On Sunday, November 9, at roughly 8pm Greenwich Mean Time (GMT), Cassini executed a wonderfully nominal maneuver, viz. Trajectory Correction Maneuver #1 (TCM-1), starting it on it's way to Saturn. 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.

A rendition of where Cassini is in space now is available from SPACE.

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

Anyway, without giving a complete treatise on interplanetary navigation (which, if you can find anywhere, I'd love to know) I present the following TCM-1 data. This data represents one of the few ways that we can observe the spacecraft in action. Actually, the Deep Space Network (DSN) does the observation; in particular, we're looking at changes in speed (specifically, changes in velocity along a line from Earth to Cassini). And quite appropriately so since maneuvers like TCM-1 alter Cassini's path by changing its velocity (delta-v, a change in speed and direction of travel).

Cassini's main engine (MEA right now), a liquid rocket motor, was used for the TCM-1 burn. In a nutshell, by choosing the length of time to fire the engine we can specify the change in speed. By choosing what direction to orient the spacecraft while MEA is on, we can specify the new direction of travel. Of course, all this is complicated by the fact that objects in space don't generally travel in straight lines; they're really tracing out conic sections, but I digress :)

Now, in order to execute such maneuver, we have to first rotate the spacecraft to the burn attitude. This is accomplished using the RCS Thruster Clusters. You can see the real measurements of this in the image below (a larger, annotated image is available).

These measurements are doppler measurements, they show the frequency shift due to changes in the antenna's speed. (Since the antenna stays with spacecraft, this should also give the spacecrafts change in speed; however, the antenna rotates when we rotate the spacecraft, so you see some interesting patterns in the plot. How a given initial orientation & rotation rate of the spacecraft determines the pattern in the plot is left as an excercise for the reader :)

Moving along, once we're sure we have the right orientation, we fire the engine for the specified time. The resulting change in speed can be seen in the image below. (a larger, annotated image is available)

Once the burn is complete, we rotate the spacecraft back to its nominal, required, orientation (HGA to the Sun, LGA2 towards Earth). (a larger, annotated image is available)

What's amazing about all this, the fact that this is my job notwithstanding, is that the DSN gives such precise measurements. Look at how closely the data points are clustered during any particular time interval! Rotating the spacecraft causes an overall change in speed of only nine (9) millimeters per second, yet every detail of the turn is so clear in the plots! In case you're curious, this data was taken on X-band.

You may not find this as exciting as I do, but I somehow feel better now that I've made this information available. Finally, you might be curious about the software that produced these plots. It's called ARGOS (The document describing it requires Acrobat reader) and it's a LabView Virtual Instrument.

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.