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True Accuracy of the Best Handheld GPS System – An Overview
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.
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Reliable Portable GPS – Problems in Advertising the Accuracy of 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.
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How to Understand the True Accuracy of the Best Handheld GPS System
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.
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Even the most reliable portable GPS devices fail to deliver maximum accuracy of GPS due to certain factors that include obstructions as well as the process of the US Defense System making regular changes to the quality of the Civilian Frequency of GPS. As a workaround to this process, Selective Availability in GPS, several algorithms are used, including the WAAS and DGPS. The WAAS is a must have feature in the most reliable portable GPS apart from high memory for constant mapping. Mapping at regular intervals is a feature of most reliable portable GPS that helps in case of any GPS Signal Blackout.
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CAUTION: Extra Channels CAN NOT Enhance the Accuracy of GPS
Some of the GPS vendor ads claim that their chipsets can receive "n" number of channels, offering an impression that with the increase in the number of channels, the accuracy of GPS will also increase. Some GPS vendors are circulating ads that say that their handhelds can receive 20 channels at a time. Some are claiming access to 40 channels and more!
However, a GPS device receives signals from four to six satellites from each orbit. The US GPS has six such orbits. Each satellite sends out two channels: L1 one for civilians and L2 for defense access only. Thus a GPS device is capable of receiving at most 24 channels at a time. Since the L2 is useless for civilian GPS, the maximum number of usable channels come down to 12, that too, when even the best reliable portable GPS is outdoors - under a very clear sky with no obstructions around... which is possible only in some desert, probably in the Sahara (Guess if these GPS vendors went all the way to test the accuracy of their GPS chipsets).
Now, even if we believe that the GPS vendor has some logic that allows it's chipset capable of accessing more channels even under disturbing environment, the GPS device needs time to collect the data points. With the best receivers, it takes a minimum of 5 minutes to receive all data points when it is turned on. Using this as a base, to receive all data points from 20 channels, it would take a minimum of 15 minutes on the best receiver. However, 15 minutes is a huge time period. By the time the RF Front collects these points and passes it for noise filtering, the GPS antenna will collect more fresh samples and send it to the RF Front, where it is bound to interfere with the older signals still being filtered. Thus, the data will be corrupted even before it receives the DSP for processing. For details on the internal processing of GPS signals and how the receiver components handle a GPS signal, check out this article.
It is strongly advised that you do not fall for the number of channels advertised for any handheld GPS. It does not make any sense even for the most reliable portable GPS as to how many extra channels the GPS vendor advertises, because the accuracy of any GPS cannot be traced beyond a certain limit: based on the standard strength as shown in the figure.
With this point in mind, check out the next section that offers important information, on the GPS technology, to be considered while determining the most reliable portable GPS.
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Factors Contributing to the Accuracy of Most Reliable Portable GPS
Selective Availability: The US controlled GPS system sends signals at two different frequencies, L1 for civilians, and L2 for military only. Other than reserving access of L2 for military, the US Defense system also alters the quality and bandwidth of L1 signals at regular intervals. This affects the accuracy of GPS considerably. The workaround for this is to use Differential GPS. Otherwise too, the DGPS helps in offering greater accuracy of GPS. The most reliable portable GPS devices employ DGPS in conjunction with WAAS (see next point) to get around the problem of S/A.
Spatial Decorrelation: This is the degradation of GPS signals as they travel from satellite to the GPS device. To achieve better accuracy of GPS for faintest signals, the handhelds use several techniques. The most reliable portable GPS contain a minimum of two receivers to incorporate WAAS (Wide Area Augmentation System), which helps in providing better accuracy of GPS even if the signal is very weak or contains unclear data.
Memory for Mapping Needs: You need to store mappings as such and also in case of GPS signal blackout. In case of GPS signal blackout, the GPS device studies the trends of previous mappings to offer further navigation. The more memory your handheld GPS device contains, the more points it can store – resulting in better accuracy of GPS in absence of signals.
With the above information, you can easily guess the true accuracy of the best handheld GPS signal without having to rely on GPS vendor ads. The next section focuses on obtaining much more accuracy of GPS from the most reliable portable GPS devices.
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Tips to Obtain Even More Accuracy from the Most Reliable Portable GPS Device
The first tip is to check out your budget and go for the most reliable GPS device, keeping in mind, the factors mentioned in the article. The most reliable portable GPS device use DGPS and/or WAAS, also called SBAS.
Though the GPS system satellites send signals from different angles, your receiver may not receive signals if you stand near any obstruction such as a building, rocks, trees, or virtually that can block or alter the path of the radio signals. Avoid checking the information in such areas. Move to some location that is more open.
For greater accuracy, hold the handheld GPS at a height of your shoulder. This should be the accurate positioning for even the most reliable portable GPS device. In addition, the portable devices require certain positioning of their antennas. Check out with the GPS device to check if the antenna should be upright or flat.
Do not turn the GPS off when not using. It is recommended that it should be kept on so as to enable the GPS to collect the signals while moving so that you can refer to it as and when you need it. Keeping it on also helps in GPS signal blackout (for mapping) as explained above. Furthermore, do not keep in any enclosed area. The best way is to keep it in any attachment to your belt or baggage is you do not want to hold it in your hands.
Finally, allow enough time for the GPS handheld to warm up when you turn it on. This allows it to collect all the necessary data related to the area. Also, check if the averaging feature is on. The averaging feature allows the GPS receiver to utilize multiple signals to enhance the accuracy of even the most reliable portable GPS device.
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