Co-location of VLBI with other techniques in space: a simulation study B. Männel, M. Rothacher ETH Zürich, Geodesy and Geodynamics Lab 7 th IGS General Meeting, Madrid 212 1
Reference frame and local ties - today TRF is defined through not very well-connected networks of SLR, GPS, VLBI and DORIS offsets between different space techniques are measured by ground-based surveys at stations with two or more space techniques accuracy of the TRF is limited by the accuracy of these local ties precise and stable local ties will cause improvements in the realization of the TRF - Global Geodetic Observing System (GGOS): realization of the terrestrial reference frame (TRF) with 1mm accuracy and.1 mm/year stability 7 th IGS General Meeting, Madrid 212 2
Co-location in space - concept improvements in local ties and technique-specific effects e.g. calibration of antennas well-calibrated and designed spacecraft - baselines between sensors are measured before satellite launch (~1mm accuracy) - satellite positioning with 1cm accuracy (1mm using a satellite constellation) - extremely simple spacecraft (no moving parts, no sloshing fuel, stable center of mass) 7 th IGS General Meeting, Madrid 212 3
Co-location in space - concept GNSS GNSS antenna co-location in space retro-reflector SLR VLBI transmitter VLBI co-location on ground 7 th IGS General Meeting, Madrid 212 4
Co-location in space - satellites - GPS, GLONASS, Compass/ Beidou and GIOVE/Galileo - Jason-1, TerraSAR-X, Tandem-X, Envisat, CHAMP, GRACE, GOCE co-location in space is typically used for orbit estimation and validation Geodetic Reference Antenna in Space (GRASP) - GNSS-antenna, SLR retro-reflector, VLBI and DORIS transmitters - sun-synchronous orbit, altitude 16 25km - total mission cost app. 15M$, launch date 216 NanoGEM - GNSS-antenna, SLR retroreflector, VLBI transmitter - sun-synchronous, polar orbit, altitude 7-8km - nano-satellite total mission cost a fraction of GRASP and possibility/goal to build up a small constellation ( Bar-Sever, 29) 7 th IGS General Meeting, Madrid 212 5
VLBI transmitter onboard a satellite - curved wave front delay model for curved wave front: Sovers et. al. (1998), Sekido and Fukushima (26) implemented in Bernese GPS Software based on the VLBIversion by Ralf Schmid - known (a-priori) satellite position and fast moving source (satellite tracking) satellite - maximal length of baselines and observation time interval is defined by the orbital design crucial orbital elements: S 2 (t 2 ) k 2 k 1 - satellite altitude - inclination baseline S 1 (t 1 ) 7 th IGS General Meeting, Madrid 212 6
Changing rate of elevation and azimuth change of azimuth and elevation per second [ ].25.2.15.1.5. 5 6 7 8 1 12 14 15 16 18 2 25 satellite altitude [km] GPS satellites: ~.1 /s (Tornatore, 21) slew rates Today VLBI21 azimuth 1.5-12 /s >6. /s elevation.7-3.1 /s >2.1 /s 7 th IGS General Meeting, Madrid 212 7
Simulation of a global network - simulation using different orbital elements - satellite altitude (7, 15 and 2km) - inclination (8, 89 and 17 ) - global observing network with 17 existing or maybe future stations (Wresnik et. al. 29) - parameter estimation during data processing - troposphere zenith delay and gradients and - station coordinates - orbital elements 7 th IGS General Meeting, Madrid 212 8
Simulation of a global network 7 th IGS General Meeting, Madrid 212 9
Number of observations number of observations 35 3 25 2 15 1 5 altitude 2km, inclination 89 altitude 15km, inclination 89 altitude 7km, inclination 89 altitude 7km, inclination 8 altitude 7km, inclination 17 2 4 6 8 1 12 14 16 18 2 min. elevation [ ] one day of observations, integration time per observation 3 seconds 7 th IGS General Meeting, Madrid 212 1
