Comments

• Derek February 12, 2013 at 12:13pm

  I'm impressed that you are down to around 1.5M, and I think what you are doing is great.

  The gyro heading should be to within .5 of a degree? Are you happy that the GPS speed and distance derivation is accurate enough? I wondered about an optical or mag system on one of the wheels would give a more accurate speed and independent DR correction.

  I'm not very good at this, how do I get the @KM6VV to come up 

  I think I'd like to see a really good top drawer GPS system reading on the home point, accuracy won't improve just using say 2 ublox GPS.  
• 

  KM6VV February 12, 2013 at 10:27am

  So what about taking the same GPS receiver first at "home" then to the various waypoints, and then back to home.  Assuming home reading is the same, then the waypoints are all valid to home reading at that time.  Keep checking home against GPS, and if it changes, send the delta to the 'bot.  Basically a mini DGPS system.  So need two GPS receivers...

  Alan KM6VV
• 

  Bot Thoughts February 12, 2013 at 10:15am

  @Derek -- come to think of it, if you want high, absolute accuracy, then no, GE probably isn't the ticket, but I think it would help for us to qualify the statement "accurate ground working positions" a little.

  Also, the question is not only how accurate but also how precise whatever solution is.

  In my case, as long as GE is sufficiently precise during a given mission, then accuracy should be irrelevant--but... the big thing I forgot to mention?

  My robot uses dead reckoning from a starting point selected in GE referencing a physical landmark (like a manhole cover or such). That's what cancels any static GE error. (Any higher order error would manifest as imprecision then). Of course that assumes sufficiently accurate orientation in GE.

  Dead reckoning will help (in my case gps heading & gyro fused along with distance). Fusing in accurate GPS position information, however infrequently, will surely help.

  Next step for me is to improve measurement accuracy (as are others on diyrovers hence the discussion I linked to) so I can get a better handle on errors, causes, fixes. The robot was precise to 1.5m-ish, but there's some oddness I still have to track down.
• Derek February 12, 2013 at 9:34am

  Thanks for the discussion links and input. I know that if we setup a high quality GPS station at the start point and have good enough software to calibrate the Bot GPS offset, we can be accurate to less than 4 inches over a course. I have seen it done in professional survey. That's expensive. 

  I don't think that GE will ever be good enough to establish accurate ground working positions for some time until at least a year after the Galileo Sats go up because of the way GE is put together. That's not to say it isn't good, it's brilliant.

  Even after Galileo the sats will still only update once per sidereal second. If you are moving at 1mph thats 17.6 inches, if at 10mph that's 176 inches to each correction in course. Can we do better by integrating DR from a good heading arrangement and accurate speed measurement updating say 20times/second? 
• 

  KM6VV February 12, 2013 at 9:09am

  @Vijay

  What was the article on ublox? Alan KM6VV
• Admin

  Thomas J Coyle III February 12, 2013 at 4:52am

  Nice discussion guys. This is the kind of group action that I like to see! Maybe one of you could start a discussion in the Discussion Forum to track GPS development?

  Regards,

  TCIII
• heliboy February 12, 2013 at 4:08am

  Go to the ublox website. They have and excellent article on GPS - history, theory, limitatations and future. It appears that having two GPS units does not solve much, but next year there will be new satellites that will transmit on two different frequencies and this will bring the accuracy (hdop) < 1m.
• 

  Bot Thoughts February 11, 2013 at 11:12pm

  @CarlT : yes, I believe it was one of the 2012 AVC competitors that tried this. Unfortunately I have no information on the results.

  @Derek : I've historically tested against Google Earth and found there aren't constant offsets even for a single mission. It really depends on what satellites come in and out of view, what multipath interference occurs and where, and such things. 

  On the Google Earth topic-- GE isn't perfectly accurate to reality but because I set waypoints in GE according to on-the-ground markers, I figure the error cancels out, provided any GE to reality error remains the same for a whole day (during the AVC) which I've observed to be the case on many, many individual days of testing. (It really only seems to change occasionally)

  We had a discussion on DGPS over on diyrovers on accurately locating one's rover, and getting into DGPS, diy DGPS, and other fun -- diyrovers discussion in case it is of interest.
• 

  KM6VV February 11, 2013 at 10:06pm

  I think DGPS was used to overcome SA, which is now turned off, for the most part.  You'd have to use DGPS every time, not just record the offsets.  Is that what you're asking?

  I don't think the problem is getting greater accuracy, just reading the waypoints, and not having any changes from readings to the run.

  Alan  KM6VV
• 

  Anthony White February 11, 2013 at 9:31pm

  DGPS is beginning to be used in smaller vessels, especially in survey work environments. 

  Would the offsets calculated from running a predetermined course vary with atmospheric or timing conditions? I had discounted that as practical only when always operating in the same region.