I am planning to install a Raytheon radar R 70RC plus CRT
display/controller.which has a MARPA feature which tracks 10 targets. How well
does this MARPA feature work if you have a stable fast reacting heading ref
(KVH gyro stabilized flux gate in my case)? I may use only the 2 KW
transceiver which has so-so angular resolution. I am mostly interested in
tarcking boats that are within 3 miles of my position. I got feedback from one
user who said his worked great. He had the Ray smart heading sensor which is a
flux gate with a rate gyro. Anyone else using this Ray MARPA feature? Comments?
Anyone out there using HSB to have two radar displays working off one
transceiver? Any glitches?

Some of this is old, some recent, maybe relevant.

We put a Raytheon R41XX with MARPA and KVH compass (supplied by Raytheon for
the radar) on Swee****er (Swan 57) in 1995. The MARPA would not maintain
lock in any kind of serious offshore conditions -- downwind in the trades
for example -- as the yawing was too great and update rate not fast enough.
As a practical matter, it was usable only in quite calm water; as a result
we rarely even tried to use it.

I have spent a fair amount of time at boat shows looking hard at this
question, as we want working, reliable, ARPA on Fintry. Both Simrad and
Furuno have no question that a stabilized flux gate will not do the trick,
that we need to use a full gyro ($10k) or, now, a GPS compass ($3k).
Raymarine is more equivocal, as they "think" that the compass you cite will
work.

All of this is complicated by the fact that Fintry is steel, and while the
compass would sit above an aluminum wheelhouse, 150 tons of steel 15-50 feet
away won't help it.

At the moment, for what it worth, my choice for the radar is Furuno (12kw,
six foot antenna), with the Furuno GPS compass providing heading
information.

And, also for what it's worth, our R41XX went down in Australia. The repair
was done by an authorized dealer, well within warranty, and I had to pay
full boat for the repair -- all parties involved pointed their finger at the
others and, in effect, said, "The other guy will pay....."
Furuno, I am told by several dealers, is better about warranty out of the
USA.

--
Jim Woodward
www.mvFintry.com


..
"BOEING377" wrote in message
...
I am planning to install a Raytheon radar R 70RC plus CRT
display/controller.which has a MARPA feature which tracks 10 targets. How
well
does this MARPA feature work if you have a stable fast reacting heading
ref
(KVH gyro stabilized flux gate in my case)? I may use only the 2 KW
transceiver which has so-so angular resolution. I am mostly interested in
tarcking boats that are within 3 miles of my position. I got feedback from
one
user who said his worked great. He had the Ray smart heading sensor which
is a
flux gate with a rate gyro. Anyone else using this Ray MARPA feature?
Comments?
Anyone out there using HSB to have two radar displays working off one
transceiver? Any glitches?

Lionheart's Raymarine setup is the Smart Heading Sensor, RL70CRC Plus
color radar/chart plotter display and the 2KW radar with the circuit
board antenna spun by the tape deck motor (go look, that's what it
is!) Keep a spare rubber band....er, ah, drive belt aboard as well as
a spare O-ring (ours leaked and Raymarine replaced the whole
receiver).

You don't need the fancy heading sensor to get great resolution from
the MARPA because it uses the GPS data to find the ship's location.
All I see the magnetic sensor does is point the boat symbol and
display in the right direction when you're SITTING STILL. As soon as
you move, with no compass data, the display spins around right,
anyway, when the GPS tells it which way the boat is moving. I guess
the compass makes it start faster, though, as it's already in the
right direction. Big deal. Disconnect your fluxgate from Seatalk and
try it. I think ours works even better using the compass data off the
NMEA network from our B&G Network Pilot autopilot's fluxgate,
actually. I can switch it to either, easily, on my custom control
panel.

Anyway, it tracks MARPA targets very well, as long as the radar
doesn't miss them too often on its scan, like in heavy seas. If there
are a lot of returns in the target's area and you're rockin' and
rollin' so the target is spotty, MARPA will mistake any fairly valid
target that MIGHT be the one you want and go off tracking the big bell
bouy, instead of the boat you wanted. It'll do that. Any computer
running on spotty data from a converted analog signal (radar) would.
But, it's a great thing to have, anyway.

