/*! \page page_gpsdo Internal GPSDO Application Notes (USRP-N2x0/E1X0 Models)

\tableofcontents

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: \ref page_gpsdo_x3x0


\section gpsdo_specs 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

<b>Antenna Types:</b>

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

\section gpsdo_install 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

\subsection gspdo_install_post Post-installation Task (N-Series only)

<b>Note:</b> 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 \ref page_sync), 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> --key=gpsdo --val=internal

    -- restore original setting --
    ./usrp_burn_mb_eeprom --args=<optional device args> --key=gpsdo --val=none

\section gpsdo_use 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:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
    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.

*/
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