Frequently Asked Questions (FAQ) on Hardware

FAQ Index

Can I use a DGPS beacon receiver with my GPS unit and GPSy at the same time?

Yes, GPSy can work with a GPS with a DGPS receiver at the same time. This will give you < 10 meter accuracy in real-time. DGPS beacon transmitters are established along the entire U.S. coastline and most other nations are setting them up as well.

There are two main issues with hooking up GPS/DGPS configurations to your computer:

Note that in all of these schematic diagrams, Data GND of all of the units are grounded (connected) together. We skip this important detail in the schematics to simplify the diagrams.

There are several options:

  1. Plan A: Connect the GPS to the DGPS receiver in the normal fashion (DGPS data out connects to GPS data in; GPS data out connects to DGPS data in). Then connect the RxD/GND pins of the Macintosh to the TxD/GND pins of the GPS, in effect tapping the transmit lines in parallel. This method may not work and may damage the transmit circuitry of the GPS (depending on its current draw limitations). We've been told that Garmin permits this for their receivers, however due to liability reasons, you should double-check with them again.

    This sets up the system as a dedicated DGPS configuration. You won't be able to transfer data between the computer and the GPS unit (waypoints, etc.) but it's a good configuration if all you're interested in is real-time DGPS corrected DATA

           GPS data in  <-----------RTCM------------------<-- DGPS data out
           GPS                                                DGPS
           GPS                                                DGPS
           GPS data out -->---------NMEA-0183---------------> DGPS data in
                                       +--- Macintosh Data in
                                            (Mac Data GND connects to both GPS/DGPS data GND)
                                            (Mac Data Out left unconnected)
  2. Plan B: Connected the units as above, but install a switch (DPDT) so that you can switch between DGPS Data Out and Mac Data Out. When operating in realtime DGPD mode, flip the switch to the DGPS side, when transferring data between the Mac and GPS unit, flip the switch to the other side. You'll also have to tell the GPS unit itself to switch between NMEA/RTCM DGPS mode and Garmin/Garmin or other data transfer modes.
                                      +--- Macintosh Data Out
           GPS data in  <------------O/O------------------<-- DGPS data out
           GPS                                                DGPS
           GPS                                                DGPS
           GPS data out -->-----------+---------------------> DGPS data in
                                      +--- Macintosh Data in
  3. Plan C: If your GPS unit cannot handle two "listeners" (the DGPS receiver and the Mac) listening in parallel (as in Plans A/B above), purchase a NMEA-0183 bus splitter. Any good NMEA-0183 supply store should carry these; they're also used when you want to use an auto-pilot with a GPS/DGPS combination. One brand that has been used is the Noland N183-41 multiplexer available from Stentec Software.
           GPS <----RTCM ------------------------------------- DGPS
           GPS                                                DGPS
           GPS                                                DGPS
           GPS ---NMEA-0183-----> NMEA-0183 BUS Splitter ----> DGPS
                                  +--- GPSy/Auto-Pilot

    Note that while you use the bus splitter to split the outgoing NMEA-0183 stream to go to both the DGPS and GPSy, you take the RTCM (DGPS correction data out) of the DGPS and send it directly back to the GPS. If confused, take this diagram to your installation technician, they should be able to understand it.

  4. Future: GPSy Pro will support serial bus chaining in a future release. This will allow you to:
            GPS ------NMEA-0183--->  GPSy/Mac  ----> NMEA-0183 ----> DGPS
             |                                                          |
             |                                                          |
             +--<------------------------- RTCM <---------------<-----+

    As far as we know, this will be a unique feature not on any other product on either Mac or PC platforms.

How does DGPS-IP service work?

Our new DGPS-IP client feature in GPSy 3.30 and GPSy Pro 1.10 works by contacting Internet DGPS-IP Servers (such as These servers stream DGPS correction data in the industry standard RTCM SC-104 protocol from two server vantage points on the East and West coast of the United States. From those two servers, there is DGPS coverage of all of the United States; much of Canada and Mexico; and the western portion of Europe. We'd like to actively encourage people in Europe, South America, Africa, Oceania and Asia to also set up DGPS-IP servers so that we may have world coverage.

What DGPS-IP clients do is set up a connection through the Internet to the DGPS-IP servers and stream the data directly into your GPS unit. Most GPS units with serial ports directly accept RTCM data (set the unit to the NMEA data out / RTCM data in setting) and will immediately calculate a DGPS position. This is unprecedented accuracy, and the best thing is that it's free!

DGPS-IP Performance
Nominal Accuracy 100 meters (95%) 10 meters (95%)

GPSy Pro Satellite Monitor with DGPS Data

In our testing here at the lab, accuracy of the GPS unit will go from the 100 meter accuracy of uncorrected GPS to less than 10 meters with DGPS-IP. This is accurate enough to even tell you which side of the road you are on. Of course, this accuracy is highly dependent upon the number of satellites you share with the DGPS-IP server as well as your satellite visibility (DOP) and other factors.

Obviously, to use DGPS-IP, you need a real-time connection to the internet. Most of our customers seem to either be using DGPS-IP in a stationary location (to get high-accuracy position data through averaging) or are using some form of mobile internet connection such as cellular PPP or Ricochet.

Some tips:


Copyright (C) 1997-2007 by Global Mapping Systems and Karen Nakamura. All rights reserved. GPSy® and GPSy.COM® are registered trademarks and GPSy ProTM and GPSyLinkTM are trademarks of Karen Nakamura. All other trademarks are the property of their respective owners. Mention of a third-party's product does not represent endorsement of or by that product.

This page was last updated on September 29, 1999. We've had [N/A] hits since May 20, 1999.