Understanding True Accuracy of the Best Handheld GPS System – Guide to most Reliable Portable GPS

Those who know about the basics of GPS understand that the accuracy of GPS devices is based on several factors and varies with the location/terrain where it is being used. The accuracy of GPS is also dependant on the signal strength which becomes weaker with the distance travelled. Though the GNNS systems send signals from multiple satellites to offer ground based GPS systems with better accuracy of GPS system, several factors in the GPS receiver also affect the reliability of the GPS information.

With the type of advertisements broadcasted or otherwise displayed through many mediums, the consumer shopping for a handheld GPS becomes even confused as each vendor claims that their GPS offers the best accuracy, while at the same time offering minimal (or no) information on the testing conditions of the GPS. The following sections discuss the different issues and factors that should be considered before going for a reliable handheld GPS while also offering a take on why it is difficult for the vendors to explain the accuracy of GPS even though the device is created by them. A section on the next page also discusses about some misleading ads that you need to be aware of before purchasing a portable GPS.

The competition among the handheld GPS devices compels GPS vendors to research and create better and reliable portable GPS devices. Before launching a new handheld GPS for sale, the vendors need to test the accuracy of GPS device. They do this under different conditions and then try to explain the process to convince potential customers. However, owing to the complexity of the topic "accuracy of GPS," the ads just cannot convey all the details. It would need a full length documentary or a several page article to achieve this. Hence, most of the efforts, which explain the process and the conditions wherein the accuracy of handheld GPS was determined, do not reach the customer.

The testing process for the accuracy of handheld GPS is done at least under two conditions: a) under an open sky, and b) under a disturbed atmosphere with plenty of obstructions, such as is an urban area with high skyscrapers. If the vendor is following the standard practices, the accuracy of the GPS is measured multiple times under each condition. Based on these observations, the average accuracy should be revealed as far as research ethics are concerned.

However, each of the experiments for both of the above conditions with exact disclosure of the terrain and other factors cannot be explained in a small ad. This leads the vendors to mention only the maximum accuracy of GPS, which may not be always true (even if the vendor is not trying to mislead you). Thus, you cannot decide the most reliable portable GPS using these advertisements.

To begin with, you need to know that in order to offer you with best accuracy of GPS, the device should be able to collect several data points from the different satellites. The number of points required for proper accuracy of handheld GPS is 1000. Based on this information, the accuracy of GPS is described in the following two terms:

Circular Error Probable (CEP): The accuracy expressed using this term is based on the points that fall within a circle, that is, you get only the horizontal accuracy of the GPS. This is because, out of the total points used to compute the accuracy of the handheld GPS, half of the data-points fall outside the circle centered round the truth (estimated accuracy of the GPS device).

For example, if the ad says that the accuracy of GPS is 2 meters CEP, it means that there is 50 per cent probability that the measurement lies inside the circle of 2 meters. On the other hand, it also means that there is 50 per cent probability that the measurement lies outside the circle with 2 meter radius

95 Percent Confidence: The accuracy of GPS is also mentioned in 95th percentile. For example, if the ad says that the accuracy of the GPS is accurate to 10 meters, you can be sure that accuracy of the GPS may contain an error of 10 meter circle and a 5 per cent probability of the error being greater than 10 meters.

NOTE: It is essential to understand that there is no such thing as perfect accuracy of GPS. Each measurement, be it for GPS or anything else, has some probability of error. Also, please note that both CEP and 95 percentile are "estimated" and not "guaranteed" error. Hence, the accuracy of GPS as mentioned in the ads by GPS vendors may or may not be precise, taking into account, the above mentioned error possibilities. Try to go for a handheld whose error possibilities are minimum based on above explanations.

Now that you understand the fundamentals of determining the accuracy of GPS, read on to find an important factor used in GPS ads by some GPS vendors that is completely misleading. The following sections also describe some important selection criteria for the most reliable portable GPS devices while offering tips to reduce the probabilities of errors in the accuracy of GPS handhelds.