The DSM? What's this all about?

Ah, the Deep Space Maneuver (DSM)! This was the largest maneuver that we would execute until reaching Saturn. (Upon arrival, we'll execute a Saturn Orbit Insertion (SOI) maneuver, placing the spacecraft in orbit around the planet ­ that will be the largest maneuver).

I'm calling the DSM the big tweak because it's the maneuver that is truly required in order to reach Saturn. Under ideal conditions, you could get away without the other maneuvers, but this one really tweaks the trajectory. Without it, the second Venus swingby (V2) would've occurred much later and the Earth swingby would have been significantly altered. If you want to hear more about this, let me know and I'll add it in.

(Please note that I do not officially represent the Cassini project, JPL, or NASA. Any opinion expressed herein is mine)

The Doppler Plots

Like I mentioned before, every maneuver is preceded and followed by a sequence of roll and yaw turns. The DSM had a roughly one-hundred and eighty degree (180°) roll turn followed by a ninety degree (90°) yaw turn. These turns place the rocket engine in an orientation facing the direction of motion, so that firing it slows the spacecraft down. Slowing down this way reduces perihelion, setting up the second Venus swingby. After the maneuver the turn sequence and angles are reversed, returning the spacecraft to its original orientation.

Since the antenna we used for communications during the DSM (called Low Gain Antenna number one (LGA-1)) has a bore-sight along the roll axis, we didn't get to see the doppler shift due to the first roll turn. We did get to see the yaw wind turn, and here it is below.

The "F3" you'll see in the plots is three way doppler. The units are millimeters per second (mm/s). The 17.7814 is a conversion factor between Hertz (Hz) and mm/s.

The plot below shows the beginning of the DSM doppler residuals (actual doppler shift minus predicted doppler shift). The previous plot had no prediction for the shift due to the turn, so you see the total actual shift due to the turn. The plot below includes a predicted shift for the DSM. Why? the DSM makes for such a large shift, that it's hard to get a good look at the features of the plot. Now, I should also mention that the timing of this predict was a few split seconds off, so you see jumps at the beginning and end of the burn. (If any of this confuses you, let me know).

This plot focuses on the initial transient. You see, our rocket motor starts off thrusting a little high and then settles down to a steady-state level. You can see the behavior for yourself.

Here's an overview of the whole maneuver. The little spikes before and after are the yaw wind and unwind turns I described earlier. The hump in the middle is due to force, mass flow rate, and pointing angle variations during the maneuver.

Hmm, let's see that hump a little closer...

This is the end of the burn. The jump is due to a timing error in the predicted maneuver (remember, these are residuals: actual minus predicted). By now, you've probably noticed a signature, viz. an oscillation, in the signal. That oscillation is due to the attitude control system's mismodelling of the rocket motor gimbal system. It's a very small mismodelling, the kind that you'll never be able to get rid of. So, the controller leaves you with a little oscillation; no big deal.

The amazing thing here is that we can see the behavior in a faint signal from a spacecraft as far away as Mars!

Finally, the yaw unwind turn