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!"