本周引力探测器B没有多大的进展，它的进程比我们想象的要慢。在本周加速2号陀螺到87Hz的时候，泄漏的氦气导致4号陀螺速度从105.8降到91Hz。为解决类似问题，加速1、3号陀螺时将让陀螺比2、4号陀螺再靠近加速通道30%，而陀螺与悬浮电极和腔体之间仍然有足够的空隙。陀螺必须有足够的速度才能保证实验有较好的信噪比。

卫星高度平移控制系统仍然在进展之中，卫星已经定位到远方的导星，转速为0.52转/分。

漫长的等待。

WEEKLY HIGHLIGHTS FOR 23 JULY 2004:
GRAVITY PROBE B MISSION UPDATE
 
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.