Introduction to VLBI, SLR and permanent GNSS stations and their applications

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Introduction to VLBI, SLR and permanent GNSS
stations and their applications

• AFREF technical workshop
• University of Cape Town10-13 July 2005
• Ludwig Combrinck
• HartRAO Space Geodesy Programme

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History of Geodesy
The Greek philosopher Aristotle (384-322 B.C.) is
credited as the first person to try and calculate
the size of the Earth by determining its
circumference Around 250 B.C., Eratosthenes
measured the circumference of the Earth using the
following equation(360 ?) x (s)In this
calculation, (s) is the distance between two
points that lie north and south of each other on
the surface of the Earth and ? the angle
subtended by s. Eratosthenes computed the
circumference to be approximately 40 000 km and
the accepted figure today is 40 075.16 km at the
equator and 40 008 km over the poles.
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Surveying, geodesy, positioning, navigation and
astronomy
Figure from Peter ApiansGeographia (1553),
cross staffs were used to measure angles and
Global Positioning meant determination of
latitude and longitude
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Further developments through time

• Accurate time (J. Harrison 1693-1776),
  development of
• the marine chronometer allowed accurate GMT on
  ships
• Transit and astronomical telescopes
• Precise star catalogues, planetary motion
  (celestial mechanics)
• Fundamental astronomy defined global terrestrial
  and celestial reference systems and
  transformation between the systems
• Terrestrial ref. system was defined by
  coordinates
• of astronomical observatories

Giuseppe Megele fecit in Milano 1775
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Space Geodesy

• Dawn of the Space Age with the launch of Soviet
  Unions
• Sputnik 1, October 4, 1957
• Satellites could be used as observing platforms
  or
• as targets from the Earth surface (e.g.SLR)
• Development of Satellite Geodesy
• Developed for astronomy but possibility of using
  Very Long Baseline
• Interferometry (VLBI) for applications in
  astrometry and geophysics
• was realised in mid sixties
• During late 1960s a NASA-led consortium of
  universities and government
• agencies pioneered geodetic VLBI
• Haystack Observatory has a long history of
  contributions to VLBI, contributed
• to development of state-of-the-art VLBI
  instrumentation and techniques

Haystack MKIV correlator
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Collocated Space Geodetic Techniques
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Modern space geodesy equipment at HartRAO
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International Association of GeodesyAssociated
Space Geodesy Services

• International Earth Rotation and Reference
  Systems Service (IERS) (1988)
• International Laser Ranging Service (ILRS) (1998)
• International VLBI Service (IVS) (1999)
• International GPS Service (IGS) (1994) now Int.
  GNSS Service
• International DORIS Service (IDS) (2003)

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Space Geodesy basic techniques, satellite based

• Satellite based
• Satellite Laser Ranging (SLR)
• GNSS (GPS, GLONASS, GALILEO)
• DORIS (1992-) (Doppler Orbitography and
  Radiopositioning Integrated by Satellite)
  system, designed
• and developed by CNES in collaboration with
  GRGS and IGN, has a dual purpose.
• It is used to determine the orbit of satellites
  equipped with DORIS receivers with cm
• accuracy using a network of ground stations as
  reference points on Earth. Via this system,
• it is also possible to precisely tie points to
  the ITRF. Every 10 seconds, it measures the
  Doppler shift in the frequency of radio signals
• transmitted by beacons at 400 MHz and 2 GHz.

GRGS Groupe de Recherche en Géodésie Spatiale
IGN Institut Géographique National
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Satellite Missions

• Geodetic - inert massive spheres designed solely
  to reflect laser light back to its source LAGEOS
  1/2, Starlette, Stella
• Earth Sensing - large, irregular shaped objects,
  equipped with radar altimeters or other
  scientific instruments TOPEX/Poseidon, ERS
  1/2, EnviSat 1
• Radio Navigation - large, irregularly shaped, and
  in relatively high orbits (approximately 20,000
  Km) GPS, GLONASS
• Experimental - carry special experiments that do
  not fit into one of the other mission
  classifications Gravity Probe B, Grace A/B,
  LRE.

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Lunar Laser Ranging
Lunokhod
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SLR History

• Laser ranging to a near-Earth satellite was
  initiated by NASA in 1964 with the launch of the
  Beacon-B satellite. Since that time, ranging
  precision, spurred by scientific requirements,
  has improved by a factor of a thousand from a few
  meters to a few millimeters. Similarly, the
  network of laser stations has grown from a few
  experimental sites to a global network of 43
  stations in more than 30 countries. Most of these
  stations (33) are funded by organisations other
  than NASA.

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ILRS Network map
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Satellite Laser Ranging
Source DEOS
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How does SLR work?

