Internal GPSDO (USRP-N2x0/E1X0 Models)

Table of Contents

• Specifications
• Installation Instructions
  • Post-installation Task (N-Series only)
• Using the GPSDO in Your Application

This application note describes the use of integrated GPS-disciplined oscillators (GPSDOs) for the USRP N-Series and E1xx. For information regarding the GPSDO that is compatible with the USRP X-Series, please see: Internal GPSDO Application Notes (USRP-X3x0 Models)

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: -80 dBc/Hz
  • 10Hz: -110 dBc/Hz
  • 100Hz: -135 dBc/Hz
  • 1kHz: -145 dBc/Hz
  • 10kHz: < -145 dBc/Hz

Antenna Types:

The GPSDO is capable of supplying a 3V for active GPS antennas or supporting passive antennas.

Installation Instructions

Instructions for mounting the GPSDO kit onto your USRP device can be found here: http://www.ettus.com/content/files/gpsdo-kit_2.pdf

Post-installation Task (N-Series only)

Note: The following instructions are only necessary for UHD 3.4.* and below.

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 10 MHz and PPS signals for reference or MIMO use (see Device Synchronization), 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 <install-path>/lib/uhd/utils
./usrp_burn_mb_eeprom --args=<optional device args> --values="gpsdo=internal"

-- restore original setting --
./usrp_burn_mb_eeprom --args=<optional device args> --values="gpsdo=internal"

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 10 MHz 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_gprmc, and gps_gpgga sensors. Location information can be parsed out of the gps_gpgga sensor by using gpsd or another NMEA parser.