From: LARRY KLAES (ljk4_at_msn.com)
Date: Sat Jul 24 2004 - 10:30:07 PDT
----- Original Message -----
From: Bob Kahn<mailto:kahn_at_relgyro.stanford.edu>
To: gpb-update_at_lists.Stanford.EDU<mailto:gpb-update_at_lists.Stanford.EDU>
Sent: Friday, July 23, 2004 10:11 PM
Subject: Gravity Probe B Update -- July 23, 2004
=========================================
GRAVITY PROBE B MISSION UPDATE -- July 23, 2004
=========================================
Please Note: During the Initialization & Orbit Checkout (IOC) Phase
of the GP-B mission, we update our Web site and send out this email
update once a week (usually on Thursday or Friday) to keep you
apprised of our progress. From time to time, we may send out extra
updates, as warranted by mission events.
One day #94 of the mission, Gravity Probe B is poised to enter the
home stretch of the Initialization and Orbit Checkout (IOC) phase of
the mission. The spacecraft is in excellent health, and all
subsystems are continuing to perform well. All four gyros are
digitally suspended, with gyros #2 and #4 spinning at science mission
speed-greater than 80Hz (4,800 rpm). Gyros #1 and #3 are spinning at
less than 1.5 Hz (90 rpm) and are ready for full-speed spin-up next
week. Fine-tuning of the Attitude and Translation Control system
(ATC) is still in progress, and the ATC is performing well. The
spacecraft's roll rate is 0.52 rpm, and the science telescope is
locked onto the guide star, IM Pegasi.
Last Friday, the full-speed spin-up of gyro#2 went smoothly, with a
final spin rate of 87 Hz (5,220 rpm). Helium gas leakage from the
spin-up of gyro #2 caused gyro #4 to slow down from 105.8 Hz (6,348
rpm) to 91 Hz (5,460 rpm). We had hoped that gyro #2 would achieve a
spin rate above 100 Hz (6,000 rpm), with less leakage effect on gyro
#4. Thus, rather than spinning up gyros #1 and #3 as originally
planned, we spent the past week doing analysis and running tests-both
on the spacecraft and here at Stanford-in order to ensure that the
upcoming spin-up of gyros #1 and #3 will result in higher speeds,
with less leakage effect on the remaining gyros.
The spin rate of the gyros during the Science Phase of the mission
affects the signal-to-noise ratio in the SQUID readouts of the
experimental data. The noise level is quite small, but constant. The
higher the gyro spin rate, the larger the London moment (magnetic
field created by a spinning superconductor), and thus, the greater
the signal-to-noise ratio. In ground testing prior to launch, we
determined that a spin rate of 80 Hz (4,800 rpm) or greater for each
gyro would provide a good signal-to-noise ratio for the science
mission. However, the threshold of 80 Hz (4,800) rpm is not a hard
and fast limit, so if the final spin rate of one or more gyros falls
slightly below this value, this will not appreciably compromise the
science data.
One way to potentially increase the spin-up rate of the remaining two
gyros, while reducing the amount of helium gas leakage during spin-up
is to use the Gyro Suspension System to position the gyro rotors
closer to the spin-up channel in the gyro housing. Tests and analysis
performed this past week indicate that we can move the rotors of
gyros #1 and #3 up to 30% closer to the spin-up channels than gyros
#2 and #4, and still have a safe margin of clearance from the
suspension electrodes and the gyro housings. We have also determined
that opening a second exhaust valve during spin-up may help to reduce
the pressure in the probe caused by helium leakage, thereby reducing
the spin-down effects on the remaining gyros. Both of these changes
will be implemented in the spin-up of gyros #1 and #3 next week.
Also, this past week, we continued fine-tuning the drag-free software
used by the Attitude and Translation Control system (ATC) to optimize
its performance at the current and final spacecraft roll rate of 0.52
rpm. Tests from parameter changes we made to the ATC system indicate
that we have reduced the time it takes to re-lock onto the guide star
from as much as 15 minutes to less than 2 minutes.
