(Larry) wrote in message ...
How are you going to filter out white light from the stars? It
contains red and green, too.

I was imagining determining the difference in magnitude between the
red and green signals, 'though there are other possibilities for
differentiating between starlight spectra and nav light spectra.

What about coming up on the stern of that 950' containership at
anchor?

Unlikely mid ocean :-)

Better stick with the radar and get a shaft alternator or
wind bugger, instead.....please?

Well maybe, but what I've learned from this thread, my own
observations and others remarks is that nothing (including radar and
an alert watchkeeper) will detect everything which may cause a
problem.

.... Lucky for them it's a big ocean.

Indeed. But it really is a big ocean - I can recall three two week
stretches at sea when we saw absolutely no other vessels.

In article ,
says...
(Larry) wrote in message ...
How are you going to filter out white light from the stars? It
contains red and green, too.

I was imagining determining the difference in magnitude between the
red and green signals, 'though there are other possibilities for
differentiating between starlight spectra and nav light spectra.


If your boat is moving, you could well be on a collision course that
would allow you to see only the red or only the green light. That
makes the problem more difficult.

Furthermore, on a moonlight night, you will have to contend with
reflections off the wave tops. Those will have transient levels
of red and green that will complicate the detection problem.

Overall, it looks like this type of sensor would require quite a bit of
spectral discrimination and tracking intelligence.
What about coming up on the stern of that 950' containership at
anchor?

Unlikely mid ocean :-)

Better stick with the radar and get a shaft alternator or
wind bugger, instead.....please?

Well maybe, but what I've learned from this thread, my own
observations and others remarks is that nothing (including radar and
an alert watchkeeper) will detect everything which may cause a
problem.

.... Lucky for them it's a big ocean.

Indeed. But it really is a big ocean - I can recall three two week
stretches at sea when we saw absolutely no other vessels.


IIRC, on one crossing from Tahiti to LA, we saw no other vessels
at all. I suppose some may have popped up on radar, though, that
I didn't know about. Our radar had quite good range, too. But then
I was aboard a 400' missile range tracking ship at the time!

Mark Borgerson

Armond Perretta wrote:

In any case a small sonar seems like a good possibility ...

Sound waves bend downward creating a blind zone near the surface.

Vito wrote:
Armond Perretta wrote:

In any case a small sonar seems like a good possibility ...

Sound waves bend downward creating a blind zone near the surface.

Quite, but the sound waves generated by the target vessel do themselves
propagate in a uniform direction from the source, so they _will_ be picked
up.

--
Good luck and good sailing.
s/v Kerry Deare of Barnegat
http://kerrydeare.tripod.com

"Armond Perretta" wrote in message ...
Vito wrote:
Armond Perretta wrote:

In any case a small sonar seems like a good possibility ...

Sound waves bend downward creating a blind zone near the surface.

Quite, but the sound waves generated by the target vessel do themselves
propagate in a uniform direction from the source, so they _will_ be picked
up.

The trouble is that the original poster was concerned about power consumption
on his radar. I can't imagine that a sonar solution could get away with anything
less than a fairly powerful PC running to do the signal processing, you need to
filter out all the false stuff including noises from your own vessel. Even so, for
boats with the power to keep it running, it would be very interesting technology.
The obvious use is for single-handers but, even for crews maintaining a 24
hour watch, humans can get tired and have lapses of concentration so an extra
"safety net" would be very useful.

- Julian.

Julian wrote:
"Armond Perretta" wrote ...

... the sound waves generated by the target vessel do
themselves propagate in a uniform direction from the source, so
they _will_ be picked up.

... for boats
with the power to keep it running, it would be very interesting
technology. The obvious use is for single-handers but, even for
crews maintaining a 24 hour watch, humans can get tired and have
lapses of concentration so an extra "safety net" would be very
useful.

I have many times, in clear conditions and with "unlimited" visibility, had
the radar pick up targets that I probably would have missed using only
binoculars and a steady hand (even assuming that I was at the time being
diligent). I am not suggesting that one should rely _only_ on electronic
aids, but if they can be supported financially, maintenance-wise, and
power-wise, I think the decision gets fairly obvious.

--
Good luck and good sailing.
s/v Kerry Deare of Barnegat
http://kerrydeare.tripod.com

In article ,
says...
Mark Borgerson wrote:
If your boat is moving, you could well be on a collision course that
would allow you to see only the red or only the green light. That
makes the problem more difficult.

I've described the colour discrimination method idea poorly. I'll try
again.

Imagine two photosensors side by side, identical except that one has a
green filter in front of it and the other has a red filter in front of
it.

Imagine a cylindrical lens in front of these photosensors, positioned
such that light from a distant source at a particular azimuth falls on
both photosensors, and light from any other azimuth falls on neither.