• SLR uses special, passive spacecraft in very
  stable
• orbits. A low area to mass ratio minimizes
  the effects
• of atmospheric drag and solar radiation
  pressure on
• the satellites orbit.
• An example of such a satellite is the Laser
  Geodynamics Satellite (LAGEOS-1), which was
  launched by the United States in 1976 into a 6000
  kilometer circular orbit with a near polar
  inclination. Lageos-I, and its sister satellite
  Lageos-II (built by the Agenzia Spaziale Italiana
  of Italy and launched in 1992 into a 6000
  kilometer orbit, but with an inclination of 51
  degrees) have a 60 centimeter spherical shape and
  weigh 411 kilograms. The exterior surface of both
  satellites is covered by 426 retroreflectors.
• Other satellites for SLR include Starlette (1000
  km) and Stella (800 km) developed and launched by
  France Etalon-I and -2 (19,000 km) developed and
  launched by the former USSR and Ajisai (1500 km)
  developed and launched by Japan. To obtain data
  for precise orbit determination, retroreflectors
  are also mounted on the US/France Topex/Poseidon
  spacecraft, the European Space Agency Earth
  Remote Sensing satellites (ERS-1 and 2), and
  GLONASS, GPS35 and 36. (What about new GPS sats?)

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SLR how does it really work?

• 1. A telescope equipped with a laser fires a beam
  to the satellite at time t0
• 2. The beam bounces off the satellite
  retro-reflector (t1) and is received by the
  telescope at time t2
• 3. The time of flight is calculated and using the
  speed of light, plus other corrections, the range
  is determined

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Moblas 6 in action
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SLR equation

• D(?t/2) x c
• D is two way range, t the time of flight and c
  the speed of light.
• Easy no?

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No..

• Range needs to be corrected for
• Atmospheric refraction
• Station eccentricity (site ties..)
• Solid Earth tides
• Pole tide
• Ocean loading
• Relativity
• Station motion due to tectonic velocity
• Station time bias, range bias

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and

• Satellite orbit integrator needs to correct orbit
  for
• Earths gravity field (at least 20x20) e.g.JGM3,
  EGM96
• Acceleration from
• Atmospheric drag (LEOs)
• Solar pressure (shadow model)
• Earth albedo
• Gravity of Sun, Moon and planets (e.g.
  JPL DE405)
• Relativity

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O-C

• Using the corrected Observed (by SLR) range
  minus the Calculated range (using orbit
  integrator)
• Fitted in least squares sense
• Some parameters can be estimated e.g.
• EOP, station position, coefficient of
  reflection etc.

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• Date UTC MJD
  Station Sat obsm
  compm o-cm H Press. Temp. El.
  Atmos. Rel. E-tide Shadow P_tide X-pole
  Y-pole dPsi dEpsilon
• 2005/12/09 192307.255 53713.808 7090
  9207002 7909792.161 7909792.154
  0.0072 90 978.2 28.4 23.8 5.837 0.007
  0.07659 1.000 0.01847 0.06598 0.38983
  -0.05721 -0.00188
• 2005/12/09 192501.054 53713.809 7090
  9207002 7736536.584 7736536.576
  0.0080 90 978.2 28.4 26.2 5.345 0.007
  0.07572 1.000 0.01847 0.06598 0.38983
  -0.05721 -0.00188
• 2005/12/09 192644.853 53713.810 7090
  9207002 7593377.282 7593377.276
  0.0065 90 978.2 28.4 28.3 4.985 0.007
  0.07490 1.000 0.01847 0.06598 0.38983
  -0.05721 -0.00188
• 2005/12/09 192856.852 53713.812 7090
  9207002 7433838.060 7433838.051
  0.0087 90 978.2 28.4 30.7 4.626 0.007
  0.07385 1.000 0.01848 0.06598 0.38982
  -0.05721 -0.00188
• 2005/12/09 193117.451 53713.813 7090
  9207002 7294163.141 7294163.137
  0.0037 90 978.2 28.4 33.0 4.340 0.007
  0.07272 1.000 0.01848 0.06598 0.38982
  -0.05721 -0.00188
• 2005/12/09 193314.051 53713.815 7090
  9207002 7203865.224 7203865.219
  0.0050 90 978.2 28.4 34.6 4.167 0.007
  0.07176 1.000 0.01848 0.06598 0.38982
  -0.05721 -0.00188
• 2005/12/09 193442.850 53713.816 7090
  9207002 7151489.168 7151489.164
  0.0043 90 978.2 28.4 35.6 4.068 0.007
  0.07102 1.000 0.01848 0.06598 0.38982
  -0.05721 -0.00188
• 2005/12/09 193636.050 53713.817 7090
  9207002 7106065.578 7106065.577
  0.0011 90 978.2 28.4 36.5 3.980 0.007
  0.07007 1.000 0.01848 0.06598 0.38982
  -0.05721 -0.00188
• 2005/12/09 193919.250 53713.819 7090
  9207002 7083899.965 7083899.964
  0.0018 90 978.1 28.5 37.1 3.925 0.007
  0.06868 1.000 0.01848 0.06598 0.38982
  -0.05722 -0.00188
• 2005/12/09 194102.850 53713.820 7090
  9207002 7096752.982 7096752.979
  0.0029 90 978.1 28.5 37.0 3.932 0.007
  0.06778 1.000 0.01848 0.06598 0.38982
  -0.05722 -0.00188
• 2005/12/09 194216.850 53713.821 7090
  9207002 7118725.327 7118725.323
  0.0041 90 978.1 28.5 36.8 3.957 0.007
  0.06713 1.000 0.01848 0.06598 0.38982
  -0.05722 -0.00188
• 2005/12/09 195742.657 53713.832 7090
  9207002 8198324.491 8198324.506
  -0.0151 90 978.1 28.5 22.3 6.215 0.007
  0.05878 1.000 0.01848 0.06598 0.38981
  -0.05722 -0.00188
• 2005/12/09 195843.658 53713.832 7090
  9207002 8312183.034 8312183.044
  -0.0109 90 978.1 28.5 20.9 6.586 0.007
  0.05822 1.000 0.01848 0.06598 0.38981
  -0.05722 -0.00188
• 2005/12/09 200220.061 53713.835 7090
  9207002 8746003.764 8746003.776
  -0.0126 90 978.1 28.5 16.1 8.434 0.008
  0.05622 1.000 0.01848 0.06598 0.38981
  -0.05722 -0.00188
• 2005/12/09 233114.445 53713.980 7090
  9207002 6348416.994 6348416.979
  0.0154 90 978.7 29.0 53.7 2.945 0.006
  0.10163 1.000 0.01850 0.06597 0.38972
  -0.05724 -0.00187
• 2005/12/09 233225.644 53713.981 7090
  9207002 6274314.719 6274314.708
  0.0111 66 978.8 29.0 55.9 2.864 0.006
  0.10197 1.000 0.01850 0.06597 0.38972
  -0.05724 -0.00187
• 2005/12/09 233520.444 53713.983 7090
  9207002 6136102.503 6136102.492
  0.0107 66 978.7 29.1 60.9 2.716 0.006
  0.10277 1.000 0.01850 0.06597 0.38972
  -0.05724 -0.00187
• 2005/12/09 233717.643 53713.984 7090
  9207002 6079855.686 6079855.680
  0.0062 66 978.8 29.1 63.4 2.655 0.006
  0.10328 1.000 0.01850 0.06597 0.38972
  -0.05724 -0.00187

