GPS / GNSS Antenna Loading – Alarm States

All ViaLiteHD GPS RF over fiber products come with a GPS mode that senses and mimics the operation of an active GPS antenna as standard.  This allows the GPS antenna to be current sensed by the RF over fiber transmitter. If there is no current loading on the RF connector port, either due to no connection or a fault with the amplifier in the antenna, the transmitter switches off its laser, forcing an alarm condition at the receiver end.  This allows the user to know there is a fault in the system.

Fig. 1 A typical GPS system set-up  Click here to expand

The transmitter’s laser will only deactivate on detecting a fault condition if the module’s GPS mode is enabled – all ViaLite GPS transmitters are shipped with this mode enabled by default, unless otherwise requested.

If the current sunk by the active antenna falls below an alarm threshold of 10 mA (i.e. in the case of the amplifier failing) the unit will generate an internal alarm. With GPS mode enabled, the transmitter laser will turn off. This will generate an alarm in the connected receiver module, as the connected unit will have a Received Light Level (RLL) alarm. The GPS mode can only be enabled and disabled when the unit is under software control. Regardless of GPS mode setting, a ViaLite GPS transmit module will always have a live LNA feed on the RF port.

Fig. 2 Horizons M&C GUI software status display  Click here to expand

If an alarm is generated at the receiver module, you must first check whether there is M&C access to the transmitter module to disable and re-enable GPS mode. If not, a developer kit might be required to access the software depending on the hardware format, such as a standalone Blue OEM module.

If the receiver is in a rack solution, check the RLL into the receiver via M&C card, e.g. HRC-3. If the receiver module is standalone, check the LED is green. If the LED is red, then the RLL is too low.

Fig. 3 Horizons M&C GUI software control to toggle GPS mode  Click here to expand

The first thing to check is whether the receiver is working. This can be tested by disabling the GPS mode in the transmitter’s software control, which should reactivate the laser. If the laser is reactivated but the receiver continues to alert, this means the receiver is likely faulty, or there is break or interruption with the fiber optic cable connecting the transmitter to the receiver.

Fig. 4 Horizons M&C GUI software status with GPS mode disabled  Click here to expand

If enabling the GPS mode disables the laser, this strongly indicates that there is a fault with the antenna, or a problem with the RF cable continuity between the transmitter and antenna.

Fig. 5 Horizons M&C GUI software showing alarm state with GPS mode enabled  Click here to expand

However, the same result may be attained if no power is going to the antenna. Measure the voltage on the SMA RF connector to confirm that power is being supplied – if it is, then it is likely that there is a fault with the GPS antenna or the cable.

When a customer is using a GPS transmitter module, the laser will be disabled until the antenna is connected to the RF port (SMA connector). If the laser continues to operate (green LED) when there is no antenna connected, and the unit is in GPS mode, then there may be a masking of the current/voltage alarm causing the laser to continue operating. This mask may need to be cleared at the factory to enable GPS mode to operate correctly.

If other components are introduced between the transmitter and antenna, for example an additional Low Noise Amplifier (LNA) passive or active, then current draw needs to be taken into account when selecting GPS mode.

For more information, please contact our dedicated Technical Support team, by emailing technicalsupport@vialite.com or by phoning +44 (0) 1793 784389.