| Date: | 1997-08-07 18:00:00 | |
| Sender: | Bill Purdy, Naval Research Laboratory <DSGJEH@cdslr3.atsc.allied.com> | |
| Subject: | [SLR-Mail] No. 76: TiPS Newsletter | |
| Author: | Bill Purdy, Naval Research Laboratory | |
| Content: | ******************************************************************************** SLR Electronic Mail 1997-08-07 18:00:00 UTC Message No. 76 ******************************************************************************** Author: Bill Purdy, Naval Research Laboratory Subject: TiPS Newsletter Hello, Things are still going well with TiPS. TiPS is now over 13 months old. The libration amplitudes are still stable at about 7 to 8 degrees in plane and 7 degrees cross plane. We have two newsworthy items: 1)The dynamics studies that we have had done, and 2) the use of radar data. DYNAMICS STUDIES We enlisted the support of several tether experts to study the long term dynamics of TiPS. In particular they were tasked to try to make analytical simulations that matched the observed behavior of TiPS. Their simulations came remarkably close to the observed behavior. We found that the cross plane libration only damps when the in plane libration is at high amplitudes. The in plane libration, however will continue to damp no matter what the cross plane libration amplitude is. So, there are two distinct phases of libration damping. The early phase is marked by slow damping of the in plane libration with rapid damping of the cross plane libration. The later phase is marked by steady in plane damping wit h no cross plane damping. This two phase damping matches what we observed. The cross plane damping had stopped within two months of the deployment while the in plane libration continued damping through the first 8 months. The lone disagreement between the theory and observations is about the transition between the two damping phases. unfortunately we didn´t have enough data during this time period to resolve this conflict. The source of the damping was energy dissipation within the tether. An interesting thing that came out of the study was that the actual type of dissipative mechanism within the tether didn´t matter. The analysts got the same behavior from their simulations whether they used friction damping, viscoelastic damping or a generalized dissipative factor. RADAR DATA We have had continuing success supplementing our SLR data with radar data from the Altair station at Kwajelein Atoll. Radar data is significantly less accurate than SLR data, however it is much easier to obtain. The radar will typically be able to track every tasked pass almost from horizon to horizon. This gives us two things we don´t get from SLR: 1) High quantities of data, and 2) continuous data for over 15 minutes. Item one makes our job of accurately determining the orbit much easier, Item two enables us to see one quarter of a full libration period which helps in figuring out the amplitudes. By contrast SLR provides highly accurate data but over much shorter periods, with 5 minutes being fairly typical. Thus we have the age-old quantity versus quality debate. Well, the simple answer to that question is... both. We have tasked the radar only during good terminator periods and have been using both data sources to produce very regular tether solutions and good targeting data for the SLR sites. We will do some tests where we retroactively analyze only the radar data to see how solutions obtained without SLR compare to those produced with the combination of SLR and radar. NOTES FOR FUTURE TETHER APPLICATIONS For future tether system designers and users they should note the following things. Tethers can be made survivable Tethers are stable Tethers librations are lightly damped when the tether is constructed from compliant, lossy materials - The TiPS tether is very soft and spongy at the on orbit tension You will probably want to actively set the libration amplitudes to their operational value Once you set the libration amplitude, you will not need to adjust it very often The easiest way to know the amplitude and phase of the libration is by measuring it on board one or both of the end bodies if this is practical You can determine the libration from the ground using radar and SLR Remember to check out our website at http://hyperspace.nrl.navy.mil/TiPS/. We are continuing to update the website. These things we have learned from TiPS are entirely dependent on the many people whose hard work and intelligence made the experiment, the flight hardware and the data gathering and analysis possible. Once again, thanks, Bill Purdy From: DSGJEH@cdslr3.atsc.allied.com ******************************************************************************** |