1. The Deorbiting of GOCE – A Spacecraft Operations Perspective
C. Steiger, M. Romanazzo, P.P. Emanuelli, R. Floberghagen, M. Fehringer
5th GOCE User Workshop, 27th Nov 2014, UNESCO, Paris

2. GRAVITY FIELD AND STEADY-STATE OCEAN CIRCULATION EXPLORER
1700 days of flight (launch 17/03/2009, re-entry 11/11/2013)
Measuring gravity: S/C designed for drag environment in very low Earth orbit
“Drag-free Mode” for science operations to cancel out non-gravitational forces (air drag)
4.8m×2.4m, weight 1050kg

3. MISSION PROFILE 2009–2013 Low orbit operations campaign Commissioning
“Routine” mission at 260 km mean altitude
21/10/2013 fuel depletion
Mean altitude [km]
11/11/2013 re-entry

4. OPERATIONS IN DRAG-FREE MODE –GEOMAGNETIC STORM AT 229 KM

5. RE-ENTRY OPERATIONS OVERVIEW
Rapid orbital decay once fuel for ion engine is exhausted
S/C remains functional, but re-entry can’t be controlled
About 40 pieces of debris (total mass 250 kg) to impact ground
“[..] Approach is to operate GOCE as long as possible in the decay phase, i.e. up to when [..] the performance of S/C or G/S does not allow to continue.”
[GOCE Deorbiting Operations Plan]
Preparations and special measures:
IADC re-entry campaign; close coordination with ESOC Space Debris Office
S/C and G/S performance evaluations (e.g. orbit prediction accuracy)
Flight control team double shift, 2 orbit determinations runs/day, ground station engineering support, extended station coverage..
Extreme drag below 180km was expected to lead to an end of operations, yet..
S/C and G/S functioned until the end in conditions far outside the specs
Last contact only 1.5h before re-entry on 11th Nov 00:16 UTC

6. ALTITUDE PROFILE

7. Along track drag force [N]
ATMOSPHERIC DRAG
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Drag levels on 10th Nov, last day before re-entry
Last contact with S/C at 10th Nov 22:43 UTC
Along track drag force [N]
00: : : : : : :00

8. TIME OFFSET VALUE (TOV) EVOLUTION
Time Offset Value TOV [sec]: difference between expected and actual time of signal acquisition = measure for along track orbit error
TOV within ±2 sec: S/C within antenna beam, automatic signal acquisition

9. S/C ATTITUDE CONTROL PERFORMANCE
View from sun
Attitude control with magnetic torquers (weak actuator)
S/C designed to be aerodynamically stable
S/C attitude nominal until the end (drag >> specs)
Passive aerodynamic stabilisation played a major role
Attitude error around Z over 1 orbit (90 min) X Y
Sun Z
Earth

10. S/C THERMAL EVOLUTION Central computer (CDMU) Battery (BAT)
Nitrogen storage tank (NST)
Power distribution unit (PCDU)
Star trackers (STR)
Ion propulsion electronics (IPCU)

11. 10 NOV 2013: FINAL OPERATIONS
All S/C FDIR disabled in the morning to keep going until the very end
Last Kiruna pass at 18:56Z, after that only Troll (Antarctica) passes
S/C nominal in last Troll pass at 22:43Z

12. GOCE re-entry captured by Bill Chater on 11th Nov 00:20 UTC
11 NOV 2013: RE-ENTRY
GOCE re-entry captured by Bill Chater on 11th Nov 00:20 UTC