Adaptive Missile Guidance Using
Pankaj Kumar Yadav
• 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
• On ethical grounds, one prays that each warhead deployed during a
battle will strike only its intended target, and that innocent civilians
will not be harmed by a misfire.
• From a tactical view, 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
• Guided missile systems help full fill all of these demands.
• Our main aim is to know how GPS is used TO GUIDE THE MISSILE!
• What is GPS?
• How Does it work?
• How is it useful in Guiding Missiles?
What is GPS?
• GPS-stands for Global Positioning System
• Initiated by AEROSPACE Corp in 1960 for the purpose
of applying advanced technology to space and ballistic
• In 1963 the company started working on Project 631-
GLOBAL POSITIONING SYSTEM, a scheme for replacing
the astro-navigation systems with satellite navigation.
• Astro-Navigation requires clear sky, Satellite Navigation
uses micro-waves and satellite distributed master clock
thereby providing all weather option and superior
• GPS (the full description is: NAVigation System with
Timing And Ranging Global Positioning System,
• Using the Global Positioning System the
following two values can be determined
anywhere on Earth:
• 1. One’s exact location (longitude, latitude and
height co-ordinates) accurate to within a range
of 20 m to approx. 1 mm.
• 2. The precise time accurate to within a range
of 60ns to approx. 5ns. Speed and direction of
travel (course) can be derived from these co-
ordinates as well as the time. The coordinates
and time values are determined by 29
satellites orbiting the Earth.
• During the development of the GPS system,
particular emphasis was placed on the
following three aspects:
1. It had to provide users with the capability of
determining position, speed and time, whether
in motion or at rest.
2. It had to have a continuous, global, 3-
dimensional positioning capability with a high
degree of accuracy, irrespective of the weather.
3. It had to offer potential for civilian use.
How Does GPS work?
• The operational GPS Constellation uses 29 satellites of
which 5 are spares, orbiting in precise 12 hour orbits.
• The orbit geometry is adjusted so that these orbits
repeat the same ground track once per day, and at any
point on earth’s surface and at any given time the same
configuration of satellites should be seen.
• The satellites are grouped in sets of four into six orbital
planes, each of which is inclined at approx. 55 degrees
to the polar plane.
• Each satellite transmits its exact position and its precise
on board clock time to Earth at a frequency of 1575.42
MHz. These signals are transmitted at the speed of light
(300,000 km/s) and therefore require approx. 67.3 ms to
reach a position on the Earth’s surface.
• A GPS Receiver measures time of signal propagation from four
or more satellites, and uses this information to calculate the
receivers position in 3 axes, using the WGS-84 earth model.
*WGS-World Geodetic System
• The satellites are controlled via a worldwide network of
tracking stations of which the main base station is situated in
Falcon Air Force Base in Colarado.
• The Master Control station measures signals from the
satellites to incorporate into precise orbital mathematical
models, which are then used to compute corrections for the
clocks on each satellite.
• These corrections, and orbital data are then uploaded to the
satellites, which then transmit them to GPS user's receivers.
• A GPS receiver can then use these signals to compute its
geographical coordinates, measure time, and also then
• The distance S to the satellite can be
determined by using the known transit time τ:
• distance = transit time * speed of the light
• S = τ * c
• The simplest geometrical model to use is the sphere model -
knowing the range to any given satellite places the receiver on
the surface of a sphere centred upon the satellite, with a
radius equal to the measured range.
• Knowing the range to two satellites places the receiver on the
curve where the two respective spheres intersect.
• Knowing the range to a third satellite places the receiver at
the intersection point common to all three spheres.
• In practice, however, a fourth range measurement to yet
another satellite will be required to compensate for the
inaccuracy in the receiver's clock.
• The result is a set of equations, which if solved yield the
position of the receiver and the time.
In reality, a position has to be determined in three-dimensional space, rather than on a
plane. As the difference between a plane and three-dimensional space consists of an extra
dimension (height_ Z), an additional third satellite must be available to determine the true
position. If the distance to the three satellites is known, all possible positions are located
on the surface of three spheres whose radii correspond to the distance calculated. The
position sought is at the point where all three surfaces of the spheres intersect
• The GPS system provides two navigational
services, the military Precise Positioning
Service (PPS), and the civilian Standard
Positioning Service (SPS).
• PPS provides a accuracy of 17.8 m horizontally,
27.7 m vertical accuracy and time accurate to
• SPS provides 100 m horizontal accuracy, 156 m
vertical accuracy and time accurate to 167
nanoseconds, and is available to civilian users.
• The simplest of receivers are single channel receivers, which
share a single channel of receiver hardware across the satellites
in view.(cheap but performance wise not good enough)
• Most high performance receivers are 5 channel receivers. The
strategy used here is for 4 channels to track satellites and one
channel to look for the next satellite to come into view.
• Normally receivers use an antenna , receiver hardware.
• Rockwell Commercial 5-Channel GPS Receiver(in the image
below) fits in a 4*2.5 in PCB.
DGPS- Differential GPS
• Technique called differential correction can yield accuracies
within 1 -5 meters, or even better with advanced equipment.
• Differential correction requires a second GPS receiver, a base
station, collecting data at a stationary position on a precisely
• Because physical location of base station is known, a correction
factor can be computed by comparing known location with GPS
location determined by using satellites.
• Differential correction process takes this correction factor and
applies it to GPS data collected by the GPS receiver in the field.
-- Differential correction eliminates most of errors.
MISSILE GUIDANCE USING GPS
• Missile guidance concerns the method by which a missile receives its
commands to move along a certain path to reach its target.
• On some missiles these commands are generated internally by the
missile computer auto-pilot. On others it is generated by some external
• The missile sensor or seeker, 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
• 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.
• The next incremental step is then to update the
weapon before launch with a DGPS derived
position estimate, which will allow it to correct its
GPS error as it flies to the target, such weapons
are designated "precise" and will offer accuracies
greater than laser or TV guided weapons.
• For an aircraft to support these guidance
systems, it will require a DGPS receiver, a GPS
receiver and interfaces on its multiple ejector
racks to download target and launch point
coordinates to the weapons.
• The development of purely GPS guided missiles
will produce substantial changes in how air
warfare is conducted.
• Unlike a laser-guided weapon, a GPS guided
weapon does not require that the launch
aircraft remain in the vicinity of the target to
illuminate it for guidance – GPS guided
weapons are true fire-and-forget weapons,
which once released, are wholly autonomous,
and all weather capable with no degradation
• Existing precision weapons require a clear line
of sight between the weapon and the target
for the optical guidance to work.
• This technology promises a revolution in air
warfare not seen since the laser guided bomb,
with single bombers being capable of doing the
task of multiple aircraft packages.
• In summary, GPS 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.