Imagine an electronic circuit which calculates the output from the red
photosensor minus the output from the green photosensor. Call this
result the "difference signal".

If light from a red source falls on both the photosensors then the
difference signal will be positive and similar to the signal strength
from the red sensor, as the green sensor will produce little output.

If light from a green source falls on both the photosensors then the
difference signal will be negative and similar to the signal strength
from the green sensor, though opposite in sign, as the red sensor will
produce little output.

If light from a white source falls on both photosensors then, (with the
relative sensitivities of the two sensors set correctly) the difference
signal will be zero since the red signal equals the green signal.

Thus a non-zero difference signal is indicative of a red or green light
(alarm!), and a white light produces a zero difference signal (no
alarm).

Sounds much closer to workable. Now all you need is some hardware
or software to discriminate against those beautiful tropical
sunsets, the green flash, and the occasional green tropical island
with floodlights over the beach resort. ;-)

I guess that could be done with an imaging sensor set to respond
only to point sources.

Depending on the sensitivity you desire, you might also have to
worry about your own running lights reflecting off waves or nearby
flotsam. To avoid that you may have to make the sensor auto-leveling
so it can ignore stuff near the boat. Hmm, it's getting a bit more
complicated now.

Mark Borgerson

What I don't understand is why a ship would turn off their radar. Are they trying to save the few watts of power? Are they trying
to reduce wear on the radar? Does it interfere with their satellite TV? It seems the cost of operation is so low compared to the
potential consequences that there would be no motive to turn it off.

On 20 Aug 2003 08:11:15 -0700, (Jim Woodward) wrote:

I don't know about 50% at fault, but the Colregs are pretty clear:

Rule 4: Rules in this section apply to any condition of visibility.

Rule 5: Every vessel shall at all times maintain a proper lookout by
sight and hearing as well as by all available means....

Rule 7(a) Every vessel shall use all available means appropriate to
the prevailing circumstances...

Rule 7(b): Proper use shall be made of radar equipment if fitted and
operational,...

In other words:
1) Keep a visual lookout.
2) If you have radar, you shall use it.

Jim Woodward
www.mvfintry.com


--
Chuck Cox
SynchroSystems Motorsport Computers
Hopped/Up Racing Team

http://www.synchro.com

You'll always have a few who work under the assumption that less usage
is better for longevity.
However, many companies now require at least one radar on at all times,
underway (the other on stby), and some require both.
Maybe someday you'll be able to rely on all ship's having at least one
radar on, but you'll never see the day when someone will be assigned to
constantly monitor it, although, with the newer bridge consoles and
seating arrangements, this is improving, EG to the detriment of the
walk around visual watch.

otn

Charles Cox wrote:
What I don't understand is why a ship would turn off their radar. Are they trying to save the few watts of power? Are they trying
to reduce wear on the radar? Does it interfere with their satellite TV? It seems the cost of operation is so low compared to the
potential consequences that there would be no motive to turn it off.

On 20 Aug 2003 08:11:15 -0700, (Jim Woodward) wrote:


I don't know about 50% at fault, but the Colregs are pretty clear:

Rule 4: Rules in this section apply to any condition of visibility.

Rule 5: Every vessel shall at all times maintain a proper lookout by
sight and hearing as well as by all available means....

Rule 7(a) Every vessel shall use all available means appropriate to
the prevailing circumstances...

Rule 7(b): Proper use shall be made of radar equipment if fitted and
operational,...

In other words:
1) Keep a visual lookout.
2) If you have radar, you shall use it.

Jim Woodward
www.mvfintry.com



--
Chuck Cox
SynchroSystems Motorsport Computers
Hopped/Up Racing Team

http://www.synchro.com

On many ships, you will find the alarms turned off (too many false
alarms) although this too is improving.
To avoid many of these false alarms, either the inner guard ring is
moved out to a range where small boats may not be detected, or sea
return is turned up to a point that close in targets can be and are lost.
Much of what is needed, is training and procedures. The Mates need to be
trained to frequently check the screen (and not just rely on alarms) but
also to scan the various ranges .... especially the lower ones.
In most cases at sea, shipboard radars will be set on either the 12 or
24mi. range, for early detection (with "sea return" minimized for best
reception) .... problem with this is that many small close in targets
can be missed.

otn

Vito wrote:
otnmbrd wrote:

Maybe someday you'll be able to rely on all ship's having at least one
radar on, but you'll never see the day when someone will be assigned to
constantly monitor it, ....


True. FWIW, the US Navy did studies that showed a skilled alert operator
could detect almost all new contacts but that one's alertness quickly
waned so that, after an hour or so detection became iffy. That's why we
spent megabucks to develop automatic detection and tracking sustems.

Also, the higher the antenna the longer the range but the poorer its
ability to "see" small craft in the sea clutter, especially close
aboard. So never, ever depend on a big ship seeing you.

73, K3DWW