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!"