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I have heard that the GPS system is more accurate, and less accurate on some specific hours on the day.

If this is true, how can I get the information on when GPS is more or less accurate at my location? And how big is the difference?

I guess this depends on where in the world you are, I'm living very north, so it's maybe has a bigger effect here. I also think it has to do with how many GPS satellites you get signals from at the moment.

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6 Answers

up vote 17 down vote accepted

There are a lot of factors influencing GPS accuracy. James Ryan is right about night time and clear atmospheric conditions. Then there is the satellite "layout" on a specific time at at specific position on earth. Depending on how many you can "see" (4 at least for a 3D fix) and their distribution (all in one row is bad, evenly spread out better) your accuracy will change. An index for accuracy is the Dilution of Precision which can be calculated from the satellelite positions. The best value this index can get is 1 (pretty much only in theory) but usually is somewhere around 2-4 when everything is normal. Times when the DOP is greater 5 should not be used for field recordings (if you want good results).

DOP can be calculated ahead of time using recent ephemeris data. This way field campaigns can be planned for maximal accuracy. Trimble offers an free planning software for that.

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DOP is the measurement of how spread apart the satellites are on the horizon. If all 4-5 satellites are directly overhead, the DOP will be high (low accuracy of readings). If they are spread out on the horizon, the DOP will be low and the readings will be more accurate. –  bsigrist Jul 23 '10 at 12:30
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Great Question: Can Time and Position affect your GPS accuracy.

Short Answer: YES

Why?

First lets place your GPS Receiver in perfect conditions where the atmospheric conditions are perfect, there are no multipath effects, no radio interference, and line of sight to GPS Satellites is clear.

Your Time and Position on Earth will determine where the GPS satellites will be in relation to you. Relative to your earth postion, the GPS satellites have their own positions that will give you a maximum accuracy and precision GPS Location. Any deviation from this theoretically perfect alignment between you and the GPS satellites will degrade accuracy to a point where no accurate position can be computed.

Besides the Windows Tools provided by other very helpfulls, I really love the NAVCOM online website tool for quickly checking GPS satellite availability and predictive DOP in TIME relative to co-ordinates given.

NAVCOM GPS Satellite Predictor Tool

Great tool if you require a certain level of (x)DOP for GPS Data Collection, to prevent spending an entire day in the field due to poor (x)DOP of GPS Satellites. Of course this tool can not calculate other external factors which will affect (x)DOP.

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had not come across the Web based NAVCOMTECH.com GPS Satellite Predictor Tool. Great alternative to loading the Trimble Planning software. Thanks! –  V Stuart Foote Jan 13 '11 at 18:14
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Accuracy is better at night and in clear weather as the state of the atmosphere is more consistant and predictable. This is particularly the case if you are using WAAS or EGNOS to improve accuracy.

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Almanacs are files that list the orbit information for GPS satellites. You can download almanac information here: http://www.navcen.uscg.gov/?pageName=gpsAlmanacs

Trimble has a free Planning software ( http://www.trimble.com/planningsoftware_ts.asp ) which uses these almanacs to graph the locations of the satellites, along with times of day with high DOP. You can use this information to plan the best time to collect data.

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yes and no ;)

It may be more accurate if a Gps receiver can 'see' more sattelites and/or they are high above horizon. Position of Gps sattelites on sky depends on time of day and position of receiver.

Gps receiver requires at least 4 sattelites to calculate position, they are usually 5 or more of them visible (in open area), but sometimes even 8.

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There are too many factors to consider to answer this question.

  • Whether you have access to more than one Satellite System. GPS
  • L1/L2 Carriers: Correct for Atmospheric Errors
  • Selective Availability (now turned off)
  • Satellite Coverage
  • Sky Masking
  • Dynamic GPS - RF to a nearby base station.
  • WAAS/EGNOS

All these affect your Dilution of Precision or DOP

The access to more observations, the better and of course the position of where the observations come from. Least Squares throws out outliers and increases your precision.

It really doesn't depend on where you are in the world. It depends on if you are in the city, in the forest, on the prairies or on a bald mountain top. The satellite constellation is designed to provide the best coverage around the entire globe (at an apogee of 63.9 degrees). What you will see is that at a certain time of day you will see the same satellites. The US has control of the GPS satellites and can speed up or slow down the satellites for their purposes.

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