Orange Jelly

GPS Accuracy

With good satellite coverage, the location displayed by your GPS receiver should be accurate to within 50 meters (164 feet) about half the time. Most receivers show "Estimated Position Error" (EPE), "Dilution of Precision" (DOP), or both, but these are only esimates based on satellite coverage and geometry.

Many things can degrade the accuracy of GPS positions. The most well-known of these is the intentional error known as "Selective Availability", but other errors can be far worse.

Make sure your GPS receiver is set up properly. If you'll be using GPS with a map, or for map-making, the datum used by the receiver should match the datum specified in the margin of the map.

GPS accuracy can be degraded by:
  • Operator error:
    GPS receiver datum and coordinate-system settings, data entry errors like bad UTM Zone or degrees West instead of East, transcription and map-measurement errors, and magnetic/true bearing errors.

  • Obstruction of satellite signals:
    When the signal from a satellite is suddenly lost, the receiver can get "garbage" data, or make bad assumptions or calculations. The same can happen as a satellite signal is restored. GPS receivers are designed to handle these obstructions seamlessly, but rare errors can be hundreds of miles.

  • Few useable satellites:
    Less than 4 available satellites leads to a "2D mode" position, which is less accurate. Altitude must be assumed, leading to horizontal errors which can exceed a mile.

  • Selective Availability (SA):
    See below - SA is "off", but may reappear. This is an intentional error to "deny accuracy" to recreational GPS users. SA is described below, along with some suggested solutions.

  • Poor satellite geometry:
    When the available satellites are in one part of the sky, or nearly lined up, the receiver's calculations are less accurate.

  • Multipath interference:
    Reflection of GPS signals from buildings or cliffs can change signal timing, or garble GPS data, causing position errors.

  • Atmospheric conditions:
    Rain or snow can weaken signals, but are usually not a problem. Atmospheric refraction can change signal timing, causing position errors.

  • Satellite position errors:
    Orbit and position data for GPS satellites are subject to "ephemeris" errors.

All GPS errors can "double up" if you store a position, then attempt to relocate it later with GPS. This is because the error when a location is marked can add to GPS errors while re-locating the mark.

GPS altitude errors can be 1.5 times as large as horizontal errors. On some older receivers, a "stale" position can be shown even with no navigation signals received.

Most of these errors are either small or easily avoidable by knowing and operating your GPS receiver well.

Orange Cap

Selective Availability

On May 2, 2000 SA was "switched off", giving GPS users about 10 times better accuracy, subject to all the other factors above. Some of the following can thus be skipped, but SA can still reappear - anytime, anywhere.

While standing still, your GPS readings can "wander" around your location because of "Selective Availability" (SA). Timing and position information from each satellite is "dithered" randomly, within predetermined limits. The intent is to "deny accuracy" to non-military users of GPS, but mounting pressure from business and recreational GPS users may soon put an end to SA.

With SA, accuracy is limited to 100 meters RMS (Root Mean Squared - a statistical term). One hundred meters (1/16 mile or 328 feet) is about the size of a smaller city block.

So what does this really mean for outdoor navigation? In practice, 95% of the time the error will be less than 100 meters. Half the time, the error will be less than 50 meters, but for one out of a thousand readings, error can exceed 300 meters!

SA is bothersome, but with a little care, uncorrected GPS is fine for most recreational uses. You can almost always tell which mountain, stream, or trail you're near, and finding your way to a destination is usually easy once GPS gets you within a hundred feet or so.

Selective Availability is a great reminder to never grow too dependent on any one method of navigation, especially GPS.

GPS Errors on gpsMaps

Even with SA and other errors, trails plotted on our gpsMaps are surprisingly clear and readable. The Adventures section has lots of good-looking gpsMaps with only a few obvious GPS errors.

Here are two examples of larger, less-frequent GPS errors. On the top map, poor coverage caused two "spikes" which cross the river. Selective Availability caused the bottom track to be offset from the road.

On large-scale gpsMaps, SA is visible, but easily accounted-for. The shape of a trail or road is usually correct, and SA puts little "knots" in the plot to show where you stopped.

For critical mapping and navigation, SA can be a problem. Fortunately, you can improve the accuracy of your GPS receiver, and there are also ways to refine your position and track data. There are high-tech solutions, and some less-expensive low-tech things you can do.

Ocean Lake
Differential Corrections

Along the U.S. shorelines and major waterways, the Coast Guard broadcasts free Differential GPS (DGPS) corrections. Properly-equipped receivers can use these signals to calculate a more accurate GPS location (about 10 meters / 33 feet RMS). Amateur Radio operators also broadcast free DGPS corrections in some areas.

You can buy a DGPS-equipped receiver, or connect a separate DGPS add-on to many handheld receivers supporting the RTCM standard. A DGPS setup costs more, and is usually less portable than a standard recreational GPS receiver.

DGPS corrections are available by subscription to a radio or satellite service, and there have been discussions of providing free nationwide DGPS.

If your business is splitting hairs, a GPS receiver costing about $40,000 can combine DGPS and a carrier-phase measurement to achieve one-millimeter accuracy.



It has been suggested that using two GPS receivers: one stationary, the other in motion, can reduce the effects of Selective Availability. The stationary receiver records SA displacements, which are then subtracted from the readings of a second "survey" receiver.

This is only partly true because SA dithers each satellite signal independently. By the time your receiver has calculated a position, the timing and position errors from each satellite have been lumped together into one offset. The two receivers can also have differences in coverage, math, and software, so the results could conceivably be worse than no correction at all.

The traces at right are from two different stationary receivers, recording the same 30 minute interval. There are similarities, but the paths are too different to suggest this type of correction would work well.

NEXT: GPS-PC Interface Cables & Software.

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