“Endless Distance, Wildlife and Stars, Blanket the Night…”
The last lesson was about Field of View (FOV) and Field of Regard (FOR). It was intended to help with understanding the next few lessons regarding satellites in geosynchronous orbit (GEO). All mentions of GEO on this blog, unless otherwise stated, refer to a particular type of orbit: it is an orbit above the Earth’s equator matching the revolution, or rotation, of the Earth.
Just in case there are people reading this blog poised with a “well, actually”—yes, yes—there are other orbits associated with GEO. You can go here to read all about those—the article is mercifully short, so if you’re curious, go ahead and read it. But we’re not going to cover those other geosynchronous orbit types in these particular lessons. The geosynchronous orbit type we will be focusing on is the geostationary orbit. As stated before, we will use GEO as the term for that orbit type.
“…You lying beside me darling, Eyes open wide…”
Hopefully the concept of a GEO weather satellite being able to see more with its “eyes” in its FOR than a Low Earth Orbiting (LEO) satellite is an easy concept to grasp. This main FOR distinction means a few things: with GEO satellites, you can see the patterns of the clouds, instead of just cloud cover, which LEO satellites will give you. With GEO weather satellites, you can see where a weather pattern is trending towards—so they are important for hurricane warnings and such. The GEO satellite’s “eyes” cover a wider area. The resulting images from such a vantage point are like the next image:
But one of the most important advantages is associated with the GEO’s orbital period (how fast it goes around the Earth).
“…While the wide arc of the globe is turning, We feel it moving through the dark…”
In the LEO satellite lessons, you found there were some variations in the orbital period of LEO satellites. This has to do with the variations in altitude of the different satellites and you can just go to these posts to read more about them. A GEO satellite is, obviously, at a much higher altitude and directly “above” the Earth’s equator: 35,786 km (22,236 mi) from Earth’s surface—or 26,199 miles if you go to the Earth’s core. This altitude, and its position above the Earth’s equator, means the GEO satellite’s orbital period will match the Earth’s rotation. Below is a decent animation of what’s happening:
The satellite will arc through the sky, matching the globe as it turns. So, what is the benefit of this orbital characteristic?
It means a weather satellite (or any kind of satellite, really) in a GEO position will observe the part of the Earth the satellite is orbiting above 24 hours a day. This one aspect describes the concept of “persistence” in satellite operations. Persistence is how weather satellites in GEO can “track” a weather system. Instead of seeing small, swiftly passing “weather trees” that a LEO weather satellite can see, a GEO satellite sees the entire “weather forest.”
“…On a voyage between dusk and dawn, Space and time…”
And the GEO satellite can observe that “forest” for 24 hours a day, seven days a week, etc. A LEO satellite, because it’s moving so quickly, and the Earth is rotating as much as 2,200 km (1,367 miles) per 90-minute low earth orbit, doesn’t have this kind of persistence. Is it possible for a LEO satellite to have persistent observation of a single point of the Earth? Well, kind of—you have to have more than one LEO satellite to accomplish persistence. But, as discussed in previous lessons, this kind of LEO satellite constellation introduces complicated ground system requirements, communications interlinks, etc., which is why using only one GEO satellite is the option selected by many organizations to do the job.
There is another advantage regarding this orbit, though, and we’ll get into that next week.
The interspersed lyrics are from the B-52’s song, “Revolution Earth.” Disappointing video, but great song from their album “Good Stuff.”
- SpaceX successfully launches its first attempt at geosynchronous orbit (arstechnica.com)
- SBIRS Geo-2 Missile Defense Early Warning Satellite Certified For Operation (spacewar.com)
- Iran to send telecoms satellite into orbit by 2018 (theiranproject.com)
- To Infinity… and Back! (alexanderszewczak.wordpress.com)
- Geek Answers: What is geostationary orbit, and why is it so important? (geek.com)
- Hillbilly Tracking for Low Earth Orbit Satellites (amsat-uk.org)
- Missile Warning Sensor for Third Space Based Infrared System (SBIRS) Geosynchronous Orbiting (GEO) Satellite Completed (matthewaid.com)
- Panel Warns of ‘Catastrophic’ Gap in Weather Satellite Data (climatecentral.org)
- Nasa reveals breathtaking image of the Earth on Christmas morning (thisismoney.co.uk)