On Wednesday, February 25, 1998 8pm Greenwich Mean Time (GMT), Cassini executed Trajectory Correction Maneuver #2 (TCM-2), getting ready for it's first Venus swingby. Now, I realize the information I give below is of rather limited appeal, but I think the excessively curious should be rewarded :)

Please note that this is not an official JPL web-page.

There are three topics here:

• Planning TCM-2
• Results/Data from the Execution of TCM-2
• Upcoming Venus Flyby

Planning TCM-2

We started planning TCM-2 right after TCM-1. Really, it's been in the planning stages since well before launch. After the orbit determination (OD) and reconstruction effort (figuring out what change in velocity was really applied by TCM-1 and where we're going because of it), we finally started to see what this maneuver was going to look like. The informal post-TCM-1 OD solution, named LP47D, indicated that a 60 mm/s to 90 mm/s (yes, that's millimeters per second) maneuver would be required. More recently, the LP88D OD solution indicates that a 140 mm/s to 180 mm/s maneuver is needed.

I must admit that I'm continually amazed at both the accuracy and precision that JPL works with. And, the accuracy off these estimates gets better as we get more data. (I'd point you to our DSN-tracking schedule, but I don't know if it's on-line). Right now, we get data (tracking passes) about once-a-week. We'll start getting daily passes around February 9.

Now, why would we want to execute such a small maneuver?? First of all, a 160 mm/s maneuver on February 25 will make an 875 km (525 mile) difference in where the spacecraft flys by Venus on April 26.  That can be the difference between a safe flyby and a Venus-impacter! Second, the earlier a maneuver is performed, the greater influence on trajectory it has, and, therefore, the smaller it generally needs to be; therefore, it makes sense to correct the trajectory early instead of waiting until TCM-3 (April 8) or TCM-4 (May 14). (TCM-4 is after the flyby; flybys magnify trajectory errors, so waiting until TCM-4 to make this correction is a pretty bad idea.)

Cassini has two propulsion systems, RCS and the Main Engine. Which will be used for this maneuver? It's a small maneuver (less than 0.7 m/s) so we'll use the RCS system.

(February 4, 1998) According to the latest orbit determination solution, noted for Venus-1 minus 88 days or V1M88D, TCM-2 will be a 169 mm/s maneuver. To put that into English units: we're changing the spacecraft's speed by about 0.38 miles per hour! I'll wager that you can't maintain that kind of correction in your car, no matter how much you paid for it :)

On February 2nd, I gave Navigation's presentation at the TCM-2 strategy meeting. We (navigation) suggested that TCM-2 should be executed as planned; however, executing TCM-2 may preclude the execution of TCM-3. Why? We can predict, statistically, the execution errors that will occur from the execution of TCM-2. We can also predict how much of a velocity correction will be needed from TCM-3, statistically, to correct those errors. As of the V1M88D solution, TCM-3 would only be about 60 mm/s, if we executed it. That's really not much at all.

That's not the end of that story, because another important question arises. How much bigger will TCM-4 be if we skip TCM-3? Again, not much. Using V1M88D, TCM-4 would grow from 0.4 m/s to 0.6 m/s. That's an extra 50%, but we see it as only a 0.2 m/s cost. Again, that's not much.

The maneuver kick-off meeting is scheduled for February 18th. At that meeting, the final design for TCM-2 will be presented. The sequence (of computer instructions for the spacecraft) will be finalized and uploaded shortly thereafter.

As an added bonus, I'm giving you a picture of the TCM-2 B-plane! Hatched in red, you see a portion of the limb of Venus. The sizes of the ellipses represent the sizes of the navigation uncertainty around each point. If you have any questions this stuff, feel free to email me.

When you look at this plot, imagine that your sitting on top of the spacecraft as it approaches Venus.  This is about as fancy as my graphic-design gets :)  The ovals (ellipses) in the lower left show where Cassini was headed before the maneuver.  The arrow shows the shift in the trajectory due to TCM-2.  At the head of the arrow, you see our predictions for where TCM-2 will take us.
 

Results/Data from the Execution of TCM-2

(10-April-1998) I intend to write up more about TCM-2, but until I do, just know that these plots are analgous to what I've shown my TCM-1 page.

First, the roll & yaw turns:

 

The plot on the left is 3-way doppler. On the right, you see 2-way doppler. I want to show you both of these so you can see the dramatic improvement that 3-way is over 2-way. The improvement I'm referring to is the variation in the data points. The green line in each plot is a moving average of 20 points. The yellow circles are the data points. The 2-way doppler data points are much more spread out than the 3-way.

Unfortunately, after these turns, we lost lock and did not gain it back until the maneuver was over. Why did we lose lock? Because, in order to perform the maneuver, the spacecraft has to rotate to a specific orientation. The orientation required for TCM-2 put the spacecraft's antenna in "a bad place" for telemetry...

The next plot shows all the data points right up to loss of lock:

We didn't get lock back until after the burn, but we can still see the effect of the maneuver in the so-called doppler shift.

Venus-1 Flyby

A rendition of where Cassini is now is available from SPACE.  On the other hand, you may be interested in checking out Venus as seen by Cassini during the flyby or now. Just put today's date and desired picture size into the form below, and SPACE will generate an image for you.

If you're looking for results from the Venus-1 swingby, you've got them!

The Venus-1 swingby was slated for April 26, 1998 at 1:44 PM UTC (already entered below). The swingby distance (closest-approach) was planned to be about 6,400 km, or a Venus-relative altitude of 337 km (about 200 miles). However, I don't think they loaded SPACE with the latest trajectory; check out the Venus-view and range on April 26, 1998 at 1:00 PM UTC).

(By the way, Feel free to send me e-mail about this page!) 

Again, feel free to send me e-mail about this page. While you're out and about, visit my home-page. Finally, please realize that this is not an official JPL web-page.