GPS Page

GPS Page

• What is GPS
• GPS inaccuracies
  • What are the causes
  • How to improve accuracy
  • The Future
• Our GPS
• Garmin eTrex Vista Specs

What is a GPS

A GPS is a Global Positioning System Unit designed by the US Military to pinpoint its location anywhere in the world. It uses a low frequency signal from High Atmospheric Satellites. There are 24 satellites, 21 in use and 3 spares.

The satellites all give out a signal at exactly the same time, stating their position and the time it was sent out. The receiver receives these signals and compares the sent time to the current time (from its internal clock). This time difference allows the GPS to calculate the distance between the GPS and the satellite as it knows the speed that the signal travels at (speed of light) as distance = velocity / time. It needs to see a minimum of 4 satellites for the four dimensions - three for a 3 dimensional fix and 1 for time. As can be seen from this, the more satellites the GPS receives a signal from, the more accurate the GPS can calculate its location.

Shows how more satellites increase accuracy

The satellites - known as the Space Segments - are constantly updated about their location from a ground station - known as the Control Segment and owned by the US Government. This ensures that they know where they are so they can transmit it to the GPS Receiver - also known as the User Segment.

GPS Segments

As can be seen form the above, there are three parts to GPS

• Control Segment - Ground unit informing the satellites of their location
• Space Segment - The GPS satellites
• User Segment - The GPS receiver

Additional Parts non-GPS

• DGPS Transmitter
• DGPS Receiver
• WAAS Satellites (in North America) or Euro Geostationary Navigation Overlay Service - EGNOS (in Europe)

As GPS (or NAVSTAR which is now its official name) was initially invented by the US Air Force in the 1960s but is now owned by the US Military. It only became fully functional in 1995. Originally they restricted its accuracy by imposing Selective Availability by adding interference to the Civilian signal so that the satellites did not give out a signal at the same time (it varied by a fraction of a second). This interference was controlled again by the US Military. This interference caused the receiver units to be randomly up to 100 metres out. Fortunately, this interference has been turned off so accuracy has drastically improved.

There are two signals given out by the GPS Satelites, P codes and CA codes. The P code is scrambled and only the US Military receivers can unscramble it. The CA code is not scrambled and is therefore available to Civilian Users. 

CA codes are transmitted on one frequency, L1 = 1575.42 MHz. P codes are transmitted on two frequencies, L1 and L2 = 1227.6 MHz. Military receivers can receive the P code on both L1 and L2 frequencies and as the L1 signal is a slightly higher frequency, it is not affected by the Ionosphere and Troposphere (see GPS Inaccuracies) quite so much as the L2 frequency therefore with a mathematical calculation, they can reasonably accurately work out the ionospheric delay due and increase the accuracy. As a result of this delay, Civilian Receivers (using CA codes) are accurate to approximately 15 metres horizontally (36m vertically) while the military receivers (using P codes) are accurate to about 1 metre horizontally because they can compensate.

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Why are GPSs Inaccurate?

"Hmmmm!" I hear you say. "I thought GPSs were supposed to be highly accurate accurate!" Well they are, or at least can be. 

The reasons that can be inaccurate are as follows

• Selective Availability - Up until May 2000 the US Government, quite rightly considering it was their military system, put restrictions on the civilian GPS signal known as Selective Availability (SA). this reduced accuracy to 100 meter horizontal accuracy and 156 meter vertical accuracy. This restriction is no longer in place so the accuracy is now less than 15 metres horizontal, 95% of the time.
• Receiver Clock Errors - The clocks in receivers are not very accurate. To make it accurate, it would have to have an atomic clock (very big and very expensive so this will not happen in the near future)
• Number of Visible Satellites - The more there are, the more accurate the triangulation becomes. It is also worth noting that generally GPS units cannot receive a signal underground, underwater, indoors or become inaccurate under thick glass as the signal travels slower through glass
• Ionosphere and Troposphere Delays - These are the delays in the GPS signal passing through the atmosphere. This is somewhat counteracted by a model that the system uses to calculate these delays and correct them but is uses averages so it it not too accurate.
• Satellite Geometry - The wider the angle between the satellites the more accurate the GPS will become
• Signal Multi-path - this is where a signal gets bounced off buildings or cliffs causing slight errors
• Orbital Errors or Ephemeris Errors - These are caused by the satellites slightly mis-reporting their locations

This accuracy can be increased with the use of extra methods

• WAAS - Wide Area Augmentation System - (currently under development in North America) can increase the accuracy to under 3 metres.
• EGNOS - Euro Geostationary Navigation Overlay Service -  (again under development in Europe)
• DGPS Differential Global Positioning System (a corrective signal from known reference points, correcting the GPS signals - generally requires an extra signal receiver unit) can also increase the accuracy to between 1 to 5 metres.

Most modern GPS units display how accurate they believe they are (this is usually very accurate). This is based on all of the above criteria.

What does the future of GPS hold?

For Europe, the future holds a new GPS system called Galileo. This new system is much the same as the existing GPS with some major differences. It will not be available until 2012 at the earliest but offers some major advances.

The new system offers 30 satellites although again 3 are (hot) spares. There are however 3 orbital planes.

There are 3 available options

Open Service is available free to all users. It delvers a signal on 2 frequencies (1164–1214 MHz and 1563–1591 MHz). This allows much greater accuracies, typically less than 4m horiziontally and less than 8m vertically. Existing GPS units will be able to receive one signal so the accuracies will be the same as the existing GPS system (15m horizontal approximately). Modern receivers should be compatible with both GPS and Galileo, improving accuracies and availablitity.

Commercial Service broadcasts on a third frequency (1260–1300 MHz) increasing the accuracy further down to only 10cm. This is an encrypted signal and is only available to paying coustomers.

The Public Regulated Service and Safety of Life Service are again an encrypted signal but the accuracy is the same as the Open Service, however it is designed to be more robust against jamming. It is mainly designed around the police/emergency services and the military and safety-critical transport applications .

For mor information, read Wikipedia.

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Our GPS

We have recently purchased a Garmin GPS eTrex Vista and I'm learning more and more about it as I use it more. What I know about this model is that it uses a huge amount of battery power. The operation manual says that the battery life is up to 12 hours of typical use in 'Battery Saver' mode. This means that in 'Normal' mode it lasts even less time!

Battery saver mode simply means that it reduces the rate of satellite signal position updating. This is OK when travelling at a constant velocity but not quite so good if you don't. I do not have the update rate specs but to be honest it will actually make very little difference!

There are a number of ways to to improve the battery life. The System Set-up page can be used to reduce the power consumption.

• GPS option

• Normal - Uses a large amount of battery power
• Battery Saver - Gets less satellite updates so uses less power
• Use With GPS Off - This uses much less power but renders the device pointless in the field
• WAAS option
• Enable - Uses more power (there is generally no point in using this yet as is is currently under development and does not work in most of the world yet)
• Disable - Conserves power
• Electronic Compass
  • On - Uses a huge amount of power. 
  • Off - Turning this off can increase battery life nearly double. It still uses the GPS to discover the direction of travel but it is necessary to move the get a value and this is not very accurate as you may not be sure the exact line of travel.
• Altimeter
  • Automatic Calibration On - Calibrates the Barometric Altimeter with the GPS height, therefore making the Barometric Pressure more accurate but does use more power.
  • Automatic Calibration Off - Uses slightly less power.

See the Specifications of our GPS below.

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Specifications (from eTrex Family Specifications - Garmin)

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