#Better# is a relative term.

The things that make a particular GPS receiver #better# are, number of
channels , 8,12 or more. Aquisition time ie the time to get a fix,
either warm, that is following a normal power on sequence or cold,
that is following a prolonged period of being powered down or being
moved several hundred kilometers, and perhaps channel sensitivity.

Most equipment these days use the same basic manufacturers 1/2 dozen
receiver 'engines' so there is little beteween them.

A minimum of 8 channels is prudent and contrary to what the salesman
will tell you having 24 channels will NOT get you more accuracy, it
will deliver a very nominal amount of additional resilience to signal
interruption, multi channel receivers were implemented to try to
overcome the challenges of operating in an urban environment with the
signals being interrupted, and reflected.

Aquisition time is the ability of the system to get an accurate
position after power up or complete loss of signal. Anything less
than 30 seconds is ample, and pretty standard these days. After a cold
start most receivers will take a couple of minutes to read all the
data from visible sattelites, make some decisions about which ones to
use and come up with a 'fix'. Again contrary to what some salesmen
will tell you you cannot magically make this time shorter. GPS is a
broadcast, unidirectional system, GPS receivers DO NOT talk ( exchange
data ) with the satellites. It's the same principle as TV, you can
yell at it all you like, the ball game umpire cannot hear you! GPS
receivers only listen to broadcast data and then use it to form a fix.
The 'ephemeris' data comes down at 50 bits per second and takes 12.5
minutes to cycle - PERIOD!

Receiver sensitivity relates to how able the unit is to pick up weak
signals. On a day with heavy cloud at several levels we are asking
quite a lot from a piece of £20 electronics ( the receiver section )
to listen in to a signal generated by what amounts to a large
microwave situated 20,200 km 'up' and travelling at about 14000 km/hr.
This is why you should always use the right antenna for your
particular receiver.

Anything else is just whistles and bells. Display, memory, types of
output, inputs, user interface, button size etc etc etc!

A few things to consider.

Use ONE source of nav at a time. One in the cockpit and one in the
saloon may lead to slight differences in position due to different
signal paths, locations, selected constellation.

Laptop type USB mouse GPS's are great for what they are but generally
they are not rugged ( nor are the laptops! ) not waterproof and use
lightweight, unarmoured cables.

I always have a couple of hand helds on the boat. In an emergency its
nice to have one around, they are independant of the boats electrics,
usually safe from lighting strikes or near misses, can be chucked into
a liferaft and are a boon in a strange country to get around
unfamiliar towns.

Fit plugs/sockets/wires to allow you to use either receiver for your
nav. Any handheld should be selceted with a NMEA interface ( the GPS
72 does have one ) If the USB device goes 'phut' you can connect the
72 to the laptop.

Getting a NMEA or RS232 output from your laptop to drive your DSC
radio, autopilot or cockpit repeater can be a challenge and may be
restricted by your software. USB is just NOT compatible with NMEA or
RS232. It can be solved in software but you are reliant on the laptop
then.

With a USB GPS the biggie is that you are using the laptop for
navigation, usually this will demand 1.5 or more Amps from your power
supply. A handheld will run all day on rechargeable AA's

Ian M


On 10 Feb, 10:01, "Dennis Pogson"
wrote:
Shaun Van Poecke wrote:
cross posted to rec.boats.cruising

Hi all,
Im looking at getting a cheap(ish) setup for my boat, i have a couple
of laptops with navigation software and charts, so what im looking at
is a couple of GPS units to hook up to them. Ive seen GPS receivers
with serial or USB connections that plug straight into a laptop, but
these things cost as much as or more than a basic hand held GPS. are
they any better?

What im probably thinking is one basic handheld unit (maybe garmin
GPS 72) and one receiving antenna to plug straight in to the laptop.
any ideas on this setup?

You only need an antenna for a handheld if you intend to use it where signal
strength is weak. As this is a sailing NG, I assume you intend to use it on
a yacht.

My own GPSMap 60C will pick up down below at the chart table without an
external antenna, but you would be better to expiment to see if you need the
antenna, unless you think the few bucks they cost is worth investing anyway,
just in case.

Your last paragraph reads as if you intend to plug the antenna into the
laptop. I assume this is a typo?

Dennis.



Thanks,
Shaun