Plan on buying an external ALARM! Some idiot thought it was funny to
make it beep like a cellphone that just found a tower when it alarms.
It isn't even going to wake up the sleeping helmsman slumped over the
display. It's just WEAK. Because of the widely variable nature of
analog radar in the pitching and yawing, you'll get some false alarms,
no matter how fast the compass input is. Those darn bouys change
course when that following sea pushes up on the stern and you're
trying to keep it straight hard over and losing the battle.... This
also might be caused by false data from the compass sensors at high
heel angles. Notice how the warning on the gyro unit and fluxgate say
they must be with in TEN DEGREES of vertical when you mount them.
What the hell sailboat is THAT gonna work on, a 100' trimaran?! No,
he's all sideways, too on the face of that swell.....more than 10
degrees, easy!

We used to have the original compass sensor Raymarine sold for the
system. It's in a cabinet as we installed the much-more-expensive
SHS. I don't really see any operational differences......

If you're interested in the compass sensor, just answer me here. My
cap'n will sell it at a bargain. It's the NMEA model but has seatalk
plugs on it to confuse everyone. We got the cable to plug into it if
you're interested. It's never been abused was mounted in a custom
made teak box in the cabin of the Endeavour 35 he had last. The new
system is in an Amel Sharki 41 ketch, "Lionheart". Save ya a few
bucks....(c;



On 10 Nov 2003 23:19:32 GMT, (BOEING377) wrote:

I am planning to install a Raytheon radar R 70RC plus CRT
display/controller.which has a MARPA feature which tracks 10 targets. How well
does this MARPA feature work if you have a stable fast reacting heading ref
(KVH gyro stabilized flux gate in my case)? I may use only the 2 KW
transceiver which has so-so angular resolution. I am mostly interested in
tarcking boats that are within 3 miles of my position. I got feedback from one
user who said his worked great. He had the Ray smart heading sensor which is a
flux gate with a rate gyro. Anyone else using this Ray MARPA feature? Comments?
Anyone out there using HSB to have two radar displays working off one
transceiver? Any glitches?


Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"

On Mon, 10 Nov 2003 18:45:31 -0500, "Jim Woodward" jameslwoodward at
attbi dot com wrote:

All of this is complicated by the fact that Fintry is steel, and while the
compass would sit above an aluminum wheelhouse, 150 tons of steel 15-50 feet
away won't help it.

Sorry you're so far away and our degaussing range and degausser are
all gone when they closed the Charleston Naval Shipyard, Jim. I used
to know someone who could neutralize Fintry's field for a few beers or
a harbor cruise. It would have been fun to see her plotted magnetic
field all printed out. That thing was so sensitive it could detect an
out-of-place electronics tool box in a minesweeper....and tell you
where you lost it!

The degausser is still right next to the marina at the south end of
the old base. I've been inside its amazingly intense field many
times. Those a big cables laying across the deck and even then they
got really HOT!



Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"

That's not what I experienced on Swee****er, or what the three
manufacturer's experts say -- but I could very well be missing something.
I've tried hard to really understand the issues, because they have led me to
think I need a $3,300 GPS compass for successful ARPA use, and I think hard
before spending that kind of money.

As I understand it:
1) the ARPA wants, in effect, a north-up display, so that it can compute the
actual bearing from your boat to the target, not a relative bearing. I'm not
saying the ARPA actually works off the display, or that you can't use it in
heading-up mode, but that it needs to be able to act like a north-up
display.
2) in order to get a north-up display, it needs heading data. As the boat
yaws in a seaway, it needs heading data frequently -- several times a
second. It's true that an ordinary small boat radar is providing new data
only about every two seconds (around 25rpm) and an update rate of once every
two seconds might be adequate if you could sync it to the radar, but I
suspect (I don't know) that the ARPA computer updates the bearing
information for each target as it's swept, not for the screen as a whole.
3) the GPS heading update rate (single antenna GPS, not a GPS compass) is
relatively slow -- on the order of once a second. The flux gate and GPS
compass can do much better -- ten times that.
4) the ARPA's ability to hold a lock is limited by the update rate and the
amount of junk on the screen, but it's much more sensitive to the former
than the latter. In effect, if the update rate is too slow as the boat yaws,
it's looking for the target along the wrong bearing and may lock on
something else that is close to the spot it's looking at.
5) my various conversations suggest that a properly installed ARPA will
maintain lock under almost all conditions, even when the returns are hard to
figure out visually. If it has a good heading lock, "all" it has to do is
keep looking for the center pixel in the middle of the return at that spot
and then update the range and bearing on each sweep. If it misses the return
on a sweep or two, it keeps looking in the same spot, as long as it's
looking at the right spot.

