Astronomical Institute University of Bern 64 th International Astronautical Congress 23-27 September...

Astronomical Institute University of

Bern

64th International Astronautical Congress

23-27 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)


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ng 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


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ng Observation concept (Cibin et al. 2011)

Fly-eye telescope

1m, 6.7 x 6.7 deg2, 1.5“/px

Complex optical system (splitter, lenses)

Dynamic fences

Fields close to shadow border

Fields in low phase angle region


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ng 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


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ng 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


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ng 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°


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ng 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°


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ng 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

Dec. Jun. Sep.

TEN. 312 989 661

TEN. AZR.

456 1286 895

Missed objects are: Visible only below the minimal elevation In the twilight region

Neglecting twilight constraints and assuming 0° for minimal elevation

=> 1953 objects


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ng Coverage during night

0

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-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Ob

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Hours after midnight UTC

Tenerife, September

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Tenerife, December

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°


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ng Coverage during night

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Tenerife, June

No gap in summer (3 months) Only reductions due to:

Minimal elevation Twilight constraints

Almost full coverage with: No twilight constraints 0° minimal elevation

0

0.5

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1.5

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Tenerife, June, 0 deg min. elev., no twilight constraints


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ng 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


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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


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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“


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Observation intervals: 5 min, 2 h, 4 h , 6 h, ... , 24 h Assuming a perfect coverage from all longitudes (12 or

more sites)


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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


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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


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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


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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


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ng 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 64 th International Astronautical Congress 23-27 September...

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Transcript of Astronomical Institute University of Bern 64 th International Astronautical Congress 23-27 September...