Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

150 marine electronics installation and troubleshooting

typically identify minimum radius bends

for cabling in their instruction manuals.)

• Secure flaked cable a sufficient distance
  (usually 18 to 36 inches) from potential
  sources of electrical interference.

The matter of flaking the cable versus coil-

ing is one of the most common problems seen

in the field. Coiling these cables creates an

undesirable ancillary antenna—i.e., separate

from the actual antenna at the end of the

cable—that easily receives interference.

If the GPS has an additional differential

antenna hookup onboard, interference can

become a real concern. DGPS operates in the

150 kHz to 500 kHz spectrum and is suscep-

tible to the noise generated by the various

sources of RFI already discussed. Appropriate

separation (again, typically 18 to 36 inches)

needs to be maintained between these emitters

and the differential antenna.

In contrast, the operating frequency of

nondifferential, or WAAS-enabled, GPS is

1.5 GHz. Most small boats don’t have any-

thing on board that approaches that frequency,

except for cell phones. As noted earlier, cell

phones operate at either 850 MHz or 1.9 GHz.

Basic GPS operates roughly in the middle of

that spectrum, so a cell phone interfering with

a GPS is a real possibility.

Although still rare on anything but the

largest boats, satellite television is becoming

increasingly popular, and its 1.5 GHz to

1.66 GHz frequency spectrum is close enough

that it could also interfere with GPS.

Volts, Amps, and Temperature

GPS is much more forgiving of variations in

the DC power supply than some of the other

equipment we’ll discuss. The voltage operating

range is usually from about 10 V to 16 V

(although you should always confirm this with

the manufacturer’s installation documentation).

Current draw typically ranges from milliamps

for the smaller units to as much as 4 amps on

some of the larger, color chartplotter units with

bright backlighting. Therefore, wiring for

power to GPS systems does not impose any

unusual demands. Some units have a chassis

ground lug, and others don’t. Use it if it’s there,

and don’t worry about it if it isn’t.

Likewise, cooling isn’t much of a problem on

most smaller units, as they do not generate much

heat. On some of the larger displays, the LCD

backlighting can generate a fair amount of heat

buildup. I use a simple rule of thumb: If there are

GPS can be used on board with a computer. The

computer serves the chartplotting function with higher

power than a typical integrated chartplotter. The GPS unit

can be a stand-alone fixed sensor without its own display,

or a GPS chartplotter. The computer accepts data from a

variety of sensors in this configuration through a multiplexer

(MUX). The multiplexer sequentially forwards the data to

the computer from each source. Wind, depth, water

temperature, radar, and GPS data can all be presented

on the computer screen using navigation and other soft-

ware. The computer then can direct information and com-

mands to the autopilot. Note that power and ground lines

are omitted in the diagram for simplicity. (Reprinted with

permission from GPS for Mariners by Bob Sweet)