You say that Lionheart's ARPA loses lock in heavy conditions. I doubt that
either of us will ever know for sure whether this is mistaking the target or
inadequate heading information, but I'd bet on the latter unless, for
example, your target passed very near to another target of similar radar
size -- say a good sized boat near a large radar reflectored buoy.

In response to de-gaussing -- it would be nice for the big steering compass
in front of the driver if the boat were degaussed, but my understanding of
flux-gates is that they deal with residual magnetism pretty well. The
problem for ARPA use comes from the fact that acceleration errors while
yawing are not easy to correct because of all the soft iron under you
bending the field around. This, in effect, weakens the field, so that a flux
gate, even a rate stabilized flux gate, can't really keep up. Hence the need
for a real gyro or GPS compass to get solid ARPA locks.

I'd love to be wrong here -- around $2,500 worth (cost of GPS compass less
cost of rate stabilized flux-gate) -- but I need facts to contradict half a
dozen experts.....


--
Jim Woodward
www.mvFintry.com


..
"Larry W4CSC" wrote in message
...
Lionheart's Raymarine setup is the Smart Heading Sensor, RL70CRC Plus
color radar/chart plotter display and the 2KW radar with the circuit
board antenna spun by the tape deck motor (go look, that's what it
is!) Keep a spare rubber band....er, ah, drive belt aboard as well as
a spare O-ring (ours leaked and Raymarine replaced the whole
receiver).

You don't need the fancy heading sensor to get great resolution from
the MARPA because it uses the GPS data to find the ship's location.
All I see the magnetic sensor does is point the boat symbol and
display in the right direction when you're SITTING STILL. As soon as
you move, with no compass data, the display spins around right,
anyway, when the GPS tells it which way the boat is moving. I guess
the compass makes it start faster, though, as it's already in the
right direction. Big deal. Disconnect your fluxgate from Seatalk and
try it. I think ours works even better using the compass data off the
NMEA network from our B&G Network Pilot autopilot's fluxgate,
actually. I can switch it to either, easily, on my custom control
panel.

Anyway, it tracks MARPA targets very well, as long as the radar
doesn't miss them too often on its scan, like in heavy seas. If there
are a lot of returns in the target's area and you're rockin' and
rollin' so the target is spotty, MARPA will mistake any fairly valid
target that MIGHT be the one you want and go off tracking the big bell
bouy, instead of the boat you wanted. It'll do that. Any computer
running on spotty data from a converted analog signal (radar) would.
But, it's a great thing to have, anyway.

Plan on buying an external ALARM! Some idiot thought it was funny to
make it beep like a cellphone that just found a tower when it alarms.
It isn't even going to wake up the sleeping helmsman slumped over the
display. It's just WEAK. Because of the widely variable nature of
analog radar in the pitching and yawing, you'll get some false alarms,
no matter how fast the compass input is. Those darn bouys change
course when that following sea pushes up on the stern and you're
trying to keep it straight hard over and losing the battle.... This
also might be caused by false data from the compass sensors at high
heel angles. Notice how the warning on the gyro unit and fluxgate say
they must be with in TEN DEGREES of vertical when you mount them.
What the hell sailboat is THAT gonna work on, a 100' trimaran?! No,
he's all sideways, too on the face of that swell.....more than 10
degrees, easy!

We used to have the original compass sensor Raymarine sold for the
system. It's in a cabinet as we installed the much-more-expensive
SHS. I don't really see any operational differences......

