Differential Global Positioning System (DGPS)
Differential GPS (DGPS), also known as M823 DGPS and SC104 DGPS, is a supplementary correction signal used by GPS receivers to increase the accuracy of GPS based positioning.
DGPS came in two formats, MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.) and QPSKQuadrature PhaseShift Keying (2 bits per symbol). QPSKQuadrature PhaseShift Keying (2 bits per symbol) was used in the higher frequency bands (13 MHzMegaHertz (MHz) 10^6 Hz) but the vast majority of them have stopped broadcasting, while MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.) based systems in the 284 kHzKiloHertz (kHz) 10^3 Hz325 kHzKiloHertz (kHz) 10^3 Hz region are plentiful and expanding.
MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.) DGPS uses Minimum Shift Keying (MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.)) and has typical baudBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. rates of 100 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. or 200 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second.. The 100 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. DGPS signal has about 150 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz). of bandwidth and 50 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz). shift, and the 200 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. DGPS signals has about 250Hz of bandwidth with 100 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz). shift. There are some reports that suggest some DGPS stations run with 300 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. and a shift of 200 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz)..
The QPSKQuadrature PhaseShift Keying (2 bits per symbol) DGPS used Quadrature Phase Shift Keying (QPSKQuadrature PhaseShift Keying (2 bits per symbol)) and had ~330 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. with 450 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz). of bandwidth.
Click Here to Hear DGPS Stations Live at WebSDR Univ. of Twente[edit]
Message Format[edit]
DGPS transmits data in frames. Each frame passes along a certain type of information. These are categorized by Message Types. They can be see here on page 7.
The most common message type transmitted is Type 9, which transmits satellite corrections for a few satellites, often up to three at at time. Although Message Type 1 is the standard type used to send satellite corrections for all satellites in view of the DGPS radio beacon, the Type 9 message is more resistant to noise and allows for faster resync for GPS receivers, so most DGPS beacons transmit Type 9 messages over Type 1.
Samples[edit]
MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.)  QPSKQuadrature PhaseShift Keying (2 bits per symbol) 

MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.) 100 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second.  MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.) 200 BdBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second. 

Additional Samples[edit]
200Bd MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.) DGPS
Frequencies[edit]
A full and updated list of DGPS frequencies worldwide can be found here. For United States Coverage, the USCG has an updated list of stations across the US as well as coverage maps of stations (Nationwide DGPS Network).
Most MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.) DGPS signals are in the marine radiobeacon 285 kHzKiloHertz (kHz) 10^3 Hz  315 kHzKiloHertz (kHz) 10^3 Hz band. Some QPSKQuadrature PhaseShift Keying (2 bits per symbol)based DGPS signals had been reported a while ago to be in the 13 MHzMegaHertz (MHz) 10^6 Hz range, although nowadays only two such signals exist, in Argentina, @ 2570 kHzKiloHertz (kHz) 10^3 Hz and 2950 kHzKiloHertz (kHz) 10^3 Hz. It is unknown if they use MSKMinimumShift Keying (When Shift/Bd = 0.5. It is impossible to get this ratio to be lower than 0.5, hence it is called the 'Minimum' shift.) or QPSKQuadrature PhaseShift Keying (2 bits per symbol), but QPSKQuadrature PhaseShift Keying (2 bits per symbol) is clearly being phased out of DGPS usage.
Decoding Tutorials[edit]
 RTLSDR.com Decoding Differential GPS Beacons with an RTLSDR, Speclab and SDR#
 DGPS decoder in Spectrum Lab's Digimode Terminal
Decoding Software[edit]
 Hobby Level Software
 MultiPSK
 DSCdecoder
 Spec Lab (using Digimode Terminal for DGPS decoding)
 MultiMode for Mac OS X
 DGPS Decoder for iPad and iPhone
 DGPS Decoder for Android
 DGPS Decoder for Kindle
 Professional Equipment/Software
Video Examples[edit]
 Decoding NDB DGPS beacon
 DGPS Decode w Spec Lab & Sony ICF2001
 Decoding a DGPS beacon with SDR# & MultiPSK
 M823 Differential GPS Decoder with Krypto500
 Differential GPS (DGPS NDGPS) with TitanSDR
Additional Links[edit]
 RadioRaft DGPS
 Utility World DGPS Info
 WAVECOM Database: DGPS
 shoc Database: DGPS
 WUN Jan '96 Description of DGPS
 DIFFERENTIAL GLOBAL POSITIONING SYSTEM BROADCAST STANDARD, United States Coast Guard Technical Reference
 TRANSMISSION CHARACTERISTICS OF MARINE DIFFERENTIAL GPS (DGPS) STATIONS
 United States Department of Transportation NDGPS Assessment Final Report
Additional Images[edit]

