The GPS System. Brief history of GPS.

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1 The GPS System. Some time ago, driven partly by commercial interest and partly by curiosity, I decided to do some research into how the GPS system actually works. To understand it at grass roots level requires some understanding of time vs. speed and the ability to visualise things in 3 dimensions, but broken down it s not that difficult to grasp. In the following series of articles, I ll try and explain the nuts and bolts of the system, and outline some of the more unusual applications for GPS in the modern world. Much of the matter in these articles will be obvious to the more technically savvy, but I ve included basic material for those of you who are unfamiliar with the base concepts. GPS is an acronym for Global Positioning System, a satellite based navigation system. It is capable of providing accurate 3 dimensional positioning information anywhere on earth (or above it for that matter), together with an extremely accurate timing reference. Brief history of GPS. The GPS system is widely credited to be the brainchild of 3 individuals; Dr Ivan Getting (American physicist and electrical engineer), Roger L Easton (American physicist/scientist) and Bradford Parkinson (American engineer/inventor and US Air Force colonel). The project to develop the system was launched in 1973 to overcome the limitations of existing navigation systems, and was spearheaded by the US Department of Defence, becoming fully operational in Originally the accuracy of the system was deliberately downgraded to prevent hostile forces from using it for precision guidance targeting purposes, and accurate positioning was only available to selected military organisations. This was known as Selective Availability. In the year 2000, an executive order by US president Bill Clinton saw removal of Selective Availability following pressure from the FAA (among other groups), hence the full accuracy of the system became available to the general public. The US military currently still has the ability however to deny GPS to hostile forces in specific areas of crisis on the planet, without affecting the rest of the world. Overview of the system today. The GPS system consists of 3 major parts known as Segments as follows. Space Segment Ground Control Segment User Segment

2 Space Segment The Space Segment consists of 31 satellites, of which 24 are always operational (orbital diagram and a typical satellite Figure 1). The satellites have been placed into a series of orbits around the earth, such that there is always a minimum of 4 within line of sight of a GPS receiver anywhere on earth. The satellites fly in Medium Earth Orbit (MEO) at an altitude of approx 20,200km, which sees each one orbit the earth approximately twice a day. Among the more crucial elements of choosing this type of orbit are; 1) wider coverage is guaranteed over the entire surface of the earth and 2) the orbital parameters mean the position of each satellite can be predicted with an extremely high degree of accuracy (more on this at a later stage). Figure 1. Ground Control Segment Monitoring the orbits and controlling the general health of the satellites is the task of a series of ground control stations, situated at various geographical locations around the world. This network of stations consists of a Master Control Station located at Schriever Air Force Base in Colorado Springs, Colorado (ground antennas and station locations Figure 2), an alternate Master Control Station at Vandenberg Air Force Base in California, and a further series of monitoring stations located in Hawaii, Ascension Island (mid Atlantic Ocean), Kwajalein Atoll (North Pacific Ocean), Diego Garcia (Indian Ocean) & Cape Canaveral (Florida). Further stations were added to the network in 2005 and 2006 and now include one located in Wellington. The Ground Control Segment is able to adjust the orbits of satellites (necessary periodically due to orbital drift), bring spare satellites online and take others offline for maintenance, apply software upgrades, carry out atomic clock calibration and perform a host of other duties necessary to keep the satellites functioning correctly. Figure 2

