Today it is extremely easy and inexpensive to get a GPS signal to your chartplotter or laptop. You can buy a small GPS button antenna measuring about 1" by 1" for as little as $47 that plugs straight into your USB port.
On fiberglass boats the button antenna will usually work fine below decks, as long as it is placed reasonably high up in the cabin.
On Just Imagine we have two Raymarine LP120 antennas (about $270) mounted on deck. The Seatalk version was supplied with the Raymarine radar/chartplotter that is mounted in the cockpit. The other is a NMEA version and connects to the serial port of the laptop or a serial to USB converter if no serial port is available.
You can remove the pole mount base from these small Raymarine antennas (see below)and mount them, almost flush on the cabin top or anywhere on the deck where they will not get crunched.
It is not necessary to mount GPS antennas high in the rigging. As long as they have a reasonably unobstructed view of the sky and and horizon they will work fine. The antennas are receiving up to 12 different satellites that are rising and setting and zipping across the sky all day long, and you only need to see 4 of them to get an accurate fix. I have one GPS antenna mounted on the deck, outside the cockpit coaming under the stern pushpit rail and it works fine.
These GPS antennas draw very little current, so you don't have to worry about how many you have running.
You should have at least one GPS running when you are underway. In case of emergency you need to be able to tell someone where you are and you can't wait for the unit to boot up and get a fix.
For the same reason, at anchor, you should at least write down your position if you choose to turn off the GPS.
The disadvantage of using laptops and chartplotters to show the GPS position is that they draw quite a bit of current from your batteries when you are not actually navigating. An alternative is to keep a small handheld GPS running (or a GPS with a small display) if you choose to shut down the chart plotter or laptop to save batteries when chart navigation is not needed.
Handheld units are so small and inexpensive these days ($100) and it doesn't hurt to have some redundancy on the instrument that we rely on the most.
It is true, that on any vessel the GPS reigns supreme in the instrument hierarchy.
It gives you an accurate course over the ground (COG) that is True, and needs no Magnetic correction, making your old magnetic compass reading a curiosity, rather than the essential that has been for centuries past.
It gives you an accurate Speed over the Ground (SOG) that needs no correction for currents and tides and the input from your speed log takes less importance, unless you are curious about how the currents are affecting you.
Apart from all this, it tells you exactly where you are, making sextants, RDFs and other navigation instruments a thing of the past.
The danger is that it also reduces our need to be competent navigators, and anyone who can "drive" a boat is now out on the water relying heavily on this little device to keep them on track.
One can carry several GPS units for redundancy, but this does not replace the need for basic navigation skills. Electronics can fail through water intrusion, lightening strikes, or batteries going flat and you may need to break out the hand bearing compass and paper charts when you least expect it.
Howard [ Just Imagine ] 10-Mar-2008
USB to Serial Converter (Technical)
It is hard to buy a laptop today with a 9 pin serial (COM) port as most of the new models only come with USB (Universal Serial Bus) ports.
The USB to serial converters have improved over the last few years and are now mostly quite reliable for GPS and other applications.
On Just Imagine we use inexpensive ($30) Keyspan USB to Serial converters that are readily available and so far have had good results. There are several other choices and prices available.
The GPS NMEA cable connects to pins 2 and 5 of the 9 pin connector. Look for the tiny numbers (1-9) on the connector.
Pin 5 is the ground connection and pin 2 is the NMEA signal coming from the GPS.
If your laptop software can upload waypoints to the GPS device, then you need to connect pin 3 as well, which is the NMEA signal from the laptop to the GPS.
Howard [ Just Imagine ] 10-Mar-2008