GPS Base

For GPS equipment, it is necessary to set up the base antenna and record the correct antenna height, the antenna type (for certain equipment), and the methods used for localizing (obtaining the desired coordinates). 

Much of GPS Base is common to most instruments.  For example, the Base Ant. Height would be entered in feet or meters, depending on the setting specified in Units within Job Settings.  Vertical or Slant height refers to the method used to measure the height of the base antenna.  Vertical refers to the distance from the ground point to the antenna disk, measured plumb.  Slant refers to the distance from the edge of the antenna disk down to the ground point directly beneath the center of the antenna.  Elevation Mask refers to the degrees above the horizon above which satellites will be used for GPS “correction” calculations, and below which satellites will be ignored.  Keep in mind that it is not always better to lower the elevation mask below 10 or 15 degrees.  Satellites closer to the horizon than 10 degrees will often degrade the calculation of coordinates.

The “alternative” to GPS Base is GPS Rover.  GPS receivers can usually function as either type, but must be “configured” for the role they will serve.  You will be warned when you switch receiver configuration from Base to Rover and vice versa, to confirm that is what is intended.

With all RTK GPS Equipment selections (except GPS Simulation), GPS Base will first bring up settings and then proceed to the 6-option GPS Base screen which “locates” the base position. 

After completing the GPS communications setup and clicking on the Ok button the Base Configure dialog will be presented.  These button options "locates" the base position. The 6-button GPS Base options (3 buttons per “tab”), common to all, are discussed first, followed by equipment-specific settings.

GPS Base for All RTK GPS Brands

For all brands of GPS, the GPS Base button is the command that configures the base receiver for broadcasting GPS corrections to the rover.  You must click the GPS Base button in while you are connected to the base receiver.  The base needs a set of coordinates to use as its stationary position. Given that you are configured for a particular coordinate system, there are 2 main “types” of localizations:  (1) Enter an Accurate Base Position and work from that point outward on the scale and “true north” of that system (no rover localization), (2) Enter an Accurate or Semi-Accurate (Read from GPS) Base Position and then localize to known rover points that transform to the local coordinate system (required if the local system is not true GPS north).  There is also a “mixed” base-rover approach.  To use the base in the localization, you should configure the base with the "Use Local Coordinates" option under "From Known Position".  Here, you configure the base by entering the local point (5000, 5000,100) and start a new localization file (or use an existing one if it applies).  Then, at your rover, you can add more points to the localization as necessary.  In this case, a single rover point within Localization will produce a scale factor and best fit to the local coordinates.

There are six methods to set the stationary base position, organized into 2 categories:  From New Position includes Read from GPS, Enter Lat/Long and Enter Grid System Coordinates.  The From Known Position tab includes Previously Surveyed Point, Use Local Coordinates and Read From File.  When you are starting a new job (no information in the raw RW5 file yet), always use the options in From New Position.

From New Position Options

• Read from GPS - This method takes a specified number of GPS readings from the base receiver’s autonomous position and uses it as its “true” position.  The autonomous position can be off of the actual position by 150 feet. The base will calculate corrections based on this autonomous position.  If you set up the base with this method, the rover points must be used for localization since the corrections they are using are based on the approximate position of the base antenna.  Read GPS, combined with rover-based localization, is one of the most common methods used to start a GPS survey. When Read from GPS is used, the program will present the current position, as shown:

Latitude:  N 42d21’28.35882”

Longitude:  W 71d08’12.87540”

Elevation:  116.376

Continue with Base Setup?

Yes        No

If you like the result, press Yes and continue on.  You will then be prompted for the Reference Station Number.  This is an “ID” that will store to the raw file and permit post-processing of the raw GPS data.  A typical entry is 0001.  The final prompt will say,

Base Configuration Successful.  Save Settings to File?

Yes                No

Answering Yes will bring up an entry screen for the reference file name.  Reference files are stored with a “.ref” extension.  It is important to save a reference file if you plan to return to the job at another date.  Without re-localizing to control points, you can set up over the same base position, recall the reference file, and enter the new antenna height for the base position, and start working.

The Read from GPS option is sometimes referred to as “Find Me”.  This option is accurate to about 10 to 50 meters, typically, and it enables the corrections to broadcast with enough accuracy to permit localization and sub-centimeter real-time work.  Be sure that your GPS zone in Job Settings is set correctly before using this option.  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.

