1 Introduction
1.1 What is Gpredict?
Gpredict is a real-time satellite tracking and orbit prediction program. A
satellite tracking program is a computer program that predicts the position
and velocity of a satellite at a given time using a mathematical model of the
orbit. Once the position and velocity of the satellite is known other data can
be calculated, for example bearing, distance, footprint, and visibility just to
mention a few. Figure 1.1 shows a diagram of the core functionality of a
satellite tracking program.
Gpredict, like any other satellite tracking program, takes three kinds of
input:
1. Keplerian Elements describing the satellite orbit, as well as the
position and velocity of the satellite at a given time t
0
.
2. Position of the ground station on Earth.
3. The date and time at which the satellite position and velocity should
be calculated.
Given the data above, gpredict calculates the position and velocity of any
Earth orbiting satellite using the NORAD SGP4/SDP4 algorithms. These
algorithms solve Kepler's equation of orbital motion and apply various
corrections to compensate for irregular effects like the shape of Earth and
gravitational influence from other celestial bodies.
Gpredict has several ways of presenting the calculated satellite data to the
user. It can show the position, footprint (i.e. coverage area), and ground
track of the satellites on maps, it can show detailed satellite data in tables,
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Figure 1.1: Functional overview of a satellite tracking program.