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3. GPS Receiver
3.1 WHY GPS?
In addition to location, GPS and other Global Navigation Satellite Systems (GNSS) provide time. In fact, the reason why these
systems can provide very precise position is because the satellites and the ground control stations have extremely accurate
atomic clocks.
In the world of PTP, GPS is particularly useful for the DXD Universal clock because a clock locked to GPS is much more stable
and will be in a better position to be selected by the BMCA to be the PTP Grandmaster.
Another important reason for GPS is that, in addition to providing an extremely accurate clock, GPS (or GNSS) allows units
located in remote locations to have their outputs in phase with one another when they are locked to GPS (or GNSS).
3.2 DXD/GPS RECEIVER
The DXD/GPS is a high-stability, high-accuracy Multi-GNSS timing receiver that supports concurrent reception of GPS (US), GLONASS
(Russia), Galileo (Europe), QZSS (Japan - Quasi-Zenith Satellite System) and SBAS (Satellite Based Augmentation System).
Using multiple satellites constellations concurrently increases the number of satellites in view and improves accuracy.
Simultaneously, using different frequency bands improves robustness against interferences.
3.3 ANTENNA
Once the GPS receiver is installed in your DXD, an antenna will need to be connected to the receiver's SMA connector
(an antenna is not included with the DXD/GPS). The cable with an SMA-Male connector on the DXD side is required.
Ideally the antenna should be mounted on the roof for better view of the satellites. However, the DXD/GPS incorporates
Dynamic Satellite Selection™, a new satellite signal selection algorithm developed by NTT that permits GNSS antennas to be
mounted more freely than ever before. Experimenting will be required to determine the optimal location of your antenna.
Active antennas are recommended and 5v power is provided by the DXD. For detailed recommendations, see Appendix C.
3.4 ANTENNA CABLE
Knowing the distance between the antenna and the receiver is required to implement the proper cable solution. There are 2
aspects to consider: signal loss and cable delay.
• Signal loss: Since the GPS signal is very weak, the antenna usually amplifies the signal. But the antenna cable offers some
resistance and the GPS signal strength will attenuate as it travels down the cable. Therefore the maximum cable length depends
on the gain of the antenna and the quality of the coax cable. Keep in mind that splitters and connectors will further induce
losses. The general recommendation is that after all system losses there should be 10-15dB of gain left (see appendix C).
As an example, when using a coaxial cable with a loss of 5.2dB per 100 feet at 1575MHz (GPS) and an antenna with 40dB
gain, an adequate the cable length could be about 400 feet.
• Cable delay: Signal propagation will cause delay but most likely in the nanoseconds range. If required, the DXD provides
a way to compensate for these delays in menu 6.2 (see below).
4. DXD/OCXO
The DXD/OCXO is an optional Oven-Controlled Crystal Oscillator for the DXD series. It achieves a very high frequency stability
providing greater long-term time and TOD accuracy (+/-10ppb, equivalent to less than +/-10 frames per year). The DXD/OCXO
is particularly recommended when using GPS as the greater stability of the OCXO will minimize drift during long dropouts.
After installing the OCXO, no special setting is required. The DXD will automatically use the OCXO as the Internal oscillator
and indicate INTERNAL/OCXO on the different displays.
DXD/GPS/OCXO User Manual (rev 2.10)
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