The Cosmic Positioning System - GPS for Satellites
Ben - So we've heard how we rely on satellites to tell us where we are, but how do the satellites themselves know their own location? Well, all satellites use a kind of cosmic positioning system and to find out more I spoke to Chopo Ma from NASA's Goddard Space Flight Centre...
Chopo - If you want to get information from the satellites you need to know where the satellites are and so that applies whether it's a GPS satellite or an altimetric satellite from which you're trying to find the height of the sea surface.
Ben - What options do we have? What reference frames could we possibly use for satellites?
Chopo - Well, the satellites can define their own reference frame if you have accurate tracking. But the reference frame of satellites is dependent on knowing their orbit and not everything about an orbit of a satellite can be derived.
Ben - So, the most obvious option to me would be that we have bases on earth, we know where they are, and we can use those to triangulate where a satellite is. Is that a viable option?
Chopo - Well, that's a viable option but of course, to know where you are on the earth, you have to know where the earth is. And in fact, it is a coupled problem of how to define an accurate terrestrial reference frame and an accurate celestial reference frame and in fact, the angle between these two which is the orientation of the earth has to be measured. There's no way of modelling it.
Ben - So, can we therefore look at the other local objects? So we look at the sun, look at the moon, look at the planets, and use that as our reference frame?
Chopo - The sun and the planets are not necessarily the easiest things to observe; the sun because it's rather large and the planets because there are not very many of them. What has been used in the past are the stars and the sky, and that served this purpose for many centuries.
Ben - So we're looking at the stars that we can see obviously. How do we actually use them to compile a reference frame?
Chopo - What was necessary was to observe the stars over some period of time and relate those positions to positions on the earth, and also to look at the relative positions of stars. But the difficulty with stars in the medium regime is that they also move. So, in addition to knowing their position, you need to know their velocities and this takes observations over quite a long period of time.
Ben - So, what are our other options? What else can we use?
Chopo - Well, what the astronomical community decided to do in the 1980s was to go to objects which are not moving and the simplest objects at least conceptually are those objects that are so far away that they actually can be considered fixed points in the sky. And so, these are objects that are completely outside the Milky Way.
Ben - So what are the objects we're actually looking at?
Chopo - The objects we are looking at now are radio sources. Mostly quasars which are billions of light years away and they are so distant that geometrically, you could not expect to be able to detect transfer motion, that is motion across the sky.
Ben - And how are these actually measured?
Chopo - We use large radio telescopes that are mostly 20-metre in diameter and we use a network of stations which basically cover the globe. It is necessary to have a large network because the angular resolution with which we can measure the positions depends on the physical size of the network. If we were limited to a single radio telescope, the accuracy of the positions of quasars would not be achievable at the level we need.
Ben - So essentially, this reference frame acts as a sort of GPS for things like GPS satellites. How have we improved it since we first came up with it?
Chopo - The extragalactic reference frame dates from the middle to late 1990s and the way we have improved it is to make more observations, to improve the positions of the sources that we have then, and to add quite a few more sources, quasars, to the list of objects for which we have good astrometric positions.
Ben - How many have we got now?
Chopo - The total catalogue whose positions are listed is about 3,000. So, in the count of celestial objects, this is really a drop in the bucket but the advantage is that the positions of these objects are known so much better than the positions of stars derived optically.
Ben - And so, what's the next stage for you? How are we going to make this even better?
Chopo - The project that the astronomical community is waiting on is a satellite project called Gaia which is scheduled to go up in 2011 or 2012. This satellite will be able to see the extragalactic objects, the quasars in the optical, and the hope is that the frame can be extended to hundreds and thousands of objects because there are many more quasars that have been detected in the optical than can be used in the radio.
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