Number of observations - min. elevation angle set to 5 - satellite altitude 2 km number of observations 4 35 3 25 2 15 1 5 NYALES2 - ONSALA6 GILCREEK - NYALES2 ONSALA6 - WETTZELL FORTLEZA - MAS1 HOBART12 - KWJ1 1 2 3 4 5 6 7 length of baseline [km] 7 th IGS General Meeting, Madrid 212 11
Number of observations - min. elevation angle set to 5 - satellite altitude 2 km number of observations 4 35 3 25 2 15 1 5 NYALES2 - ONSALA6 GILCREEK - NYALES2 ONSALA6 - WETTZELL FORTLEZA - MAS1 HOBART12 - KWJ1 1 2 3 4 5 6 7 length of baseline [km] 7 th IGS General Meeting, Madrid 212 12
Number of observations - min. elevation angle set to 5 - satellite altitude 2 km number of observations 4 35 3 25 2 15 1 5 NYALES2 - ONSALA6 GILCREEK - NYALES2 ONSALA6 - WETTZELL FORTLEZA - MAS1 HOBART12 - KWJ1 1 2 3 4 5 6 7 length of baseline [km] 7 th IGS General Meeting, Madrid 212 13
RMS of station coordinates 7 th IGS General Meeting, Madrid 212 14
RMS [m].5 WETTZELL 3 5 1 NYALES2 3 5 1 RMS of station coordinates.4.4.3.2.1 HEIGHT LATITUDE LONGITUDE RMS [m].5.3.2.1 HEIGHT LATITUDE LONGITUDE TIGOCENC 3 5 1 HOBART12 3 5 1 3 1.2 2.5 1 RMS [m] 2 1.5 1 RMS [m].8.6.4.5 HEIGHT LATITUDE LONGITUDE.2 7 th IGS General Meeting, Madrid 212 15 HEIGHT LATITUDE LONGITUDE
RMS of station coordinates - scaled to RMS obtained by the simulation using the orbit with 2 km altitude GILCREEK GILCREEK number of observations 5 4 3 2 1 2 15 7 changing RMS (RMS2=1) 15 1 5 2 15 7 satellite altitude [km] satellite altitude [km] HEIGHT LATITUDE LONGITUDE NYALES2 NYALES2 number of observations 1 8 6 4 2 2 15 7 changing RMS (RMS2=1) 6 4 2 2 15 7 satellite altitude [km] satellite altitude [km] HEIGHT LATITUDE LONGITUDE 7 th IGS General Meeting, Madrid 212 16
RMS of station coordinates - scaled to RMS obtained by the simulation using the orbit with 2 km altitude WETTZELL GILCREEK WETTZELL GILCREEK number of observations 5 4 3 2 1 2 15 7 satellite altitude [km] changing changing RMS RMS (RMS2=1) (RMS2=1) 415 3 1 2 5 1 2 2 15 15 7 7 satellite altitude [km] [km] HEIGHT LATITUDE LONGITUDE NYALES2 NYALES2 slow increase (or decrease) of RMS for stations with shorter baselines regional networks are a good option for 1 6 8 6 4 4 2 observing 2 satellites with lower altitude 2 15 7 2 15 number of observations satellite altitude [km] changing RMS (RMS2=1) satellite altitude [km] 7 HEIGHT LATITUDE LONGITUDE 7 th IGS General Meeting, Madrid 212 17
Consequences and outlook - co-location in space allows the estimation of local ties and will improve the realization of the TRF - VLBI observations - fast moving satellites -> high antenna slew rates necessary - the satellite altitude is crucial for the number of observations, but also baseline length and station latitude is important - to estimate the local ties with 1mm accuracy we recommend the use of baselines shorter then 2km - next steps - combination of GPS, SLR and VLBI some further simulations and maybe once using real satellite transmitted VLBI data to improve the local ties 7 th IGS General Meeting, Madrid 212 18
Benjamin Männel ETH Zurich maennelb@ethz.ch References: Thank You for listening Y. Bar-Sever et. al. (29) The Geodetic Reference Antenna in Space (GRASP) Mission Concept. EGU meeting in Vienna 29 M. Sekido and T. Fukushima (26): A VLBI delay model for radio sources at a finite distance, J. Geod, 8:137-149 O. Sovers et. al. (1998): Astronometry and geodesy with radio interferometry: experiments, models, results, Review of Modern Physics, 7:1393 V. Tornatore (21): Planning of an Experiment for VLBI Tracking of GNSS Satellites, IVS 21 General Meeting Proceedings, p. 7-74 J. Wresnik et. al. (29): VLBI21 simulations at IGG Vienna, Proceedings of the 19th European VLBI for Geodesy and Astronometry Working Meeting 7 th IGS General Meeting, Madrid 212 19