======================================================================== UHD - Internal GPSDO Application Notes ======================================================================== .. contents:: Table of Contents This application note describes the use of integrated GPS-disciplined oscillators with Ettus Research USRP devices. It pertains specifically to the Jackson Labs Firefly-1A device unless noted otherwise. ------------------------------------------------------------------------ Specifications ------------------------------------------------------------------------ * Receiver type: 50 channel with WAAS, EGNOS, MSAS * 10MHz ADEV: 1e-11 over >24h * 1PPS RMS jitter: <50ns 1-sigma * Holdover: <11us over 3h * Phase noise: * **1Hz:** -80dBc/Hz * **10Hz:** -110dBc/Hz * **100Hz:** -135dBc/Hz * **1kHz:** -145dBc/Hz * **10kHz:** <-145dBc/Hz ------------------------------------------------------------------------ Installation ------------------------------------------------------------------------ See the documentation for your device for specifics on installing the GPSDO. ------------------------------------------------------------------------ Configuring the USRP to use the GPSDO ------------------------------------------------------------------------ This is necessary if you require absolute GPS time in your application, or need to communicate with the GPSDO to obtain location, satellite info, etc. If you only require 10MHz and PPS signals for reference or MIMO use, (see the Synchronization application note), it is not necessary to perform this step. To configure the USRP to communicate with the GPSDO, use the usrp_burn_mb_eeprom utility: :: $ cd /share/uhd/utils $ ./usrp_burn_mb_eeprom --key=gpsdo --val=internal This will configure the driver to communicate with the GPSDO on startup. ------------------------------------------------------------------------ Using the GPSDO in your application ------------------------------------------------------------------------ By default, if a GPSDO is detected at startup, the USRP will be configured to use it as a frequency and time reference. The internal VITA timestamp will be initialized to the GPS time, and the internal oscillator will be phase-locked to the 10MHz GPSDO reference. If the GPSDO is not locked to satellites, the VITA time will not be initialized. GPS data is obtained through the mboard_sensors interface. To retrieve the current GPS time, use the "gps_time" sensor: :: usrp->get_mboard_sensor("gps_time"); The returned value will be the current epoch time, in seconds since January 1, 1970. This value is readily converted into human-readable format using the time.h library in C, boost::posix_time in C++, etc. Other information can be fetched as well. You can query the lock status with the "gps_locked" sensor, as well as obtain raw NMEA sentences using the "gps_gpgsa", "gps_gprmc", and "gps_gpgga" sensors. Location information can be parsed out of the "gps_gpgga" sensor by using gpsd or another NMEA parser.