CMG 3 to be offline for tests

From this morning's DPC:

CMG #3 will be taken offline for 4 days starting wednesday for tests. This
will leave the station with only 2 CMGs, with the expectation that they will
saturate more often and require russian segment thrusters to desaturate.

Mike Foale commented that taking the CMG offline would automatically send
attitude control to the russian segment, but ground then explained something
(which I missed) to the effect that the US segment would remain in control but
that the CMGs would saturate more quickly.

Does anyone know if the software automatically switches to russian segment
attitude control if there are fewer than 3 active CMGs ? (i.e. for those
tests, do they have to bypass that part of software)

Would failure of CMG system still leave the US segment software in control of
attitude control (using its GPS) with the US segment then commanding russian
thrusters, or would it be considered "failed" and the russian segment be
wholly responsible for attitude control ?

Small correction - the CMGs will not saturate more often - in fact
they shouldn't saturate at all. The software will automatically
request a desaturation if the momentum exceeds a software threshold
(50% for attitude hold controllers and 90% for momentum management
controllers).

If the Minimum ORUS Available PUI (program unique identifier)
registers "NO" (meaning less than 2 CMGs on-line and/or no source for
attitude & rate information) then the US GNC will relinquish control
and the Russian Segment will take over.

The US Segment can continue to use GPS and RGAs to provide Pointing &
Support information, but once the Minimum ORUs flag says no, the
Warning message "Loss of CMG Attitude Control" goes into alarm and the
Russian Segment becomes solely responsible for control of the vehicle.


On Tue, 13 Apr 2004 04:34:08 -0400, John Doe wrote:

From this morning's DPC:

CMG #3 will be taken offline for 4 days starting wednesday for tests. This
will leave the station with only 2 CMGs, with the expectation that they will
saturate more often and require russian segment thrusters to desaturate.

Mike Foale commented that taking the CMG offline would automatically send
attitude control to the russian segment, but ground then explained something
(which I missed) to the effect that the US segment would remain in control but
that the CMGs would saturate more quickly.

Does anyone know if the software automatically switches to russian segment
attitude control if there are fewer than 3 active CMGs ? (i.e. for those
tests, do they have to bypass that part of software)

Would failure of CMG system still leave the US segment software in control of
attitude control (using its GPS) with the US segment then commanding russian
thrusters, or would it be considered "failed" and the russian segment be
wholly responsible for attitude control ?

capbrit wrote:
Small correction - the CMGs will not saturate more often - in fact
they shouldn't saturate at all. The software will automatically
request a desaturation if the momentum exceeds a software threshold
(50% for attitude hold controllers and 90% for momentum management
controllers).

Is it correct to state that with 2 instead of 3 CMGs, russians thrusters will
fire more often ?

The US Segment can continue to use GPS and RGAs to provide Pointing &
Support information, but once the Minimum ORUs flag says no, the
Warning message "Loss of CMG Attitude Control" goes into alarm and the
Russian Segment becomes solely responsible for control of the vehicle.

Does the Russian segment have the ability to draw readings from the US
GPS/RGAs to help it determine current attitude/Position of station ? Or does
it operat 100% independantly from US segment when it has been given control ?

On Tue, 13 Apr 2004 20:40:43 -0400, John Doe wrote:


Only during attitude holds. During Momentum Management (the majority
of the time), the momentum level hovers around 18% - drop to 2 CMGs
and that becomes around 40% - far below the 90% threshold, so no
thrusters are fired.

The Russians use a strapped down inertial navigation system called
BINS. They determine a base quaternion for the vehicle and propagate
from that using a gyroscope called GIVUS to measure vehicle rates and
a suite of attitude sensors (star cameras, sun sensors, magnetometers,
and infrared horizon sensors). If GIVUS fails, there is a Russian
backup gyro called Ort. Ort, however, has a short lifetime, so the
Russians will switch to US segment data from GPS and the RGAs.

Similarly, if there are problems with GPS or the RGAs, the US segment
can use Russian navigation data.

An important thing to understand is that regardless of whom is
controlling the vehicle's attitude, both segments require valid
navigation data so that the communications antennae and solar arrays
will know where to point.






Is it correct to state that with 2 instead of 3 CMGs, russians thrusters will
fire more often ?


Does the Russian segment have the ability to draw readings from the US
GPS/RGAs to help it determine current attitude/Position of station ? Or does
it operat 100% independantly from US segment when it has been given control ?

capbrit wrote excellent description of attitude control.

Many thanks.

"capbrit" wrote
GOOD STUFF

capbrit, please visit often.

you can also reach me directly via joberg at houston dot rr dot com

Additional info from this morning's DPC.

They had have a few control problems with CMG #2 lately, and Houston explained
to Foale that during the 4 day test, it is possible that #2 might fail
(temporarily), at which point control would fall back to the russian segment,
with at least 3 alarms/warnings issued.

Yes, #2 has been having LOCs (losses of comm). If that happens while
there are only two active CMGs, the following messages will appear:

CMG 2 Failed (suppressed caution, no tone)
Loss of CMG Attitude Control (Warning)
RS Autorecovery Initiated

Once the RS Autorecovery completes and they take control, the last two
messages will go out of alarm.


On Wed, 14 Apr 2004 04:39:36 -0400, John Doe wrote:

Additional info from this morning's DPC.

They had have a few control problems with CMG #2 lately, and Houston explained
to Foale that during the 4 day test, it is possible that #2 might fail
(temporarily), at which point control would fall back to the russian segment,
with at least 3 alarms/warnings issued.