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GPS

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Background

GPS or Global Positioning System is a space based radio navigation system, which provides position data anywhere on the Earth's surface, 24 hours a day and in all weathers. It is wholly owned and operated by the United States Department of Defense. Development of the system started in earnest in 1973, with the first Space Vehicle (SV) in the constellation launched in 1978.

Services

In broad terms, GPS uses 2 types of code, plus a third encrypted code.

Coarse Acquisition (C/A) Code

C/A code, also known as Link 1, is broadcast on 1576.42 MHz, at 1.023 million BPS. In addition, a second code, known as P Code is broadcast simultaneously, on a frequency of 1227.6 MHz at 10.23 million BPS. To ensure a higher degree of accuracy, the third encrypted code, known as Y Code is coupled into the P Code, resulting in the high definition encrypted positioning signal known as P(Y) Code.

These codes are manipulated in different ways to afford two types of user service.

Standard Positioning Service (SPS)

SPS is for civil use, and uses Link 1. It can have degraded accuracy (known as 'Selective Availability' or SA), however this functionality was switched off by the DoD on 1 May 00 following a marked increase in lawsuits brought against the DoD by, most notably, the United States Coastguard.

SPS provides accuracy of 30m CEP, or 100m CEP with SA.

Precise Positioning Service (PPS)

PPS is solely for military use, and uses both Link 1 and Link 2. Techincally, it provides full system accuracy of 16m CEP (9m PE) but this is clearly highly debateable. In addition, this service incorporates certain anti-spoofing and anti-jamming functionality, assisted by the inclusion of the Y Code.

The System

The system is comprised of 3 segments.

Space Segment

The Space Segment utilises 24 SVs, consisting of 21 working SVs and 3 'working spares'. Each SV follows one of 6 near-circular orbits approximately 20200m (11Nm) above the Earth, 60 degrees longitude apart, inclined at 55 degrees to the Equator. Thus the orbital paths lie between 60 degrees North and 60 degrees South, and orbit fully in 11 hrs and 58 minutes, or twice per day with 4 minutes to spare.

SV Factoids

Each SV is part of the NAVSTAR array, and has a mass of approximately 1 tonne. They measure 5 metres across the panels, and travel on their orbits at speeds approaching 2.6km/s. Each SV transmits at 50W, and has a life expectancy of 10 years.

Control Segment

The Control Segment consists of a series of Monitoring Stations (in Colorado Springs, Hawaii, Acension, Diego Garcia and Kwajalein), plus one Master Control Station (in NORAD, Colorado Springs) and a 'fine tuning' facility (based in Cape Canaveral).

Each SV passes over some element of the Control Segment twice a day, where it receives updates and fresh positioning and time feeds if required.

User Segment

The User Segment consists of receivers that receive, decode and process the SV signal. The User Segment consists of all users, both civil and military.

GPS for Dummies

Did you understand any of the above?

No?

Well, here's how it works in layman's terms:

The Spams put lots of satellites up in space (good place for them) to help their officers work out where they are. But it's no longer limited to our American friends - you can use it too. You will need:

- A magic box
- Lots of batteries
- More batteries. And a charger if you've anywhere to plug it into.

Alternatively (and a huge saving in batteries)...

- To find out where you are, ask a policeman.

If the polis are off filling in forms and one of them isn't available, take out your magic box and replace the batteries it will have used up during the twenty minutes or so since you put them in.

Switch it back on and utter the magic invocation "Huuaretheffukahhhwee?".

The magic box will be activated by your magic words and it then "shouts" in a special way to the satellites above. They then have a bit of a discussion amongst themselves (they're very high up so they can see for miles) and give your magic box their best guess. Your magic box will then need its batteries changing again, and on switching back on it will pass on the satellites' best guess as absolute truth.

Of course, it's then up to you to work out where you are on the map/chart; the RNLI are sick of picking up Sunday sailors who've got into trouble and can tell them their exact grid reference, but have no idea where that actually is.

The magic box needs to be able to see at least three satellites, ideally four, so that there are enough of them to be able to give a decent guess. You really don't want to be relying on the opinion of just one satellite, especially if it's been on the piss.

If your magic box can't talk to enough satellites, hold it a bit higher. That's it, stand on tip-toes. Change its batteries. Try getting up on that rock. Left a bit. How's that?

Important Note:

In no way should you trust the magic box totally otherwise you may find yourself half way up a vertical mountain wondering a) where the rambling path went and b) who the blokes are in the helicopter and why they are trying to entice you off your comfy inch-wide ledge and into that dodgy looking sling.

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