Assessment of possible observation strategy in LEO regime

1. 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)

2. 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 Astronomical Institute University of Bern

3. Observation concept (Cibin et al. 2011) • Dynamic fences • Fields close to shadow border • Fields in low phase angle region • Fly-eye telescope • 1m, 6.7 x 6.7 deg2, 1.5“/px • Complex optical system (splitter, lenses) Astronomical Institute University of Bern

4. Visibility Based on dynamic fences concept Stripe around the shadow region Tenerife latitude  = ~ 30° site   φ  120° 0  90°  = ± 23° • 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 Astronomical Institute University of Bern

5. Visibility • Stripe at  = 30° allows better visibility in September • But it does not cover low-inclination orbits • Better visibility in Summer (from Northern emisphere) • Coverage like a sliding window that covers around 30° or 2 h of the moving station Astronomical Institute University of Bern

6. 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° Astronomical Institute University of Bern

7. 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° Astronomical Institute University of Bern

8. Coverage simulations LEO TLE population (~ 2000 objects) Eccentricity = 0 - 0.05 Inclination = ~ 50° - 100° Satellites at 1000-2000 km altitude Stations in Tenerife (TEN) and Azores (AZR) Stripe declination  = 30° Simulations without detection model 10° minimal elevation • Missed objects are: • Visible only below the minimal elevation • In the twilight region • Neglecting twilight constraints and assuming 0° for minimal elevation => 1953 objects Astronomical Institute University of Bern

9. Coverage during night • Reduced visibility due to Earth shadow • 4 hours idle time around local midnight • Covered range: ~ 2 h or ~ 30° • Also about 4 hours idle time • In winter the nights are longer • But the visibility is very reduced Astronomical Institute University of Bern

10. 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 Astronomical Institute University of Bern

11. 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, 21.09.2012 • 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 Astronomical Institute University of Bern

12. 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 Astronomical Institute University of Bern

13. 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“ Astronomical Institute University of Bern

14. 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) Astronomical Institute University of Bern

15. 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 Astronomical Institute University of Bern

16. 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 Astronomical Institute University of Bern

17. 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 Astronomical Institute University of Bern

18. 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 Astronomical Institute University of Bern

19. 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 Astronomical Institute University of Bern