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Esteemed Contributor
Posts: 2,192
Registered: ‎02-17-2016

Re: Mark IV GPS?

Where are you seeing the picture?


Have you tried loading the actual long and lat into google maps to see if it is correct?

Esteemed Contributor
Posts: 3,260
Registered: ‎06-11-2013

Re: Mark IV GPS?

I've never had any issues with mine... but when I saw your post I pulled out my phone and put it next to my 5D IV to compare... they both agree on the geo-location (to within a few tenths of a second).


The GPS will take a few minutes to get an initial position fix.  If you check it too soon after powering up then it may not have a good fix yet.  The solution is to just wait a few minutes (it'll typically have a pretty good fix after just 3 or 4 minutes).  However the position fix you initial get (right after it's able to get that first fix) will continue to get more accurate over time.


Something else to know about the GPS network... a GPS receiver calculates the your position by determine the fractional time offset for the signals it is receiving from each of the GPS satellites it is currently using.  It has a list of known satellites and their orbits and uses that data to determine where the satellite would have been located when it transited the signal... and uses that to work out where YOU must be located in relation to all the satellites it's using.  


But here's the rub... the satellites don't maintain consistent orbits.  They'd drift.   This means the satellite may not have been located in the spot where the receiver thinks they were and this can cause a fairly strong level of inaccuracy ... especially if it's been a while since you've used the GPS.


The organization that controls the GPS network works out the drift and updates the orbit data.  The satellites themselves periodically then transmit their new orbit data so that all GPS receivers can update their internal data.  But this takes a while.


If you have a GPS that hasn't been used in a while... or you've moved subsantially since the last time you powered it up (not a few miles... but hundreds of miles or even thousands of miles) then it will take the GPS longer to get it's first position fix.


Basically it has to listen to the network for those orbit updates that come from each satellite and once it's received enough new orbit data from enough satellites then the position fix will get better and better (you'll also find the startup time significantly improves before it gets the first fix because it internally stores those updates and it'll be powering up in roughly the same location with very recent data which means the initial data that it receives will be able to worked out to a reasonable solution much faster.


I have a sailboat with a GPS.  I keep a second hand-held GPS onboard so that if the main GPS ever fails or loses power (and that actually did happen to me personally once... and it happened to a friend of mine (different boat, different GPS) on another occasion)... then you can find yourself in the middle of a massive body of water and you can't see land in any direction.  You've been relying exclusively on this thing for your location and now it's dead.  So you need a backup.  I test the backup GPS from time to time but only about once per year.  I don't keep the batteries in the backup GPS (don't want them to leak and corrode the contacts).  That means when I power it up... it's "as if" it's never been used before (it has no stored updates of any satellites).  That system has to start from scratch.  It's EASILY a 20 minute (maybe even closer to 30) before it starts getting accurate position data.  (I also keep paper charts on the board so I can find the locations of harbors).  


Anyway... I never understood (a) why it took so long to get a fix and (b) why the fixes were always initially very poor... but got better with time... until I learned about the satellite drift and orbital data updates.




Also, you can set the update rate to preserve battery.  I want to say the default rate is 15 seconds... but you can set it to as infrequent as once every 5 minutes.   If you're primarily working in the same location then once every 5 minutes is great because you're not going anywhere.  But if you're constantly on the move (driving etc.) then in 5 minutes you can be miles away from the last position update.



Tim Campbell
5D III, 5D IV, 60Da
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