[FPSPACE] FW: Gravity Probe B Update -- January 31, 2006
LARRY KLAES
ljk4 at msn.com
Thu Feb 2 09:03:39 EST 2006
>From: Bob Kahn <kahn at relgyro.stanford.edu>
>Reply-To: kahn at relgyro.stanford.edu
>To: gpb-update at lists.Stanford.EDU
>Subject: Gravity Probe B Update -- January 31, 2006
>Date: Wed, 1 Feb 2006 17:32:58 -0800
>
>============================================
>GRAVITY PROBE B MISSION UPDATE FOR 31 JANUARY 2006
>============================================
>
>GP-B STATUS AT A GLANCE
>=============================
>Mission Elapsed Time: 651 days (93.0 weeks/ 21.3 months)
> --IOC Phase: 129 days (4.2 months)
> --Science Phase: 352 days (11.6 months)
> --Final Calibration Phase: 43 days (1.3 months)
> --Extended Science Phase: 4 days
> --Post Mission Phase: 123 days (17.6 weeks/ 4.0 months)
>Current Orbit #: 9,605 as of 5:30 PM PST
>Spacecraft General Health: Good
>Roll Rate: Normal at 0.4898 rpm (2.04 minutes per revolution)
>Gyro Suspension System (GSS): Gyro #1 digitally suspended; Gyros #2, #3, &
>#4 in analog suspension
>Gyro Spin Rates: ~0 rpm (spinning at spacecraft roll rate)
>Dewar Temperature: ~195 K and rising ~0.6 K/day
>Global Positioning System (GPS) lock: Nominal
>Attitude Control System: Nominal for post-mission operation
>Pointing Error: (XY/Pitch-Yaw Axes) 0.39 degrees RMS;
>Roll Phase (Z Axis) Error: 7.6 degrees RMS
>Telescope Readout: Pointing performance too low to lock onto guide star
>Command & Data Handling (CDH): B-side (backup) computer in control
>Multi-bit errors (MBE): 1 in CCCA Backup computer; 3 in GSS computer
>
>MISSION DIRECTOR'S SUMMARY
>=======================
>On Mission Day 651, the Gravity Probe B vehicle and payload continue to be
>in good health. All active subsystems, including solar arrays/electrical
>power, Experiment Control Unit (ECU), flight computer, star trackers and
>magnetic torque rods, gyro suspension system (GSS), and telescope
>detectors, are performing nominally. We continue to communicate with the
>spacecraft regularly, though less frequently, monitoring the Dewar and
>probe as they continue to warm up, and collecting status data from various
>instruments on-board.
>
>The temperature inside the Dewar has now warmed to ~195 kelvin, and its
>rate of temperature rise has slowed to ~0.6 kelvin per day. The temperature
>inside the Dewar will eventually reach thermal equilibrium with the outside
>temperature of ~0 centigrade (~273 kelvin), but this will occur very
>gradually, over a long period of time.
>
>As a result of the CCCA backup computer re-boot on December 21, 2005, the
>attitude of the spacecraft shifted approximately 90 degrees, so that
>instead of pointing in the direction of its orbit, the spacecraft is now
>pointed broadside or perpendicular to its orbit plane (orbit normal
>orientation). Because we are no longer tacking the Guide Star, IM Pegasi,
>there is no need to maneuver the spacecraft back to its Guide Star
>orientation. Rather, we have been stabilizing the spacecraft in its orbit
>normal orientation, and we are preparing to reduce its roll rate to 0.04
>rpm (25 minutes/revolution) in order to collect sample planet eclipse data
>from the two star trackers on-board. (See today's Mission News story below
>for more information about the planet eclipse data and other sample data we
>will be collecting over the next few weeks.)
>
>Because the spacecraft has been in orbit normal orientation for the past
>month, its two antennae have been oriented less favorably for communication
>with the NASA TDRS (Tracking Data Relay Satellite) system and with the NASA
>ground network tracking stations. Even though the spacecraft's antennae are
>omni-directional, their optimal transmission/reception path spreads out
>like a cone towards the front and rear of spacecraft, and their gain is
>somewhat diminished in the orbit normal orientation. Over the 3-day Martin
>Luther King holiday weekend of 14-16 January, this diminished antenna
>communications link, coupled with reduced weekend/holiday monitoring from
>our Mission Operations center (MOC), triggered a safemode that
>automatically re-boots the on-board CCCA Backup computer if it does not
>receive any commands from our MOC within a 36-hour period. We have since
>recovered from this re-boot.
