Differential GPS (DGPS) Configuration - iOS

The Bad Elf GNSS Surveyor uses an RTCM 2.3Radio Technical Commission for Maritime feed delivered through an NTRIP client from a reference station to differentially correct the GPS signal and improve the positional accuracy reported. Using the built-in NTRIPNetworked Transport via Internet Protocol client in the Bad Elf iOS application, one can connect directly to a reference station network. This write up outlines the configuration of a typical NTRIP connection.

DGPS is a technique to improve the location accuracy of the Global Positioning System. DGPS uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the GPS satellite systems and the known fixed positions.

Note: The GNSS Surveyor only supports corrections of the coarse-acquisition (C/A) code (broadcast in the L1 signal)

Note: Accuracy of DGPS decreases with distance from the reference station. If the user and the station are unable to see the same satellites, this problem is further exacerbated.

Note: Where the GNSS Surveyor is configured to use differential correction data (e.g. via an RTCM stream), the receiver's coordinate frame will switch to whatever frame the source of correction data is using. For example, if the reference station is using NAD83 (2011) the locations provided by the GNSS Surveyor will shift to that datum.

Configuration of the Bad Elf NTRIP Client

The following steps demonstrate the typical steps one would use to configure the NTRIP client.

  1. From the main UI of the Bad Elf app, select NTRIP Differential GPS
  2. The NTRIP client displays the current Network, Mount Point, and GPS Location.
  3. If the Network has not been configured, click on the “>” to the right of the Network & Mount Point. The UI shown below provides a means to enter in your corrections station information.
  4. Ensure the host name or IP, port number, userid and password are entered according to the credentials provided by your reference station administrator.
  5. A list of mount points is provided on the line labeled “Mount Point”. Clicking on the “>” provides a list of compatible mount points. If no mount points are show in the list, this particular host configuration does not include an RTCM 2.3 feed
  6. Upon completion of setting the credentials and selecting a mount point, click the “<” arrow at the top of the screen to return to the previous menu. To test this connection, slide the switch that is labeled “Not connected”. If everything is working properly, the label will change to “Connected” and the check marks will appear as below. If this does not occur, see the article on Advanced NTRIP Details.

NTRIP Status Dialog

  • The topmost checkmark indicates data is flowing from the reference station. This checkmark is the first verification of a good data connection.
  • The second indicator provides the status of valid messages flowing into the Bad Elf app. If a checkmark is next to this item, all of the messages a GNSS Surveyor needs are included in the data stream.
  • The third checkmark indicates that the data is flowing to the GNSS Surveyor. This checkmark informs you that a connected GNSS Surveyor is receiving the data from your corrections station.
  • The final checkmark shows the station ID of your corrections network and an indication back from the GNSS Surveyor that the corrections are being used in the solution.