NI USRP supports L1 GPS Simulation using the LabVIEW GPS toolkit. LabVIEW with a NI USRP-2920, a stable 10 MHz reference clock, and 60 dB of fixed attenuation can simulate up to 24 hours with up to 12 satellites.
The connections to the GPS are simliar to those used in a record and playback discussed in this tutorial:
90dB of fixed Attenuation (with USRP Gain set to 0)
(When using an NI USRP-292x device, an external reference clock is required (an OCXO is recommended)
NI-USRP 1.0 driver
NI GPS Toolkit 2.0
Providing Signals at the Correct Power Level to the DUT:
The RF front end of a commercial GPS unit typically expect signals between -130 and -160 dBm. This may vary by GPS DUT. Signals scaled to full range (+/- 1) in software at the L1 GPS Frequency 1.57542GHz are transmitted by the NI-USRP at approximately -14dBm (this level is not calibrated, so taking the measurement is suggested). The output signal generated with the GPS toolkit is not a +/- 1V full scale signal (you may want to measure this as well). In order to avoid clipping, the USRP gain was set to 0dB and externally, 90dB of attenuation was added to the SMA cable between the USRP and GPS using 3 fixed 30dB attenuators.
External Reference Clock:
GPS is very sensitive to frequency offset and clock jitter, so an improved frequency source is recommended beyond the standard TCXO. An OCXO is highly recommended, which provides a very precise frequency reference. The NI USRP-293x provides an internal OCXO that can be GPS disciplined or a user can connect their own external via the RefIn port on the front of the NI USRP-292x. A property node in the niUSRP driver allows you to specify the clock source in both cases.
IQ Generation Rate
The generation rate in the GPS Toolkit is 1.5MS/s. This rate needes to be interpolated in software to an IQ rate supported by the USRP. Therefore we upsample the signal from 1.5MS/s to 2 MS/s using a Modulation Toolkit VI before being sent to the USRP for transmission.
Note: It's likely that your GPS Almanac files will be located in a different folder than mine. Be sure to double check both file paths and make sure your GPS Toolkit is activated in the license manager if you're getting error messages.