R Appavu Raj, A K L Bhagat, Sunit Kumar Gupta
DD(RS), Scientist, Scientist
Integrated Test Range Chandipur,
Defence Research and Developmental Organization, India
Range Safety Wing, Integrated Test Range Chandipur,
Balasore – 756025, Orissa, India
Email: [email protected], [email protected]
Tel: 06782-272103, 06782-272032, Fax: 06782-272170
Global Positioning System (GPS) is all weather, worldwide, continuous coverage, most modern, sophisticated and state of art yet simple and user friendly Satellite based navigation system . GPS is a frontier and cutting edge Technology. Its application in Military and civilian purpose is tremendous and ever increasing. Since it’s inception it has grown to provide not only worldwide, all-weather navigation, put precise position determination capabilities to all manner of users. The resulting precision available exceeds any previously attainable without large expenditures of time and resources. Few of the many applications include precise timing, position fixation, survey, navigation, auto landing etc. If present trends are of some indication then it is amply clear that GPS is all set to invade all aspects of human life. It has inherent potential to cater much more than what it is catering today. Lots of Research and developmental activities are going on around the world to explore the avenues of opportunities pertaining to the application of GPS. In view of its global and wide range of application the more we talk about GPS the lesser it is. Unlike any other technology it has carved a niche for itself and likely to revolutionize the way of thinking.
As a technology it has got a vital role in Integrated Test Range applications. For the flight evaluation of any test object, it’s time and space position information plays a crucial role in determining its performance against pre-determined trajectory parameters. In this regard, Test Range has got a prime role in delivering the accurate time and space position information data to the designers. The capabilities of GPS technology, in terms of data accuracy, speed of data availability and reduction of test operating cost moved Integrated Test Range to make an investment and integrate itself with GPS .
2. Range Calibration methodology
Test Range has got a prime role in providing the accurate time and space position information data to the designers/users. Thus requires an accurate calibration source for tracking instruments. As performance of various test vehicles are evaluated presently using various tracking instrumentation systems like optical/Infrared, Radar, Telemetry, DGPS (Differential Global Positioning System) has a prime role in calibrating these trackers.
To fulfill the above objective, Range validation helicopter/ aircraft sortie has been conducted in Test Range. During sortie, all the tracking sensors deployed at various sites track the target. At the same time rover GPS receiver set installed in helicopter/ aircraft (i.e. on board) also provides positional information of helicopter/aircraft. The GPS reference station is installed at precisely surveyed benchmark location. In real time RTCM corrections are transmitted from reference station to rover station by UHF Trans-receiver communication antenna installed at reference station. The same antenna is receiving the rover station positional information from air born object. The positional information derived from GPS along with the other tracking sensors has been displayed. Sortie controller identifies GPS and other tracking sensor position using different color displays and accordingly he is giving instruction to pilots as per the range instrumentation requirements in real time. For this purpose, a real time DGPS data Acquisition and display software has been developed .
3. Details of system configuration and data Acquisition and display software
The basic DGPS system configuration for a typical calibration sortie is given below
The rover receiver is on the aircraft/helicopter and reference receiver is on the benchmark. In this case the two receivers communicate in real time over radio channel with the help of radio modem associated with each of them, radio modem works in half-duplex mode using TDMA, means it can either transmit or receive at any one instant. In fact, it takes a finite amount of time to change from transmission to reception mode.
TDMA Mode makes use of the precise UTC time determined by the GPS receiver as part of its Navigation solution to precisely control the transmission periods of a radio modem. This TDMA mode allows users to interleave the transmissions from multiple radios operating on the same frequency without jamming each other. In a TDMA system, the reference station transmits RTCM information in the first part of the controlled time interval and the mobiles return their information in the subsequent parts. A GPS receiver controls transmissions to make sure that signals do not interfere with each other. In addition, the radio modem is able to compress standard status messages into a compact form, which further reduces the amount of time required for transmission. The radio modem also uses addressing derived from the GPS receivers ID numbers, directing messages to the correct destinations.
a. Basic functions: –
The basic function of the software is to receive the positional data of rover GPS though reference receiver through communication port of the PC; the received data is in the ASCII format. The processing of data is done in order to extract necessary information from it, the steps involved in it can be mentioned as below:
- This ASCII message is first converted in to the different variables (integer, float and string) for further processing depending upon the message length.