If you're interested in the compass sensor, just answer me here. My
cap'n will sell it at a bargain. It's the NMEA model but has seatalk
plugs on it to confuse everyone. We got the cable to plug into it if
you're interested. It's never been abused was mounted in a custom
made teak box in the cabin of the Endeavour 35 he had last. The new
system is in an Amel Sharki 41 ketch, "Lionheart". Save ya a few
bucks....(c;



On 10 Nov 2003 23:19:32 GMT, (BOEING377) wrote:

I am planning to install a Raytheon radar R 70RC plus CRT
display/controller.which has a MARPA feature which tracks 10 targets. How
well
does this MARPA feature work if you have a stable fast reacting heading
ref
(KVH gyro stabilized flux gate in my case)? I may use only the 2 KW
transceiver which has so-so angular resolution. I am mostly interested in
tarcking boats that are within 3 miles of my position. I got feedback
from one
user who said his worked great. He had the Ray smart heading sensor
which is a
flux gate with a rate gyro. Anyone else using this Ray MARPA feature?
Comments?
Anyone out there using HSB to have two radar displays working off one
transceiver? Any glitches?


Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"

I'm interested in the sensor... take the proper word out of my reply address
and send details. I need one for a Raymarine RL70C, about a year old.

"Larry W4CSC" wrote in message
...
Lionheart's Raymarine setup is the Smart Heading Sensor, RL70CRC Plus
color radar/chart plotter display and the 2KW radar with the circuit
board antenna spun by the tape deck motor (go look, that's what it
is!) Keep a spare rubber band....er, ah, drive belt aboard as well as
a spare O-ring (ours leaked and Raymarine replaced the whole
receiver).

You don't need the fancy heading sensor to get great resolution from
the MARPA because it uses the GPS data to find the ship's location.
All I see the magnetic sensor does is point the boat symbol and
display in the right direction when you're SITTING STILL. As soon as
you move, with no compass data, the display spins around right,
anyway, when the GPS tells it which way the boat is moving. I guess
the compass makes it start faster, though, as it's already in the
right direction. Big deal. Disconnect your fluxgate from Seatalk and
try it. I think ours works even better using the compass data off the
NMEA network from our B&G Network Pilot autopilot's fluxgate,
actually. I can switch it to either, easily, on my custom control
panel.

Anyway, it tracks MARPA targets very well, as long as the radar
doesn't miss them too often on its scan, like in heavy seas. If there
are a lot of returns in the target's area and you're rockin' and
rollin' so the target is spotty, MARPA will mistake any fairly valid
target that MIGHT be the one you want and go off tracking the big bell
bouy, instead of the boat you wanted. It'll do that. Any computer
running on spotty data from a converted analog signal (radar) would.
But, it's a great thing to have, anyway.

Plan on buying an external ALARM! Some idiot thought it was funny to
make it beep like a cellphone that just found a tower when it alarms.
It isn't even going to wake up the sleeping helmsman slumped over the
display. It's just WEAK. Because of the widely variable nature of
analog radar in the pitching and yawing, you'll get some false alarms,
no matter how fast the compass input is. Those darn bouys change
course when that following sea pushes up on the stern and you're
trying to keep it straight hard over and losing the battle.... This
also might be caused by false data from the compass sensors at high
heel angles. Notice how the warning on the gyro unit and fluxgate say
they must be with in TEN DEGREES of vertical when you mount them.
What the hell sailboat is THAT gonna work on, a 100' trimaran?! No,
he's all sideways, too on the face of that swell.....more than 10
degrees, easy!

We used to have the original compass sensor Raymarine sold for the
system. It's in a cabinet as we installed the much-more-expensive
SHS. I don't really see any operational differences......

If you're interested in the compass sensor, just answer me here. My
cap'n will sell it at a bargain. It's the NMEA model but has seatalk
plugs on it to confuse everyone. We got the cable to plug into it if
you're interested. It's never been abused was mounted in a custom
made teak box in the cabin of the Endeavour 35 he had last. The new
system is in an Amel Sharki 41 ketch, "Lionheart". Save ya a few
bucks....(c;



On 10 Nov 2003 23:19:32 GMT, (BOEING377) wrote:

I am planning to install a Raytheon radar R 70RC plus CRT
display/controller.which has a MARPA feature which tracks 10 targets. How
well
does this MARPA feature work if you have a stable fast reacting heading
ref
(KVH gyro stabilized flux gate in my case)? I may use only the 2 KW
transceiver which has so-so angular resolution. I am mostly interested in
tarcking boats that are within 3 miles of my position. I got feedback
from one
user who said his worked great. He had the Ray smart heading sensor
which is a
flux gate with a rate gyro. Anyone else using this Ray MARPA feature?
Comments?
Anyone out there using HSB to have two radar displays working off one
transceiver? Any glitches?


Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"

On Tue, 11 Nov 2003 07:04:52 -0500, "Jim Woodward" jameslwoodward at
attbi dot com wrote:

That's not what I experienced on Swee****er, or what the three
manufacturer's experts say -- but I could very well be missing something.
I've tried hard to really understand the issues, because they have led me to
think I need a $3,300 GPS compass for successful ARPA use, and I think hard
before spending that kind of money.

As I understand it:
1) the ARPA wants, in effect, a north-up display, so that it can compute the
actual bearing from your boat to the target, not a relative bearing. I'm not
saying the ARPA actually works off the display, or that you can't use it in
heading-up mode, but that it needs to be able to act like a north-up
display.

We always use MARPA in heading-up mode. Once the GPS tells it which
way we are heading, I know it ignores the compass input because I can
screw the compass heading up 180 degrees and we still have the correct
heading up. The only thing I see different is the little boat symbol
on the charge is pointing backwards...(c; It uses compass information
for heading up only when we're sitting still. If it doesn't find a
compass, the RL70CRC Plus simply points us where it had a heading from
GPS last time. If the boat were swinging on anchor, it might be the
wrong way, but not when we're underway and GPS is telling it our
course. MARPA works fine with the little boat pointed backwards, too.
Where MARPA fails is when it looses the target for more than about 3
sweeps, which are painstakingly slow when you know it's there and it
doesn't show on the display, then MARPA starts beeping for help, "I
Lost HIM!" If there is a nearby target, like the target is passing a
bouy, MARPA locks onto the bouy and starts tracking it.

Too bad boats don't all have a beacon transponder like airplanes with
any sense do. The beacon system does great tracking with its powerful
old transmitters. Navaids should all have beacon transponders, too,
instead of idiotic blinking light bulbs as if this were 1925. Nothing
in Navaids makes any sense. The damned Charleston Jetties should be
topped with towers full of airport strobe beacons and an ILS, too.
Maritime navigation lives in the stone age.... Even Morning Dew's
crew of boys would be alive if the damned jetties weren't black and
invisible, nostalgia or no nostalgia.

2) in order to get a north-up display, it needs heading data. As the boat
yaws in a seaway, it needs heading data frequently -- several times a
second. It's true that an ordinary small boat radar is providing new data
only about every two seconds (around 25rpm) and an update rate of once every
two seconds might be adequate if you could sync it to the radar, but I
suspect (I don't know) that the ARPA computer updates the bearing
information for each target as it's swept, not for the screen as a whole.

The yaw rate always exceeds the radar sweep rate on any boat I've been
on except ships. Head up is a joke on a small sailboat in a following
sea...an illusion. With the antenna pointing first towards Andromeda
then at Atlantis any chance of actually seeing a surface target is
about the same as winning the lottery. The antenna is lucky if it's
pointing at the target the brief instant it's actually horizontal.
The beacon system I spoke of above eliminates this problem. Beacon
transponders work in microseconds not 10s of seconds. They respond
much faster than any roll rate and there's no chance of missing the
signal no matter how fast your antenna passes horizontal on its way to
your next roll peak.

Of course, boating, ships and the USCG would resist a change to a
beacon nav system like it would bubonic plague. Any change for the
better is to be resisted at all costs....like GMDSS was.

3) the GPS heading update rate (single antenna GPS, not a GPS compass) is
relatively slow -- on the order of once a second. The flux gate and GPS
compass can do much better -- ten times that.

Look at the compass/gyro. The Raymarine Smart Heading Sensor is
useless in heavy seas. "The unit must be installed vertically within
10 degrees of plumb." A compass sensor or fluxgate's data is useless
when it's laying on its side. The little gyro has words like that,
too. 10 degrees? Are they kidding?! When the radar antenna is
pointed at Andromeda, so is the fluxgate, rendering it useless. A
stable gimbal mount must eat into company profits way too far. None
of these little gyros are actually REAL gyros. I agree with what
you've been told. The real gyro would give you accurate course info
no matter if the damned boat were pitchpole. It's always pointed at
the horizon.

4) the ARPA's ability to hold a lock is limited by the update rate and the
amount of junk on the screen, but it's much more sensitive to the former
than the latter. In effect, if the update rate is too slow as the boat yaws,
it's looking for the target along the wrong bearing and may lock on
something else that is close to the spot it's looking at.

