Astronomical Institute University of Bern 64 th International Astronautical Congress September 2013, Beijing, China Assessment of possible observation strategy in LEO regime A. Vananti, T. Schildknecht Astronomical Institute, University Bern (AIUB) G.M. Pinna, T. Flohrer European Space Agency (ESA)

Slide 2 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Introduction European Space Situational Awareness (SSA) system:  Network of optical telescopes  Established concepts for GEO/MEO  Few studies for LEO LEO regime:  Traditionally covered by radars  Telescopes for upper LEO is more cost efficient Assessment of LEO strategy:  Visibility of LEO objects  Coverage simulations  Orbit determination simulations

Slide 3 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Observation concept (Cibin et al. 2011) Fly-eye telescope 1m, 6.7 x 6.7 deg 2, 1.5“/px Complex optical system (splitter, lenses) Dynamic fences Fields close to shadow border Fields in low phase angle region

Slide 4 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Visibility Based on dynamic fences concept Stripe around the shadow region Tenerife latitude = ~ 30° 120° 90°  φ site   = ± 23°  0 Limitation is the minimal elevation Reduced visibility around midnight in September With stripe at  = 0° no visibility Station at high latitude needed for better coverage

Slide 5 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Visibility Better visibility in Summer (from Northern emisphere) Coverage like a sliding window that covers around 30° or 2 h of the moving station Stripe at  = 30° allows better visibility in September But it does not cover low-inclination orbits

Slide 6 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Phase angles Phase angles show a gap around midnight similarly to visibilities In summer, phase angles are slightly better reaching around 90° In general, when visibility is allowed are the phase angles around reasonable values < 60°

Slide 7 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Phase angles For the fixed declination stripe in the visibility region the phase angles show big variation Smallest phase angles are well below 20° High phase angles exceed 100°

Slide 8 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Coverage simulations LEO TLE population (~ 2000 objects) Eccentricity = Inclination = ~ 50° - 100° Satellites at km altitude Stations in Tenerife (TEN) and Azores (AZR) Stripe declination  = 30° Simulations without detection model 10° minimal elevation Dec.Jun.Sep. TEN TEN. AZR Missed objects are:  Visible only below the minimal elevation  In the twilight region Neglecting twilight constraints and assuming 0° for minimal elevation => 1953 objects

Slide 9 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Coverage during night Also about 4 hours idle time In winter the nights are longer But the visibility is very reduced Reduced visibility due to Earth shadow 4 hours idle time around local midnight Covered range: ~ 2 h or ~ 30°

Slide 10 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Coverage during night No gap in summer (3 months) Only reductions due to:  Minimal elevation  Twilight constraints Almost full coverage with: No twilight constraints 0° minimal elevation

Slide 11 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Orbit determination simulations Simulated 100 orbits in LEO regime:  Altitude: 1000 km – 2000 km  Eccentricity 0 – 0.01  Inclination 60° - 85° Simulated observations (0.5“ error) from Tenerife, midnight UTC, Orbit determination with observations at different time intervals, assuming tracklet correlation Examined angular position error  after 24 hours Examined radial and along-track components of position error after 24 hours Requirements for orbit accuracy:  Radial component: 4 m  Along-track component: 30 m

Slide 12 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Orbit determination simulations Object discovery at plot origin Observations after 5 minutes The error strongly diverges after only 1 follow-up Histogram of angular position error Δ after 24 hours Observations after 5 min and 2 hours After 5 min: object observed from same station on a second stripe After 2 hours: object observed after one revolution from same station

Slide 13 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Orbit determination simulations Observation intervals: 20 min, 2 h After 20 min: object observed from site at same longitude in the opposite hemisphere Slight improvement compared with the intervals 5 min, 2 h Observation intervals: 5 min, 2 h, 4 h Assuming observations after 4 h from a different longitude (> 30° shift) Error for most of the orbits < 1“

Slide 14 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Orbit determination simulations Observation intervals: 5 min, 2 h, 4 h, 6 h,..., 24 h Assuming a perfect coverage from all longitudes (12 or more sites)

Slide 15 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Orbit determination simulations Analysis of the position error Required accuracy: radial (4 m) and along-track component (30 m) Observation intervals: 5 min, 2 h Radial error < 600 m Along-track error ~ 7 km Follow-up after 5 min and 2 hours: => not enough to satisfy requirements

Slide 16 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Orbit determination simulations Observation intervals: 20 min, 2 h Required accuracy: radial (4 m) and along-track component (30 m) Requirements are partly satisfied:  ~ 50 % radial  ~ 35 % along-track

Slide 17 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Orbit determination simulations Required accuracy: radial (4 m) and along-track component (30 m) Observation intervals: 5 min, 2 hours, 4 hours Requirements are partly satisfied:  ~ 45 % radial  ~ 50 % along-track

Slide 18 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Orbit determination simulations Required accuracy: along-track component (30 m) Observation intervals: 5 min, 2 h, 4 h, 6 h,..., 24 h Requirement is well satisfied: => > 90% orbits within the required along-track accuracy

Slide 19 Astronomical Institute University of Bern Assessment of possible observation strategy in LEO regime, IAC 2013, Sep., Beijing Conclusions Ideal strategy follows the contour of the Earth shadow Visibility window ~ 30° along the stripe During 9 months, 4 hours idle time per night Additional sites at higher latitude are an advantage, but not indispensable 2 sites: 25% - 65% of objects covered depending on season For orbit determination 2 considered situations:  1 site North. and 1 site South. Hemisphere, same longitude => observations after 20 min and 2 hours  2 sites same Hemisphere, > 30° longitude separation => observations after 5 min, 2 hours, and 4 hours On average 40 % - 50% objects with required accuracy after 24 hours