| Date: | 1996-10-31 10:00:00 | |
| Sender: | Bill Purdy <[Mailed EDC <edc@dgfi.badw-muenchen.de>]> | |
| Subject: | [SLR-Mail] No. 29: TiPS Newsletter 10/30/96 | |
| Author: | Bill Purdy | |
| Content: | ******************************************************************************** SLR Electronic Mail 1996-10-31 10:00:00 UTC Message No. 29 ******************************************************************************** Author: Bill Purdy Subject: TiPS Newsletter 10/30/96 Hello again, Its been a while since my last newsletter, however I have a lot of good stuff to report. We made significant strides forward in the past month and made good use of our week of copious terminator observation opportunities. We are now getting regular solutions for the in plane tether motion based on analysis of the SLR (Satellite Laser Ranging) data. We are giving this information to the SLR network to improve their targeting. This has helped them get some passes without terminator assist but not as much as we would like. We are continuing to work towards our goal of easy acquisition of TiPS with lasers targeted by our solutions. The out of plane tether motion is very difficult to obtain from the SLR data, however we´ve developed methods for obtaining this data from visual observations of TiPS. LIBRATION DATA TiPS is presently librating with a amplitude of about 10 to 15 degrees in-plane and 5-10 degrees out of plane. Initially the libration was 30 to 50 degrees in plane and 15 to 30 degrees out of plane, in other words, it was really moving. We´ve seen a steady downward trend in our solutions for the libration amplitude. We don´t yet have enough data to say whether or not this is damping, this will take more time and more data points. I leave you the reader to make your own interpretations. Our method for analyzing the SLR data and several of our solutions in late September was confirmed by visual observations of the tether state from video taken at the Air Force Maui Optical Site (AMOS). I´ll be writing more on the visual solutions in the next paragraph. We´re now working on methods to assess the accuracy of a given SLR solution. The accuracy improves when there are: 1) lots of data, 2) from lots of sites, 3) for both end bodies , 4) over a several day long time span, and 5) when we have a long time to analyze the data. Each solution has its own accuracy, which we know qualitatively and are working towards estimating the quantitative accuracy of a solution. In general our solutions early in the mission are accurate to +/- 10 degrees or so, and our solutions recently are accurate to +/-5 degrees. Obviously some solutions are better than others. We know that the amplitude of the libration will vary from cycle to cycle since the in plane and out of plane motions are coupled. When we get a better handle on the accuracy of the solutions we´ll be publishing a history of the solutions and their accuracy on our webpage. VISUAL SOLUTIONS We´ve developed a method to determine libration angles from visual observations of the tether. We compare an observed orientation of the tether relative to the velocity or the nadir vector with a simulation of how that observation would a appear for a given pair of in-plane and out of plane libration angles. We then adjust the libration angles in the simulation until it matches the observation. We compare all possible solutions for several observations within a single pass and determine which of these solutions is/could be valid. With this method we can determine the angles of the tether for certain times within the pass and we can often determine the motion of the tether. For example if your viewing line to the tether is perpendicular to the plane of the orbit, then only in plane libration is apparent to you and can be measured accurately. Similarly if your viewing line to the tether is in the plane of the orbit, then only out of plane libration is apparent to you and can be measured accurately. These observations can be as accurat e as +/-3 degrees. We are going to start making regular use of these visual solutions. AMOS will begin taking daily observations of TiPS next week. AMOS will be able to observe the tether day or night whether or not the tether is illuminated. This will be an excellent complementary data source for the SLR observations. That is because it is easiest to make measurements of the out of plane libration from the visual data, while it is far easier to observe in plane libration than out of plane libration with the SLR data. The reasons for the relative ease or difficulty of obtaining a solution with a given method are too difficult to explain without pictures and discussion, so I will not even try here. If you´re curious get in touch with me and we can talk about it. COMPARISON OF SLR AND VISUAL SOLUTIONS We make comparisons between the predicted in plane libration angle from the SLR solution with the libration angle observed visually at the same time as the prediction. As often as not these two independent approaches agree to within 1 degree. We have always had agreement to within 3 degrees. A nice thing about this correlation is that it has been done in a double blind manner, that is each of the two solutions is obtained independently with no communication between the two analysts. We are obviously very happy with this correlation. You´ll note that the repeated correlation shows the validity of both methods. ORBITAL ELEMENTS We are now publishing the TiPS orbital elements on the website. This is done every day. The website has a list of all elements sets dating back to the deployment. OTHER FINDINGS The observations we took at AMOS in late September showed that the bow has decreased significantly. In fact the bow is barely visible now. We are now using an effective tether length of 20 meters shorter than nominal in our SLR solutions, while earlier in the mission we were using an effective tether length of 80 meters shorter than nominal. This is a good way of characterizing the magnitude and effect of the bow. We also determined that the bow is rotating in a skip rope mode. This rotation takes place with a period of between 1 and 24 hours. We have insufficient data to determine the period of rotation other than to bound it. We took high resolution images of the tether with 5 to 10 meters of tether shown per pixel. These images showed that the tether was locally straight and taut. There were no bends or discontinuities seen in about 10 high resolution surveys of the tether. We´ve seen that the spin of the end bodies has slowed to less than 1 rpm from their original 4 rpm spin at deployment. This is not a big finding, but interesting data. PRESENT ACTIVITIES We are now working primarily on three thrusts: 1) Putting out daily SLR tether solutions, 2) integrating radar data with SLR data for our numerical tether analysis, and 3) making the visual solutions from data taken at AMOS a regular data source. WEBSITE I recommend that you check our Website periodically. We´ll be posting more data, images and papers on there. WEBSITE ADDRESS http://hyperspace.nrl.navy.mil/tips VIEWING As for viewing TiPS yourselves, it is pretty difficult. The people who have seen it are fairly good amateur astronomers using star charts, the satellite ephemeris and good binoculars. For dummies like me it is nearly impossible. There should be another TiPS article in next week´s Aviation Week so keep your eyes peeled. I´ll be writing this newsletter more or less monthly now, since the pace of things is getting more routine. See you later, Bill Purdy bpurdy@space.nrl.navy.mil [Mailed From: EDC ******************************************************************************** |