Gaia Astrometry Upkeeping by GNSS - Evaluation Study [GAUGES]

Transcription

1 Gaia Astrometry Upkeeping by GNSS - Evaluation Study [GAUGES] M. Gai, A. Vecchiato [INAF-OATo] A. Fienga, F. Vakili, J.P. Rivet, D. Albanese [OCA] Framework: Development of High Precision Astrometric Techniques from Ground Technique proposed: Observation of artificial satellites against stellar fields Issues for 21 st century Meridian Circle concept: Can we find a good reference outside atmosphere? Can we factor out Earth motion irregularity? M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 1

2 Satellite visual tracking (+ laser ranging) astrometry Apparent motion of satellite over background stellar field Individual measurement error: ~10 mas for both satellite and stars Best fit of subsequent positions to orbit track estimate of average satellite position AND velocity star positions referred to satellite orbit M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 2

3 Project submitted to OCA BQR Proposal approved for feasibility tests [1 month visit by M. Gai] at the Observatoire de la Côte d Azur (OCA) Telescope site: Calern plateau Observing equipment: MéO (1.5 m ) + C2PU (2 1 m ) Test goal: verification of the potential performance of the proposed technique (GAUGE) as a tool for direct connection between ITRF and ICRF OCA teams involved: Lagrange (F. Vakili) + GéoAzur (A. Fienga) MeO observations included in schedule of orbit monitoring of Galileo satellites by laser ranging over 18 months ( ) C2PU dedicated observations M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 3

4 Why investigation at OCA? Different context, goals and methods: focus on LEO satellites and debris LAGEOS 1 orbit reconstruction attempted Satellite tracking, trailed star images M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 4

5 Combined techniques already in progress Astronautical / geodetic application M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 5

6 Satellite orbit tracking: established astronautical practice (I) Adv. Sp. Res. 47 (2011) M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 6

7 Satellite orbit tracking: established astronautical practice (II) M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 7

8 Framework Long term goal: Verification and maintenance of Gaia Catalogue Medium term goal: development of high precision astrometry methods from ground Approach: differential astrometry of GNSS satellites against field stars with conventional, ground-based, small size telescopes Additional applications: Satellite orbits monitoring; geodesy Experimental results: Astrometric performance analysis on selected sky regions M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 8

9 Satellite tracking for astrometry Astronomical usage of existing satellites Star positions perfectly known precise orbit determination Satellite orbit perfectly known large angle determination of star positions Practical implementation: - start from approximate star positions and satellite orbit; - perform precise relative measurements; - derive more accurate determination of star positions and satellite orbit M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 9

10 Astrometric catalogue verification & maintenance Scientific application: astrometry + geodesy Reduction approach: best fit between measurements, star coordinates and satellite orbit Zonal errors of catalogue result e.g. in apparent deviations of satellite orbit Stellar astrometry satellite orbit M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 10

11 Figures & facts (I) Satellite Laser Ranging (SLR) uncertainty: few mm on normal points (15 s 5 min) Orbit consistency between optical (SLR) and microwave (GNSS): ~10 cm Height of Galileo satellites: ~23,000 km σ θ $.& ( )*+ ( 3e-9 rad 0.5 mas Comparable with precision of first release of Gaia catalogue Average over ~ 100 measurements: ~50 µas Also corresponding to mm level point precision Comparable with precision of final Gaia catalogue M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 11

12 Figures & facts (II) Galileo satellite orbital period: ~14 hours Sidereal speed: 25.7 arcsec/s 25.7 deg/hr CCD field size = 3 arcmin exposure time = 3 x 60 / 25 7 s CCD field size = 5 arcmin exposure time = 5 x 60 / s Laser impulse frequency: 10 Hz Satellite images spaced by 2.6 arcsec Assumed resolved vs. seeing Arc of +/-25 deg vs. meridian covered in 2 hours M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 12

13 Expected star distribution (average sky) N -./ strip of average sky (<2 hours) Field width: 5ʹ Technique limited by magnitude and precision Small improvement on geosynchronous satellites (+30% exposure time) M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 13

14 Atmospheric limitations on sky strip (I) Telescope 1 m (C2PU): Limiting precision on star set center: σ : ; <:=> 3 mas Limiting precision on star RMS distance: σ C 0(- σ 45-3 mas Telescope 1.5 m (MeO): Limiting precision on star set center: σ : ; <:=> 2 mas Limiting precision on star RMS distance: σ C 0(- σ 45-2 mas M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 14

15 Photon limited astrometric noise on stars Dependent on star magnitude and field size exposure time 3ʹ 7 s, 5ʹ 12 s Bright stars: few mas Adaptive optics: significant improvement on limiting magnitude atmospheric residual individual star precision Issue for future development M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 15

16 Differential astrometric noise from atmospheric turbulence Strongly dependent on star separation Kolmogorov model Wind speed V = 10 m/s Turbulence layer height h = 1000 m Transit level error: D = 1.0 m 8.05 mas D = 1.5 m 5.19 mas [Pravdo & Shaklan, ApJ 1996] Most useful laser shots: closest to each field star Additional information on atmospheric turbulence: further investigation? M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 16

17 Atmospheric limitations on sky strip (II) Milli-arcseclimiting precision level achievable with ~4 transits (MéO) ~ 10 transits (C2PU) Realistic requirements: few ten nights M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 17