The spacecraft is being controlled from the Gravity Probe B Mission
Operations Center, located here at Stanford University. The
Stanford-NASA/MSFC-Lockheed Martin operations team is continuing to
perform superbly.
TOUR THE GP-B SPACECRAFT & PAYLOAD ON OUR WEB SITE
BUILD A PAPER MODEL OF THE GP-B SPACECRAFT
Both versions include two pages of instructions and six pages of
LEARN MORE ABOUT OUR GUIDE STAR, IM PEGASI
In addition, you'll find information in the Guide Star FAQ on our Web
LEARN MORE ABOUT GRAVITY PROBE B & IOC
If you are interested in following the IOC procedures more closely,
TRACK THE GP-B SATELLITE ON THE WEB OR WITH YOUR PDA
Also, Big Fat Tail Productions (http://www.bigfattail.com FOLLOW THE GP-B MISSION ELSEWHERE ON THE INTERNET
NASA's Marshall Space Flight Center Gravity Probe B.com Web page
ABOUT THE GPB-UPDATE EMAIL LIST
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: Sat Jul 24 2004 - 11:59:45 PDT
============================================
You can take a visual tour of the GP-B spacecraft and payload by
using the navigation menu along the left edge of our Web pages.
Choose the second item, The Engineering Story, and from that
sub-menu, choose Visual Tour, or simply enter the following URL into
your Web browser:
http://einstein.stanford.edu/content/vehicle_tour/index.html
=====================================
We have created a 1/20 scale, paper model of the GP-B spacecraft that
you can download as a PDF file, print out, and assemble. There are
two versions of the PDF file-a 6 MB high-quality version and a 2.2 MB
standard-quality version. The only visible difference between the
versions is that the colors are not as bright and saturated in the
standard version, but it will download in much less time for people
with low-speed Internet connections. The URL for these PDF files is:
http://einstein.stanford.edu/p_model
images to cut out and assemble. You'll need scissors, an Exacto
knife, a straight edge, glue (glue sticks and hot glue guns work
well), Scotch tape, two 9.5" long, 1/8" diameter wooden dowels
(shish-kabob skewers work well) about 3-5 hours, and patience to
assemble the model. For best results, we suggest printing the six
cut-out pages on heavyweight paper. Although it is a bit hard to work
with, glossy photo paper will yield a very realistic looking model.
=====================================
The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York
University (Toronto) are studying the guide star to provide crucial
measurements of its motion relative to far away quasars. These
measurements are needed to relate the tiny changes in the gyroscopes'
spin direction to the distant universe, so that general relativity
can be tested. Learn more about these measurements by going to the IM
Pegasi web site: http://www.yorku.ca/bartel/guidestar/
site: 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
==================================
Our GP-B Web site, http://einstein.stanford.edu
information about the Gravity Probe B experiment, general relativity,
and the amazing technologies that were developed to carry out this
experiment.
you'll find a schedule and description of them on pages 12-14 of the
"Gravity Probe B Launch Companion." You can download a copy of this
document, in Adobe Acrobat PDF format, from
http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf
=============================================
You can 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/
created shareware satellite tracking software that runs on Personal
Digital Assistants (PDAs) using either the Palm OS or Pocket PC
operating systems. Like all PDA shareware, you can try out either
version for free. If you like the software and decide to use it, Big
Fat Tail asks that you pay a nominal shareware fee.
==============================================
In addition to our Stanford GP-B Web site, the ELV Missions Virtual
Launch Center Web page on the John F. Kennedy Space Center Web site
has information and several streaming video clips covering the GP-B
mission. The URL is: http://www.ksc.nasa.gov/elvnew/gpb/vlcc.htm
(You can view these video clips free of charge, but you will need to
have either the Real Media Player or Windows Media Player installed
on your computer to view them.)
(http://www.gravityprobeb.com
GP-B launch, including photos of the spacecraft separation, as well
as other information about Gravity Probe B.
============================
The email distribution list for this GP-B Weekly Highlights update is
maintained on the Stanford University email lists server. These email
updates are automatically sent to GP-B program internal email
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--
**********************************
NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu