In order to synchronize to external GPS signals you will need the National Instruments PXI-6608 board.
NI PXI-6608
http://sine.ni.com/apps/we/nioc.vp?cid=3590〈=US
There are a number of tutorials, knowledge bases, and examples programs written for this board that explain it's GPS features. I have linked one great tutorial below.
Synchronizing and Correlating Measurements to a Global Timebase with GPS
http://zone.ni.com/devzone/conceptd.nsf/webmain/2FAD3B4E67BA766186256B9C00679F75?opendocument
Additionally, if you go to http://www.ni.com and search for 6608 you will find a number of examples written for this hardware. Just one of these examples I have linked below.
Timestamping Multiple Hardware Events using a PXI-6608 and a GPS Receiver
http://sine.ni.com/apps/we/niepd_web_display.DISPLAY_EPD4?p_guid=B45EACE3E8EF56A4E034080020E74861&p_node=DZ52327&p_submitted=N&p_rank=&p_answer=&p_source=External
As for your question about adjusting the sampling rate for your DAQ device when the system frequency alters continuously, you should not have to alter your sampling rate. In order to capture the frequency of a signal we need to follow the Nyquist Theorem and sample at a rate of at least two times the maximum frequency component. If you want to accurately represent the shape of the signal you should sample at 5 to 10 times the signal's frequency. Therefore, following the Nyquist Theorem, if you know that your system frequency is changing, but will never be higher than 100kHz, then you need to sample at a rate of at least 200kHz. Frequency analysis can be performed after the signal has been captured.
Regards,
Justin Britten
Applications Engineer
National Instruments
