In article ,
says...
"Shaun Van Poecke" wrote in
:

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?




http://www.pcmaritime.co.uk/leisure/...eshooting.html

The handheld GPS units are made for NMEA, which is an RS-422 clone using
data that switches between 0V and +5 (or 12, I forget which). A serial
port on your computer is RS-232 standard which switches between -12 and +
12, which is more reliable over long lines as the -12 doesn't get buried
in the noise like 0VDC does, making false data. However, MOST, not all,
serial ports will read RS-422 data fairly reliably.

NMEA is also slower than dirt at 4800 baud, but that's fast enough for
GPS data streams, which are also slower than dirt.

Is your laptop old enough to have a serial port? New ones don't have
them any more as everyone else has gone to USB, up to 2Mbps in ver 2.x.
It just works better and doesn't have those nasty "what speed is this
supposed to be set to by me" problems old serial ports had. USB is
automatic, right down into the operating system, plug n play. It takes
care of itself.

How cheap is cheap? Here's a 20 channel, WAAS-enabled GPS receiver
Froogle at Google lists for $36 that plugs right into the USB port and
folds for storage. Is that cheap enough? It uses the same chipset as
the $150 Garmins and the spec says it will track at -159dbm which is
below the noise level, so I believe impossible....

Not at all. GPS receivers do receive signals that are below ambient
RF noise levels. They do this through the magic spread-spectrum
signal processing.


http://www.mwrf.com/Articles/Index.c...rticleID=14303

"For civilian L1-band applications, the GPS system is actually a simple
spread-spectrum communication system.4 Figure 2 shows the signal
generation block for civilian applications. First, the 50-b/s navigation
message is repeated 20 times to produce a 1000-b/s bit stream, then the
repeated signal is spread by a unique Coarse/Acquisition (C/A) code with
a length of 1023 chips (the rate at which the pseudorandom noise code is
applied). The result is a baseband signal of 1.023 Mchips/s. As a result
of this spread-spectrum approach, the total processing gain (G) of the
GPS system can resolve a signal well below the thermal noise level."




http://store.luluusa.com/nausbgpsrewi.html
I know someone who is using one of these stuck to the top of his SUV for
Streetmaps. 12 channels, WAAS accurate, same chipset as the rest, nicely
made, simple with a cable for $50, marked down from $199 since the
Chinese took over the market, which is depressed at the moment, flooded
with GPS. I see its only good up to 1000 knots, which might be a problem
for racers...(c;

Get the USB if you're using a laptop and quit screwing around with
handheld nonsense with tiny screens hard to see. You don't need to buy
another computer.

Before you do so, make sure that the 6-foot cable on the GPS unit will
allow you to position the antenna where it will get a clean signal. You
will probably want it to be as far as possible from other electronics
that might generate interfering signals. Strong signals can overload
the front end amplifier on the GPS antenna. You can usually prevent
this type of problem by keeping the antenna 3 or 4 feet from other
electronics.

If your're going to be using the system in the Continental US, make sure
that you get a receiver with WAAS capability. That will usually reduce
positionion errors from around 15m to about 3m.


Mark Borgerson