Ulysses Nutation anomaly in 2000-2001 Andy McGarry ESA Ulysses Spacecraft Systems Engineer

Outline of talk Introduction to Ulysses & the Nutation anomaly What is the impact of the Nutation anomaly? How do we control Nutation? Nutation operations requirements - ESOC & Kourou’s role How is Nutation in 2000-1 different from 1994-5? Preparations for 2000-1 Nutation operations ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Ulysses mission Launch: 6 Oct 1990 on Shuttle STS-41 Discovery. First Orbit: Oct 1990 - Sep 1995. Jupiter Flyby: 8 Feb 1992. Second Orbit: Oct 1995 - Dec 2001. Third Orbit: Jan 2002 - Sep 2004. Note the unique Ulysses orbit. It is inclined approximately 70o to the ecliptic plane, allowing it to survey the Sun at Solar latitudes from 80oS to 80oN. Contrary to ones first expectations, Ulysses is further away from the Sun than Earth! The distance from the Sun varies from approximately the orbital distance of Mars to that of Jupiter! The one way light time (OWLT) varies from less than 20 minutes, up to 53 minutes at maximum Earth range. A Third Orbit has recently been approved and Ulysses is expected to operate until September 2004. The mission would therefore end before another period of Nutation would begin (approx. 2006 ) ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Spacecraft Dimensions Length 3.2 m (booms stowed) Width 3.3 m Height 2.1 m Weight Total Spacecraft 370 kg Scientific Payload 55 kg Stabilization Spin-stabilized 5 rpm ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Science payload Suite of 9 instruments Fields, particles & waves Radio Science No camera! ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Nutation Anomaly First occurred in 1990 after deployment of axial boom. Caused by solar pumping of 7.5m axial boom + underperformance of passive nutation dampers. Nutation Forcing Function (NFF) is function of SAA & 1/R2. Successfully controlled in 1994-5 to < 0.7 deg half-cone. Returning in 2000-1. ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

72.5 m (~221ft) across wire booms Impact of Nutation Spin rate: 5 rpm If left uncontrolled => wire booms wrap around s/c or detach damage or loss of s/c data loss due to s/c off-pointing 72.5 m (~221ft) across wire booms ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

The s/c Closed-loop Conscan (CLC) function Continuous U/L from Earth provides beacon Conscan electronics calculate s/c off-pointing error and thruster firing phase CLC provides damping component Process repeats until s/c spin axis is inside Earth off-pointing deadband (0.125o or 0.23o) Conscan very sensitive to U/L disturbances t s/c AGC t spin DC bias ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Nutation Operations Goals Maintain s/c safety Try to prevent Nutation starting Keep any Nutation which does start as low as possible ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Nutation ops depend on steady, continuous uplink When CLC is disabled, Nutation can grow. To provide maximum Nutation damping, CLC is continuously enabled. CLC only disabled during scheduled U/L disturbances e.g. U/L transfers, U/L sweeps, CMD MOD On/Off. We must carefully schedule all ground and s/c operations to avoid U/L transients affecting Conscan measurements,or producing erroneous thruster firings. S/c CLC off for 20mins around U/L transfers. S/c CLC off for (35mins+gap), around gap in coverage. ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Basic g/s requirements for Nutation operations Continuous (24 hr) ground station coverage providing: Steady uplink (TX) as beacon for s/c CLC Telemetry (TM) to monitor critical s/c parameters Telecommand (TC) capability to respond to contingencies High resolution Doppler data (from DSN stations only) to provide additional ARGOS monitoring of Nutation levels. ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Antenna coverage High latitude Solar Polar orbit =>gaps in DSN coverage. ESA providing Kourou station to fill in gaps in DSN coverage from 1 Feb - 31 Mar 2001. Unique orbit geometry causes scheduling and tracking problems. ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Effect of Ulysses Solar Polar orbit on antenna tracking Ulysses Nutation track during high latitudes Typical s/c track Antenna path Antenna path 10.x deg limit (Tx ON/OFF) 6 deg limit (RISE / SET) No uplink No uplink Continuous view, but large gap in uplink ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Kourou fills in gaps in DSN coverage

Antennas suitable for Ulysses Nutation support Notes: F FULL Can support Nutation Operations standalone. P PARTIAL Can provide part of Nutation Operations requirement. E EMERGENCY Uplink beacon only. * Maximum during nutation period. ** Available Feb-Mar 2001. Noisier Conscan, no Doppler, reduced TM rates. ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Differences in 2000-1 Nutation Compared to 1994-5 the following points are different: NFF greater, Earth drift rate lower => greater ops challenge. NFF is continuously high for one year - no break in Nutation ops. Metal fatigue - wire booms have accumulated ~15months of flexing vs ~3 months before 1994-5. Kourou is a shared resource. Used for XMM-Newton & LEOP support. Kourou will provide lower bitrates => reduced monitoring capability. Kourou is operated remotely from ESOC - any considerations? ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

2000-1 Nutation Forcing Function is greater Nutation ops 1994-5 nutation threshold 2000-1 nutation threshold Peak value of NFF2000-1 is 34% greater ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Tx elevation waiver requests DSN antennas supported Ulysses Nutation operations in 1994-5 with uplink down to 6o. Since then, station upgrades have resulted in hardware & software changes limiting transmission to above 10o elevation. 10o elevation limit caused large gaps in Tx coverage Low elevation Tx Waiver requests submitted to DSN complexes in March-May 2000. Radiation surveys performed at all sites & waivers granted ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Nutation Coverage without Tx ops at low elevation. ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Kourou support and waiver requests fill the gaps in Nutation coverage. ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Current status of Nutation preparations Kourou data flow tests completed 6 Sep 2000 Santiago uplink stability confirmed on 29 Aug, 6 Sep, 11 Sep. Kourou Mission Readiness Tests underway (26 - 28 Sep). DSN antenna coverage allocated through March 2001. Work has commenced on h/w & s/w modifications for DSN Low elevation Tx (available for testing at Canberra on 1 Nov 2000). 2.5 month Fuzzy Logic study (PHAEACIAN) commenced 11 Sep 2000. Will assess feasibility of FL techniques in analysing Nutation, and in recommending corrective action. ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Important dates 6 Sep 2000 - 16 Jan 2001 Second South Polar Pass (Solar Latitude > -70°) 26 -29 Sep 2000 Start Kourou MRTs, Nutation operations training Nov 2000 Nutation operations training at JPL and Canberra 1 Dec 2000 Begin Nutation operations 1 Feb - 31 Mar 2001 Kourou antenna provides Nutation support 23 May 2001 2nd Perihelion (1.34 AU) 25 May 2001 Ecliptic crossing 26 May 2001 Maximum value of NFF 31 Aug - 10 Dec 2001 Second North Polar Pass (Solar Latitude > 70°) 1 Dec 2001 End Nutation Operations 31 Dec 2001 End of Second Orbit 1 Jan 2002 Begin Ulysses Third Orbit ESA Ulysses Flight Control Team Ulysses Nutation 2000 - 2001

Latest Nutation information is available at Questions Latest Nutation information is available at http://ulysses-ops.jpl.esa.int/ulsfct/Spacecraft_ops_index.html