PROBA 2 orbits and conditions of EUV observations V. Slemzin P. N PROBA 2 orbits and conditions of EUV observations V. Slemzin P.N. Lebedev Physical Institute, Moscow

Preliminary parameters of the Proba 2 orbit (from ESA Science Management Plan, 27/04/06) “Eurokot” launch opportunity, co-passenger of the SMOS satellite Sun-synchronous, stabilized at 06:00 LT Inclination: 98.41920 <H> = 755 km 14.5 orbits/day with a period of 99.2 min Launch date: 1 Feb 2007 > 1 Sept 2007 Mission duration: > 1 year 284 days without visible eclipses, 80 days with eclipses <18 min/orbit

Tools for evaluation and data sources: STK - Satellite Tool Kit (universal) – stand alone package TRAKSTAR code (T.S. Kelso, http://www.celestrak.com (Position and Velocity, Latitude, Longitude, and Altitude, look angles, right ascension and declination, etc.) SPIRIT software (based on TRAKSTAR code)

Calculation of orbital parameters Two-Line Element (TLE) files are produced from orbital measurements by the NASA Orbital Information Group (http://www.space-track.org) PROBA2-755 Sat. N year,doy 1 99999U 07032.00000000 .00000000 00000-0 00000-0 0 00000 2 99999 098.4192 41.50000 0000001 000.0000 000.0000 14.45500000000019 #orb incl.an. RAAN Eccentr. orb/day #orbit H=700 km Period= 98.5 min H=755 km Period= 99.2 min H=800 km Period=100.6 min

EUV occultations by absorption in the Earth’s atmosphere Averaged data for quiet atmosphere Transmission of the Earth’s atmosphere (based on SPIRIT measurements): Hlos, km 175 A 304 A EUV occultation 450 km 0.976 0.954 penumbra 350 km 0.760 0.574 250 km 0.177 0.038 <200 km <0.07 umbra

UV and VIS Eclipse periods and their duration Sun-Synchronous Non-Sun-Synchronous PROBA2 H=755km TRACE H=600 km CORONAS-F H=500km VIS eclipses: 14/05-30/07 (77 d) UV eclipses: 12/04-30/08 (141 d) Period without UV eclipses: <2 month/year Period without UV eclipses: 3 weeks/year

First day on orbit (1Feb 07) No eclipses Beginning of UV eclipse period (~2 weeks before VIS eclipse) UV eclipse: from 450 to 50 km – 10,5 min Total - 24% of all time in eclipse

End of UV eclipse period 01 Aug 2007 (-35.5, 85.6) (-34.6,-127.8) (-71.5,170.6)

20 km

Precession of the orbit Sun-synchronous orbit: max of time is available for observations of the Sun. Stabilized at 6:00 LT - the orbital plane is near normal to the Sun direction. Precession of the orbit : non-sphericity of the Earth Variation of beta-angle between the orbital plane and direction to the Sun (H=755 km) The curves for Hlos are similar!

Optimization of total time of UV eclipses The orbit can be optimized on total UV eclipse time by variation of the RAAN angle The angle from the inertial X-axis to the ascending node. The ascending node is the point where the satellite passes through the inertial equator moving from south to north. Right ascension is measured as a right-handed rotation about the inertial Z-axis. RAAN(t1)  RAAN(t0)+10· (t1-t0,days) Launch 1 Feb 2007 RAAN=32.50 1 Sep 2007 244.50 Each 1% = 3.65 days!

RAAN=41.50 RAAN=51.50