Meindert Sprang wrote:

"maxlynn" wrote in message
news:9tXKb.103004$pY.50340@fed1read04...
What you say is true in a fixed one-g environment. You can derive a crude
measurement of angular rotation in any fixed, uni-directional acceleration
environment in the manner that you suggest. But as I understand the
proposed application, the accelerometer(s)/instruments are to be mounted
in
a dynamic environment. The accelerometers mentioned are designed to
measure
linear acceleration. That is what the manufacturer designed them to do.
They are NOT designed to measure rotation or rotational rates. That is
generally a domain reserved for gyroscopes and related insruments or
systems.

Well, read the datasheets. AD says specifically in their datasheets that the
ADXL's can be used as an inclinometer. Apart from that, a boat seems an
'almost fixed' one-g environmet to me.

Meindert

Stand on your bathroom scales while under sail in rollers and
then tell me your local gee is stable at one. The boat bobs and
weaves. The local gee variation is one of the things that cause
seasickness. Your simple accellerometer pitch and roll sensor
will also get sick.

The sensors could be used as roll, etc sensors, but not if the
platform is heaving or weaving, unless you use twice as many as
you might otherwise out of phase, and then combine their outputs
to enable nulling the elevator effects.

A pendulum and potentiometer rig seems better though it too will
sense unwanted displacements. A gyro setup, even a stationary one
with pendulum type sensing via bar coded markings, is likely the
best. How much accuracy do you need? To resolve satellite
pointing issues, a fixed 3 axis setup (six sensors) will be
required, with integration of the outputs to derive azimuth and
elevation, unless you servo the yaw efficiently and then sense
the elevation, then you only need juggle two indications. You
might use an electronic compass to control the yaw of the antenna
platform. Daily adjustments of magnetic variation might be needed
in passage, and as well, you would need to integrate compass
variations due to local ferrous masses at various headings.
Perhaps a gyrocompass?

This is not so simple as it sounds. If it was, a one chip
controller would be produced by now, and our airliners and cars
might all have satellite reception available.

Alternate methods might be use of gps position, time of day, and
star sight information during clear sky moments.
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On Mon, 12 Jan 2004 17:52:40 GMT, Terry Spragg
wrote:

How much accuracy do you need? To resolve satellite
pointing issues, a fixed 3 axis setup (six sensors) will be
required, with integration of the outputs to derive azimuth and
elevation, unless you servo the yaw efficiently and then sense
the elevation, then you only need juggle two indications. You
might use an electronic compass to control the yaw of the antenna
platform. Daily adjustments of magnetic variation might be needed
in passage, and as well, you would need to integrate compass
variations due to local ferrous masses at various headings.
Perhaps a gyrocompass?

The (Inmarsat D+) satellite transceiver uses an omnidirectional antenna, so
no moving parts. According to the specs it has an elevation mask angle of 5°
so one can theoretically work out where in the world it will have coverage of
the Inmarsat satellites. However, the certifying body wants practical tests
done in real-life conditions before certifying it for marine use. We aren't
the manufacturer, only end-users, but need certification for our work.

The units may work up to the theoretical limits in perfect weather, but the
aim is to see how much degradation of performance there is in bad weather -
when the units would be more likely used.

Dave