• Enter Lat/Lon - Requires you to enter the latitude and longitude for the position of the base antenna.  This is useful if you are setting up over a monument whose latitude and longitude is known.  It can also be used over a control point whose position is known from GPS post-processing.  This will put you on the NAD83, NAD27, UTM or other coordinate system specified in Job Settings, GPS Tab.  You can still do a rover-based alignment to transform to a local system.  Rover-based localizations are always improved by more accurate base positioning.   If the GPS detects that your entered Lat/Long is not near the current GPS readings, the program will warn the user.

This method should be used if you are reasonably sure of the accuracy of the latitude and longitude—it should be at least as accurate as the “Read from GPS” option, or otherwise you will have difficulty getting a “fix” on the rover.  If it is a very accurate latitude and longitude, you will get the best results. Pressing OK leads to the option of store the base position as a reference file, similar to Read from GPS.

• Enter Grid System Coordinates: Requires you to enter the grid system northing and easting for the point that the base is occupying.  This applies to any projection that you have configured, including U.S. state plane, worldwide UTM or any individual country or user-defined grid system.  This is useful if you are setting up over a monument whose coordinates you know.   An accurate base position, either by entry of Lat/Long or Grid System Coordinates, will enable immediate data collection without rover-based localization, but also will improve rover-based localization.

The Coordinate method is similar to the Lat/Lon method, since Lat/Longs convert to Grid Coordinates based on the current  coordinate system set in Job Settings, GPS option.  The Grid Coordinates will convert back to the Lat/Long coordinates needed by SurvCE to configure the base. 

From Known Position Options

• Previously Surveyed Point: This requires you to enter the coordinates, on the configured coordinate system, of a known, surveyed point.  This will transform and localize to the local coordinate system, and optionally can be followed by rover-based localization.  The known point must be found in the RW5 file in a form that includes its Lat/Long (a previous GPS measurement).  This Lat/Long, just as with New Position options, is used to establish the base position.  This method requires that you are working within an existing job that has a raw file including Lat/Long positions for the points being used. This next figure is an example of the prompting.

Like the other options, this continues on by showing the corresponding Lat/Long for the coordinate and allowing the user to save the result in a reference file.

• Use Local Coordinates: If you have “localized” using option 5 (Localization) within the Equip menu, you obtain a “dat” file.  This localization file is used to convert any Lat/Long reading into a local coordinate.  By the same token, you can take a local coordinate, run it back through the dat file and obtain a Lat/Long for the base receiver setup.  That is the procedure used in this option. 

When OK is pressed, you will be asked to load the associated “.dat” file, in a dialog similar to below:

• Read From File - Reads a previously saved base position file. All of the other methods of setting up the base let you save the base position at the end of setup.  If you return to a site, set up the base in exactly the same position, use Read From File to use the same base position and you don’t have to re-align the rover: the old alignment is still valid.

This option recalls a reference file in a dialog similar to that above.  Then you will receive the standard prompting.

This is followed by the Reference Station ID prompt (showing the default ID).  The “new” base antenna height, which will change with each new set up, is entered back at the first set of dialogs. A message is displayed after successful configuration from a file.   Only if you set the base antenna on the same horizontal and vertical position each day would the base antenna height remain fixed.  If the base antenna height and x,y position is the same from day to day, then you do not need to do GPS Base each day.  You would simply power up the base, power up the rover and start working in that case.  Keep in mind that in certain power surge conditions, receivers can lose their settings, and a re-configuration may be necessary, even if the base receiver antenna has not been moved.

Note: In SurvCE 1.50.006 or later, a “quick connect” is performed when you exit and re-enter the SurvCE program.  In earlier versions of SurvCE (before August 20, 2004) and after the release of SurvCE 1.50 in May, 2004, it was necessary to conduct a fresh GPS Base after exiting SurvCE.

Note: One method of obtaining Latitude/Longitude and state plane coordinates in the U.S. is to occupy a point, any point, for two hours or more, logging the static data, then submit the Rinex file to the NGS “OPUS” program accessible on the web.  You must log the data with dual frequency equipment.  The OPUS program is fully automated, and will return the x,y,z, lat, long, ellipsoidal height and orthometric height.  This data can then be entered in during base localization.  Though most post-processing programs (Trimble Geomatics Office, Leica SKI-Pro) use proprietary post-processing file formats, they will output the ASCII  “Rinex” format needed by OPUS.

GPS Rover

For many brands of GPS, receivers will perform differently depending on whether they are set as base or rover receivers. Even though the receivers are identical, they perform differently depending on whether they are configured as a base or a rover.  Multiple rover receivers can communicate with a single base receiver.  The GPS Rover command is used to set the rover receiver to the correct parameters and to instruct the receiver that it is, in fact, a rover.