The ACE spacecraft is operating nominally, in science data collection mode.

On April 3, 2000 the Ultra Low Energy Isotope Spectrometer (ULEIS)
instrument experienced an anomaly involving a portion of the sensor's
operation. The anomaly was detected from processed science data.  The simplest
description of the anomaly is that the START-1 system response showed a very
large decrease in gain at that time.  This decrease has persisted through the
time of the most recent data available.  The START-1 system produces the start
pulse for ULEIS time-of-flight (TOF) system 1. ULEIS has two redundant TOF
systems, and the primary system is TOF system 2, which derives its start pulse
from a separate MCP in the sensor.  However, this anomaly does degrade the
capability of the instrument to resolve isotopes, since data from TOF system 1
is used to fine tune the data from TOF system 2. The effect on the mass
resolution is TBD, however 3He rich events and events with enriched 22Ne should
still be identifiable. It is likely that another important effect will arise
from background issues since we lose the ability to require consistency
between the two times-of-flight.

No other aspect of the instrument's operation has been affected.
All of the rate channels and pulse height channels with atomic species
resolution are fully functional. All other electronic functions, voltages, and
data processing operations are nominal.

We have performed a number of tests to stimulate and/or "jar" the system,
including the following:

1) activating the in-flight calibrator to test electronic functionality
2) cycling the high-voltage command settings (both START-1 and START-2)
3) turning off the high-voltage supplies
4) power cycling the telescope and analog electronics box
5) moving ULEIS closable cover to introduce microphonics near the START-1

The problem has remained unchanged throughout these tests.  The tests have
revealed that the TOF-1 electronics are functioning properly when
stimulated electronically.

The greatly lowered response of the START-1 system to particles is
consistent with a decrease of bias voltage;  however, the START-1 HV
housekeeping voltages have remained normal throughout this period.  We have
been unable to identify a plausible single point failure that accounts for
this change, except perhaps an opening of one of the HV cables -- we
consider this very unlikely due to the manner of construction, and the
difficulty of having small break in a cable or connection be of a nature
that would not allow current to arc across it under high potential.

The START-1 system appears to have some small response to particle triggering;
however it will take a reasonably sized particle event to get the statistics
to really characterize the response.

We have now pretty well exhausted the list of tests that would appear relevant
to examining or fixing this problem.  We might turn off for an extended period
to cool the telescope, but in a prior turn-off for the C&DH problem in June
1998, we cooled down only a few degrees and so this would not be much of a
change.  Meanwhile, we are somewhat reluctant to carry out such tests with the
instrument for fear of causing other problems.

Our best current guess is that this is a permanent problem.  If the START-1
response in the next particle event is such that the science return is
minimal, we may then decide to lower the START-1 HV to 0, in order to remove a
potential source of arcing in the instrument.  We might try a turn-on in a few
months to see if there was any change.


All other ACE science instruments are functioning normally.