{"id":12771,"date":"2017-03-12T22:09:02","date_gmt":"2017-03-13T03:09:02","guid":{"rendered":"http:\/\/gisgeography.com\/?p=12771"},"modified":"2025-04-09T06:28:51","modified_gmt":"2025-04-09T11:28:51","slug":"gps-accuracy-hdop-pdop-gdop-multipath","status":"publish","type":"post","link":"https:\/\/gisgeography.com\/gps-accuracy-hdop-pdop-gdop-multipath\/","title":{"rendered":"GPS Accuracy: HDOP, PDOP, GDOP & Multipath"},"content":{"rendered":"\n
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\"GPS<\/figure>\n<\/div>\n\n\n

Have you ever wondered about your GPS accuracy<\/strong>? <\/p>\n\n\n\n

In short, a well-designed GPS receiver can achieve a horizontal accuracy of 3 meters or better<\/strong>. <\/p>\n\n\n\n

For vertical accuracy, it can achieve an accuracy of 5 meters or better<\/strong> 95% of the time. Augmented GPS systems can provide sub-meter accuracy.<\/p>\n\n\n\n

But it’s still not perfect. What are the sources of GPS errors that can leave you meters off your mark?<\/p>\n<\/div><\/div>\n\n\n\n

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What Lowers GPS Accuracy?<\/h2>\n\n\n\n

Here are possible ways that can lower the accuracy of a GPS receiver.<\/p>\n\n\n\n

For example, the geometry, atmospheric conditions, and even nearby objects can reduce the quality of a GPS signal.<\/p>\n\n\n\n

Here are the main causes of GPS error and degradation<\/strong>:<\/p>\n<\/div><\/div>\n\n\n\n

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GDOP\/PDOP – Geometric\/Position Dilution of Precision<\/h3>\n\n\n\n

GDOP (geometric dilution of precision) or PDOP (position dilution of precision) describes the error caused by the relative position of the GPS satellites. <\/p>\n\n\n\n

Basically, the more signals a GPS receiver can “see”<\/strong> (spread apart versus close together), the more precise it can be.<\/p>\n\n\n

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\"GPS<\/figure>\n<\/div>\n\n\n

From the observer’s point of view, if the satellites are spread apart in the sky, then the GPS receiver has a good GDOP.<\/p>\n\n\n

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\"GPS<\/figure>\n<\/div>\n\n\n

But if the satellites are physically close together, then you have poor GDOP. This lowers the quality of your GPS positioning potentially by meters.<\/p>\n\n\n\n

READ MORE: <\/strong>Trilateration vs Triangulation \u2013 How GPS Receivers Work<\/a><\/p>\n<\/div><\/div>\n\n\n\n

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Atmosphere Refraction<\/h3>\n\n\n
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\"GPS<\/figure>\n<\/div>\n\n\n

The troposphere and ionosphere can change the speed of propagation of a GPS signal<\/strong>. Due to atmospheric conditions, the atmosphere refracts the satellite signals as they pass through on their way to the earth\u2019s surface.<\/p>\n\n\n\n

To fix this, GPS can use two separate frequencies to minimize propagation speed error. Depending on conditions, this type of GPS error could offset the position anywhere from 5 meters.<\/p>\n<\/div><\/div>\n\n\n\n

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Multipath Effects<\/h3>\n\n\n\n

One possible error source in GPS calculations is the multipath effect. Multipath occurs when the GPS satellite signal bounces off of nearby structures<\/strong> like buildings and mountains.<\/p>\n\n\n

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\"GPS<\/figure>\n<\/div>\n\n\n

These reflected signals can create interference and introduce errors in the calculation of the receiver’s position, leading to inaccuracies in the GPS measurements.<\/p>\n\n\n\n

In effect, your GPS receiver may also detect the same signal twice at different ranges. However, this error is a bit less concerning and could cause anywhere from 1 meter of position error.<\/p>\n<\/div><\/div>\n\n\n\n

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Satellite Time and Location (Ephemeris)<\/h3>\n\n\n\n

The accuracy of a GPS satellite\u2019s atomic clock is one nanosecond<\/strong> for each clock tick. That’s pretty impressive stuff. <\/p>\n\n\n