Hi all,
Im new to the group, but I did something similar with some radio modems I
had from work. Here's a (rather long) review of several options I looked at
for wireless navigation links on boats.
I used a set of radio modems from a2bcommunication (RLM1000's -
www.a2bcommunication.co.uk) which simply replaced the NMEA cable from the
back of the GPS to the laptop PC, and transmitted the NMEA sentences all
round the boat from GPS, echo etc.I am using this to get chart software
working on flybridge without running cables up, and some homebrew
anchorwatch software in the master cabin.

It is a one-way link (at the moment) so I can't use the autopilot output
from chart software from the flybridge, but I can have receivers dotted
round the boat for various things (flybridge chart software on laptop,
anchorwatch program in cabin etc). I had to take care in choosing the right
modem as most of them packetise data and need flow control from the RS232
port to prevent their buffers filling up - and NMEA doesn't have flow
control lines! Most radio modems I looked at packetise the data and stuff it
with error correction\encoding for efficient transmission, but they require
the input data be buffered in the serial port for when it is crunching
numbers and transmitting a packet. Raw NMEA won't stop sending as it has no
flow control lines so packetised transmission would cause NMEA sentences to
be missed (unless your radio link speed is so fast it transmits the buffer
contents really quickly, but most buffers I have seen are only 96 bytes
wide, and would invariably lead to lost characters out of the longer NMEA
sentences)

This way avoids having two PC's to communicate via wirelss LAN (or as Larry
did it, a WiFi router to a WLAN card - neat!). The whole lot runs off 12V
which I have dotted round the place. Ive got 240V knocking about but others
might not...

A third, even easier way of doing it is with a 'class A' bluetooth rs232
dongle
(upto 100m range).
There are probably others out there, but this is the one ive found.
http://www.expansys.com/product.asp?code=BTP_RS232
It's plug and play then - simply fashion a socket for the dongle to connect
to the back of your GPS, or plug it directly into your NMEA multiplexer, and
hey presto (if you have bluetooth on your PC that is!) a COM port is added
to your PC. Great for sending NMEA sentences to PDA's

I did a quick review of three wireless navigation implementations when
choosing what to go for:

Method 1: WirelessLan+virtual COM port
Pro's: widely available hardware. WLAN card has other uses. Error
correction. Encryption. Easy duplex link. Compact solution at remore
(laptop) end.
Cons: needs 2 PCs or PC and router. Virtual COM ports may not work with some
older PC\chart software. Relatively power hungry having (240V) router\PC.
Tricky to transmit to more than one data receiver.

Method 2: Unbuffered (non-packetised) radio modems
Pro's: broadcast mode easy - one tx and many rx stations easy. Range depends
on transmitter module so pick the one to suit you. True cable replacement
requires no PC hardware or software addition\modification so no
compatibility issues with Virtual com ports etc. Runs off 12V~200mA at both
ends (could be powered from USB port?).
Cons: dedicated hardware needed - not as common as WiFi. Unsecure (open
transmission). Twice as expensive for two-way (duplex) operation. Error
correction tricky to implement if no flow control lines are used (as is for
NMEA format out the back of a GPS)

Method 3: Bluetooth cable replacement
pro's: easiest of the three to implement - one thing to plug in and that;s
it. works out cheaper than methods 1 and 2
cons: limited range compared with others. tricky to have multiple receivers.

I chose the radio modem approach as I like this kind of thing, and want
multiple receivers. I also have visions of plonking the transmitter in the
RIB when it is moored up to a buoy (transmitter runs off 12V low current
easily) and having anchor watch program on laptop in bedroom on land keeping
an eye on position and depth as transmitted by echo and GPS on the rib so it
doesnt float off\dry out\get nicked (could do with ethernet approach but
would need internet access and more stuff on RIB).It works out about the
same cost as the rs232-ethernet-WiFi route, and I wanted a one-box cable
replacement.

I think if I did it again I would go the WiFi route as I would use the
duplex capabililties more (to get waypoint info from the GPS, and talk to
the autopilot from the chart software) than the fact I use it to transmit to
more than one receiver.

My next project is a webcam pointing at my echo sounder, and have this in a
window next to chart software on the laptop so I can do wreck hunting from
the flybridge (did I say I was a diver?) - unless anyone can wirelessly
transmit an echo trace (not just a depth number)?

Happy wireless navigating!

Steve Cowley
Manchester, England.




"Larry W4CSC" wrote in message
...
Aboard Lionheart, I recently decided it was crazy to have the Dell
Latitude running The Cap'n bolted to the serial cable at the chart
table. I want to take it up in the cockpit during my watch in the
night so I can operate the Cap'n without leaving my helm/watch duties.

Ok, so I can easily put a second parallel cable up in the cockpit near
the helm. Our helm on the Amel Sharki is on the forward bulkhead
right over the Noland Multiplexer's cabinet location. But cables
laying out in the weather (shudder)....there's gotta be a better way!

I found it.....a virtual serial port in the computer connected to its
802.11b wireless modem in the PCMCIA port of the Dell Latitude,
connected through a DHCP enabled Netgear wireless router to the serial
port on the NMEA multiplexer. Oh, oh, wait....there's no LAN port on
the NMEA multiplexers....a minor setback. But wait! Here's one now!

http://www.stayonline.com/serial_to_ethernet/3311.asp

This little yellow box is a serial port to network adapter. It acts
just like the Ethernet card in your desktop computer. A router
assigns it a LAN IP address via DHCP protocol and the little box
packets up the serial port I/O data for transmission to anyone
connecting to it. The LAN is from a Netgear MR814 router that has 4
ethernet LAN ports and an 802.11b wireless router in it. The yellow
box and router are mounted up behind the helm, right over the top of
the NMEA multiplexer. Cables are short!

Ok, the NMEA network is on the air, protected from the prying eyes of
that damned sloop we can't stop from passing us by 128-bit encryption
and WEP passwords. (No sense giving your competitor control of your
helm, eh?)..(c; Now, how do I get The Cap'n to think it's connected
to the serial port?? Use "virtual serial port" software, of course!

The WebFoot came with a CD that had "serial port redirector software",
nothing but a driver, to install into the notebook. The notebook only
had COM1 in it so I told this driver to use COM2. Tell the Cap'n
software to connect to COM2 serial port and it doesn't know any better
than it's talking to this virtual serial port software, which handles
the TCP/IP connection to the little yellow box. All for $149.

Hmm...ok, how's it work.

Well, anywhere on the boat, you simply set the notebook down and you
have full control with The Cap'n just as if you were connected
directly to the multiplexer's serial port. In our system, the GPS
data comes through the Raymarine RL70CRC Plus radar/chartplotter
display from the Raystar 120 SeaTalk WAAS-GPS. I don't see any
changes in the operation, at all, just because it's all going through
the router/wireless/redirected serial link. I can sit in the cockpit,
on the bow, even in the head, if you like, and have total control....

I left it all running at the dock to see how far it would go. The
wireless link started getting intermittent about 2 docks away, but
that may be because Ashley Marina has a master station and TWO
repeaters on 802.11b that provides internet service to all the slips
for free to its customers. I might be seeing some interference in
this RF environment......

Hee hee.....The "remotely controlled 41' ketch"......someone bring me
my beach chair. I'll sail it around the island while we're
ashore....(c;

He who dies with the MOST toys WINS!

Hmm....I COULD put that wireless repeater on top of the mainmast......

Hmm....I also might be able to leave it running, connect it also up to
the marina's WiFi link and read the wind/depth/etc. instruments from
the house via the internet......Gotta play with that!


Larry W4CSC

NNNN