Adaptive Missile Guidance Using GPS

By Pankaj Kumar Yadav 100112 NIT DELHI

INTRODUCTION• 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 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 accuracy.

• Guided missile systems help full fill all of these demands.• Our main aim is to know how GPS is used TO GUIDE THE MISSILE!

GPS

• 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 missile problems.

• 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 accuracy.

• GPS (the full description is: NAVigation System with Timing And Ranging Global Positioning System, NAVSTAR GPS).

• 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 calculate velocity.

• 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 100 nanoseconds.

• SPS provides 100 m horizontal accuracy, 156 m vertical accuracy and time accurate to 167 nanoseconds, and is available to civilian users.

GPS RECEIVERS

• 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 known point.

• 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 source.

• 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 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.

• 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 in accuracy.

• Existing precision weapons require a clear line of sight between the weapon and the target for the optical guidance to work.

CONCLUSION

• 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.

REFERENCES• http://www.trimble.com/gps• http://

ausairpower.net/TE-GPS-Guided-Weps.html• http://

www.spaceandtech.com/spacedata/constellations/navstar-gps_consum.shtml

• http://en.wikipedia.org• http://

www.aerospaceweb.org/question/weapons/q0187.shtml

THANK YOUU!