GPS accuracy and reliability

When you buy one of our GPS tracking devices you want one that provides accuracte information all the time everytime. However, the quality of the information is based on GPS data collected.

PDOP, HDOP, VDOP... what is DOP? DOP is the term used to describe how reliable (or accurate) the location, speed and heading are for any given GPS record. DOP is an acronym for Dilution of Precision.

As more satellites are visible to a GPS receiver, the higher the precision, and thus lower dilution. The ultimate goal for accuracy is for the receiver to see as much of the sky as possible and gain a fix from as many GPS satellites as possible.

However, there are many circumstances that can compromise a clear view of the sky. Just as you may have perfect vision, if something like a billboard is obstructing your view, you can't see what's behind it. GPS is no different.

When a GPS receiver can see a broad view of the sky from horizon to horizon and everything above, it has the best chance of a high precision. If the receiver can only see straight above it will result in poor DOP, or if something is obstructing a view, it can result in poor results.

GPS Police is analyzing the embedded data behind the GPS signals...

...and producing a value in HQ of how accurate (or reliable) the DOP is. We are going to score every event from 1 to 5. * 5 out of 5 – Location accuracy is +/- 5 metres and it's speed accuracy is +/- 3kph @ 100kph
5 out of 5 is the highest level of accuracy available for a civilian GPS. * 4 out of 5 – Location accuracy is +/- 10 metres and it's speed accuracy is +/- 5kph @ 100kph * 3 out of 5 – Location accuracy is +/- 20 metres and it's speed accuracy is +/- 10kph @ 100kph * 2 out of 5 – Location accuracy is +/- 100 metres and it's speed accuracy is +/- 25kph @ 100kph * 1 out of 5 – Location and speed accuracy is unreliable probably due to total blocking coverage of the GPS device/antenna .

Wikipedia has a great article for those who wish to understand DOP better here:

What is GPS? The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use.

How does GPS work? GPS uses a syncronized time code that is broadcast from satellites in space. When a GPS receiver gets a GPS signal it decodes the time code. A GPS receiver does this with numerous GPS signals and determines which satellites are further away, and which are closer and then by using basic math the receiver can plot it's point on Earth. It's a like echo location, something we see in nature.

How reliable is it? GPS at the military level is good enough to put a missile traveling at 2000mph through the 8" target. On the civilian level, it's possible to be within less than one meter (with a low DOP) of accuracy.

What is DOP? Dilution of precision (DOP), or geometric dilution of precision (GDOP), is a term used in satellite navigation and geomatics engineering to specify the additional multiplicative effect of navigation satellite geometry on positional measurement precision