L3 - Orbit Perturbations, Groundtracks and Applications Flashcards
List 7 Earth applications orbits.
- Low-Earth
- Medium-Earth
- Geostationnary
- Geostationary Transfer
- Sun-Synchronous
- Highly Elliptical
- Molniya
Give the range of altitude of a Low-Earth orbit.
180 - 3000 km
Give the range of altitude of a Medium-Earth orbit.
~ 20000 km (between LEO & GEO).
Give the altitude of a Geostationary orbit.
35790 km and I=0°
What is a Geostationary Transfer Orbit (GTO)?
An inclined elliptical orbit connecting LEO and GEO.
What is a Sun-Synchronous orbit?
A polar orbit for which the orbital plane’s orientation remains stable with respect to the Sun.
What is a Highly Elliptical orbit?
An inclined orbit with e > 0.2
What is a Molniya orbit?
A high elliptical orbit (500km; 35790km) with a period of 12 hours and inclination of 63.4° so that its perigee doesn’t rotate.
- 11 hours hanging
- Can be launched from higher latitudes
- Better coverage than GEO sats at high latitude
What is a Ground Track?
It is the path taken by a satellite over the Earth’s surface.
How can you derive orbit configurations from a ground track?
- Earth rotation: 15°/hour
- Node displacement = ΔN = track shift between two consecutive rotations.
We can then get the orbit period and from that the semi-major axis. (T then a)
- highest latitude of track = Inclination (or 180 - highest latitude if 90 < I > 180)
- Eccentricity: 0 if symmetrical track, not 0 (eccentric) if lopsided
- Perigee location: point where track is most spread out
What are the primary perturbations to Earth orbiting satellites?
- Third body gravity (sun, moon, planets…)
- Non-Spherical mass distribution of the planet (Earth)
- Atmospheric drag
- Pressure of solar radiation
- Solar wind
- Earth’s magnetic field
- Unexpected thrust from outgassing
What impact does the non-spherical nature of Earth have on Ω and ω?
The Earth’s bulge at the equator affects Ω and ω. The gravitational force is offset from the centre of the Earth. Both Ω and ω precess by an amount Δ. Westward for direct orbits (I < 90°) and eastward for retrograde orbits (I > 90°).
What advantage can the non-spherical nature of Earth bring to a Sun-Synchronous orbit?
A circular orbit with inclination of 98° and altitude 655km will have a node that moves eastward (i > 90°) by 0.986° per day. This is equivalent to the rate at which the Earth moves around the sun. Hence, the orbital plane can maintain the same orientation to the sun.
(more acurate comparisons of images over time)
When can atmospheric drag be considered important?
< 1000km (also day/night cycle, solar activity)
