Differential GPS

[MTU_Cache_Spot]

Working at a surveying firm, I get to play with (and use) all the cool GPS stuff we get (that's worth more than my life!), and it kind of got me to thinking.....

 

Is there a way to use differential GPS with handhelds in a sub-meter fashion? Would this include post-processing/real-time processing (i.e. laptop/radios/cell phones)? Would you need carrier phase, or could you just use code?

 

Since I'm new to handhelds (and geocaching), I don't know the capabilities of them and the kind of data you can get out of them...

 

Seems to me you could use one reciever as a base station on a known point (NGS/CoastGaurd/etc.), and transfer the computed errors to another "rover" reciever. (This is SOP for surveying uses). This data could then be used in the rover (either in the field during realtime - with cell phones or radios, or post processed in the laptop to determine the exact lat/long wherever you wanted....say within 100' of where you know the cache is).

 

Obviously, this is about a million dollar question, so it's not likely that this is possible and no one has thought of it yet (especially with companies like Leica and Trimble working on this continuously), but it's not like we need to be sub-centimeter here.

 

Just a thought, any other techno-weenies feel free to chip in....

 

P.S. I know that this is a moot point anyway unless the cache was set with as much accuracy as you go looking for it with (that's why I haven't tried to convince some of the guys from our office to go looking for caches with our Leica SR530's)

[+parkrrrr]

Given that your requirements for surveying are much stricter, you may or may not already know that the US Coast Guard operates a number of DGPS transmitters already. Black-box hardware with integrated DGPS and GPS receivers have been available for some years now; when I was in the precision ag business, back in the dark days of Selective Availability, I had access to a DNAV-212 from StarLink for a while and found it to be a nice little receiver. It's not very portable, though: it's about the size and weight of a brick and needs an external antenna, 12V power source, and a computer or other device to do something with the NMEA data. It's also quite a bit more expensive than a WAAS-enabled eTrex, but not as expensive as your average subcentimeter survey-grade receiver. A dedicated cacher could carry the whole thing in a backpack and hook it to a Palm Pilot or PocketPC for submeter precision. As you noted, though, that isn't very useful unless everyone has one.

 

[Kerry.]

With certain handhelds there are ways to post process against data from something like a CORS but off course the hardware/software in a handheld isn't as refined as some of the more "clever" receivers.

 

From these handhelds the data does need to be recorded in rinex to a computer using some specific software. Sub-metre is still a big ask especially when one considers the satellite URE is probably generally more than a metre but some of the Block IIR's are now less than a metre but of course there's only about 6 of them. But one could process against precise emphemeris but that's not available until at least 2 weeks after the event and when it's all said and done one is probably spending 98% of the time to improve the accuracy by 2%

 

It may be surprising but even with the 530's in standalone pure navigation mode the SPS accuracy isn't going to be all that much better than some handhelds. In other modes now that's a different story.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

With certain handhelds there are ways to post process against data from something like a CORS but off course the hardware/software in a handheld isn't as refined as some of the more "clever" receivers.

 

From these handhelds the data does need to be recorded in rinex to a computer using some specific software. Sub-metre is still a big ask especially when one considers the satellite URE is probably generally more than a metre but some of the Block IIR's are now less than a metre but of course there's only about 6 of them. But one could process against precise emphemeris but that's not available until at least 2 weeks after the event and when it's all said and done one is probably spending 98% of the time to improve the accuracy by 2%

 

It may be surprising but even with the 530's in standalone pure navigation mode the SPS accuracy isn't going to be all that much better than some handhelds. In other modes now that's a different story.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+park2]

I think Garmin has a DGPS beacon for the coast guard frequencies, as does Trimble - BOB (Beacon On a Belt) for their handheld line (pretty expensive, meant for commercial asset mapping/inventory)

 

Don't forget though, we're at the mercy of the placer and the unit they used when placing the cache.

[sc_surveyor]

I read a article where (www.uspositioning.com) had developed post processing software using Garmin GPS receiver to obtain sub-meter accuracy.

 

Their site provides additional information. Also see post on April 24, concerning this.

[sc_surveyor]

I read a article where (www.uspositioning.com) had developed post processing software using Garmin GPS receiver to obtain sub-meter accuracy.

 

Their site provides additional information. Also see post on April 24, concerning this.