Connect the GPS 1PPS signal

Connect the 1PPS signal from your GPS receiver to one of the input channels of your soundcard. Use the -p option to specify which input channel is used. For a stereo soundcard the left channel is usually input 1 and the right channel is input 2. For a multichannel soundcard the inputs and usually labelled numerically. In the example below I am using input 8 for the 1PPS signal.

[james@swan UltraMSK-1.0.2-F17]$ ./msk -p 8 -d -f 1
% Your evaluation license will expire on 2012-12-31.
% Connected to VLF signal on input system:capture_2
% Connected to PPS signal on input system:capture_8
First ppsOffset = 7.834010, rate = 26182.834010, mPPSPhase = -8.16043e-05
1346387662.002 96005.939784 6.18727e-05 1 -0.999997
1346387663.001 96006.021465 5.69062e-09 1 -0.999996
1346387664.002 96006.006321 -7.77683e-07 1 -0.999995
1346387665.001 96005.993134 -1.40359e-06 1 -0.999994
1346387666.002 96005.981632 -1.93804e-06 1 -0.999994
1346387667.001 96005.971685 -2.33104e-06 1 -0.999998
1346387668.002 96005.963192 -2.5855e-06 1 -0.999998
1346387669.001 96005.955996 -2.74974e-06 1 -0.999998
1346387670.001 96005.949847 -2.90485e-06 1 -0.999999
^C% Terminating...
[james@swan UltraMSK-1.0.2-F17]$

Terminate using Ctrl-C. For each GPS pulse received the program outputs one line of 5 numbers. The first number is the unix time stamp of the pulse. The second number is the current estimate of sound card sampling rate. The third number is the time difference between an internally generated 1 Hz signal and the received GPS 1PPS signal. The fourth number is the number of seconds since the previous GPS pulse was received. The fifth number is the correlation coefficient of a linear fit to the phase of the GPS pulse versus frequency. This value must be very close or equal to -1.0.

Setting up the 1PPS detection is one of the most important steps in the configuration of your UltraMSK receiver. It is critical that the 1PPS pulse is correctly detected for the receiver to function optimally. Use alsamixer to adjust the gain of the 1PPS channel. Reduce the gain until the 1PPS is not detected and then increase the gain just enough so that the pulses are reliably detected each second. Not all soundcards are suitable for use with ultramsk.

Back 19 Aug 2012