3.1 GNSS Basic Principles
Learning Objectives
After successfully completing this chapter you should be able to:
- explain the basic principles of satellites’ motion using Kepler’s laws of planetary motion
- describe satellites’ position in space using ECEF Cartesian coordinate system
- explain the different satellite orbits parameters for different GNSS
- define and explain the basic components of a GNSS signal structure
- explain the fundamental concepts and importance of GNSS time systems
- explain the function and structure of navigation data messages
- differentiate between the almanac and an ephemeris, and explain how and why they are used
- list error sources in GNSS positioning
- list accuracy levels in GNSS.
In the beginning…
In 1942 the German rocket (technically it was a guided long range ballistic missile) V-2 was the first man-made object to reach space, reaching around 80 km above MSL. In 1957, the Union of Soviet Socialist Republics launched the Sputnik 1 (“Satellite 1” in English) successfully, and it orbited the Earth at around 250 km. Sputnik 1 had two radio transmitters on board, and it emitted beeps that could be heard on radios across the planet. The radio signals that were transmitted were used to gather information about the Earth’s atmosphere, as well as understanding temperature and pressure in space.
The idea that we could use radio signals from satellites for positioning wasn’t that big of a leap from the use of radio waves on the ground for measurement. And once Sputnik 1 carried transmitters successfully, it was really only a matter of time before a military started experimenting with the idea. As with most technological advances, the military involvement in GPS meant a scale of investment that would have been almost impossible in the civilian sector.
It turns out the key ingredients for GPS were Scottish & German physicists, a German mathematician, a World War, a Cold War, some astrophysicists and electrical engineers, a space race and a huge amount of money!