All the more reason to press for a beacon system like aircraft use.
It's been working great since the Korean War! Isn't it time we
upgraded??

5) my various conversations suggest that a properly installed ARPA will
maintain lock under almost all conditions, even when the returns are hard to
figure out visually. If it has a good heading lock, "all" it has to do is
keep looking for the center pixel in the middle of the return at that spot
and then update the range and bearing on each sweep. If it misses the return
on a sweep or two, it keeps looking in the same spot, as long as it's
looking at the right spot.

You say that Lionheart's ARPA loses lock in heavy conditions. I doubt that
either of us will ever know for sure whether this is mistaking the target or
inadequate heading information, but I'd bet on the latter unless, for
example, your target passed very near to another target of similar radar
size -- say a good sized boat near a large radar reflectored buoy.

What target? It doesn't see a target unless the antenna is within
15-20 degrees of horizontal AND happens to be pointed the right way on
its narrow horizontal beamwidth just when the boat is level enough AND
isn't in a wave trough hiding the target in seawater....all
simultaneously. It won't see the target, so can't track it, if the
antenna isn't pointed just right at the precise moment all at
once......

As with most marine systems, it fails when you need it most...heavy
seas. The spinning beacon IFF antenna would have interrogated the
target a thousand times every time he poked his head up above the
wave, making tracking very accurate. Ask any Navy flyer how it finds
the carrier deck for a safe touchdown from 400 miles out.

In response to de-gaussing -- it would be nice for the big steering compass
in front of the driver if the boat were degaussed, but my understanding of
flux-gates is that they deal with residual magnetism pretty well. The
problem for ARPA use comes from the fact that acceleration errors while
yawing are not easy to correct because of all the soft iron under you
bending the field around. This, in effect, weakens the field, so that a flux
gate, even a rate stabilized flux gate, can't really keep up. Hence the need
for a real gyro or GPS compass to get solid ARPA locks.

The fluxgates would be flawless mounted on a stabilizing gyro to hold
a horizontal platform for the electronic sensor. But, alas, we don't
have that. The fluxgate is only temporarily horizontal on it way to
the next peak of pitch and yaw, so that's the only time its data means
anything. You can hear Raymarines fluxgate banging against the stops
on its internal spring mounts as it tilts over towards vertical. Sure
glad the GPS knows what our course is, even if it's slow.....

I'd love to be wrong here -- around $2,500 worth (cost of GPS compass less
cost of rate stabilized flux-gate) -- but I need facts to contradict half a
dozen experts.....

Until they solve the pitch/yaw problems and give the fluxgate a stable
platform and real gyro to work from, I think it's a waste of money,
personally. It isn't anywhere near perfect and isn't usable when the
going gets really tough.

The sooner a mandatory beacon transponder program starts, the better.
But don't worry about it. We can't even get our CG station to install
the Channel 70 DSC equipment, yet. They blew the money on a new front
gate to make the base look pretty. They don't have any direction
finding equipment, either. Ask the dead of the Morning Dew who could
have been saved.



Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"

On Tue, 11 Nov 2003 06:46:29 -0600, "Keith"
wrote:

I'm interested in the sensor... take the proper word out of my reply address
and send details. I need one for a Raymarine RL70C, about a year old.

Got cap'n's voicemail when I called him. Sent you an email. He'll
call me sometime today, probably, if he's in the country and I'll
email you as soon as I hear from him. It's not doing him any good in
that box. On the old boat, it was nicely hidden in a teak box in the
main cabin real near centerline. Worked great but he thought the new
sensor was neater. I'm not so sure, either way....


Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"

"Larry W4CSC" wrote in message
...

Too bad boats don't all have a beacon transponder like airplanes with
any sense do. The beacon system does great tracking with its powerful
old transmitters. Navaids should all have beacon transponders, too,
instead of idiotic blinking light bulbs as if this were 1925. Nothing
in Navaids makes any sense. The damned Charleston Jetties should be
topped with towers full of airport strobe beacons and an ILS, too.
Maritime navigation lives in the stone age.... Even Morning Dew's
crew of boys would be alive if the damned jetties weren't black and
invisible, nostalgia or no nostalgia.

snip

Of course, boating, ships and the USCG would resist a change to a
beacon nav system like it would bubonic plague. Any change for the
better is to be resisted at all costs....like GMDSS was.

snip

All the more reason to press for a beacon system like aircraft use.
It's been working great since the Korean War! Isn't it time we
upgraded??

snip

The sooner a mandatory beacon transponder program starts, the better.

