TCM-11: On the Mark...

(Please note that I do not officially represent the Cassini project, JPL, or NASA. Any opinion expressed herein is mine) Feel free to email me with any comments about this page.

Design

Cassini's Trajectory Control Maneuver Number 11 (TCM-11) was executed on 
Monday, August 2nd, 1999 at 21:30 UTC.

First, let's mention the design:

Maneuver Location:           8/2/1999, 21:30 GMT, E-15d,(8/2/1999 14:30 PDT)
Distance to Sun:             0.88 AU (Astronomical Units)
Maneuver Strategy:           Correct Three B-Plane Conditions 
Engine Type:                 Main Engine
Turn Type:                   Normal (.25 degrees per second) 
Total Delta-V magnitude:     36.309 meters per second
Turn Delta-V magnitude:      21.4 millimeters per second
Burn Duration:               389 seconds
Turn Angles:                 roll: -171.70 degrees; yaw: -64.86 degrees

What does all that mean? First of all, these are design parameters for the maneuver. The next section will deal with the actual execution.

Earth is roughly 1 AU from the Sun. Therefore, the Cassini spacecraft was about 88 % as far from the Sun as we are when it executed this maneuver. By the so-called corrections to the B-plane conditions, we mean that our target is a certain Earth-relative position and a particular time for closest approach.

Delta-v refers to the change in velocity made by the maneuver. Velocities are vectors, they have magnitude and direction. We desire the total delta-v, but to attain the correct direction, we must roll the spacecraft about its long axis (the Z axis) and then yaw (turn) it about one of the lateral axes (the Y axis). We must use the RCS thrusters to perform the turns -- these thrusters also change the spacecraft's velocity. We therefore have turn delta-v and a separate burn delta-v.

Execution

The green dots (circles) in the plot, below, represent actual measurements of Cassini's speed towards Earth. Unfortunately, the orientation required for the maneuver places the antenna we're currently using (Low Gain Antenna #1, LGA-1) on the far side of the spacecraft, causing a brief communications black-out.

The plot above is designed to show only the effects of events related to the maneuver. Before the maneuver starts, the measurement residuals (predictions subtracted from actual measurements) show an average of zero -- the trajectory is as predicted. Now, these predictions purposely omit maneuver-related events, therefore, the first such event, deadband tightening, causes a change in residuals. (It looks like a stair-step on the left side of the plot).

After a several-minute pause, the spacecraft begins turning. This is the first turn of the sequence: the roll turn (that we talked about above). As the spacecraft turns, the antenna moves to the far side of the spacecraft. As It's signal gets weaker and weaker, our measurements start to become less precise (a.k.a. noisy measurements). Finally, the signal is too weak to receive...

Of course, this was no surprise, the cognizant telecommunications engineer, Randy, predicted this situation some time ago. He also assured us that the signal would come back after the burn, during the process of returning the spacecraft to its pre-maneuver orientation.

And he was right! Again! The plot above shows the data we received during what we call the unwind roll turn. To return the spacecraft to its pre-maneuver orientation, we simply unwind the roll and yaw turns -- we perform them in reverse. Pretty slick, huh? Well, maybe not the most sophisticated approach, but simple is often better.

What I haven't mentioned before was the scale of this plot. If you're not famaliar with Doppler shift, skip this paragraph :) We expected a 71.8 Hertz shift in the carrier signal. We instead received about 71.07 Hertz. A 1 Hertz shift represents about 17.78 millimeters per second change in speed.

In order to know if that's good or not, you need to know that the angle between the delta-v (velocity change) and the line from Cassini to Earth (a.k.a Earth line-of-sight) was about 92 degrees. All in all, we see in this data indicates a 0.36 milliradian (or 0.021 degrees) error in orienting the spacecraft. I call that hitting the mark :)

Quick Look Results

About 4 hours after each maneuver, a quick-look is presented. Here's a summary of what was reported:

Reconstruction