/*! \page page_gpsdo_x3x0 Internal GPSDO Application Notes (USRP-X3x0 Models)

\tableofcontents

This application note describes the use of the board-mounted GPS
Disciplined OCXO, as used with the USRP X300/X310. For information
regarding the GPSDO that is compatible with the USRP N2xx or E1xx,
please see \ref page_gpsdo.

\section gpsdox_specs Specifications

-   **Receiver type**: 50 channel with WAAS, EGNOS, MSAS
-   **10 MHz ADEV**: 5e-11 over \>24h
-   **1PPS RMS jitter**: \<50ns 1-sigma
-   **Holdover**: \<20us over 3h

<b>Phase noise</b>:

 Offset  |   OCXO Phase noise power
---------|----------------------------
1Hz      |-75dBc/Hz
10Hz     |-110dBc/Hz
100Hz    |-132dBc/Hz
1kHz     |-142dBc/Hz
10kHz    |-145dBc/Hz
100kHz   |-150dBc/Hz

<b>Antenna Types:</b>

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

\section gpsdox_install Installation Instructions

To install the GPSDO, you must insert it into the slot on the board near
the 10 MHz Reference SMA. Keep in mind that the two sides of the GPSDO
have a different number of pins. When inserting the GPSDO, make sure to
press down firmly and evenly. When turning on the USRP X3x0 device, a
green LED should illuminate on the GPSDO. This signifies that the unit
has successfully been placed.

<b>NOTE: The pins on the GPSDO are very fragile. Be sure to press down
evenly, or the pins may bend or break. Once the GPSDO is in place, we
very highly discourage further removal, as this also risks damaging the
pins.</b>

\section gpsdox_using 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:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.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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