| Date: | 2004-10-09 01:06:00 | |
| Sender: | Julie Horvath, NASA SLR <Horvath, Julie E. <Julie.Horvath@honeywell-tsi.com>> | |
| Subject: | [SLR-Mail] No. 1268: Gravity Probe-B Status | |
| Author: | Julie Horvath, NASA SLR | |
| Content: | ******************************************************************************** SLR Electronic Mail 2004-10-09 01:06:00 UTC Message No. 1268 ******************************************************************************** Author: Julie Horvath, NASA SLR Subject: Gravity Probe-B Status Dear Colleagues, As you may already be aware, the Gravity Probe-B satellite has proven to be a very difficult target for SLR. This is exactly opposite of our initial estimates of GPB laser acquisition. As of August 27th, the GPB program placed the satellite in ”drag-free” operations. This indicated that there would be no drag or other outside forces on the satellite, and therefore very easy to predict. Unfortunately, the program has admitted (with some urging) that the satellite is not really in ”drag-free” operations, and there are several external forces affecting the satellite´s orbit that they have analyzed and have attempted to correct. During this period, we have tried to improve the quality of the predictions by modeling these constantly changing forces. Because we had varying degrees of success with this, we also requested, and received, fresher (GPB project supplied) GPS derived state vectors, and are currently working with the project on a form of a drag function file for distribution to the systems, to help improve the situation. As of Sept. 27th, the project sent a message indicating that the satellite went into a ”safe mode” and therefore turned off the drag-free mode. They quickly turned it back on, but are using a new method for drag-free and have indicated that acquisition should become easier. Here is the message received from our Stanford GPB representative: Message 9/27/04: ”As of this weekend, GP-B switched to a different gyro for control which should improve our situation quite a bit. From this point on, we expect our semi-major axis to remain relatively fixed (with small oscillations of a couple of meters about a constant value). I think we can go back to our modeling of a drag-free orbit, and hopefully our prediction accuracy will drastically improve.” Unfortunately, we have no control over the project and when they decide to ”fix” things on the spacecraft or the forces on the orbit. Many of these ”fixes” are probably changing things drastically in relation to laser acquisition. It appears that there has been more data in the last week, so we are hopeful that the gyro switch did make a difference in the predictions. If you have any questions, or would like to add any tracking experience to this message please contact the Central Bureau. The GPB project acknowledges the effort that all the stations are making in laser acquisition attempts, and are appreciative of all of your efforts. Best Regards, Julie Horvath HTSI, NASA SLR ILRS Central Bureau julie.horvath@honeywell-tsi.com From: ”Horvath, Julie E.” ******************************************************************************** |