Satellite Tracking 

October 4, 1957, Sputnik, the first artificial earth satellite, was launched by the USSR, and thus began the space age. Now, 45 years after, there are more than 5,000 satellites in outer space, and tracking and controlling them is big work. Thousands of observatories, computers, communication systems and human beings work round the clock, around the earth, to keep their satellites in proper condition and on the right path. Telemetry, tracking and control are three major functions that make this happen.

Satellite telemetry is measuring the conditions in the atmosphere and oceans from outer space using artificial satellites, and monitoring the performance and condition of rockets, spacecraft and satellites that send information about their own performance, flight path and other data to earth. With this, the earth stations keep track of the spacecraft’s condition in outer space and are able to control it from the ground. This processing has to be done in real time using real-time systems (systems that guarantee the execution of a job in the specified time period with no delays). This means developing high performance and robust systems that will work in extreme conditions and constraints of space, weight and power in satellites. Earth stations, too, demand a high degree of expertise and planning while developing such systems.

A basic telemetry system consists of a measuring instrument, transmitter and receiving station. The measuring instruments are sensors (transducers) that measure the amount of a physical attribute and transform the measurement to an engineering unit value. This is multiplexed as a single data stream, formatted and then transmitted using radio or microwave signals. Hence, measurements can be made at remote or inaccessible spots, and the data collected is transmitted to the receiving equipment for monitoring, display, recording and analysis.

On the ground, the receiving station receives the signals transmitted by the satellite, which are then sent for processing. The ground system makes sense of the enormous amount of data received and presents it in a user-friendly manner to the operators. They use a variety of display objects–like strip charts, bar charts, vertical meters, cross plots–and analysis programs–like Excel or high-level analysis languages (Matlab)–to evaluate archived data, extract results and generate reports. A satellite’s conditions is also monitored in real time so that decisions regarding its operation and path can be taken instantaneously in case of a problem. The display systems run on both UNIX (GUI using MOTIF and X-Windows) and PC platforms. The communication systems for interfacing with ground stations network and the real-time systems for real-time processing of satellite data generally use UNIX. The communication system uses TCP/IP protocol for data transfer between computers on the ground station networks. The data is stored on disks or tapes as files, but other alternatives are relational database systems like MS Access and Oracle. Scheduling systems generate operation schedules and allocate network resources for satellites supported by the ground station.

Web-based telemetry solutions enable such activities using a Web browser, and can be used from any machine on the ground system network. You can get some from www.netacquire.com. To keep track of an orbiting satellite over the Net for free, check
http://liftoff.msfc. nasa.gov/RealTime/Jtrack.

In India the ISRO telemetry, tracking and command network (ISTRAC) does these. It provides mission support to satellites and launches vehicle missions, and has a network of ground stations across the country and abroad that provide valuable inputs for controlling India satellites.

Anoop Mangla