After extensive discussions and evaluation of impacts by TRMM Science Team members and spacecraft engineering support personnel, and after approvals by NASDA and NASA management, the decision was made to move the average operating altitude for TRMM from 350 kilometers to 402.5 kilometers. This will significantly extend the mission lifetime for TRMM. The schedule for the boost segments is shown below:
| Burn Pair Number | Burn Start Date | Burn Start Times | Mean Altitude After Burn Pair (km) |
|---|---|---|---|
| 1 | 8/7/2001 completed |
14:54 15:40 |
360 |
| 2 | 8/8/2001 completed |
16:50 17:36 |
372 |
| 3 | 8/9/2001 completed |
15:47 16:33 |
383 |
| 4 | 8/21/2001 completed |
14:24 15:10 |
393 |
| 5 | 8/22/2001 completed |
18:04 18:50 |
402 |
| 6 (Fine Adjust) | 8/24/2001 completed |
13:42 14:13 |
403 |
The first three burn pairs were completed nominally, bringing TRMM more than half way to the new operating altitude, however two events delayed the further boosts. First, a projected conjuction of the TRMM orbit to uncomfortable proximity to the International Space Station delayed the fourth boost. Then, after holding around the 383 kilometer altitude for a couple days, an anomaly occurred induced by Sun interference in the horizon sensor which caused the spacecraft to exceed certain attitude flight data checks and to enter sun acquistion mode and shut down the instruments. Engineers spent some time carefully analyzing the anomaly and alternate control options before the spacecraft was commanded back to normal Earth pointing. On Thursday August 16, while still in sun acquisition mode, engineers enabled and checked out a contingency kalman filter control option which does not rely on the horizon sensor. On Friday August 17, this kalman filter mode was used in controlling the spacecraft back to Earth pointing mode, and the science instruments were turned back on. On Monday, August 20, a planned orbit maneuver was cancelled to avoid a close conjuction with a piece of space debris. The fourth burn pair was completed on August 21. The final large burn pair was completed on Wednesday August 22. Each burn in burn pairs one through four were approximately 175 seconds in length. The two burns in burn pair five were approximately 150 seconds in length.
The fifth burn left TRMM just below the target altitude, at about 401.7 kilometers average altitude, and with a slightly smaller eccentricity (~0.00042) than the target (0.00054). The two burns in the final "fine adjust" burn pair were short (about 30 seconds), bringing the spacecraft close to the top of the box for the target orbit parameters (see below). The orbit was expected to take at least a week to decay to the bottom of the box at the new altitude.
It is possible that further adjustments to the target orbit parameters will be made after this boost is completed and the PR instrument performance at this new altitude is evaluated. However, nominally the new frozen orbit box parameters will be as follows:
Predicted Ephemeris Accuracy During Boost
Users of certain TSDIS products should have noted that the predicted arc of our ephemeris files had limited accuracy during this boost period. Our daily ephemeris file was delivered late in the day during the orbit boost period.
The ephem file during the boost period included:
Overflight Predictions
The TRMM Overflight Finder was not reliable for any future orbits
during the boost period, and that web
site posted a notice when predictions could be used again.
More Information
More information about the TRMM altitude change will be added at this site as it becomes available.
Questions about the information on this page can be directed to
Steve Bilanow.