18 Test goals Check overall feasibility aspects Satellite brightness Availability of bright stars Pointing of fast moving objects Trade-off on observing parameters Datation / time stamping issues Simultaneous observation from several stations Usage of Laser Ranging for satellite illumination M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 18

19 Observations performed at Calern Nights: 25, 26, 28 April 2016 MéO: Galileo 201, 202 [ongoing observing program] C2PU Omicron (IXON888a): Lageos 1, 2; Galileo 101, 102, 201, 202, 204, 208, 209; ASTRA 1L Remarks: Strong wind & turbulence on nights 25, 26; C2PU: failure of CCD camera on night M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 19

20 Galileo 201, C2PU, night 25/4, UT 23:45:00, exp.: 1s M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 20

21 Galileo 202, C2PU, night 26/4, UT 20:20:00, exp.: 0.5s Also observed: Galileo 204, Galileo 208, Galileo M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 21

22 LAGEOS 1, C2PU, night 26/4, UT 21:47:00, exp.: 0.5s M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 22

23 ASTRA1L, C2PU, night 28/4, UT 20:22:00, exp.: 2s M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 23

24 LAGEOS 2, Laser Ranging on MéO, night 26/4 Exposure time: 0.5 s Fast moving object: 90 /s Laser pulses: brighter than solar reflection freeze satellite motion well separated ( 9 ) on image Optical scale: 0ʺ.8/pixel Field: 14ʹ Back-scattering of laser beacon in the atmosphere [Toyoda et al., 2001] M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 24

25 LAGEOS 2, C2PU, night 25/4 preliminary results - I Photon limit Preliminary check Dispersion of star positions over frame sequence Seeing limit Consistent with qualitative expectations M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 25

26 LAGEOS 2, C2PU, night 25/4 preliminary results - II Linear fit of satellite motion from frame sequence, using raw center-of-gravity positions and correction by asterism arithmetic photocenter Data dispersion [pixels]: Uncorrected: Referenced: RMS contribution removed: [Reduction of atmospheric noise] M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 26

27 Galileo 202, long range MéO, night 28/4 Pointings: Serie 1 Serie 2 Serie 3 Serie 4 Serie 5 UT 22h20 22h25 22h50 23h10 23h25 α 8h 59m 24s 9h 20m 58s 11h 07m 05s 12h 18m 20s 13h 01m 12s δ ' 45" ' 49" ' 49" ' 16" ' 09" Arc ʹ 21 37ʹ 32 22ʹ 45 16ʹ Arc: distance from initial pointing Remarks: Field rotation evident (alt-az mount), requires correction Slower camera, smaller number of distinct trails available. processing in progress to preliminarily evaluate precision on orbit reconstruction M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 27

28 Data reduction aspects & future steps Focused on astrometric analysis of intrinsic data dispersion Investigation on atmospheric residuals, imaging performance, Based on early Gaia catalogue release for data comparison Further investigation on Gaia connection by Torino team (DPCT access) Investigation on technique potential for astrometry & geodesy Investigation on technique potential for laser-less satellite monitoring Performance optimisation by custom hardware (CCD clocking) Evaluation of proposal feasibility for technique exploitation (EU?) M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 28

29 Working with elongated images of moving objects Distinguishing features of CCD astrometry of faint GEO objects V. Kouprianov Adv. Sp. Res. 41 (2008) (a) CCD frame; sample star and target space debris object are indicated by arrows; (b) intensity along star trail: 3D image and contour plot Tracking on GEO object Precision reported: 0.01 pixel, consistent with photon limit Model line-shaped PSF Intensity residuals after PSF fitting M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 29

30 Example: Triangulation (1) Simultaneous / time referenced imaging satellite motion common mode to both telescopes Baseline and satellite orbit known at first order in advance Astronomical usage of existing satellites as out-of-atmosphere references Atmospheric turbulence common mode to satellite and field stars Geometry calibration in data reduction Intermediate angle (few deg) feasible Earth motion irregularity vanishing in the differential measurement M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 30

31 Triangulation for asteroid astrometry Observation: 2014 January 2 UT T1: 0.9-m at Kitt Peak National Observatory T2: 0.6-m at Cerro Tololo Int. Observatory The Minor Planet Bulletin (ISSN ) Vol. 42, No. 1, pp (2015) M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 31

32 Example: Triangulation (2) Baseline related to satellite orbits Non-simultaneous, time referenced measurement Large angle (~1 rad) feasible Benefit: single station required, common mode instrumental parameters Satellite as reference outside atmosphere Catalogue zonal errors evidenced by apparent orbit irregularities Case most suited to satellite laser ranging ~0.5 mas M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 32

33 Potential targets: Geosynchronous satellites (Clarke s belt) Large number of available targets: M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 33

34 Distribution of satellites Polar view of GEO and LEO satellites M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 34

35 Further applications (1): Satellite orbit tracking for geodesy M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 35

36 Further applications (2): Astrometry of main planets SAS: large dynamic range M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 36

37 Further applications (3): Celestial Reference to GPS Technical difficulty: high speed apparent motion of satellites Brighter limiting magnitude Benefit: astrometry with full sky coverage M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 37

38 Distribution of baselines and arcs N Telescopes Baseline centres 1 N N 1 2 # arcs simultaneously sampled = # baselines between telescope pairs M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 38

39 Advertisement page Instrument size: 40,000 km Project timescale: many centuries Deployed infrastructure: 400+ satellites Past investment: many billion Minimum add-on required! M. Gai Gaia Astrometry Upkeeping by GNSS - Evaluation Study 39