Image of the Week

Gaia Orbit Reconstruction

 

 
  Differences in km between two deliveries of the Gaia orbit. The left part shows that the final reconstructed orbit is the same in both deliveries and the two predicted orbits on the rightmost panel differ by less than 1000 km, well below the requirements. The orbit is updated every week by the ESOC Flight Dynamic Group and made available to DPAC. It is processed at ESAC into a Chebyshev representation and accessible through the DPAC Software Library (the so-called GaiaTools) for the whole mission. Reliable observation predictions of solar system objects depend very much on the quality of the predicted orbits.  
 

The astrometric analysis of the Gaia observations can only be performed to the highest accuracy if one knows where the spacecraft was at the time of observation. Requirements are very stringent for the orbit tracking and highly challenging for the teams involved at ESOC. At the very end one expects the position of Gaia to be known within one hundred metres uncertainty and its velocity at the mm/s level. We know today that this challenge will probably be met thanks to the combination of the radio, Doppler and optical tracking of the spacecraft.

But the data processing is on-going and orbital data are available at any time, based on a preliminary solution using the tracking up to the date close to the orbit release (typically the orbit is updated every week), and extended with a predicted orbit covering the whole mission. The requirement was such that the true orbit should depart from the planned orbit determined a few days after launch by no more than 7000 km. This limit was set by the design of the relativity experiment requiring the apparent direction of Jupiter as seen from Gaia could be ascertained at launch time to be within 1/10th of the planet radius.

The delivered orbit file comprises three time-segments: (i) a finally reconstructed orbit fitted on tracking data and ending typically one week before the release; (ii) a provisionally reconstructed orbit covering about the last week of tracking and likely to move into the first segment at the next delivery; (iii) a predicted orbit extending from the delivery date to the nominal mission end, which will change from delivery to delivery. The first segment is final and will not be updated in the subsequent deliveries, until late into the mission when all the optical observations processed with the Gaia catalogue can be integrated. In the following discussion the first two segments are merged into a 'reconstructed orbit' for the sake of simplicity.

The plot shows the positional difference between two orbit releases. One from 12 May 2015 and a more recent one delivered on 1 September 2015. The first one represents the reconstructed solution until around 12 May (and a finally reconstructed until 5 May) and a predicted orbit afterwards, and similarly for the September-solution. Therefore they have a common part, until 12 May, followed by an interval until 1 September, during which we have a predicted orbit and a true orbit fitting. Finally beyond this date, we have two realisations of the predicted orbit, which have no reason to be identical. These intervals are identified at the top of the diagram along with the kind of orbits which are compared.  The orbits are given with the origin at the barycenter of the Solar System in equatorial coordinates. The coordinate differences are plotted in km.

  • During the overlap of the fitted periods, the two orbits are fully identical (and almost identical in the last week, the difference being not appreciable in the plot). This was expected and seeing it in real is more than reassuring.
  • In the intermediate period one sees that the (nearly) final orbit reconstruction departs gradually from the predicted orbit by 300 km at most, a very nice achievement compared to the requirements. One should notice that the numbers should not be understood into 'how well one can follow a targeted orbit'. This is not the role of the predicted orbit.
  • Finally one sees in the last interval that the two predicted orbits differ from each other by less than 600 km, and the separation remains in this range over a longer timespan.

A previous comparison with a much earlier initial orbit (October 2014) shows that the difference between the predicted and the final orbit has dropped by a factor 3, down to 400 km from 1500 beforehand. Although it can go differently in the future, it is nice that we can rely on a predicted orbit so close to the final orbit for the assessment of observations of solar system bodies and to make observation predictions long in advance with a sufficient reliability. Unlike the stars, the apparent direction of nearby minor planets is very sensitive to the exact location of the observing platform. Likewise events like the passage of the Moon on the solar disk can be predicted more safely with a reliable predicted orbit.

 

Credits: ESA/ESOC/DPAC, F. Mignard (Observatoire de la Côte d'Azur), S. Klioner, A. Butkevich, (Lohrmann Observatory, Dresden), F. Budnik (ESOC) and the Orbit Determination Team

[Published: 18/09/2015]

 

Image of the Week Archive

2017
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
 
Please note: Entries from the period 2003-2010 are available in this PDF document.