radius equal to r = c x (tr - tto). This
defines the outer surface of a sphere of radius r.
Where r = radius c = speed of light, t is the time at the origin tr is the time at the
receiver
Figure 5.1: Basic Trigonometry - Single Satellite.
From two measurements we know the receiver must be anywhere on the line of the outer
edge of a circle of intersection between the two spheres shown as a shaded ellipse below:
Figure 5.2: Basic Trigonometry - Two Satellites.
[19]
A third measurement reduces this to the intersection of a plane with the circle. This reduces
the possible location to two points. Of the two possible point of intersection only one point
lie on the Earth’s surface.
Figure 5.3: Basic Trigonometry
Unfortunately, the above description is an oversimplification. This method of triangulation
requires the receiver to know the precise time that the signal was transmitted and received.
Even though time at the satellite (tto) is known precisely because it is time stamped by the
atomic clock on board the satellite, time at the receiver (tr) is not known because this is
generated by the internal receiver clock. To determine positional fixes to metre accuracy
-10 requires the GPS receiver to measure time accurately to -10 of a second. To keep the cost
of GPS receivers below several thousand dollars per unit, atomic clocks are not used in the
handsets. Due to these inaccuracies in timing the margins of error in calculated positions are
very large. The way GPS receivers circumvent this problem is by using an additional
measurement. The internal clock of the receiver will measure tr incorrectly for all satellites.
Therefore, because the offset is the same for all satellites, the receiver can use an additional
satellite to bring all the points to one location.
5.2 GPS Accuracy
The signal transmitted by the satellites has a potential accuracy of <1 m but several factors
influence this and reduce the actual resolution. The US military designed the end user of the
SPS to be able to resolve a position 95.4% of the time (two standard deviations) to an
accuracy of 100 m in X and Y (longitude and latitude) and 156 m in Z. Using the PPS service
the end user should be able to resolve 22 m in X and Y and 27 m in Z. These are very
conservative estimations and actual accuracy will lie between the theoretical resolution and
these design schematics.
[20]
5.2.1 Factors affecting GPS accuracy
The reason why the actual locational position is significantly less accurate than the data
transmitted by the satellite is due to various influences on the signal. These can be
collectively termed local and atmospheric effects. Local effects are detrimental conditions on
the ground near the receiver or in the receiver’s software while atmospheric effects are
problems with the medium through which the signal passes.
1. Atmospheric delay
The assumption is that radio signals travel at the speed of light, and that the speed of light is a
constant, but this is only true of light if it is in a vacuum. Atmospheric delay is largest during
the heat of the day when ionosphere activity is greatest. Furthermore, weather patterns in the
troposphere can be different at the base and rover receivers.
2. Clock errors
Timing is critical to GPS, and the GPS satellites are equipped with very accurate atomic
clocks, but they are not perfect and slight inaccuracies can lead to errors.
3. Ephemeris errors
Satellites are launched into a precise orbit well above the Earth's atmosphere. The
Department of Defence constantly monitors the exact altitude, speed and position of each
satellite. Small changes are caused by gravitational pulls from the moon and sun and by the
pressure of solar radiation on the satellites. Slight ephemeris errors over such large distances
can make a difference.
[21]
Chapter 6
DGPS (Differential Global Positioning System)
Differential Global Positioning System (DGPS) is an enhancement to Global Positioning
System that provides improved location accuracy, from the 15-meter nominal GPS accuracy
to about 10 cm in case of the best implementations.
DGPS uses a network of fixed, ground-based reference stations to broadcast the difference
between the positions indicated by the GPS (satellite) systems and the known fixed positions.
These stations broadcast the difference between the measured satellite pseudo ranges and
actual (internally computed) pseudo ranges, and receiver stations may correct their pseudo
ranges by the same amount. The digital correction signal is typically broadcast locally over
ground-based transmitters of shorter range.
