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Old 01-06-2006, 10:51   #1
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AIS Antenna Height

Just received a NASA AIS and was considering the install. As VHF is line of sight, the higher the antenna, the further the horizon and the longer the range the saying goes.

But if the signal originals from a very tall and distant antenna you may still receive a good signal if YOUR antenna is not especially high. This is one of the reasons why we can receive weather VHF on a handheld.

Since the NASA is a receive only and the purpose is to track "large" vessels, which presumably have a tall mast and their AIS transmitter is quite high, the range at which their signal can be received (sq rt of (ht x 1.5) = range miles).

So a commercial ship with a mast at say 80' tall will have a range to the horizon of 1.5 x 80 = 120. Sq rt of 120 = 11± which you add to the horizon distance of YOUR receiving attenna 10'
1.5 x 10 = 15 Sq rt 15 = 4±. So yoo can receive the signal at 14 miles which is close the the range of most yacht radars and more than enough for collision avoidance.

So does it make sense to mount YOUR AIS antenna at the masthead as opposed to on the stern rail, for example? I think not.

What say you?
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Old 02-06-2006, 06:46   #2
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Okay, but at 28 knots, that ship could meet you in 30 minutes. With a higher antenna you get a little more warning. The frequency of transmission, if you have a single channel AIS receiver could also be a factor cutting into your window of warning. We opted for an antenna splitter going to the masthead VHF antenna. Should have the Milltech Marine receiver and splitter installed in the next two weeks. Will then report back on the range.

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Mark
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Old 02-06-2006, 14:00   #3
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defjef,
Not all of the ships out there you want to avoid have tall masts. For example I couldn't pick up the high speed (20 - 25 kt) Lisbon ferries that were about 12 miles away until I put the antenna at the top of my mizzen mast (about 30'). So my recommendation is to put it as high as possible. For a single masted boat I would use the splitter and the existing antenna.
That said, your analysis seems to be correct and you should get adequate range with the antenna on the rail.

John
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Old 02-06-2006, 18:17   #4
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I mounted mine on the rail. There is enought traffic without AIS out there to require a sharp lookout anyway. My radar is mounted on a stern pole, so visible/radar/AIS horizon are all pretty close. All are more useful for in-close collision concerns-there are so many targets that are not visible to any of these until they get within 5 miles or so. I find AIS a big improvement over ARPA (if AIS is available), and it is particularly useful for identifying ships by name so you can call them if necessary.
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Old 03-06-2006, 19:56   #5
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Another way to view the question is to consider what you want to get from the AIS receiver.

Are you concered about ships that are so far away that you could not see them in ideal conditions? If yes, you need a high antenna. If no, an antenna as high as your head is enough.
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Old 04-06-2006, 10:45   #6
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Having used a pushpit mounted aerial for a cross channel. We were picking up shipping causing alerts (i.e. cpa within 1 mile) when they were more than 10 miles away. that was sufficient distance to be easly able to alter course before being visual and change the CPA. Any ore would have been wasted information. Therefore I have installed my own antenna at the same height.

I looked long and hard at the use of aerial splitters, and decided against it. I mounted my AIS engine adjcent to the VHF and I also bought adaptors, so I can use my AIS aerial as an emergency VHF aerial, but if I want the extra AIS range I can swap over aerials and use the masthead one for the AIS. It is also a lot cheaper than buying a splitter.
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Old 06-06-2006, 00:38   #7
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Jef, I like several of Talbot's points: versatility of using antennas and the advance warning that even a low mounted antenna provides. Personally, I would like the antenna mounted a bit higher than the pulpit/pushpit because the reg not only mandates active transponders down to 300 tons (which may not be mounted all that high) but also because in time many of the larger recreational boats will have active transponders and, IME, large motor yachts have atrocious radar returns.

As one benchmark, we mounted ours about 20' above the waterline (spreader of mizzen mast, which I realize you don't have) and when skirting inside the TSS off Cabo St. Vincente, I was seeing a clean representation of the N and S bound lanes with ship contacts >40 NM. FWIW in heavy traffic areas (e.g. the Channel) I wouldn't want to see anything outside of 12NM; it would be data overload and unnecessary since you'd be managing your boat's course relative to the nearest (threatening) targets. OTOH I like the idea of a watch out to 20-30 NM when at sea as it will snare the rare contact running at 25+ kts well in advance. It's that versatility that makes mounting it a bit higher more appealing to me. OTOH I think a masthead location is unnecessary tho' not something one should avoid.

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Old 06-06-2006, 15:33   #8
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Nother issue:

NASA AIS engine is supplied with a rs232 cable... persumably meant for connection to a PC.

I intend to connnect the engine to the NMEA port on a RayMarine C80 which is a propriatary I/O cable connector with splice ends for connection to a buss block etc.

So..... which wires on the NASA do I use for this purpose. They have NO MANUAL to speak of. Anyone have a clue? Awaiting a reply from NASA... but thought on of you salts might have been there already.