>
>Another unsurprising consequence of the spacecraft's recent position
>changes, as well as thermal changes in the quartz block where the gyros are
>housed, is that the Gyro Suspension System (GSS) automatically transitioned
>gyros #2, #3, and #4 from digital (highest control) to analog (safe and
>secure) suspension after the computer re-boot two weeks ago. The
>ultra-sensitive GSS interprets spikes in rotor position due to thermal
>stresses during warm-up as excessive gyro motion, and it automatically
>transitions the suspension mode from digital to analog to ensure the safety
>of the rotors. In due course, we will return these three gyros to digital
>suspension.
>
>All of these recent spacecraft behaviors--spotty communication, computer
>re-boots, gyro safekeeping transitions--are the expected results of the
>spacecraft operating outside the limits of its controlled experimental
>environment, along with reduced communications and monitoring from our MOC.
>As we've stated many times in recent status updates, our main focus now is
>analyzing the science data we have collected. However, we will continue to
>perform minimal maintenance on the spacecraft, so that it is ready and
>available for other post-mission experiments, as described in the Mission
>News story below.
>
>GP-B MISSION NEWS--SPACECRAFT READY FOR OTHER EXPERIMENTSŠOR HIBERNATION
>============================================================
>Our GP-B team has now completed all planned, as well as some extended
>post-mission analyses on our spacecraft and its component systems. We are
>in the process of stabilizing the spacecraft in its current orbit normal
>orientation, and the spacecraft is now ready and available for use by other
>scientists to perform various types of experiments. The GP-B spacecraft is
>a state-of-the-art orbiting laboratory, and it has performed
>extraordinarily well throughout the GP-B mission and beyond. While it no
>longer maintains the cryogenic environment necessary for testing the
>geodetic and frame-dragging effects of general relativity, the on-board
>star trackers, magnetometers, and science gyros are still functioning
>perfectly, and they can be used individually or in combination for a number
>of other types of experiments.
>
>Thus, we are actively seeking scientific partners around the world who
>would be interested in using this space borne laboratory to perform
>additional post-mission experiments. To this end, for the next few weeks we
>will be collecting sample data to demonstrate and validate the spacecraft's
>post-mission experimental capabilities in the following five areas.
>
>1. Occlusion of Stars by Planets
>Within the next few days, we will begin the process of slowing down the
>spacecraft's roll rate to 0.04 rpm (25 minutes per revolution). At this
>very slow roll rate, the on-board star trackers can be switched into a more
>sensitive tracking mode, in which the light from stars can be integrated
>over much longer time periods. By observing brightness variations of
>various star systems, the star trackers can detect the presence of orbiting
>planets in those systems.
>
>2. Measuring Residual Drag on the Spacecraft
>The four GP-B science gyroscopes can be used as 3-axis accelerometers that
>are capable of measuring solar pressure and upper atmospheric drag on the
>spacecraft to an accuracy of 5 x 10-12 g.
>
>3. GPS Satellite Accuracy Measurements
>Because the orbit plane of our GP-B spacecraft is very well established, we
>can use the four science gyroscopes as 3-axis accelerometers to determine
>the spacecraft's precise inertial position without GPS reckoning. We can
>then compare this internally-calculated spacecraft position information
>with corresponding position information generated by various GPS satellites
>to determine their level of accuracy.
>
>4. Subtle Aurora Borealis Effects
>Using a combination of the on-board magnetometers, the proton monitor, and
>the four science gyros as accelerometers, we can investigate what happens
>when the upper atmosphere heats up as a result of bombardment by charged
>particles. We can also measure the buffeting effects of the upper
>atmosphere as the spacecraft passes through a region containing charged
>particles.
>
>5. Latitude Axis Gravity Gradient
>As a complement to the GRACE mission that measured gravity gradients along
>a longitudinal axis between a pair of orbiting spacecraft, the GP-B science
>gyros can be used as 3-axis accelerometers to measure latitude-axis
>(cross-orbit) gravity gradients.