Figure 6.1: Differential GPS
6.1 Operation
A reference station calculates differential corrections for its own location and time. Users
may be up to 200 nautical miles (370 km) from the station, however, and some of the
compensated errors vary with space: specifically, satellite ephemeris errors and those
introduced by ionospheric and tropospheric distortions. For this reason, the accuracy of
DGPS decreases with distance from the reference station. The problem can be aggravated if
the user and the station lack "inter visibility"—when they are unable to see the same
satellites.
[22]
6.2 Accuracy
The United States Federal Radio navigation Plan and the IALA Recommendation on the
Performance and Monitoring of DGNSS Services in the Band 283.5–325 kHz cite the United
States Department of Transportation's 1993 estimated error growth of 0.67 m per 100 km
from the broadcast site but measurements of accuracy across the Atlantic, in Portugal, suggest
a degradation of just 0.22 m per 100 km.
6.3 Post Processing
Post-processing is used in Differential GPS to obtain precise positions of unknown points by
relating them to known points such as survey markers.
The GPS measurements are usually stored in computer memory in the GPS receivers, and are
subsequently transferred to a computer running the GPS post-processing software. The
software computes baselines using simultaneous measurement data from two or more GPS
receivers
The baselines represent a three-dimensional line drawn between the two points occupied by
each pair of GPS antennas. The post-processed measurements allow more precise positioning,
because most GPS errors affect each receiver nearly equally, and therefore can be cancelled
out in the calculations.
Differential GPS measurements can also be computed in real-time by some GPS receivers if
they receive a correction signal using a separate radio receiver, for example in Real Time
Kinematic (RTK) surveying or navigation.
[23]
Chapter 7
Applications of GPS
The Global Positioning System, while originally a military project is considered a dual-use
technology, meaning it has significant applications for both the military and the civilian
industry.
7.1 Military
• Target tracking: Various military weapons systems use GPS to track potential ground and
air targets before they are flagged as hostile. These weapon systems pass GPS co-ordinates of
targets to precision guided munitions to allow them to engage the targets accurately.
• Navigation: GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and
to coordinate the movement of troops and supplies. The GPS-receivers commanders and
soldiers use are respectively called the Commanders Digital Assistant and the Soldier Digital
Assistant.
• Missile and projectile guidance: GPS allows accurate targeting of various military weapons
including ICBMs, cruise missiles and precision-guided munitions. Artillery projectiles with
embedded GPS receivers able to withstand accelerations of 12,000G have been developed for
use in 155 mm howitzers.
• Search and Rescue: Downed pilots can be located faster if they have a GPS receiver.
• The GPS satellites also carry a set of nuclear detonation detectors consisting of an optical
sensor (Ysensor), an X-ray sensor, a dosimeter, and an Electro-Magnetic Pulse (EMP) sensor
(W-sensor) which form a major portion of the United States Nuclear Detonation Detection
System.
Figure 7.1: Military application of GPS
[24]
7.2 Civilian
Many civilian applications benefit from GPS signals, using one or more of three basic
components of the GPS: absolute location, relative movement, and time transfer. The ability
to determine the receiver's absolute location allows GPS receivers to perform as a surveying
tool or as an aid to navigation. The capacity to determine relative movement enables a
receiver to calculate local velocity and orientation, useful in vessels or observations of the
Earth. Being able to synchronize clocks to exacting standards enables time transfer, which is
critical in large communication and observation systems. An example is CDMA digital
cellular. Each base station has a GPS timing receiver to synchronize its spreading codes with
other base stations to facilitate inter-cell hand off and support hybrid GPS/CDMA positioning
of mobiles for emergency calls and other applications. Finally, GPS enables researchers to
explore the environment including the atmosphere, ionosphere and gravity field. GPS survey
equipment has revolutionized tectonics by directly measuring the motion of faults in
earthquakes.
Figure 7.2: Civilian use of GPS
[25]
Chapter 8
Missile Guidance and Control
A guidance and control system meets the requirements of a servomechanism as it is implied
in the above definition. The guidance portion of the system develops the variable input signal.
The input signal represents the desired course to the target. The missile control system
operates to bring the missile onto the desired course. Therefore, you can say that the output of
the guidance and control system is the actual missile flight path. If there is a difference
between the desired flight path (input) and the one the missile is actually on (output), then the
control system operates to change the position of the missile in space to reduce the error.
