SPECIFICATION OF THE INVENTION
1. Title of the invention
A novel system and method for computing the height accuracy of Radio altimeter using differential global position system (DGPS).
2. Field of Invention
The present invention relates to compute the accuracy of height computed by Radio altimeter unit. In particular, it relates to a methodology of using the altitude data, measured simultaneously by DGPS systems (DGPS airborne system, DGPS ground system) and Radio altimeter unit.
3. Prior art and draw backs of prior art
Aircraft is equipped with variety of sensors. Sensors which are under development are required to undergo performance evaluation. Prior to this invention, the performance evaluation of the sensor such as Radio altimeter was being done using Laser based sensors. Laser based sensors provides height above ground level, but laser based sensors are expensive and its operation for low altitude is not eye safe due to usage of LASER.
4. Aim of Invention
The main objective of this invention is to provide a novel method to find accuracies of height computed by Radio altimeter using differential global position system (DGPS).
5. Summary of the Invention
This invention presents a system employing a novel method to find the accuracy of height data, obtained from Radio Altimeter using DGPS. The system consists of a Data analysis system, Radio altimeter unit and DGPS system (DGPS airborne system, DGPS ground system).
DGPS system consists o f a ground station and airborne unit installed on the aircraft. The static ground based Reference Receiver positioned at a known location that has been previously surveyed. Both the airborne and the ground based receiver records that position data, which can be collected and stored for later processing. DGPS is based on the principle that receivers in the same vicinity will simultaneously experience common errors on a particular satellite ranging signal. In general, the airborne r eceivers us e measurements from the ground

based to remove the common errors. The DGPS positioning equations are formulated so that the common errors are cancelled. With this method DGPS gives corrected altitude information along with latitude and longitude. Each time the aircraft moves slowly across the runway and runway altitude is monitored, this method is called runway profiling. After runway profiling, aircraft flies above runway i.e level flight and corresponding altitude of aircraft is monitored The difference of runway profiling altitude and level flight altitude gives the point to point, across runway terrain height. These heights are compared point to point with a radio altimeter computed altitude. The difference computed this way gives error in height computation by radio Altimeter.
6. Brief Description of the Drawings
Figure 1 is Block diagram of systems for computing the height accuracy.
Figure 2 is DGPS altitude data across runway on ground.
Figure 3 DGPS altitude data across runway in flight.
Figure 4 is Difference between DGPS ground and airborne data.
Figure 5 Radio altimeter height across runway .
Figure 6 is difference between DGPS and radio altimeter data.
(Note :ln all graph X axis is time in terms of samples and Y axis is altitude in feet)
7. Detailed Description of the invention
The present invention performs evaluation of accuracy of the height data using a Data analysis system methodology which receives altitude data from Radio altimeter using DGPS as reference altitude sensor. This evaluation is done to achieve the best possible estimates for height accuracy.
The DGPS system consists of ground and airborne station. The ground DGPS station is located at previously surveyed position. Hence it is fixed at particular position. Other DGPS system is installed on the aircraft. Its position moves according to movement of aircraft. A ircraft will fly in nearby vicinity of DGPS ground station. Hence the errors due to multipath reflection, signal noise in data will be of same nature in airborne and ground station.

At first step, runway profiling will be done. This can be done by moving the aircraft slowly across the runway. In this way each position of runway like latitude, longitude and baro altitude will be recorded in DGPS unit fitted in aircraft This data will be download and processed with ground based DGPS data. So this data set will be free from noise. In the same time in other unit called Flight test instrumentation, Radio altimeter height data will be recorded. Radio altimeter gives height of the aircraft above terrain. This time since aircraft is in the ground its height above ground will be zero.
At step two, the aircraft does level flight above the same runway. At this time airborne DGPS system records latitude, longitude and altitude information. Also flight test instrumentation system will records height above terrain computed by Radio altimeter.
At step three, airborne DGPS data will be downloaded and processed with ground DGPS data. By performing this corrected latitude, longitude, altitude information above runway will be available. Similar way height computed by radio altimeter will be download from flight test instrumentation system.
At step four, difference between DGPS data during runway profiling and during flight will be computed. This difference gives the height of the aircraft above terrain.
At step five, this height will be compared with height computed by Radio altimeter. The data recorded during this exercise will have many samples w.r .t to time position. For each recorded sample difference of DGPS altitude is compared with height computed by Radio Altimeter. This error will be for each recorded data.
The total error is expressed by the deviation in error obtained by taking the square root of the sum of individual error:
sx = (sj2 + (sb)2 + (sc)2 + ... By doing this accuracy of the height computed by Radio altimeter will be verified.

CLAIMS
We Claim
1. A data analysis system and methodology for evaluating the accuracy of height
computed by the Radio Altimeter unit by using the various altitude sensor
systems.
The system comprising of airborne DGPS system, ground DGPS system, Radio Altimeter unit, satellites, Flight test instrumentation system.
2. A data analysis system, as claimed in Claim 1, have a method to perform runway profiling, downloading altitude information from DGPS, RADALT and filtering of data based on runway.
3. A data analysis system, as claimed in Claim 1, has a method to compute the point to point error between Radio altimeter and DGPS height and RMS for identifying the height accuracy of radio altimeter.