4.4.8.
Station Shift
This function is called up via the menu item GPS
> Station Shift.
If the environment of the point with known coordinates marked as reference
station (Base) is unsuitable for the installation of a GPS antenna, the antenna
must be erected on a point with unknown coordinates. In order to perform a
precise positioning nevertheless, a reference station adjustment is required. To
this end, after Initialisation of the reference station a point with known
coordinates must be observed with the Rover.
If you
work with a Base that is transmitting Base coordinates which are off for more
than 5 – 10 m from the true Base position you will get systematical errors in
your RTK solution even if you do a standard transformation to fit yourself into
the local system (Calculate > Polar/GPS transformation).
Therefore TPS and Ashtech rovers support a feature to ignore the Base coordinate
transmitted in the differential corrections and to replace it with a user
defined Base coordinate.
The Station Shift function supplies the neccessary
tools to compute and apply a corrected (shifted) Base coordinate on the Rover
side.
The following steps must be performed:
1. Initialisation
of the Base via GPS > Start Base with the aid of an approximate solution (<Nav.Sol.>,
<OK>).
2. Initialisation
of the Rover via GPS > Start Mobile. Here, wait for the resolution of the
ambiguities (display: "fixed").
3. Proceed
with the Rover first of all to a point with known coordinates.
4. Perform the adjustment via GPS > Station Shift as described below.
5. Observe
all other points in the customary manner.
By selecting the menu item, the dialog opens up to calculate the reference
station adjustment.
The
comparison point is to be specified in this first dialog. This is the point with
known coordinates currently observed with the Rover. Make
sure that the GPS-Measurements dialog is opened simultaniously. Enter
this point in the field Pt-No via the graphic, via the <<
button or via the keyboard.
The nominal coordinates are displayed in the fields East,
North and Height. In the
fields dE, dN and dZ the computed
alterations in the reference station coordinates for East, North and Height are
displayed after the comparison has been successfully completed.
When the point has been selected, the calculation can be started with the button
<Comparison>.
The calculation is started via the button <Start>. In the fields dE,
dN und dZ the current results of the comparison are displayed for East,
North and Height as well as their standard deviations in the fields +/-.
These
shift values correspond to the offset between navigation solution and true
position of the Base..
The field Data displays the observed
quantity of data. If the attained accuracy is sufficient, you can end the
comparative calculation with the buttons <End>
or <OK>.
The result of the comparison is displayed
once again in a further window. To accept the result, confirm with <Yes>.
The new (aligned) coordinates of the reference
station can subsequently be saved. To do so, enter a point number in the dialog
that opens up. In the field Comment a
corresponding remark is entered automatically. Save the coordinates with <OK>.
You are now returned to the initial dialog. The word "on"
appears in the field Status Station
Shift . In the fields dR, dH and dZ the computed
modifications for East, North and Height are now displayed.
Quit
the dialog with <OK> in order
to apply the reference station shift. Now all measurements made with these settings
will be shifted according to the displayed values.
In the menu GPS
the activity of the reference station alignment is shown by a tick against the
corresponding submenu.
To quit the dialog without applying the reference station shift hit the <X>
button.
If you wish to invalidate the reference station alignment, in order to
re-perform the calculation via a different point, press the button <Reset Shift>. The coordinates of the points observed up to
that point are retained. Please note, however, that any subsequently observed
points are calculated using the approximate coordinates of the Base, which are
considerably more imprecise. When setting up a new Base the reference station
shift is resetted automatically.
ATTENTION: Even if you leave the program GART-2000® CE in the meantime, the reference station shift remains active for the individual project. |