When the missile has been steered to the desired course, the guidance system will detect no
error and the control system will not move the control surfaces in response to a guidance
error, because there isn't any.
The units of the guidance system may be carried in the missile (as in active and passive
homing), or they may be distributed between the missile and the launching ship (as in beam-
rider and semi active homing missiles). The principal functions of the guidance system are to
detect the presence of the target and track it; to determine the desired course to the target; and
to produce electrical steering signals which indicate the position of the missile with respect to
the required course.
The units that respond to the guidance signals and actuate the control surfaces make up the
major division referred to in figure 4-2 as the CONTROL SYSTEM. For convenience we will
include gyros in this system. The units in the control system may be considered as consisting
of two groups: the COMPUTER, and the CONTROL-SURFACE SERVOSYSTEM.
Specific computer units vary widely in design because of basic differences in the type of
guidance used. But in most cases this section contains damping instruments (accelerometers
and rate gyros), summing networks (electrical circuits that add and subtract voltages), and
servo amplifiers as principal components. In general, these units originate information about
missile motion and flight attitude; add this data to incoming guidance signals, and produce
output voltages suitable for operating the control-surface servo.
A typical control-surface servo section is made up of hydraulic units. This section serves as a
power stage of the control system; it releases large amounts of energy under accurate control.
The principal parts of this section are electrically operated servo valves and the wing or tail
hydraulic actuator units which make the adjustments to guide and stabilize the missile. In the
newest missiles, hydraulic actuators are replaced by electric systems. This saves considerable
weight and space in the missile.
[26]
Figure 8.1: Missile guidance and control
As indicated by the feedback loops shown in figure 4-2, the basic operation of the guidance
and control system is based on the closed-loop or servo principle. The control units make
corrective adjustments of the missile control surfaces when a guidance error is present, in
other words, when the missile is not on the correct course to the target. The control units will
also adjust the control surfaces to stabilize the missile in roll, pitch, and yaw. You must keep
[27]
in mind that guidance and stabilization are two separate processes, even though they occur
simultaneously.
Figure 8.2: Essential parts of basic guidance and control system
To make this idea clearer, think of you throwing a dart with its tails removed. It is possible
for you to hit the bulls-eye of a target because your arm and brain guide the dart onto the
proper trajectory to score a hit. But without its tail surfaces to stabilize it, it is very possible
that the dart will land on the target in some position (attitude) other than point first. Well,
missiles are like darts or arrows in this respect, and must be stabilized about the three (roll,
pitch, and yaw) axes we talked about in chapter 3, so that the missile will fly nose first and
will not oscillate about its direction of flight. So in summary we can say that, if there is an
error in missile heading due to guidance or stabilization, the corrective actions taken by the
control system are such that any error present in the system is reduced to zero. This is true
servo action, as you have learned in previous Navy courses on basic electricity and
electronics.
[28]
Chapter 9
Missile Guidance using GPS
The basic idea behind GPS is to use satellites in space as reference points for locations on
earth. With GPS, signals from the satellites arrive at the exact position of the user and are
triangulated. This triangulation is the key behind accurate location determining and is
achieved through several steps.
9.1 Determining Your Position
Suppose we measure our distance from a satellite and find it to be 11,000 miles (how it is
measured is covered later). Knowing that we're 11,000 miles from a particular satellite
narrows down all the possible locations we could be in the whole universe to the surface of a
sphere that is centered on this satellite and has a radius of 11,000 miles. Next, say we
measure our distance to a second satellite and find out that it's 12,000 miles away. That tells
us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles from
the second satellite, i.e. somewhere on the circle where these two spheres intersect. If we then
make a measurement from a third satellite and find that we're 13,000 miles from that one, that
narrows our position down even further, to the two points where the 13,000 mile sphere
cuthrough the circle that's the intersection of the first two spheres.
So by ranging from three satellites we can narrow our position to just two points in space. To
decide which one is our true location we could make a fourth measurement. But usually one
of the two points is a ridiculous answer (either too far from Earth or moving at an impossible
velocity) and therefore can be rejected without a measurement.
