I have always been in the habit of setting up the base the very first thing on the job and let it just collect for awhile as I go look around, find some corners or whatever and come back to do the "Find Me" or "Read from GPS" set up. But NOT every time. Sometimes I am just in a hurry and go right to it.
Looking through the Carlson SurvCE manual tonight I came across this:
Also, give the base enough time to find itself. When first set
up, the base receiver has to “collect” satellites and “resolve ambiguities”. If you
Read from GPS too early, you may find a position, but one that is not fully
resolved and ends up slowing down the speed at which the rover fixes.
How long is, "enough time to find itself"?
How long do you wait?
How many reading do you take?
That last part of the statement has me wondering, because we always have to many trees and often slow fixes.
Scott McLain
> How long is, "enough time to find itself"?
Typically well under a minute. An exception is if the unit has not been used for a long time - several weeks, perhaps - it may take considerably longer in that case.
> How long do you wait?
Until I get initialization at the rover. Typically 10 seconds. Rarely more than 30 seconds.
> How many reading do you take?
That varies with circumstances. If you are attempting to collect RTK in very bad sky/multipath conditions forever may not be enough. In good conditions 3-5 epochs is usually good for topo. Boundary / control ties need more. Test by staking out previously tied points after some little time has passed and checking repeatability. If you need to occupy topo points more than 3-5 seconds to achieve acceptable repeatability you should probably be using the total station.
if you are using the equipment every day, the time is very short.. it's only if the unit has been on the shelf for a while that you want to make sure it sits for 5 or 10 minutes. IMHO.
Not sure how Carlson does things, I use Leica with the OEM SmartWorx, all the base needs is to track ALL the SV's and then I start it logging and broadcasting. That time is pretty short, certainly nothing like the time required for PP a static session.
SHG
Unless you travel far, then again I'd wait a couple of minutes.
If you can, watch the results live, they will let you know.
We usually fire up the base and rover once we get on site. By the time we get the legs setup over a control point, I get rolling on getting the base configured. With SurvCE the first thing I do is switch to GPS Base and hit the monitor key. Looking at the HRMS, VRMS, and PDOP will tell you a lot about how long you should wait. If we happen to be in a PDOP spike, I'll let it set until the numbers settle down. Once I'm ready to go, I let the base read 100 shots, and save the reference file. Then I configure the rover. Our service range is about 200 miles from our office, and I have never had problems driving that far, getting the gear out and up and running. I think if you are traveling farther, you might want to wait 10-15 minutes to the let the receivers collect a new ephemeris.
About 10 minutes after I turn it on. Hell, it takes 10 minutes to set the base up and another 10 to get where I'm going and start. The base flashes fast when it doesn't see many birds for oh, about, 30 seconds, before I start the base.
The issue is not the RTK, but how long before you let it sit there, before you make your autonomous (here) solution. The only reason you would care, if any, would be so that the here position is within 5 feet of the OPUS solution, thereby negating, in a practical sense (even though Loyal will correct me) any rotation of bearings and a straight translation can be used to correct the here position data to true NAD83 coordinates via an OPUS solution or processing between CORS stations.
Generally by the time you set up the rover and drive or walk to your first point (I like to start away from the base and work my way back, especially if I'm walking), the base has had plenty of time to resolve an autonomous position. Even on a small site, it should only be a matter of a few minutes.
The big thing is to think about the age of the almanac, and any dissimilarities in the "expected" constellation geometry. If it's been several months since I've fired up that base unit, or you've moved a long distance from the previous setup, then you need to give it time to download the new almanac and find the SVs.
I turn on both units before I setup so the updated almanacs can be downloaded and the GPS units can 'find themselves' based on an autonomous solution.
After I setup the base, I then ensure the rover has a fixed solution.
Once it does, I have check into a point to make sure things are in order.
Then, away I go...
Thank You for all the great input.
I often go weeks without using it and move around a lot.
Today, I went right to the monitor screen and just watched it. Within a few minutes it went from 10 feet down to about 3 and stayed there. I will check this screen every setup from now on and know what is happening.
> Thank You for all the great input.
>
> I often go weeks without using it and move around a lot.
>
> Today, I went right to the monitor screen and just watched it. Within a few minutes it went from 10 feet down to about 3 and stayed there. I will check this screen every setup from now on and know what is happening.
Scott, I like your conclusion.
I usually use 100 epochs, but your conclusion, I think is better. Also, James Meis comment above seems to have merit.
Nate
