05 - Orbit Visualization
This tutorial is a brief overview of various satellite concepts by means of visualization using STK. Examples include visualization of sun angle relative to geostationary and sun-synchronous satellites. The goal of this tutorial is not to show how to do the modeling but convey orbital concepts by showing rendered animations.
CONTENTS
1. Summary
Sometimes it can be difficult to convey how satellites operate in space. One such example is how the solar panels are oriented toward the sun while a satellite is in orbit. One way to describe how this is done is through drawings or various forms of animations. Here we have chosen to illustrate concepts with AGI's Systems Tool Kit which is excellent at modeling satellites in orbit without any simplifications.
The on orbit operation of a satellite is an intricate optimization problem of power, thermal, communications, mission, and lifetime management. Many subsystems need to be accounted for with numerous inter-dependencies. The modeling shown below focuses on relatively simple single issue items, in this case how solar panels are oriented toward the sun and to what extent satellites in different orbits experience solar eclipses which the satellite need to be able to operate through.
What we will see is that the even a limited problem focusing just on the solar panel is not entirely trivial and depends heavily on the chosen orbit and varies throughout the year.
This is a living document and will be added to as ideas for additional examples come up.
2. Examples
Example 1: Geostationary Orbit Satellite Solar Power
One may wonder if it is possible to put a satellite in geostationary orbit and keep it in constant sun light to provide constant electrical power. In the video below, we see that it is indeed possible for a geostationary satellite to remain in constant sunlight.
Figure 1. A Satellite in Geostationary Orbit
However, we must also consider what happens to the satellite throughout an entire year. Turns out that because the Earth is tilted at 23.5 degrees, the sun oscillates around the equator by +/- 23.5 degrees. The result is that we have two Equinoxes.
Figure 2. Earth Seasons (svg)
If we look at the geometry of the geostationary orbit relative to the Earth, we see that for a certain angle, 8.6 degrees specifically, the geostationary satellite will experience some amount of eclipse. It turns out this eclipse period corresponds to approximately 28 Feb - 11 April and 2 September - 14 October. The eclipse is however relatively short, lasting at most 70 minutes when the orbit plane is aligned with the equatorial plane.
Figure 3. GEO Angles (png)
We can visualize this eclipse if we specifically look at the period between 28 February - 11 April as shown in the video below. Keep an eye on the "beta angle" which is the angle between the geostationary satellite's orbital plane and the ecliptic plane. From figure 3 above, we expect to see the eclipse occur for a beta angle between +/- 8.6 degrees
Figure 4. Satellite in Geostationary Orbit During Solar Equinox
Example 2: Sun-synchronous Orbit Satellite Solar Power
One may wonder if it is possible to put a satellite in Sun-synchronous orbit and keep it in constant sun light to provide constant electrical power. In the video below, we see that it is indeed possible for a Sun-synchronous satellite to remain in constant sunlight.
Figure 5. Satellite in Sun-synchronous Orbit
However, we must also consider what happens to the satellite throughout an entire year. Turns out that because the Earth is tilted at 23.5 degrees, the sun oscillates around the equator by +/- 23.5 degrees. The result is that we have two Equinoxes, but also a summer and winter solstice (i.e. middle of summer and winter). The solstice occurs when the ecliptic plane reaches a latitude of +/- 23.5 degrees (also known as the Tropic of Cancer and Tropic of Capricorn)
Figure 6. Earth Seasons (svg)
We can visualize what happens during the summer solstice and if we look at at figure 7, we indeed see that even for a relatively high altitude satellite (1,000 km) there is still a brief eclipse of the solar panel.
Figure 7. Satellite in Sun-synchronous Orbit During Solstice.
The Sun-synchronous satellite in Figure 7 not only experienced eclipsing, but it did so even with an orbital plane with ideal orientation relative to the sun. In practice a satellite's orbit is not defined by its ideal orientation relative to the sun. Instead the key driving criteria is what the satellite's payload needs to see and when.
This brings us to the reason for why you would use a sun-synchronous orbit in the first place. It has nothing to do with finding an ideal orbit with respect to the sun, but to provide a payload with the same sun angle of the ground every day of the year. The reason this is possible is that because the Earth is not perfectly spherical, it is possible to select orbital parameters that makes the satellites orbital plane precess exactly one revolution per year. The consistency allows for simplified scientific analysis of payload collection. An example of such a satellite is the AURA satellite that is part of the NASA A-Train. The A-Train is shown in figure 8 below.
Figure 8. NASA A-Train (jpg)
In Figure 9 below, we can see what a typical sun-synchronous satellite orbit looks like. We can clearly see that the satellite sees a relatively long eclipse every orbit. Consequently reliable power design for a satellite can be quite a challenge. A typical cell phone battery is rated for 1,000 charge/discharge cycles. However, a satellite in sun-synchronous orbit would experience approximately 4,745 charge/discharge cycles every year. With an expected lifetime of over 10 years for many satellites, we are looking at ~50,000 cycles total. Hence the space industry has been slow to adopt Li-Ion batteries and stayed with Nickel-Hydrogen batteries for a long time.
Figure 9. Typical SSO Orbit Example: AURA in NASA A-Train