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Posted by baalke on March 26, 2007, 8:19 pm
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http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_current.php
The PI's Perspective
Alan Stern
Trip Report
March 26, 2007
New Horizons tripped up but recovered itself without a nasty spill
last
week. This event occurred on the afternoon of March 19, precisely 14
months to the day since we launched.
What do I mean by saying that the spacecraft "tripped?" What actually
happened was that an uncorrectable memory error was detected in the
memory of our primary Command and Data Handling (C&DH) computer, which
is the "brains" of New Horizons. Although onboard error detection
routines can and did recognize such an error within seconds of its
occurrence, the error was so severe (a double-bit error in a single
memory word) that there was no definitive way for our error-correction
algorithm to unambiguously restore the correct series of 1's and 0's
in
this memory location. (Our memory, like that on many other spacecraft,
is encoded such that a single-bit error can both be detected and
corrected; a double-bit error can be detected, but there isn't
sufficient information encoded to be sure how to correct it.)
Since a bad word in C&DH memory could invoke an unpredictable
spacecraft
action, C&DH is programmed to command itself to reboot (and thus
restore
memory from a boot PROM whenever such an double-bit error is
detected.
But whenever C&DH resets, our onboard autonomous fault-detection and
-protection system declares an emergency and commands the spacecraft
to suspend all current activities and go to a "Safe Mode."
Given the spacecraft mode and state on the afternoon of March 19, the
result was that the bird was spun up from 3-axis control to a stable
5-RPM spin, and its antenna was pointed to Earth and commanded to call
home for help. New Horizons also commanded itself to shut off
unessential power loads (like the PEPSSI and SWAP instruments, which
had
been collecting Jupiter magnetotail data) and go to an emergency (low)
bit rate for downlink.
In an amazing stroke of luck, the NASA Deep Space Network and our
control center at the Johns Hopkins Applied Physics Lab were actually
in
contact with the spacecraft when this even occurred, so our ground
team
saw - in real-time - the double-bit error, the resulting C&DH computer
reset, and the spacecraft commanding itself to "Go Safe."
The Go Safe maneuver itself resulted in a temporary loss of contact
with
our baby, but within about 90 minutes, New Horizons was back in
communication with Earth, and shortly thereafter, the ground control
team at APL had re-established commanding capability.
This was the first time New Horizons had commanded itself to Go Safe
in
flight, and both the spacecraft and the APL ground team responded
expertly. As a result, we regained spacecraft control quickly, and we
were back in a nominal operations configuration - taking science data
again - in less than two days.
What actually caused the spacecraft C&DH memory to be corrupted with a
two-bit error in a single C&DH address? We're still trying to
determine
that, but early indications are it was related to a burst of four bit
errors within a short time that may have been due to RTG or natural
space-environment radiation. Such multi-event bursts have not been
uncommon in flight, but they have only once before resulted in a
double-bit error in the Guidance and Control (not the C&DH) processor.
The event is less critical in the G&C processor, because the
spacecraft
can operate through such an event, so no Go Safe was required.
Will such Go Safes happen again? Quite possibly. Can we find a way to
better protect against such events so they don't occur as frequently
as
they might otherwise have? Maybe, and we're looking into it. Will the
spacecraft take care of itself as it did this time? Our confidence is
high that it will - extensive ground testing of the autonomy system
and
its Go Safe response paid off on March 19, and because of the test of
the Go Safe function in flight last Monday, we have even greater
confidence in our "autopilot" than we did from ground testing alone.
Of course, no one - and most particularly this mission PI - wanted
such
an in-flight test of contingency procedures. But New Horizons didn't
ask
our permission, and we got our Go Safe test, like it or not. What we
learned as a result is that our flight system - both the silicon part
in
space and the carbon part in Maryland - responded with grace and
precision to recover without causing any real injury.
This event is a reminder of the very real risks of space flight and
the
long journey we have ahead in order to accomplish our goal of
reconnoitering the Pluto system at the far end of the planetary
frontier. So we proceed with both confidence and a renewed sense of
the
fact that we are playing for keeps. We are now also back to
downlinking
Jupiter encounter data; back to taking Jovian magnetotail
measurements;
and back to preparing to initiate hibernation operations this summer.
Onward we go, into the cold, yawning abyss that is the outer solar
system, with our eyes, minds and hearts firmly fixed on our goal of a
history-making scientific exploration of worlds where no one has gone
before!
Well, that's all I wanted to tell you about this time. I'll be back
with
more news in another update in April. In the meantime, keep on
exploring, just as we do.
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