Larry, your prayers have been heard. By the end of 2004, all SOLAS vessel
are required to have an AIS system, which does exactly what you want. It
transmits position, course, ROT and other info on a VHF packet radio-like
network.

Meindert

I think (underline "think") that your various radar troubles -- "Head up is
a joke on a small sailboat in a following sea...an illusion." and your
trouble with MARPA -- are problems with heading update rate. With a good
enough compass, any of the radar modes should be stable ("should" -- big
word).

Maybe (I'm speculating here) if you set the radar up so that it didn't have
any GPS heading data, just the compass, you'd get better results.

As Meindert says below, AIS will help a lot. It's about $5,000, but that's
for a bulletproof GMDSS version and soon I would expect that a recreational
version will be available for less money.



--
Jim Woodward
www.mvFintry.com


..
"Larry W4CSC" wrote in message
...
On Tue, 11 Nov 2003 07:04:52 -0500, "Jim Woodward" jameslwoodward at
attbi dot com wrote:

That's not what I experienced on Swee****er, or what the three
manufacturer's experts say -- but I could very well be missing something.
I've tried hard to really understand the issues, because they have led me
to
think I need a $3,300 GPS compass for successful ARPA use, and I think
hard
before spending that kind of money.

As I understand it:
1) the ARPA wants, in effect, a north-up display, so that it can compute
the
actual bearing from your boat to the target, not a relative bearing. I'm
not
saying the ARPA actually works off the display, or that you can't use it
in
heading-up mode, but that it needs to be able to act like a north-up
display.

We always use MARPA in heading-up mode. Once the GPS tells it which
way we are heading, I know it ignores the compass input because I can
screw the compass heading up 180 degrees and we still have the correct
heading up. The only thing I see different is the little boat symbol
on the charge is pointing backwards...(c; It uses compass information
for heading up only when we're sitting still. If it doesn't find a
compass, the RL70CRC Plus simply points us where it had a heading from
GPS last time. If the boat were swinging on anchor, it might be the
wrong way, but not when we're underway and GPS is telling it our
course. MARPA works fine with the little boat pointed backwards, too.
Where MARPA fails is when it looses the target for more than about 3
sweeps, which are painstakingly slow when you know it's there and it
doesn't show on the display, then MARPA starts beeping for help, "I
Lost HIM!" If there is a nearby target, like the target is passing a
bouy, MARPA locks onto the bouy and starts tracking it.

Too bad boats don't all have a beacon transponder like airplanes with
any sense do. The beacon system does great tracking with its powerful
old transmitters. Navaids should all have beacon transponders, too,
instead of idiotic blinking light bulbs as if this were 1925. Nothing
in Navaids makes any sense. The damned Charleston Jetties should be
topped with towers full of airport strobe beacons and an ILS, too.
Maritime navigation lives in the stone age.... Even Morning Dew's
crew of boys would be alive if the damned jetties weren't black and
invisible, nostalgia or no nostalgia.

2) in order to get a north-up display, it needs heading data. As the
boat
yaws in a seaway, it needs heading data frequently -- several times a
second. It's true that an ordinary small boat radar is providing new data
only about every two seconds (around 25rpm) and an update rate of once
every
two seconds might be adequate if you could sync it to the radar, but I
suspect (I don't know) that the ARPA computer updates the bearing
information for each target as it's swept, not for the screen as a whole.

The yaw rate always exceeds the radar sweep rate on any boat I've been
on except ships. Head up is a joke on a small sailboat in a following
sea...an illusion. With the antenna pointing first towards Andromeda
then at Atlantis any chance of actually seeing a surface target is
about the same as winning the lottery. The antenna is lucky if it's
pointing at the target the brief instant it's actually horizontal.
The beacon system I spoke of above eliminates this problem. Beacon
transponders work in microseconds not 10s of seconds. They respond
much faster than any roll rate and there's no chance of missing the
signal no matter how fast your antenna passes horizontal on its way to
your next roll peak.

