MultiMode for Mac OS X can decode DGPS (Differential Global Positioning System) transmissions. DGPS stations transmit the difference between positions indicated by GPS satellite systems and the known fixed position of the station. This allows higher accuracy. DGPS transmissions are 100 or 200 baud and are transmitted on frequencies from 285 kHz to 325 kHz. They can be interesting DX targets.
A copy of MultiMode can be downloaded here: http://www.blackcatsystems.com/download/multimode.html
To decode the transmission, tune your radio to a DGPS frequency. You can either tune directly to the frequency in CW mode, in which case you set the center frequency in MultiMode to that for your radio’s CW mode, or use USB mode, tune 1 kHz low, and set the center frequency to 1000 Hz.
You can listen to an example DGPS audio recording
Select the baud rate, either 100 or 200 baud, using the button. Also be sure to set your location so that the correct distance and bearing is calculated. Eventually, if you have tuned into a DGPS transmission that is strong enough, you will start seeing decode messages printed:
The Short Demod button can be toggled on, in which case MultiMode will look at a smaller part of the DGPS packet. This often allows decodes of weaker transmissions.
Note that since no error checking is performed on the packet, it is possible to get false decodes. To help determine if you are actually receiving the correct station, compare the printed frequency for that station to what your radio is tuned to, to verify they match. Also look for several decodes from the same station in a row, that indicates that you probably are really receiving that station.
Here’s a list of some stations I have received here with a modest 200 ft random wire antenna:
[15:44:47 11/19/15] 008 804 008 009 286.0 kHz Sandy Hook, NJ United States 40.4747 -74.0197 235.632 km 45.2895 deg
[19:27:49 11/19/15] 198 772 198 199 306.0 kHz Acushnet, MA United States 41.7492 -70.8886 529.571 km 53.1416 deg
[19:28:52 11/19/15] 190 782 190 191 305.0 kHz Dandridge, TN United States 36.0225 -83.3067 723.745 km 245.071 deg
[19:29:12 11/19/15] 156 863 156 157 311.0 kHz Rock Island IL United States 42.0203 -90.2311 1245.06 km 290.156 deg
[19:32:00 11/19/15] 012 806 012 013 289.0 kHz Driver, VA United States 36.9633 -76.5622 231.719 km 192.449 deg
[19:33:00 11/19/15] 184 788 184 185 291.0 kHz Hawk Run, PA United States 40.8889 -78.1889 280.839 km 319.079 deg
[19:33:24 11/19/15] 006 803 006 007 293.0 kHz Moriches, NY United States 40.7944 -72.7564 340.978 km 53.1725 deg
[19:33:37 11/19/15] 196 771 196 197 294.0 kHz New Bern, NC United States 35.1806 -77.0586 434.825 km 192.789 deg
[19:33:50 11/19/15] 092 843 092 093 295.0 kHz St Mary's, WV United States 39.4381 -81.1758 448.281 km 277.867 deg
[19:33:54 11/19/15] 136 792 136 137 297.0 kHz Bobo, MS United States 34.1253 -90.6964 1414.92 km 252.075 deg
[19:33:59 11/19/15] 058 847 058 059 301.0 kHz Annapolis, MD United States 39.0181 -76.61 52.734 km 272.373 deg
[19:36:40 11/19/15] 046 824 046 047 303.0 kHz Greensboro, NC United States 36.0694 -79.7381 463.251 km 226.48 deg
[19:40:01 11/19/15] 218 777 218 219 304.0 kHz Mequon, WI United States 43.2025 -88.0664 1110.64 km 298.697 deg
[19:41:11 11/19/15] 130 834 130 131 307.0 kHz Hagerstown, MD United States 39.5553 -77.7219 160.52 km 293.159 deg
[19:43:59 11/19/15] 312 929 312 313 296.0 kHz St Jean Richelieu, QC Canada 45.3244 -73.3172 736.38 km 16.5642 deg
[19:44:05 11/19/15] 154 862 154 155 322.0 kHz St Louis, MO United States 38.6189 -89.7644 1190.3 km 272.301 deg
[19:55:36 11/19/15] 112 836 112 113 292.0 kHz Cheboygan, MI United States 45.6556 -84.475 1013.8 km 319.521 deg
[20:32:19 11/19/15] 017 808 016 017 314.0 kHz Card Sound, FL United States 25.4417 -80.4525 1560.45 km 196.764 deg
[20:34:44 11/19/15] 340 942 340 341 288.0 kHz Cape Ray, NL Canada 47.6356 -59.2408 1650.3 km 49.1252 deg
[22:10:54 11/19/15] 168 869 168 169 290.0 kHz Louisville, KY United States 38.0175 -85.31 816.337 km 265.238 deg
[22:11:34 11/19/15] 192 778 192 193 292.0 kHz Kensington, SC United States 33.4906 -79.3494 681.801 km 207.126 deg
[22:20:26 11/19/15] 320 925 320 321 313.0 kHz Moise, QC Canada 50.2025 -66.1194 1464.05 km 28.7438 deg
[11:34:56 11/20/15] 262 881 262 263 302.0 kHz Point Loma, CA United States 32.6769 -117.25 3697.45 km 272.2 deg
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