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How Accurate is my GPS?


New hand held GPS receivers costing more than $200USD should regularly give an accuracy of better than 10m. For most marine science applications (e.g. water quality or seabed sampling) this should be fine. A competent diver will also find seabed installations with this accuracy.

To increase your chances of a good position fix, ensure the antenna has a clear all around view of the sky, you have 5 or more satellites in view and these satellites are spread throughout the sky (not bunched up, or all low on the horizon). Check the satellite status page for all this information. You may also note an estimated error that is expressed as meters or DoP on this page. If you see DoP (Dilution of Precision) it should be less than 5. If your errors are high you will normally have to wait for 30 minutes to an hour before the satellites become more favourable in your area.

It is possible to increase the accuracy to less than 2m using by using either of the two free to air GPS correction systems, free Satellite Based Augmentation Systems (SBAS) or ground based RTCM signals or paid SBAS subscription services. Free SBAS systems currently include EGNOS in Europe and WAAS in the US, if your hand held receiver is capable of processing the WAAS signal (note WAAS and EGNOS are compatible signals) simply switch on the function in your GPS’s menu. You will know it is working by looking at the Satellite status page and noting “3D Diff” and/or a small “D” symbol on the signal strength bar for each satellite in view. Similar systems should be coming on line in Japan, India and SE Asia soon. It is important that if you are not in a WAAS or EGNOS designated service area, this function is switched off on your GPS as stray or test signals may confuse your GPS.

The second way to increase your accuracy is to look for differential broadcast stations. These stations broadcast RTCM signals and are usually operated by governments or non-profit organisations as an Aid To Shipping. Your hand held receiver will need to be able to accept RTCM signals via a serial port and you will normally have to buy (or hire) a decoder box for about $1000USD. Stations are widely available in the Arabian (Persian) Gulf, Australia, SE Asia and parts of Africa. The setup process for this system is more complicated and you will need to consult the manuals for your decoder box and GPS or contact us for help.

Finally it is possible to purchase survey grade corrections usually from a SBAS provider such as Omnistar, Veripos or C-Nav. We are happy to advise on any of these services.

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