Matt wrote:
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Cool stuff and nice blog!
Is your GPS module capable of logging raw data and, if so, have you tried post-processing that for better accuracy? I came to the conclusion that "on the water" accuracy is of less importance when mapping than than the accuracy of logfiles - which can be improved by post-processing if raw GNSS data is logged. We have been experimenting with L1 RTK (single channel) which has been giving really impressive results - perhaps in the order of a few cm. Running 2 x receivers can give extremely accurate headings also. Take a look at
this forum post for an example. We have written a small utility that takes the sonar logfile and an independently logged set of RTK logfiles and synchronises them (brute force, minimising differences which works surprisingly well). Eventually this will be part of RM2, but not the first release. The GPS data we have been logging is accurate enough to use for tidal adjustments. Certainly, accurate headings (in particular) and position can make a very big difference to the quality of sidescan mosaics.
It is (or I should say was) a NEO-6M so it could if I reprogrammed it. What I have been doing before I got the Elite 7 Ti last year was logging several NMEA sentences especially the one that has the PDOP, VDOP, and HDOP data and then run the data through a Python script that I got from a friend who works for the USACE Rock Island District which covers half the Upper Mississippi River (The part with the locks and dams) and you basically input your acceptable levels for each setting and it goes through and culls out the high xDOP sentences along with everything else with that GPS timestamp as invalid data because the GPS fix just wasn't good enough
However I just got a NEO-M8N module which is capable of picking up the Russian GLONASS 'birds' and in my test setup which simulates less than ideal conditions ( Basically an active puck antenna sitting on the sill of my west basement wall with a big maple tree out front and an even bigger oak tree, like I said less than ideal conditions) and I average 8 US birds and 3 Russian birds. This thing is going to freaking rock out in the open in my boat. This module is also able to accept RTK correction data and I just found out the state runs a RTK network with close to 100 stations covering the state for the farmers. I also found out AT&T Wireless is offering a prepaid SIM for IoT applications (Like a remote weather station) for $10 bucks and it lasts for 6 months or you use 500 MB of data over the 2G network, basically enough to last me an entire season since the correction data does use up a lot of bytes. I'm going to get one of those, stick it in an old smartphone I have and then send it via Bluetooth Serial (SPP) to my unit (I knew there was a reason I stuck that Bluetooth module in their last year)
If this works as I expect it should I'll basically be out 30 bucks for the GPS module, and 10 bucks for the SIM and with the stuff I already have I'll have a centimeter accuracy RTK system on my boat . I can pretty much set up and test everything via Wifi here at home and work all the kinks and bugs out and then get the SIM and just have it grab the internet from the cellular data connection instead of the Wifi (Actually I should be able to use both) You can also buy GSM shields for Arduinos to replace the cellphone but I'm currently using it for Bluetooth controlled robots and robotic arms I make with my 3D printers anyways. I'm really temped to get another one of these with a built-in patch antenna (Even cheaper, around 20 bucks) and put it on the WASP II which has a Class 1.5 (40-50 meter range) Bluetooth 4.0/LE dual Bluetooth module which is an upgrade to my original mapping aquabot ....
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