Why is the boat not positioned if the VHF is off? Aragon-Technologies


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Electronics  -  Software

Why is the boat not positioned if the VHF is off?

The NMEA output of your GPS is connected to your computer (on a serial port, a USB-NMEA adapter) or any other device that allows the computer to receive NMEA data for positioning the boat on the map using your navigation software. You also have a VHF DSC which must also receive the NMEA data to know the position in case of need to trigger a distress call. Other devices may also be connected to the same NMEA output of the GPS. You have noticed that if you turn off the VHF your boat is no longer positioned and your software sends you an alarm unknown position. In the best case, positioning occurs from time to time but the position is not refreshed as it should do every 2 seconds.

This is because the NMEA input of your VHF is not opto-coupled (isolated) and so it becomes very low impedance when its circuit is no longer powered (in fact your GPS is trying to power your VHF with its NMEA output on the entry NMEA of the VHF which of course can not succeed). The NMEA signal is therefore "overloaded" to such a level that it becomes unusable by any other device connected to this output.

The only solution to this problem is the installation of a NMEA amplifier.

How does a NMEA amplifier work?

A quality NMEA amplifier has a NMEA input that perfectly meets the specifications of the standard. This input is opto-coupled (isolated) so that it is completely insensitive to what happens on the other side of the opto-coupler. It is totally insensitive to a possible changing of the mass level. The GPS is perfectly protected against any voltage return that may come from a fault in a device, wiring, radio-frequency radiation from a wrong tuned transmitter etc ... On the other side the optocoupled input stage is a signal calibration stage to rectify the flanks and the level faults. Finally are the buffers (usually 4, 1 per output). These buffers will generate the signal that will be sent to the devices. While respecting the voltage levels of the standard, the buffers are able to provide much more current than the GPS can provide. It is now possible to apply this signal to devices whose input impedance is much lower than the NMEA standard. The buffers being completely independent, whatever happens on one output can not act on the others.

The NMEA amplifier AMN010-2:

The AMN010-2 is a high efficiency NMEA amplifier. It corrects and amplifies the NMEA signal and send it on 4 independent NMEA outputs.

The AMN010-2 is a high performance NMEA amplifier. Its powerful, independent outputs can be used with any device that has an isolated NMEA input or not, regardless of its input impedance. It has an auxiliary output and input to couple as many AMN010-2 modules as needed to achieve the desired number of NMEA outputs without mobilizing any of the power outputs. Its amplification ratio is greater than 1000.

See the complete information about the NMEA amplifier AMN010-2