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Old 06-06-2006, 18:11   #9
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Jef:
Jack’s (“Whoosh”) friend John Stevenson (s/v “Sarah”) seems to have dealt with these issues. Take a look at:
http://www.svsarah.com/Sarah/Upgrades/Electronics.htm
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Old 07-06-2006, 00:18   #10
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Not quite Gord
I have finally connected my AIS Engine to the NMEA input on the C120 plotter, but so far i have been unsuccessfull getting any data to the plotter.

jef,
I haven't had a chance to go back and check all my connections. i also haven't loaded the latest version of the firmware (3.18), I'm still running 3.16. So i have a lot to look at. However, the pinouts i documented on the web page are correct. i have run the NASA supplied PC cable to a terminal block with the SD, RD, and SG wires. I then have another PC cable from that block to my PC and a wire pair with RD and SG that goes to another terminal block to which the NMEA cable from the C120 is connected.
Right now I'm getting AIS data to the PC, but not the plotter. So the pinouts are correct and as you can see from the description above I've got a number of connections to checkout for the C120. The NMEA input to the plotter was working at 4800 Baud when I had it connected to that network, so the problem may be in the firmware.
One thing I suggest is to not cut the NASA supplied cable if you think you might want to use the GPS input. I bought a standard PC serial data cable and cut it close to the Male end. I used that short cable to connect the NASA cable to the terminal block and the longer female end to connect my PC to the terminal block. So the NASA cable is intact and I can reconnect it directly to my PC to trouble-shoot issues wihin all these connections. I use the GPS input on the NASA cable as a backup GPS connection should the Raymarine GPS on the SeaTalk network fail.

Oh, and if you get a response from NASA technical support please post it here. You will have made history
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Old 07-06-2006, 05:34   #11
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Nasa Said

John,

I planned to buy a cable as you suggested and cut that and wire it to a C80 NEMA buss and save the NASA cable.

I have not done the antenna yet and I still am using the C80 NEMA in @4800 which is too slow for the NASA. So will have to change the port speed to 38400 for the NASA, but this may ALSO change the NEMA out port speed... no sure as I haven't tested it. And I don't know if the C80 will actually "read" the NASA AIS as it has not be connected yet.

I don't have or use a laptop on board for navigation... that may change in the future. Right now I am trying to get the C80 to be the MFD it is supposed to be and get all the data in and out of it from all talkers and listeners.

The C80 has only one NEMA port for Listening so you can't mix baud rates of the talkers.

Brookhouse suggest a second MUX for the INPUT side of the c80... this one would handle multiple inputs and varying baud rates. I am considering the Miniplex lite or another Brookhouse.

I am using one MUX to deal with B&G issues and that worked out fine. But I am still unable to "read" TTG and XTE on my KVH and B&G repeaters. Either these sentences are not sent, or not computed by the repeaters or sent "corrupted"... Frankly, I don't understand because BOTH of these data are displayed on the C80. Not terribly important, but curious. My old NavTrac was sending this data to the KVH and B&G fine.

I found this interesting discussion about AIS and C80 online:

http://groups.google.com/group/rec.b...07d3daba3790ae

I am running the 3.18 now.

The pins are #2 and #5 which is what you determined.

Jef
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Here is the NASA comment to my email query:

Specification for NASA Marine AIS Engine

Electronic Interface

Power input: +10Volts to + 16Volts.
RS232 interfaces:
data format: 8 bits, 1 start bit, no parity, 1 stop bit.
Connector: 9-pin D-type socket mounted on the board.
pin 2: 38,400 Baud output from Engine to PC
pin 3: 38,400 Baud input to Engine from PC
pin 5: common (ground).
pin 9: 4800 Baud NMEA input from GPS receiver to Engine
Amplitudes: standard PC RS232 (0 to 5V)
Message headers: see below.

Data format (AIS sentences from Engine to PC)

The signals sent by the Engine shall comprise strings conforming with the NMEA 2000 specification at 38,400 Baud. AIS message types 1, 2, 3, 4, 5, 11, and 21 shall be formatted as UAIS VHF Data-link Messages, as defined in NMEA 0183 Version 3, and having the header code !AIVDM. Whereas the NMEA 0183 specifies a channel number of 1 or 2 for the AIS receive channel, the AIS Engine signifies the channel by A for channel 1, and B for channel 2.

Data format (Status sentences from Engine to PC)

Status sentences will be transmitted following transmission of a valid !AIVDM message or following detection of an AIS message with reception errors. Erroneous reception causes an increment of the threshold setting, which is shown by a status sentence without a preceding !AIVDM sentence. For valid !AIVDM sentences, the status sentence always follows the AIS Encapsulation sentence it applies to.

Status sentences sent by the Engine shall comprise strings conforming with the NMEA 2000 specification at 38,400 Baud. They will have the proprietary format $PNMLS,ss,tt,r*cc<CR><LF>.

