Different GPS coordinates for the same point using base-mb-rover configuration

Q&A forumCategory: QuestionsDifferent GPS coordinates for the same point using base-mb-rover configuration
tseco asked 3 weeks ago

Hello,
We use a base station – moving base – rover configuration.
We have observed that if we place the base station at different positions in different days, the rove gps value of the same point changes. 
Does it mean that we have to place always the base station at the same position? Is there any way to configure the initial position of the base station?
Thanks in advance,
Teresa
 
 
 

Ardusimple Staff replied 3 weeks ago

Dear tseco,
that’s probably because you are using “survey in” mode and not calibrating the base station location. “Survey in” process locks the base station location but with a permanent offset. If subsequent runs want to be done with the same results, it is important that both the location of the base station and its position are the same on subsequent days. the base station position can be manually set with ubx-cfg-tmode3

tseco replied 3 weeks ago

But if every day I run the base station with survey-in mode, the offset should be modified each time the base station is switch on, shouldn’t it?

Ardusimple Staff replied 3 weeks ago

if every day the base station starts a new survey-in, even if installed in the same location, the survey-in final position will be different every day, within the defined target error.

tseco replied 3 weeks ago

Thank you very much for the quick answer.
I have just configured the base station with fixed mode. I need to set the position accuracy. I used the position accuracy provided by the base station, but, what is the real meaning of that accuracy. I mean, when the base station in swithed on, it fixes its position with tha configured values, but wthat does imply to use one or another accuracy? We need that a same point of the rover will be always the same from day to day. If we set the position accuracy of the base as 2 m, then the diference in the rover position could be up to 2 meters from one day to another?

tseco replied 3 weeks ago

Thank you very much for the quick answer.
I have just configured the base station with fixed mode. I need to set the position accuracy. I used the position accuracy provided by the base station, but, what is the real meaning of that accuracy. I mean, when the base station in swithed on, it fixes its position with tha configured values, but wthat does imply to use one or another accuracy? We need that a same point of the rover will be always the same from day to day. If we set the position accuracy of the base as 2 m, then the diference in the rover position could be up to 2 meters from one day to another?

Ardusimple Staff replied 3 weeks ago

– setting the accuracy it’s just for having the correct information then output by the receiver.
– if you set always the same base position, the tests will be repeatable from day to day.

1 Answers
clive1 answered 3 weeks ago

Survey-In results on an unaided receiver will usually be poor, figure ~1 metre, and not repeatable.
If you are going to the same site on multiple days, mount the receiver’s antenna to a pole you’ve driven into the ground, or bolted to a concrete post.
Ideally do a proper survey, but if you can’t do that, do it ONCE, and then use the SAME coordinates developed over subsequent uses.