GPS accuracy question

dingbat

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Just wondering how big a difference there actually is out there.

What kind of antenna do you have on your GPS unit, built in or remote? How many satellites do you typically lock on to and what’s your typical postioning accuracy?


I have a remote antenna and receive a full compliment of “birds” (12) including WAAS but understand that different locations see different things. My typical accuracy bounces around between 6’ and 12’.
 

bruceb58

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Re: GPS accuracy question

First of all, the accuracy that you are saying " 6' to 12' " is what the unit is estimating that you have based on the WAAS info. That isn't necessarily what you actually have due to multipath error.

Are you worried that you aren't accurate enough?
 

dingbat

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Re: GPS accuracy question

Yep, projected accuray taking into account DOP. Close enough to find offshore wrecks and such with.

Have heard many people complain of the lack of WAAS signal, limted birds, ect and was wondering how wide speard of a problem it is if at all.
 

bruceb58

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Re: GPS accuracy question

You are not going to to know DOP unless you know which satellites are actually being used to solve the "equation". The guy who wrote the software for the GPS unit will know this but you and I won't.

I would think that you are definitely going to be good enough. I am going to assume you are a diver or fisherman. The coordinates for the wrecks you are tring to find were probably originally calculated with equipment that was not as accurate as what you are using now.
 

dingbat

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Re: GPS accuracy question

You are not going to to know DOP unless you know which satellites are actually being used to solve the "equation". The guy who wrote the software for the GPS unit will know this but you and I won't.

It's interesting that you say that because the status page on my unit lists satellite numbers, gives a DOP number as well as a DGPS signal to noise ratio. Thought they where commom status outputs.

I'm a fisherman so it's certainly good enough for my usage.:D
 

bruceb58

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Re: GPS accuracy question

It's interesting that you say that because the status page on my unit lists satellite numbers, gives a DOP number as well as a DGPS signal to noise ratio. Thought they where commom status outputs.

I'm a fisherman so it's certainly good enough for my usage.:D

I stand corrected on the DOP info.

Yep..definitely good enough for your use. If you were flying an instrument approach, it might be a different story.
 

JB

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Re: GPS accuracy question

My Garmin 276C with internal antenna spends most of its time in my vehicle and part time in the field or in the boat.

It typically locks on to 8-12 satellites plus WAAS.

On the road it is spot on because it locks on to the nearest road on the map.
In the field it typically finds a waypoint within 3'.
On the water it finds waypoints within 6'-8' with WAAS, 10'-20' without WAAS.
 

Big Keepers

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Re: GPS accuracy question

As a land surveyor, I've used GPS many, many times. The kind of GPS that surveyors use requires an FCC license and can obtain centimeter accuracy. The GDOP and PDOP numbers that show your quality of a signal are statistical, meaning that you may possibly have an error no greater than the quality you're reading. But you don't necessarily obtain those errors. Example, the unit I am most familiar with would usually lock in to about a 1" accuracy. But every time I took a reading on a known point, I was always better than 1" with my check. This was with an RTK (real time kinematic) unit that is constantly receiving and converting the GPS signals to track your movements. It communicates with a base station of known origin to compute position.

Handhelds and navigation aids should get you to within 5' on average. That's what I've found. On water I've had better than that with some models to the point that I trolled right along side buoys that are shown on my maps and the distance to it is within 3 feet.
 

dchris

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Re: GPS accuracy question

That 5-6 foot mark is probably about the best you can get. The survery unit is using an external radio signal from a ground based unit that has been measured exactly. GPS was built for the military.
 

Big Keepers

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Re: GPS accuracy question

The Federal government still has control over the satellites. If there is a plane crash or other crisis, they can make GPS satellites less accurate for civilians and only military and emergency personnel will be able to accurately tie into the system. We had this problem at the company I worked at on 9/11.
 
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