From: Mike Machado [Michael.J.Machado.1@gsfc.nasa.gov]
Sent: Tuesday, May 08, 2007 11:13 AM

Subject: FW: Polar Power Situation

Importance: High
Polar instrument teams,

Due to high sun angles and degraded solar array output, Polar is experiencing
negative power situations. Two weeks ago, the FOT altered the load shed levels
to account for the higher sun angles. Last night due to negative power
conditions, the FOT turned off PWI main power and set load shed levels to
"long eclipse" values which reduce the under voltage condition that triggers
load shedding to 19.1 volts. As we have not yet reached the maximum sun angle,
we may need to turn additional loads off to prevent a load shed. We are still
evaluating the current load profile and will let you know as soon as possible
if we need to turn your instrument off. As the high sun angle period extends
through June 6, we will need to determine the best operations approach for
maintaining health and safety of the spacecraft. Please see more details from
the spacecraft engineer below. 

The order for turning off instruments was determined with project coordination
based on previous eclipse seasons, operating power requirements versus heater
power draw and current operations plans:

(CAMMICE/MICS, PWI, SEPS, PIXIE already off)

CAMMICE/HIT
TIDE
TIMAS
Hydra
CEPPAD
MFE (1 side only)
UVI
MFE (2nd side)
VIS
EFI


Thanks,

Mike Machado
Wind/Polar Flight Operations Team Manager 
301-286-0666

At 08:10 PM 5/7/2007, Steve Hearn wrote:

  Mike,

  Today, at 21:45z, I was informed by Aurora of the Polar battery voltages
reaching a low point of 19.8V, nearing the 19.6V load shed level.  I directed
Aurora to immediately take the following steps to prevent an unnecessary load
shed:

  1.  At 127/21:49:48z, turn PWI main power off.  The saves approximately one
      half ampere in bus load.
  2.  At 127/21:56:11, set load shed levels to 'long eclipse' values, i.e.,
      UV-1 is now set to the 19.1V threshold.  This is the lowest (least
      sensitive) setting available.  The only remaining action in this area is
      to disable UV1 protection.

  These actions together reduced the immediate threat of a load shed.

  Polar's sun angle exceeded 150 degrees on May 2 and will stay above 150 until
June 6 (based on the latest FDF predicts), a period of 5 weeks (this is
typical).  Polar has not experienced sun angles above 150 since the first week
of 2000.  The sun angle will peak on May 18 at 154.9 degrees.  The Polar sun
angle has not been this high since the second week of 1998.

  At these higher sun angles, the solar arrays do not provide sufficient power
to account for the real-time load increase due to the comm downlink transmitter
and amplifier (2.0 Amps combined load) and the DTR in playback mode (0.5 amps).
Hence, the batteries discharge during real-time contacts.

  In addition, there is a significantly increased power bus load due to
thermostatic heaters turning on (and staying on).  The majority of the heaters
and subsystem components are located at or above the geometric midline of the
spacecraft.  This area is now limited to solar heating at highly oblique angles
on the sides and constant shadow on the top of the spacecraft.  The total load
increase from the average minimum thermal load is approximately 2.3 amps.

  While this is consistent with past high-sun angle excursions, solar array
degradation over time has decreased the total amount of solar array input power.

  The two steps taken above will remain in place until the sun angle situation
has corrected itself sufficiently, probably two or more weeks from now.  In
addition, I am working with Joyce Ross to limit real-time contacts of over one
hour duration that are not required to fill data gaps.  As you are aware, Joyce
tries to schedule a two-hour day-shift support every weekday to aid real-time
commanding and science operations.  By limiting this, we can reduce the maximum
battery discharge during real-time contacts.

  Additional steps may be needed to reduce the Polar power load over the next
few weeks.  I would like to have an instrument turn-off sequence in-hand in
case one or two instrument loads need to be shed. At this time PWI, PIXIE,
SEPS, and CAMMICE/MICS are powered off.

  Steve