Tracking Satellites with PREDICT

A look at the development and use of an open-source satellite-tracking and orbital-prediction program.

When Sputnik 1 was launched into orbit on October 4, 1957, the space age was born and the fields of science, engineering and technology were changed forever. At last count, there were over 8500 payloads from over 30 countries in orbit around the earth. All of these spacecraft are bound to their home planet by the Earth's gravitational field, and their motions can be described by simple principles of gravity and planetary motion discovered by scientists such as Isaac Newton and Johannes Kepler hundreds of years ago.

Today, earth-orbiting satellites serve many purposes and play important roles in global positioning and navigation, communication networks, scientific exploration, earth resource research, national defense, weather monitoring and education. USSPACECOM, the United States Space Command (formerly NORAD), along with NASA, the National Aeronautic and Space Administration, use radar and optical-ranging techniques to keep close track of the thousands of man-made objects in earth orbit and provide orbital data suitable for orbital modeling and open-ended tracking of unclassified payloads. With an accurate set of orbital parameters in one's possession, it is possible to determine velocities and the past, present and future positions of a satellite in its orbit around the earth with a degree of accuracy suitable for many science and engineering applications.

The Development of PREDICT

PREDICT is an effort to bring a versatile, open-source, satellite-tracking and orbital-prediction application to the Linux operating system. PREDICT was adapted from ideas developed in earlier satellite-tracking and orbital-prediction software written nearly a decade ago for use on the then-popular Commodore 64 home computer.

The original version of PREDICT was created as a replacement for the QuickTrak satellite orbital prediction program that was also available for the C64. While QuickTrak was written in BASIC and its source code was interpreted at runtime, PREDICT was written in C and compiled into 6502 machine code. The sole reason for writing PREDICT was to be able to quickly forecast passes of amateur radio communication satellites in advance of their arrival.

For real-time satellite tracking, a separate program called SpaceTrack was written, using a combination of BASIC and hand-assembled machine code. SpaceTrack was sophisticated enough to permit the display of a satellite's position on a bit-mapped Mercator projection map of the world. It even had the ability to articulate the tracking coordinates of a satellite through a voice synthesizer connected to the Commodore 64's user port. The speech synthesizer was used to relay tracking coordinates to a visual observer in real time over a short-range radio link so that the Mir space station and other large spacecraft could be easily located and identified in the evening sky. The speech routines were written entirely in hand-assembled machine language and executed through the same address vector as the computer's hardware interrupt routines. This essentially created a multitasking environment, with the voice synthesizer receiving data through a background process that in no way interrupted the numerical processing taking place in the foreground by the BASIC interpreter.

Although neither program was ever released to the public, they served me quite well for several years until a switch from the aging Commodore 64 to a more modern MS-DOS-based PC was made. In many ways, the switch to the MS-DOS platform was a significant step backward from the C64, especially in terms of programming flexibility and the requirement to relearn the programming process under the new environment. PREDICT was eventually ported to MS-DOS, but the MS-DOS environment simply was not enticing enough to add many new features to the program. Furthermore, there was seemingly no simple way of multitasking and passing parameters between processes under MS-DOS as was possible (as odd as it sounds) on the older and less-sophisticated C64.

PREDICT was also ported to several multi-user UNIX machines around the same time, but hardware differences and the lack of a true understanding of the operating environment prevented further development of the program. Nevertheless, the DOS version of PREDICT was polished up and released to several popular Internet and dial-up software repositories as free software in May 1994, and became quite popular among amateur radio operators involved in satellite communications.

By the time Windows 95 was released, it was time to switch computing environments entirely to Linux. PREDICT was successfully ported from DOS to Linux, and has functioned well in the Linux environment for many years. A pre-compiled Linux binary of PREDICT was released as free software to several FTP sites in 1996. Then in 1999, major portions of the program were rewritten, and in an effort to enhance PREDICT's functionality, several real-time satellite-tracking modes similar to those available in the original SpaceTrack program were added to the program. Speech routines were also added, but instead of using a voice synthesizer to produce vocal announcements, audio samples were sequentially directed to the system sound card using a separate program that was invoked by PREDICT. Much like the original design of SpaceTrack, the speech routines were executed as background processes so as not to delay the execution of real-time orbital calculations while the announcements were being made.