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Geosynchronous Satellites - Low Earth Orbit - Spy Satellites & GPS
For any who have installed or worked with a TV satellite dish, we may have found ourselves asking…”how does this dish point off into the sky in one direction constantly and get my TV signal?” This is a reasonable question and worth exploring.
Typically when one thinks of a satellite the image that is conjured is one of the space shuttle or space station, careening through the sky orbiting the earth 15 or 16 times per day. If one has had the privilege of observing the miraculously bright space station in the sky just after sundown you likely where stuck by its stellar clip. Obviously a small fixed TV dish has no chance of catching a signal from such a quick moving object. So what is the TV dish pointing at?
It is here that Geosynchronous Satellites shine. A satellite of this type is in a very unique orbit known as Geosynchronous or Geostationary orbit. This orbit is at a very particular distance and location around the earth, and here is why. As mentioned above most of the satellites and orbiting objects we are familiar with are in what is known as LEO, or Low Earth Orbit. In fact all manned orbital space flight outside the Apollo missions to the moon has been LEO or sub-orbital. LEO is typically defined as an altitude of 100-1200 Miles above the earth. At this height the orbiting velocity of a satellite is around 17,000mph and whether in a retrograde orbit or otherwise the motion of the satellite does not coincide with the spin of the earth. So the result is when observed the satellite briskly scoots across the sky.
Some Geosynchronous orbits are unique. They orbit at the magical distance of 22,236 miles directly above the equator. At this distance the orbital time coincides with the 24 hour period of the spin of the earth. So from our perspective the satellite never appears to move since its motion or orbit is in lock-step with movement of the planet.
This orbital distance is highly useful for a number of applications and has some limitations. When meteorologists want to observe the prolonged progression of a hurricane, the geosynchronous satellite is ideal since it can wait out the storm from its fixed vantage point. Also as pointed out in the outset it is much cheaper for the average consumer to have a stationary satellite dish to acquire a TV signal then a complicated tracking dish to follow a quick moving lower satellite. This orbit does create some challenges though. Because of the necessity of having a geosynchronous orbiting satellite directly over the equator it is troublesome for locations at higher latitudes on the planet. At these higher latitudes toward the Earths poles, receiving dishes would have to be pointed almost directly at the horizon, and possibly be obstructed by topography or experience interference from having to receive the satellites signal through a much thicker portion of the atmosphere. Because of this problem creative minds have utilized Kepler's laws to overcome this with the Molniya orbit. In addition due to the highly crowded and limited space in geosynchronous space, the Sirius Satellites utilize what is called a Tundra orbit. Both of which allow for a communication satellite to be available for larger swaths of time over a specific region of the planet allowing consistent coverage, while still being available to higher latitudes and not crowding the already congested geosynchronous orbital plane.
What about Spy Satellites?
Spy satellites if dumbed down considerably are just telescopes in space turned toward the earth to see what is going on. But don’t be fooled by popular fictional portrayals of the utility of spy satellites. Recall from above if one wanted a permanent spy in the sky to watch over a particular area of the planet, geosynchronous orbit would be a necessity. This may seem an elegant solution but recall the distance for this unique orbit is some 24,000 miles from the surface of the planet. And due to constraints which will be covered when I get into light in other Hubs, the best resolution one could get form that distance is approximately 200 meters. Ultimately then at this distance one could identify an object the size of a football stadium, but would have no way of discerning if there was even cars in the parking lot.
That being the case, it seems obvious that spy satellites must be in a much lower earth orbit to be able to attain the incredible detailed photos from space some may be familiar with from applications such as Google earth. This is in fact the situation, but the lower altitude presents its own challenges. Recall also that a LEO satellite rips across the sky at some 17,000 mph. At this velocity a spy satellites exposure to a particular area of the planet is only about a minute, and although it may orbit around every 1.5 hours this is a still very narrow window of time. In fact many nations are very familiar with the schedule of other nations spy satellites and are sure that for the brief periods each day that the satellites are within eye-shot their secretive operations are hidden from the prying eyes in the sky. It is for this reason we hear more and more lately of the extensive research and utilization of unmanned drones that can camp over an area for hours at a time, with a considerably lower altitude then even LEO satellites, and provide incredibly high resolution photos and live video.
All of us by now are at least tangentially familiar with GPS technology. This technology utilized the signals from 3 or more GPS satellites in orbit overhead to determine ones location via basic triangulation. The USA uses the system we know the Global Position System, Russia uses their own constellation of satellites known as GLONASS, while China is working on the COMPASS system. These satellites are not found in LEO, since they need to be line of site, and they would obscured by the horizon far too often. So these satellites are in a middle orbit known as medium-Earth orbit or MEO. At this orbit of some 12,000 miles these satellites orbit every 12 hours on average. Allowing for the bulk of the GPS constellation to in line of site for most of the earth they are designed to cover.
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