- As the data received are in standard WGS-84 co-ordinate system, so this data is first converted in to the ECEF and then local Easting-Northing w.r.t the reference point by keeping the applying earth curvature.
- Finally, the required data is packed in the structure for sending it to the data processing center over e-net. Data processing center receives this data and plot it along with other sensors for real time comparison and monitoring the aircraft position with other sensors.
b. Functional modules: –
The software has been divided in to the number of modules, which runs in parallel to make the overall software functional. The brief details of these modules are given below.
- Data acquisition modules
Data input to this block is receiver identification number, date, GPS time, position in latitude, longitude, mean sea level (MSL) height w r t WGS84 datum, operating mode, horizontal accuracy, speed and best five satellites.
- Data processing and storage modules
The input rover positional data format is latitude, longitude and MSL height. In order to make compatible with other tracking sensor format for display and comparison purpose, it is converted into easting (X) and northing (Y) coordinate with respect to some reference point by applying suitable earth curvature correction. The converted data is then stored in user-defined file.
- Data display and real time plot modules
Rover GPS position is plotted in real time w r t suitable reference point location at GPS reference station. Alphanumeric display of some important parameter is shown in graphic window like GPS time, latitude, longitude, MSL height, X and Y with respect to reference point.
- Data transfer modules
The converted data is then transmitted to central computer for real time plotting along with other tracking sensors. Time synchronization of all tracking sensor with GPS time is very crucial step in this process The flow of data in to these functional blocks can be represented as:-
c. Software’s features: –
The software has been developed in VC++ environment; it has a number of features mentioned below:
- Software can be operated in real time as well as in simulation mode; in real time mode software receives the data from the communication port and in simulation mode it reads the data from the previously stored file.
- Name of data logging file and reference point co-ordinate can be entered in a dialog box through window menu.
- For real time mode, software has the option to set various parameters (Communication port number, baud rate, parity, stop bits etc.) through the menu of the window for making the communication link between the GPS receivers and communication port.
- Software can accept the data in the multi rover and multi reference station scenario, and plots the data in multi color.
4. Testing and validation
The software has been tested in Real time and simulation mode and proven to be satisfying and reliable. It has been successfully used repeatedly in the following dynamic conditions of rover station.
- Low dynamic platform (Jeep trial).
- Medium dynamic platform (Helicopter trial).
- High dynamical platform (Aircraft trial).
During the above exercise steady data link has been observed and this data in turn has been transferred to data processing center. Further analysis shows that the data is also matching with the other sensors data.
The developed real time data acquisition and display software acts as aid to Sortie controller during range calibration aircraft/helicopter sortie. The software can be executed in simulation mode, which helps in data communication link checks with data processing center. This feature makes the developed software a generalized one and it can be modified/changed depending on calibration sortie requirement before each trial of testing and evaluation of flight vehicles, missiles and air born vehicles.
The authors would like to thank the Director, ITR for his encouragement to pursue the R &D activities in Range. Invaluable contribution of Sri R Marndi and Smt Sharabani Bhattacharya, Scientist ITR enabled us to complete the assignment in time. Special thanks are extended to members of Technical Expert Committee & Human Resource Development Group, ITR for their invaluable suggestions. Last but not the least, direct & indirect contributions of all concerned Scientists & Staff of ITR are gratefully acknowledged.
R Appvu Raj, R Marndi and A K L Bhagat, “GPS and DGPS Applications to Test Range”, International Seminar on “GTS to GPS: Geodesy on Move”, Survey of India, Dehradun, 5th to 9th February 2003.
P J G Teunissen, A Kleusberg, “GPS for Geodesy”, January 1996.
Fig 3.1: A Typical Graphical User Interface to the Developed Software