>
>As noted earlier, for the next few weeks, we will be collecting sample data
>to illustrate the feasibility and limits of performing all of the above
>experiments with the GP-B spacecraft. Because the spacecraft is already in
>orbit and functioning, funding requirements for such "experiments of
>opportunity" will be minimal. The cost for using the spacecraft to perform
>any or all of these experiments amounts to the cost of a 4-5 person mission
>operations staff over the period of the experiments.
>
>In summary, the GP-B spacecraft has performed exceedingly well to date. It
>has experienced no serious failures, and all the systems required for
>performing post-mission experiments are operational and ready for use. In a
>few weeks, after we have finished collecting the sample data, if there is
>no interest or funding for performing any of these experiments, we will
>place the spacecraft in a safe, hibernation configuration and reduce our
>maintenance monitoring of its health to once a week.
>
>====================================
>NEXT SCHEDULED GP-B UPDATE ON FEBRUARY 28, 2006
>====================================
>Our next regularly scheduled update will be at the end of February. Of
>course, we will send out a timely update if there are any important changes
>in the spacecraft's status, or if noteworthy events occur here at GP-B in
>the meantime.
>
>===================
>PREVIOUS GP-B UPDATES
>===================
>If you wish to read any of our previous updates, our GP-B Web site includes
>a chronological archive of all the updates/highlights (with photos and
>drawings) that we have posted over the past 8 years:
>http://einstein.stanford.edu/highlights/hlindexmain.html
>
>=============================
>OTHER LINKS THAT MAY INTEREST YOU
>=============================
>
>Our GP-B Web site,
><http://einstein.stanford.edu>http://einstein.stanford.edu contains lots of
>information about the Gravity Probe B experiment, general relativity, and
>the amazing technologies that were developed to carry out this experiment.
>
>Visual tour of the GP-B spacecraft and payload from our GP-B Web site:
><http://einstein.stanford.edu/content/vehicle_tour/index.html>http://einstein.stanford.edu/content/vehicle_tour/index.html
>
>PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that
>you can download print out, and assemble:
><http://einstein.stanford.edu/content/paper_model>http://einstein.stanford.edu/content/paper_model.
>
>NASA's Marshall Space Flight Center also has a series of Web pages devoted
>to GP-B: <http://www.gravityprobeb.com>http://www.gravityprobeb.com
>
>Photo, taken through a telescope by Swiss physics teacher and amateur
>astronomer Stefano Sposetti, of GP-B spacecraft in orbit, passing near IM
>Pegasi:
><http://aida.astronomie.info/sposetti>http://aida.astronomie.info/sposetti
>
>
>The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York
>University (Toronto), with contributions from the Observatoire de Paris,
>have been studying the motions of the guide star, IM Pegasi for over a
>decade. To find out more, visit:
><http://www.yorku.ca/bartel/guidestar/>http://www.yorku.ca/bartel/guidestar/
>
>In addition, you'll find information in the Guide Star FAQ on our Web site:
><http://einstein.stanford.edu/content/faqs/faqs.html#guidestar>http://einstein.stanford.edu/content/faqs/faqs.html#guidestar
>and on pages 18-20 of the Gravity Probe B Launch Companion:
><http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf>http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf
>
>Track the GP-B satellite on the Web using NASA's Java-based J-Pass
>satellite tracking application at:
><http://science.nasa.gov/realtime/JPass/>http://science.nasa.gov/realtime/JPass/
>Also, you can track the GP-B satellite on Personal Digital Assistants
>(PDAs) using either the Palm OS or Pocket PC operating systems with
>software from Big Fat Tail Productions:
><http://www.bigfattail.com>http://www.bigfattail.com
>
>The Einstein Exhibition at the Skirball Cultural Center in Los Angeles has
>closed.However, you can visit the American Museum of Natural History's
>virtual Einstein exhibit on the Web at:
><http://www.skirball.org/exhibit/amnh_frame.html>http://www.skirball.org/exhibit/amnh_frame.html
>
>==========================
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>--
>**********************************
>NASA - Stanford - Lockheed Martin
> Gravity Probe B Program
>"Testing Einstein's Universe"
> http://einstein.stanford.edu
>
>Bob Kahn
>Public Affairs Coordinator
>
>Phone: 650-723-2540
>Fax: 650-723-3494
>Email: kahn at relgyro.stanford.edu
>**********************************
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