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Accuracy and challenges of GPS Device!

How accurate is a GPS device?

The accuracy of most of the GPS devices is 2 meters CEP, Circular Error Probability, which means the device is accurate within two meters. There is no such thing as a measuring gadget that is 100 percent accurate ever till now. Taking multiple measurements over several minutes yields an accuracy rate that approaches 100%. This is accomplished by increasing the number of decimal places like 99.99998 percent.

Warm-up of GPS devices

GPS receivers need to “warm-up” after being turned on, for a few minutes before they can start receiving satellite timing signals.

A kind of almanack from the GPS satellite network in orbit around the Earth and the “ephemeris data” in this almanack tells the GPS tracker device, which satellites it can anticipate to view at a certain time of day. Once the device has received this information, it can begin the process of locating itself and so the object in which the device is attached, for example, a car.

 Position accuracy of a GPS Device

To rate its position accuracy, the GPS receiver uses an internal measurement termed “Horizontal Dilution of Precision” (HDOP). If HDOP falls below a pre-determined threshold for more than a minute, it will report its position with latitude and longitude values of 0.0.

The HDOP rises above the threshold once the receiver can see 5 or more satellites again, and the device reports highly accurate positions once more. This behaviour allows the GPS receiver to send only one or two position reports with incorrect positions before the reported position “goes to zeros.”

When compared against position reports before and after any other single position report, it is fairly easy to determine whether a single position report’s correctness is doubtful. In every case, GPS accuracy varies by little more than a half-mile for a minute before returning to zeros or snapping back to the “real” position.

While driving down the road, GPS horizontal accuracy is excellent: often within 25 feet, but frequently within 10 feet. While moving, GPS speed determination is also quite good, within 1 MPH. In contrast, GPS vertical accuracy is low, with errors in elevation/altitude ranging from hundreds of feet to thousands of feet.

 Challenges of GPS receivers

While stationary, GPS speed determination is comparatively less accurate, sometimes displaying speeds of 1 to 3 MPH when speed values should be 0. Because inaccuracies are added up in an imaginary straight line, a movement of a mile or two per hour results. The inaccuracies, on the other hand, are dispersed in all directions around the actual location and cancel each other out over time, as seen in the image on the left.

GPS Drift is a phenomenon that occurs when the “view” of 5 or more satellites is occluded. As satellites pass through areas of the sky where the device’s view is impeded, the device’s location “fix” may hop around until a solid fix is re-established.

This abrupt change may lead the device to believe it has travelled further than it has, resulting in reported speeds of 10 to 100 miles per hour and higher. Vehicles parked within garages, under trees with dense foliage, or next to a building obstructing the view of the sky are the most common examples.

Through HDOP, the device is aware of these events, and PAJ GPS devices take precautions to avoid reporting Geofence or Excessive Speed events that would otherwise be reported incorrectly. For our customers who are having difficulties with this for a certain vehicle, they’re tracking that is routinely parked overnight in a region with bad GPS reception, we normally apply filters for such devices on a case-by-case basis.

Reverse Geocoding 

The software on the PAJ GPS device derives the “Location” information via a process known as “reverse geocoding.” Reverse geocoding transforms a Latitude and Longitude pair into a Street Address or other descriptive information that helps viewers figure out where the GPS device is located, such as “13 Silver Ln, Cardiff, UK.”

Even the finest map designers aren’t sure where are the exact limits of certain addresses are, thus PAJ GPS devices can’t ensure that location readings are always accurate.

PAJ GPS device’s reverse geocoding algorithm looks for the closest known place name in a database of place names, however, it frequently fails to discover a precise street address. When this happens, the software on the PAJ GPS device reverts to a road or highway name, or sometimes only a county and state name. As the result, “Location” values should always be read as “within a reasonable distance of.”

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