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I'm looking for an extra accurate GPS. It needs to be able to relocate the path of tire tracks from year to year to check for soil compaction.

Are there portable commercial units available capable of this degree of repeatable accuracy (4")? How much would such a unit cost? Any recommendations?

Many thanks.
 

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Before you spend a lot of money on anything. GPS accuracy ultimately depends on your LOS to the orbiting satelittes. If your GPS unit can't stay in constant contact with them due to overhead obstructions or masking terrain--it doesn't matter how accurate your unit is. Your accuracy will not be optimal due to your surrounding environment. If this is not a factor--then your accuracy hopefully will depend on the GPS unit alone.
 

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Some of the units we have at work cost $8,000, and they have to be down-loaded to a computer for differential correction from a CORS Station... Even then, only 50% of the points we get are within 50cm, but 75% are within 50cm to 1m, and 100% are within 1.5 meters...

If you are looking for 4" of accuracy, you will almost need real-time correction, and a base station. When you are talking about <4" of accuracy, you are darn near talking about survey grade equipment and generally speaking, to get anything with tighter than 50cm accuracy is going to cost quite a bit more than that $8,000 we spent.
 

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Yes there are portable GPS units that are capable of the type of accuracy that you are looking for. Topcon makes one and I know that there are others as well. Free bit of advice you might want to consider a unit that is rated for maritime or nautical use or four aircraft used as these offer 3D displays rather than the 2D that is common to most other units
 

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I should add that units that are capable of the type of accuracy that you seem to be looking for tend to be exorbitantly expensive. At least from the standpoint of a private individual on a budget
 

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A GPS that is suitable for cartography for map making would probably do the trick for you
 

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Cell phones are accurate to 3 meters and Iridium Sat phone was to within 1.5 meters. Under 4 inches would have to triangulate with more than 3 Sats and the refresh rate would need to be fast----like constant. I've worked with GPS clocks that were pretty accurate and just an average commercial grade one was in the 20 grand range using a quartz clock module (not the top of the line). Military grade would cost even more.
 

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A subby from a boomer told me back in 1977 that their accuracy was such that if a satellite spotted you on a life raft in the middle of the ocean the sub would be able to launch a missile at you with enough accuracy to kill you with just the impact - no explosion!
Yes they have that type of accuracy available for submarine warfare sadly you and I don't have a wallet big enough or deep enough to hold the kind of money required to purchase such equipment. Think many many many millions of dollars.
 

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A subby from a boomer told me back in 1977 that their accuracy was such that if a satellite spotted you on a life raft in the middle of the ocean the sub would be able to launch a missile at you with enough accuracy to kill you with just the impact - no explosion!
On a raft? With a missile?
Hell that's easy! Give or take a hunnerd yards and the wake from impact would swamp that raft.
 

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A subby from a boomer told me back in 1977 that their accuracy was such that if a satellite spotted you on a life raft in the middle of the ocean the sub would be able to launch a missile at you with enough accuracy to kill you with just the impact - no explosion!
The way I understand it the original purpose of GPS was to target atom bombs. Who cared it you missed your target by 100 feet, it was toast anyway.
4 inch accuracy from GPS in not for the casual hobbyist.
..
 

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A couple of years ago I was tooling across Arizona and using the GPS's MPH rather then the car's speedo. When the car said 75 the GPS said 70MPH. I had new tires and thought perhaps since they were larger than factory tires the GPS was giving me a more accurate reading.
Maybe.
The Arizona State Trooper didn't think much of my idea. It seems the lag-time for GPS can give you a false speed reading perhaps this is the same for the exact location too.

(In 1998 I rode my bicycle in the Hotter N Hell 100 bike tour in Wichita Falls, Tx. I went to the whole thing. As we got closer to the Finish Line the aid stations got closer together - about every 5 miles rather than then the tradition 10 miles apart. At 95 miles ( by my Speedo) I was sure I had only abut a 1/2 hour more to ride.
It seems the speedo was off and I rode for another 2 1/2 hours to finish the 100 mile ride. I crossed the finish line in about 8 hours and walked funny back all the way back to the car. I learned my lesson that day!)
 

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A couple of years ago I was tooling across Arizona and using the GPS's MPH rather then the car's speedo. When the car said 75 the GPS said 70MPH. I had new tires and thought perhaps since they were larger than factory tires the GPS was giving me a more accurate reading.
Maybe.
The Arizona State Trooper didn't think much of my idea. It seems the lag-time for GPS can give you a false speed reading perhaps this is the same for the exact location too.

(In 1998 I rode my bicycle in the Hotter N Hell 100 bike tour in Wichita Falls, Tx. I went to the whole thing. As we got closer to the Finish Line the aid stations got closer together - about every 5 miles rather than then the tradition 10 miles apart. At 95 miles ( by my Speedo) I was sure I had only abut a 1/2 hour more to ride.
It seems the speedo was off and I rode for another 2 1/2 hours to finish the 100 mile ride. I crossed the finish line in about 8 hours and walked funny back all the way back to the car. I learned my lesson that day!)
https://www.radarbusters.com/Police-Speed-Enforcement-Arizona-s/2527.htm


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