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What is GPS ?

GPS – Global Position System. The main GPS system which is in use today is the NavStar system which is operated and maintained by the US Air force The NavStar GPS system contains 32 satellites of which 24 are in use at any one time. The satellites orbit the earth at an altitude of 11,000 nautical miles. Other systems that are in use/development are the Russian Glonass system and the European Galileo System

How Does it work?

The GPS system uses triangulation to determine a position.

We know that a GPS satellite orbits the earth at around 11000 nautical miles. Therefore if we can receive a signal from 1 satellite, than we know our position is within 11000 nautical mile radius of that satellite

If we know the distance from a second satellite than the area where the 2 cross is our position

satellite


 

*Our Position is somewhere in the circle between the 2 satellites

Now if we know the distance from a 3rd satellite, we can narrow our position down to just 2 locations (the 3rd satellite will intersect the area between satellites 1 and 2). By knowing the position of 4 or more satellites we can narrow our position down to just 1 point.

How do we calculate distance?

We need to know the exact distance that our GPS receiver is from the orbiting satellites to be able to calculate an accurate position. By applying some basic mathematics the receiver can work out this distance.
Distance = Speed x Travel Time

  1. GPS signals are a radio signal, therefore they travel at the speed of light
  2. If we know the time the signal was sent and the time the signal was received we can work out travel time.
  3. By subtracting the sent time from the received time, we can determine the travel time
  4. Now we can multiply travel time by the speed of light and we can determine distance

 

How do we determine travel time?

Each satellite transmits it's own pseudo-code.

pseudo-code



Pseudo Code : Series of on/off pulses
The GPS receiver knows which pseudo-code belongs to which satellite

To determine distance, both the satellite and GPS receiver generate the same pseudo-code at the same time. The satellite transmits the pseudo-code which is received by the GPS receiver. The receiver is still producing the code while the satellite's code is travelling through the sky. The 2 signals are eventually compared and the difference between the 2 signals is the travel time.

signal lag


 

 GPS Accuracy

In a perfect world, GPS would provide our exact location. The world is not perfect so we need to account for errors that can degrade the accuracy of the system. The following errors are the most common.

  1. Orbital Errors – This happens when a satellite moves around in space. This problem is overcome by uploading almanac data to the receiver
  2. Atmospheric interference – distance is calculated by using the speed of light in a vacuum, which is OK when the transmitted signal is travelling in space, but as it travels through the atmosphere the speed of the RF signal changes. This problem can be overcome through few different techniques such as using dual frequencies, using DGPS (marine beacon) or using a RTK base station.
  3. Multi-pathing – Happens when the transmitted signal bounces off buildings, trees or any other large objects before it reaches the receiver. This increases the time for the signal to reach the receiver. This problem can be overcome through a few different techniques such as using dual frequencies, using DGPS (marine beacon) or using a RTK base station.
  4. Pseudo-Code Error – The width of the pulse in the code causes inaccuracies, because the receiver can only determine where a pulse starts or stops. This problem can be overcome by using a 2nd frequency or carrier frequency.
  5. Timing – The satellite has perfect time, because it uses an atomic clock, but the average receiver does not have this accuracy due to the cost of an atomic clock. This problem is fixed by using the 4th satellite. If the 4th satellite's signal does not intersect with the other 3, than the receiver knows there is a clock error. The error is calculated, thus syncing the receiver with the satellite.

Advanced Concepts – Improving Accuracy

  1. DGPS – Differential GPS
    This works by using a base station. The base station is positioned over a known point. It's purpose is now to work out the timing errors caused by local environmental conditions. It does this by receiving the GPS signal. Instead of working out distance, it's job is to work out timing errors, so it works out what the travel time should be and compares it with what the actual travel time is. This difference is the error correction factor. The error correction factor is then transmitted to a roving vehicle by a UHF radio link. The base station will work out an error correction factor for all visible satellite's


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