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========================================================================
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 <install-path>/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.
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