Another Maneuver?

Yes, another maneuver! And no, we'll never be able to "coast the rest of the way" no one can predict exactly what forces will perturb the trajectory over these years until we reach Saturn, nor can anyone provide a timetable of the exact times of the thruster firings required to maintian the spacecraft orientation. Until someone can accurately foretell the future (down to the fraction-of-a-millimeter-per-second impulsive velocity perturbations) we'll need maneuvers. Talk about job security :)

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

Trajectory Correction Maneuver Number Six (TCM-6) was scheduled to be executed at 8 PM UTC on February 4, 1999 (That's roughly noon Pacific Time). This maneuver was nicknamed "The DSM Cleanup Maneuver" because that's exactly what it did. As you may have read, the DSM was right on in magnitude (burn time) but had a roughly one degree (1 degrees) pointing error (closer to 15 milliradians).

The DSM was a large maneuver and, although it's errors were proportionately small, they really need to be cleaned up if the Venus Swingby is to be on-target.

Maneuver Design

	B-dot-R (km)	B-dot-T (km)	Arrival Time
V2-148 days Est.	8499.532718	-163338.1468142	25-JUN-1999 02:40:43 GMT
Desired Aimpoint	3255.48974	-9758.6688873	24-JUN-1999 20:30:14 GMT
Req'd Correction	-5244.042978	153579.4779269	-(6 hours, 10 mins, 29 secs)
Capability of 1 m/s	4992.6674	14003.026566	50 min

Maneuver Location	V2-140D (L+477d),2/4/1999, 20:00 UTC (12:00 PST, Thursday)
Distance to Sun	1.49 Astronomical Units
Maneuver Strategy	Correct All Encounter Conditions (B-dot-R, B-dot-T, TF)
Total DV magnitude	11.55 m/sec
Burn- DV magnitude	11.53 m/sec
Burn Duration	119.6 sec. (1 min., 59.6 sec.)
Turn Angles	-18.97° (roll), -10.71° (yaw)

Geometry of TCM-6

The geometry of this maneuver can be seen in the figure to the right. This diagram is made over a plane in space spanned by the velocity-correction vector (delta-v) and the Earth-to-spacecraft vector.These two directions are depicted in red. The total velocity increment is 11.55 meters per second (m/s). The component along the Earth-line is 9.61 m/s, due to a thirty-three and seven tenths degrees (33.7 degrees) Earth-look angle. In green are the "Magnitude" and "Pointing" directions.

"S/C" denotes the corner where you could imagine the spacecraft, "Earth" is likewise for Earth.

We model our maenuver execution errors as two independent components: magintude and pointing. Pointing errors are perpendicular to magnitude errors, the latter should be obvious. This figure shows that any error we see along the Earth-line measurement (via the doppler shift of our communication radio signal) could be due to either source. And, if you know your geometry, you can compute, for example, how much of a magnitude error would produce a 7 millimeter per second (mm/s) error in the Earth-line delta-v.

How could you get by without Doppler Plots...

The yaw wind turn. If you look closely, you'll se that it looks like the turn pauses for about 30 seconds and then finishes. Actually, the turn did finish. Don't blink! That's a third turn. We've been able to make an in-flight calibration of the main rocket engine gimbal mounts. This little tweak-turn allows us to point the rocket engine very precisely (and the proof is in the pudding).

The burn!

The unwind tweak turn -- look hard...

The yaw unwind turn.

The software that makes these plots will also estimate the doppler frequency shift due to the maneuver. Here's what it says

The Quick Look Assessment

Believe it or not, we can make a preliminary assessment of the maneuver based on these plots. The question being, did we see the Earth-line doppler shift that we expected? And just how close was it?

Nominal Total Earth-line Delta-V: 9.614 m/s (11.55*cos(33.7 degrees) m/s), 540.7 Hz

Observed Total Earth-line Delta-V: 9.607 m/s +/- 10 mm/s (doppler shift, Xardvarc), 540.3 Hz

Nominal Burn Earth-line Delta-V: 9.600 m/s (11.53*cos(33.7 degrees) m/s), 539.7 Hz

Observed Burn Earth-line Delta-V: 9.579 m/s +/- 10 mm/s (doppler shift, Xardvarc), 538.7 Hz

perturbations accounted for in maneuver design

Yaw wind: 8.9 mm/s (0.5 Hz) doppler shift

Yaw unwind 8.9 mm/s (0.5 Hz) doppler shift

perturbations not accounted for in maneuver design

Deadband tightening: 2.7 mm/s (0.15 Hz) doppler shift

Roll wind turn: 0.4 mm/s (0.02 Hz) doppler shift

tweak turn: 7.1 mm/s (0.4 Hz, 0.2Hz twice) doppler shift

By comparing the nominal and observed delta-v's, you can see that this maneuver was dead on. Everyone on Cassini was happy today!