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Space The Final Frontier

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Space - The Final Frontier!

As a kid, I never miss a sci-fi movie or space and technology documentary on TV. The gadgets/robots in the movies or shows always fascinate me. For example, the communicator that Captain Kirk used in the Star Trek Series or the scene of the complex insides of Luke's new robotic arm in Stars Wars:Return of the Jedi or inventions presented in Beyond 2000 and many more!

My interest in satellites grew when I was in the polytechnic. Satellites, mentioned in this page, refer to man made spacecrafts that orbit a heavenly body e.g. Earth, Jupiter etc. There, I learnt about the basics of a satellite system and the process of bringing the satellites into orbit. I also had the chance to use a satellite dish for receiving signals from one of the satellites in space, once in a laboratory session. I have studied on geostationary satellites and GPS systems and satellites. There are many uses of satellites such as surveying, weather forecasting, communications, space exploratory, astronomy (Hubble Space Telescope) and positioning applications (GPS).

The GPS systems are composed of three integral design parts --- the space segment, the ground control segment and the user segment.

The space segment consists of the GPS satellites, which transmit signals on the two phase modulated frequencies. These transmissions are carefully controlled by highly stable atomic clocks inside the satellites. The satellites also transmit a navigation message that contains, among other things, orbital data for computing the position of all satellites.

In the GPS system, 21 constellations of Navstar satellites plus 3 active on orbit spares are operating in six orbital planes at approximately 20,200 km above the earth surface. Four satellites will be located in each plane. There are six circular orbital planes, labeled A - F, evenly spaced in right ascension and inclined at 55° with respect to the equator. These orbital planes are spaced with 120° phasing between each plane. The orbital period of the Navstar satellites is just under 12 hours (718 minutes). The satellite will complete two orbital revolutions while the earth rotates 360°. This means the satellites will rise about 4 min. earlier each day. Each orbital plane consists of four satellites; however, to optimize global satellite visibility, the satellites are not evenly spaced within the orbital plane.

This orbit design was developed to guarantee that at least four satellites are always in view at every point on the earth's surface 24 hours a day. In many instances, however, as many as 12 are visible to a ground-based user. The circular orbit design and elevation make the system very stable in the long term with orbit variations that are relatively easy to model, in comparison, say, to low orbiting satellites.

The transmission technology employed in GPS is spread spectrum, the very concept in today's CDMA cellular systems and future 3G communications system. Topics that I am interested in are more of the user segment such as principle of GPS operations, the GPS signal, code and frequency acquisition, and code and frequency tracking. I have written two reports on GPS. One was "GPS in a nutshell" written by me for the general readers (you will need Adobe Acrobat Reader to open this pdf file). The other was "GPS in the context of Spread Spectrum Communications" written by 3 other friends and me for our Spread Spectrum Communications module in our university days.

References

Tom Logdons. The Navstar GPS. New York: Van Nostrand Reinhold, 1992.

Jeff Hurn. GPS A Guide to the Next Utility. Sunnyvale,USA: Trimble Navigation, 1989.

B. Hoffmann-Wellenhof, H. Lichtenegger, and J. Collins,Global. Positioning System Theory and Practice. Springer-Verlag Wien, New York, 1992.

Mohinder S.Grewal, Lawrence R.Weill, Angus P.Andrews. Global Positioning Systems, Inertial Navigation, and Integration. John Wiley&Sons Inc., 2001.

Alfred Kleusberg, Peter J.G.Teunissen. GPS for Geodesy. Springer.

G F Knoernschild. Global Positioning System for Vehicle Navigation and Positioning Reporting. Rockwell International Corp. 1986

Sirin Tekinay. Next Generation Wireless Networks. Kluwer Academic Publishers, 2001.

www.trimble.com

Satellites also serve the purpose of exploring our universe and helping us to better understand our universe.

Even as the Voyager spacecraft completed their initial reconnaissance of the outer solar system in the late 1970s and 1980s, we have not learnt enough on our solar system and more missions have been sent to study our heavenly neighbours. Here are some of the missions that I have been keeping tabs on through the JPLNewsletter.