3 User Segment The User Segment is a generic term used to describe all the devices capable of receiving information from the GPS satellite constellation to determine a precise position on earth. The information is also used to provide an extremely accurate timing reference. Every device from a dedicated navigation receiver to a smart phone belongs to the User Segment. In 2012 the European GNSS Agency estimated approx 2 billion units falling into this category and the current figure in 2017 is undoubtedly higher. Time, Distance and position fixing. Relationship between speed (V), time (T) and distance (D) This concept is quite straightforward where distance travelled (D) = time (T) x speed (V). Eg:- Consider a dead straight, dead flat road that passes through a desert. You set out to collect a friend who is camped beside this road, 50km from the centre of a town X. You leave the town X in your car and travel at 100km/hr. In theory you would meet your friend after travelling for a period of 30 minutes. Assuming you could accurately maintain this speed (V), minor inaccuracies in your timing (T) would likely see you either not travelling far enough or conversely travelling too far (D). At this speed however the inaccuracy would not be significantly large and there is still an excellent chance you would meet your friend as planned, probably arriving slightly early or slightly later. Now consider a different scenario. Imagine you were able to travel at the speed of light (approx 300,000km/second; currently impossible, but this is a hypothetical example). In this scenario your friend is located in space, 450,000km from earth. Travelling at the speed of light, this trip would take you 1.5 seconds, but it s obvious even the SMALLEST inaccuracy in time results in a VERY large difference in distance travelled. If your timing was out by ½ second, your distance travelled could be 150,000km either way!! Fixing a position in 2 dimensions Consider fixing a position on the surface of the earth, again somewhere in the desert referred to in the previous section. In order to fix a precise position, we would need to know the distance from at least 3 reference points. In this case, your friend knows the distance from 3 towns; X, Y & Z and he is 50km from each town. Being 50km from town X puts him somewhere on the circumference of a circle with a radius of 50km, centred on town X (Figure 3). Your friend is located somewhere on the circumference of this circle Town X Figure 3 50km radius circle

4 Adding a second reference point (Town Y) immediately narrows his location down to 1 of 2 possible places; namely the positions where the 2 circles intersect (Figure 4). Town X Town Y Figure 4 Adding a third reference point (Town Z), fixes the position precisely, as it only intersects 1 of the 2 possible positions in the previous example. In other words the point at which all 3 circles intersect (Figure 5) would be where your friend is located. Town X Town Y Figure 5 Town Z

5 Fixing a position in 3 dimensions In the previous example we considered locating a position in 2 dimensions only (in other words a flat surface, such as an area of desert). Locating a position in 3 dimensions (IE:- somewhere in space) becomes more of a challenge as we have an additional dimension to consider. Knowing the distance from an object in 3 dimensions changes all possible locations from the circumference of a circle (as in our previous example) to the SURFACE of a SPHERE. Consider now your friend is floating around in space, but only knows he is 1000km from Satellite 1. You would have to extend out a line 1000km in ALL DIRECTIONS from the satellite in order to isolate his position to somewhere on the surface of a sphere, the centre of which is located at the position of Satellite 1 (Figure 5). Location of Satellite 1 All possible locations are somewhere on the surface of the sphere centred on Satellite 1 Figure 5 Adding a distance from a 2 nd satellite (Satellite 2) narrows the location down to all possible points where the 2 spheres intersect (Figure 6). If you consider the geometry of a sphere, it will be obvious all possible locations of 2 intersecting spheres is a circle (Figure 6). All possible points where the spheres intersect forms a circle Figure 6 Location of Satellite 1 Location of Satellite 2

6 Once we know a location somewhere on the circumference of a circle (formed by the intersection of the 2 spheres as above), it immediately becomes a simpler 2 dimensional puzzle (as per the 2 dimensional example) and therefore a case of adding 2 further reference points (Satellites 3 & 4) to narrow your location to a precise position (Figure 7). To reiterate a minimum of 4 satellites is needed to locate a precise 3 dimensional fix. 2. This portion of a circle represents the 2 points from Satellite 3 that intersect all possible location points from Satellites 1 & 2 4. There is only 1 possible intersection point of these circles, which represents a precise 3 dimensional position Figure 7 1. This circle represents all possible intersection points from Satellites 1 & 2 3. This portion of a circle represents the 2 points from Satellite 4 that intersect all possible location points from Satellites 1 & 2 In practice, there are always more than 4 satellites in view of GPS receiver, which in turn helps to improve the accuracy (more is better). The receiver will analyse the signals from all satellites in view and pick the ones that give it the best fix. This decision is based on geographical location in the sky and signal strength. Signals from the satellites are transmitted in the microwave band of the radio spectrum and are relatively weak due to their low power (just above the background noise threshold). A combination of different signals from each satellite is used to compute the receiver position. Conclusion That s it for this article. Stay tuned next week when we ll discuss what goes on in a GPS receiver to resolve the signals to a precise fix, how it overcomes timing errors and much more. Dave McIver.