Of course, boating, ships and the USCG would resist a change to a
beacon nav system like it would bubonic plague. Any change for the
better is to be resisted at all costs....like GMDSS was.

3) the GPS heading update rate (single antenna GPS, not a GPS compass) is
relatively slow -- on the order of once a second. The flux gate and GPS
compass can do much better -- ten times that.

Look at the compass/gyro. The Raymarine Smart Heading Sensor is
useless in heavy seas. "The unit must be installed vertically within
10 degrees of plumb." A compass sensor or fluxgate's data is useless
when it's laying on its side. The little gyro has words like that,
too. 10 degrees? Are they kidding?! When the radar antenna is
pointed at Andromeda, so is the fluxgate, rendering it useless. A
stable gimbal mount must eat into company profits way too far. None
of these little gyros are actually REAL gyros. I agree with what
you've been told. The real gyro would give you accurate course info
no matter if the damned boat were pitchpole. It's always pointed at
the horizon.

4) the ARPA's ability to hold a lock is limited by the update rate and
the
amount of junk on the screen, but it's much more sensitive to the former
than the latter. In effect, if the update rate is too slow as the boat
yaws,
it's looking for the target along the wrong bearing and may lock on
something else that is close to the spot it's looking at.

All the more reason to press for a beacon system like aircraft use.
It's been working great since the Korean War! Isn't it time we
upgraded??

5) my various conversations suggest that a properly installed ARPA will
maintain lock under almost all conditions, even when the returns are hard
to
figure out visually. If it has a good heading lock, "all" it has to do
is
keep looking for the center pixel in the middle of the return at that
spot
and then update the range and bearing on each sweep. If it misses the
return
on a sweep or two, it keeps looking in the same spot, as long as it's
looking at the right spot.

You say that Lionheart's ARPA loses lock in heavy conditions. I doubt
that
either of us will ever know for sure whether this is mistaking the target
or
inadequate heading information, but I'd bet on the latter unless, for
example, your target passed very near to another target of similar radar
size -- say a good sized boat near a large radar reflectored buoy.

What target? It doesn't see a target unless the antenna is within
15-20 degrees of horizontal AND happens to be pointed the right way on
its narrow horizontal beamwidth just when the boat is level enough AND
isn't in a wave trough hiding the target in seawater....all
simultaneously. It won't see the target, so can't track it, if the
antenna isn't pointed just right at the precise moment all at
once......

As with most marine systems, it fails when you need it most...heavy
seas. The spinning beacon IFF antenna would have interrogated the
target a thousand times every time he poked his head up above the
wave, making tracking very accurate. Ask any Navy flyer how it finds
the carrier deck for a safe touchdown from 400 miles out.

In response to de-gaussing -- it would be nice for the big steering
compass
in front of the driver if the boat were degaussed, but my understanding
of
flux-gates is that they deal with residual magnetism pretty well. The
problem for ARPA use comes from the fact that acceleration errors while
yawing are not easy to correct because of all the soft iron under you
bending the field around. This, in effect, weakens the field, so that a
flux
gate, even a rate stabilized flux gate, can't really keep up. Hence the
need
for a real gyro or GPS compass to get solid ARPA locks.

The fluxgates would be flawless mounted on a stabilizing gyro to hold
a horizontal platform for the electronic sensor. But, alas, we don't
have that. The fluxgate is only temporarily horizontal on it way to
the next peak of pitch and yaw, so that's the only time its data means
anything. You can hear Raymarines fluxgate banging against the stops
on its internal spring mounts as it tilts over towards vertical. Sure
glad the GPS knows what our course is, even if it's slow.....

I'd love to be wrong here -- around $2,500 worth (cost of GPS compass
less
cost of rate stabilized flux-gate) -- but I need facts to contradict half
a
dozen experts.....

Until they solve the pitch/yaw problems and give the fluxgate a stable
platform and real gyro to work from, I think it's a waste of money,
personally. It isn't anywhere near perfect and isn't usable when the
going gets really tough.

The sooner a mandatory beacon transponder program starts, the better.
But don't worry about it. We can't even get our CG station to install
the Channel 70 DSC equipment, yet. They blew the money on a new front
gate to make the base look pretty. They don't have any direction
finding equipment, either. Ask the dead of the Morning Dew who could
have been saved.



Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"