The field ss is a decimal value in the range 0 to 63, signifying the signal level for the preceding message. The value tt is the present detection threshold setting. The value r is the interval in seconds (values between 1 and 7) between reductions of the threshold setting. The threshold is continually adjusted upwards in the Engine to minimise the error rate, whilst maintaining maximum signal sensitivity by reducing it at the rate specified by the r field.

Data format (GPS RMC sentences from Engine to PC)

NMEA RMC sentences received on a separate input channel at 4,800 Baud (on pin 9 of the 9-pin socket) will be copied through on the 38,400 Baud output channel whenever they are available. RMC sentences are re-transmitted only if the CRC check on the incoming data is passed, and are sent whenever they are available.


Data format (PC to Engine)

The unit is shipped with factory default settings as follows:

Alternating receive channel A, channel B every 36 seconds, threshold setting 19 (corresponding to approximately1 μVolt). These settings can be changed using the following commands:

The signals sent by the PC shall comprise the following types:

&#183;Channel setting sentence (C):

$PNMLC,c*hh<CR><LF>
Where:
c is the channel receive mode as follows:
A = Channel A (channel 1)
B = Channel B (channel 2)
S = Alternating every 36 seconds between channel A and channel B.
Note setting channel A or B cancels alternation

hh = Checksum as defined in NMEA 0183
<CR><LF> = Carriage Return, Line Feed sequence as defined in NMEA 0183

&#183;Threshold sentence (T):

$PNMLT,t*hh<CR><LF>
Where:
t is the required threshold rate setting. The value specifies the interval between decrements of the tracking threshold value. The actual threshold and rate settings are always sent after a valid message is received (see above). Values between decimal 1-7 may be sent. Any other value forces a return to the default value of three seconds.

Typical threshold values for tt correspond with signal input sensitivities as follows:
19 = 1 μVolt
29 = 10 μVolt
40 = 100 μVolt

hh = Checksum as defined in NMEA 0183
<CR><LF> = Carriage Return, Line Feed sequence as defined in NMEA 0183

Transmission priority (PC to Engine)

If the PC transmits a command to the receiver at the same time as the Engine is receiving a packet on-air, the command will not be implemented in the Engine until after the completion of the packet reception.
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Old 07-06-2006, 16:38   #12
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Jef,
You did make history if you got NASA to respond to an email inquiry. Of course it looks like they just sent you the word document that is on the CD that came with the engine.
You are correct, changing the NMEA Baudrate on the C80 changes both the output and input. I have disconnected my output for now. Still haven't started trouble-shooting the AIS input to the C-series as yet.

John
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Old 07-06-2006, 19:51   #13
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John,
I am not certain about the baud setting changing both the listening (input) and the talking (output) ports. The manual is not clear, but the implication is that the baud rate is certainly for the input side... since it refers to NavTex and AIS... I need to keep the output at 4800... so I may be screwed in using the NASA AIS with the C80... I may to use the AIS w/ the C80 and connect the cockpit repeaters a Horizon Plotter and use that for waypoint navagation and talking to cockpit repeaters. Actually.. I believe that unit has 2 input and 2 output ports which are independantly baud rate settable. Gotta give that a look.

So much for NMEA connecting everything together like a happy family.

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Old 08-06-2006, 01:09   #14
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Jef,
I'm pretty sure the setting affects both sides, but like you I haven't seen it stated clearly in the Raymarine documentation. The only evidence I have that it does change both is the green status light on the NMEA Mux. Normally it flashes on and off showing data is flowing through the box. When I initially changed the C120 Baud rate to 38,400 I forgot to disconnect the NMEA output from the C120 to the mux. With the Baud rate change the green light on the Mux came on solid and didn't blink. I disconnected the C120 NMEA data from the mux and the green light went back to the normal flashing.
In my set up the NMEA data from the Raymarine C120 is redundant as I feed the SeaTalk data directly to the mux. I had the C120 NMEA connection to the mux as a back up should the SeaTalk feed from the autopilot (closest to the mux) fail. The data from the C120 channel into the mux is normally supressed. So it appears I've lost that backup capability.
So right now I essentially have what Brookhouse recommended, two NMEA networks - one at 4800 and one at 38400. The later doesn't use a Mux as it has only one talker - the AIS engine. That is the reason I set up the GPS input to the AIS engine. If I lose my SeaTalk data I can connect the Garmin GPS to the AIS engine and continue to feed GPS data to the C120 and my PC.
Your right this just keeps getting more complicated. If there any of those paper charts and sextant guys reading this thread I can see their eyes rolling back in their heads..

John
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Old 08-06-2006, 02:22   #15
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Indeed, your gloriously perplexing correspondence has me wandering blindly through the techno’ labyrinth, with“my eyes rolled back in my head”[/i].
I’m truly humbled and humiliated by my ignorance.
NASA spent millions developing a wonderful pen that would write in the zero-gravity of space, whereas the Russians used a pencil.
My biggest problem, is cleaning the white-out off my monitor screen.
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