A SIMPLE DESCRIPTION OF HOW REAL-TIME DGPS WORKS
 
                            Reference Site Block Diagram
    / (GPS sat)
  -0-                                                            |
  /                                                              |
    ~                                                           / \
                                                               /---\
 ____/\_____                  __________                 _____/_____\____
| Reference |  RTCM Data     | Modulator |  Modulation  |  Transmitter   |
| Station   |----------------|           |--------------|                |
-------------                -------------              ------------------
 
REFERENCE STATION:
The reference station GPS receiver knows exactly the position of it's antenna 
therefore it knows what each satellite range measurement should be.  It 
measures the ranges to each satellite using the received signals just as if 
it was going to calculate position.  The measured ranges are subtracted from 
the known ranges and the result is range error.  The range error values for 
each satellite are formatted into messages in the RTCM format and provided 
to a Modulator.  The messages are generated continuously.
 
MODULATOR:
Depending on the transmission format the modulator encodes the data as 
necessary for transmission.  For example, in the free USCG system the 
modulator creates a carrier signal which varies using MSK modulation.  A one 
data bit is represented by a advancing carrier phase and a zero bit by a 
retarding carrier phase.  The modulated carrier output from the modulator is 
connected to the transmitter.
 
TRANSMITTER:
The transmitter is basically a power amplifier which is connected to an antenna 
system.  The modulated carrier is amplified and driven to the antenna.  In the 
USCG system the transmitter is usualy 500 Watts and is in the 300khz frequency 
range.  The amplified signal is radiated via the antenna to remote DGPS 
receivers for real-time position correction.
 
                          Remote Site Block Diagram
                                                         / (GPS sat)
                                                       -0-             
                                                       / 
                                                    ~  
 
          ____/\_____                        ______/\___
         |DGPS       |     RTCM Data        | GPS       |
         |Correction |----------------------| Receiver  |
         |Receiver   |                      |           |
         -------------                      -------------
 
DGPS CORRECTION RECEIVER:
The DGPS correction receiver decodes the signals receiver from the reference 
site.  The data is formatted into a serial RTCM data stream and provided to 
the remote GPS receiver.  Their are many types of DGPS correction receivers 
depending on which format and frequency ranges used by the reference station.
 
GPS RECEIVER:
The corrected GPS receiver measures the ranges to each satellite but before 
the measurements are used to calculate a position the corrections received 
from the DGPS correction receiver are applied to the measurements.  The 
position is calculated using the corrected range measurements and output for 
the user.
 
NOTES:
In the US Coast Guard system the reference station GPS receivers are made by 
Ashtech and have the modulator built in.
 
There are several companies providing units which integrate both the DGPS 
correction and the GPS receiver, none are handheld.
 
GPS without SA has an accuracy of about 100 meters.  GPS with corrections 
(DGPS) has an accuracy of  between 1 and 5 meters depending on the quality 
(price) of the GPS receiver.
 
All currently manufatured GPS receivers can accept RTCM Corrections (as far 
as I know).
 
The cost for DGPS corrections varies from free for the US Coast Guard system 
to $1000 per year for the Omnistar system.
 
-------------------------------------------------------------------
David Fowler,Vice President,Starlink Incorporated,     512-832-1331
Starlink provides products & services for professional (D)GPS users
dfowler@starlinkdgps.com  
(posted to sci.geo.sattelite-nav Feb. 26, 1995)