9.2 Measuring Your Distance
How the satellites actually measure the distance is quite different from determining your
position and essentially involves using the travel time of a radio message from the satellite to
a ground receiver. To make the measurement we assume that both the satellite and our
receiver are generating the same pseudo-random code at exactly the same time. This pseudo-
random code is a digital code unique to each satellite, designed to be complex enough to
ensure that the receiver doesn't accidentally sync up to some other signal. Since each satellite
has its own unique Pseudo-Random Code this complexity also guarantees that the receiver
won't accidentally pick up another satellite's signal. So all the satellites can use the same
frequency without jamming each other. And it makes it more difficult for a hostile force to
jam the system, as well as giving the DOD a way to control access to the system.
[29]
By comparing how late the satellite's pseudo-random code appears compared to our receiver's
code, we determine how long it took to reach us. Multiply that travel time by the speed of
light and you obtain the distance between the receiver and the satellite. However this calls for
precise timing to determine the interval between the code being generated at the receiver and
received from space. On the satellite side, timing is almost perfect due to their atomic clocks
installed within each satellite. However as it would be extremely uneconomical for receiver
to use atomic clocks a different method must be found.
GPS solves this problem by using an extra satellite measurement for the following reason: If
our receiver's clocks were perfect, then all our satellite ranges would intersect at a single
point - our position. But with imperfect clocks, a fourth measurement will not intersect with
the first three satellite ranges. So the receiver's computer will then calculate a single
correction factor that it can subtract from all its timing measurements that would cause them
all to intersect at a single point. That correction brings the receiver's clock back into sync
with universal time , ensuring (once the correction is applied to all the rest of the receivers’
measurements) precise positioning.
9.3 Error Correction
As would be expected, a variety of different errors can occur within the system, some of
which are natural, whilst others are artificial. First of all, a basic assumption, the speed of
light, is not constant as this value changes as the satellite signals travel through the
atmosphere. As a GPS signal passes through the charged particles of the ionosphere and then
through the water vapour of the troposphere it gets slowed down, and this creates the same
kind of error as bad clocks. This problem is tackled by attempting to use modelling of the
atmospheric conditions of the day, and using dual-frequency measurement, i.e. comparing the
relative speeds of two different signals. Another problem is multipath error, this is when the
signal may bounce off various local obstructions before it gets to our receiver. Sophisticated
signal rejection techniques are used to minimize this problem.
There are also potential problems at the satellites. Minute time differences can occur within
the onboard atomic clocks, and sometimes position (ephemeris) errors can occur. These other
errors can be magnified by a high GDOP "Geometric Dilution of Precision" This is where a
receiver picks satellites that are close together in the sky, meaning the intersecting circles that
define a position will cross at very shallow angles. That increases the grey area or error
margin around a position. If the receiver picks satellites that are widely separated the circles
intersect at almost right angles and that minimises the error region. Obviously good receivers
determine which satellites will give the lowest GDOP. and so they were considerably more
accurate.However, effective May 2, 2000 selective availability has been eliminated.
[30]
Chapter 10
Conclusions
GPS functionality has now started to move into mobile phones. GPS has a variety of
applications on land, at sea and in the air. Basically, GPS is usable everywhere except where
it's impossible to receive the signal such as inside most buildings, in caves and other
subterranean locations, and underwater. The most common airborne applications are for
navigation by general aviation and commercial aircraft. At sea, GPS is also typically used for
navigation by recreational boaters, commercial fishermen, and professional mariners.
Landbased applications are more diverse. The scientific community uses GPS for its
precision timing capability and position information. Surveyors use GPS for an increasing
portion of their work. GPS offers cost savings by drastically reducing setup time at the survey
site and providing incredible accuracy. Basic survey units, costing thousands of dollars, can
offer accuracies down to one meter. More expensive systems are available that can provide
accuracies to within a centimetre. Recreational uses of GPS are almost as varied as the
number of recreational sports available. GPS is popular among hikers, hunters,
snowmobilers, mountain bikers, and cross-country skiers, just to name a few.
[31]
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