Chapter 2: Orbital Aspects of Satellite Communications

Here is the work-script for chapter 2 (slides)

15 September 2021
2.1 Orbits
First we looked at the some informationfrom Voyager 1. Then we turned to satellites in geostationary orbits and their advantages and disadvantages. Then we talked about satellites in inclined orbits: the Molniya satellites with their higly elliptical orbits, military satellites, and GPS satellites. We also talked about satellites in polar orbits: Tiros- N series, NOAA satellites which also provide SARSAT service.

17 September 2021
Then we turned to  Orbital Mechanics. Important are: Kepler's laws, Newton's Universal law of Gravitation and Newton's 2nd Law of Motion.
 With Newton's laws we can describe the motion of a satellite. It is a motion in a plane. The position of a satellite in space at any time is given by the set of 6 Keplerian Elements, or Keplerian Orbital Elements.  NASA, for example, gives these Elements in a two-line format . Click here for examples.

20 September 2021
We talked about the Vernal Equinox, the zero coordinate point for the geocentric equatorial coordinate system. This coordinate system is generally used for positions of celestial sources. The coordinates are expressed in right ascension (RA) and declination (dec.). It is also the position through which the orientation of the orbital plane is linked to the celestial reference frame. The vernal equinox is also used in terms of a time. At the vernal equinox and six months later at the autumnal equinox, the Sun is at particular locations in the geocentric equatorial coordinate system. At which ones?
What are the coordinates of the Sun in RA and dec. today? Also click here. The motion of the satellite and its position at any given time are given by the solution of the differential equation derived from Newton's laws and the Keplerian Elements. We learned about the geocentric equatorial coordinate system and RA (right ascension) and dec (declination) as the celestial coordinates of a spacecraft or a celestial source). The Gaia satellite of ESA has just released a preliminary catalogue of positions of stars, galaxies and quasars in RA and dec that will include 1 billion sources. That catalogue of sources will define the most accurate celestial reference frame. Then we started on the Orbit Perturbations. We talked about the effects of the equatorial bulge and looked at numerical examples. 

22 September 2021
 The vernal equinox changes due to the precession of the Earth's spin axis. The period of precession is 26,000 years. That means that the coordinates of an object on the celestial sphere need to be referred to a particular time. RA and dec. are referred to 1 January 2000, 12:00 Terestrial Time. Then we continued with the orbit perturbations. We finished talking about the effects due to the equatorial bulge. We learned about effects of the equatorial ellipticity and then about third body effects on the Keplerian orbital parameters, the atmospheric drag and the solar radiation pressure.

24 September 2021
We finished the orbit perturbations. Look at LAGEOS for a satellite of which the orbit can be determined very accurately. Look here for the latest GRACE results in the form of gravitational deviations from a geoid. We started on the sub section 2.3 Visibility and learned about the coordinate transformations:  perifocal coordinate system, geocentric equatorial coordinate system, topocentric horizon coordinate system. In this context the relations between LST, GST, UT and standard time are important.

27 September 2021
We continued with the coordinate transformation and focused on the topocentric horizon coordinate system in which the antenna look angles are defined. We considered the equations for the antenna look angles and tried to make sense of them with particular examples for the northern and southern hemisphere.


29 September -1 October 2021
We introduced the range to a satellite and the visibility of a satellite in the geostationary orbit. During the time of the equinoxes the geostationary satellites experience earth eclipse when the satellites go into the shadow of the Earth. During these times the solar panels cannot be used and instead batteries on board have to provide the power for the satellite. Also, during the time of the equinoxes sun transit outages happen. Then we looked at aspects of the launch of a satellite or spacecraft. We talked about the launch vehicles, e.g.: Delta,  Ariane,  Proton, HV, GSLV, Long March and SpaceX
Look here for the launch of Gravity Probe B.  Look here for the first ballistic rocket launches in 1942 with the V2 the first known man-made object to reach space.  Here is the Saturn V rocket with Apollo 11 on top which took the first men to the moon and back.   Wernher von Braun, head of V2 and the American rocket program development up to Saturn V, here with US president Kennedy.  For a comparison of orbital launch systems, click here.


4 - 6 October 2021
We focused on rockets. the stages of the launch of a geostationary satellite and learned about the Hohmann transfer orbit.
We tried to remember what we learned in this Chapter and addressed our favourite subjects. That completed Chapter 2.