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French Transportable Laser Ranging Station (FTLRS)
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San Fernando, Spain
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MOBLAS-6 HartRAOpart of NASA SLR Network
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Blinking lights
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Apollo, Apache Point, 3.5m LLR
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Apollo beam to the Moon
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Why SLR ?

• The ILRS provides a service to utilize Satellite
  and Lunar Laser Ranging data to generate
  geodetic, geodynamic, and geophysical scientific
  products.
• Currently, five SLR analysis groups (ASI, DGFI,
  GFZ, JCET and NSGF) provide direct input for the
  official ILRS products (station coordinates and
  Earth Orientation Parameters, i.e. x-pole, y-pole
  and Length-Of-Day (LOD)

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SLR what else?

• Detection and monitoring of tectonic plate
  motion, crustal deformation, Earth rotation, and
  polar motion
• Modeling of the spatial and temporal variations
  of the Earths gravitational field
• Determination of basin-scale ocean tides
• Monitoring of millimeter-level variations in the
  location of the center of mass of the total Earth
  system (solid Earth-atmosphere-oceans)
• Establishment and maintenance of the
  International Terrestrial Reference System
  (ITRS) and
• Detection and monitoring of post-glacial rebound
  and subsidence.
• In addition, SLR provides precise orbit
  determination for spaceborne radar altimeter
  missions mapping the ocean surface (which are
  used to model global ocean circulation), for
  mapping volumetric changes in continental ice
  masses, and for land topography. It provides a
  means for sub-nanosecond global time transfer,
  and a basis for special tests of the Theory of
  General Relativity (LLR).

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SLR Applications
SLR provides direct, unambiguous measurement of
altimeter satellite height and permits
effective separation of altimeter system
drift from long-period ocean topography changes
at the sub-cm level. This calibration is
essential for the measurement of global mean
sea level changes of a few mm/yr and the mapping
of ice field topography used to estimate ice
volume changes.
Topex-Poseidon radar altimeter mission
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Evolution of El Nino/La Nina determined from RA
observations
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IGS Reference Frame SitesThe IGS realisation of
the International Terrestrial Reference Frame
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GNSS
Source DEOS
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How does it work?
Source Aerospace Corp
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Approximate position algorithm

• The principle behind GPS is the measurement of
  distance (or range) between the satellites and
  the receiver. The satellites tell us exactly
  where they are in their orbits by broadcasting
  data the receiver uses to compute their
  positions.
• If we know our exact distance from a satellite
  in space, we know we are somewhere on the surface
  of an imaginary sphere with a radius equal to the
  distance to the satellite radius. If we know our
  exact distance from two satellites, we know that
  we are located somewhere on the line where the
  two spheres intersect. And, if we take a third
  and a fourth measurement from two more
  satellites, we can find our location. The GPS
  receiver processes the satellite range
  measurements and produces its position.

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Global velocity field GPS
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Individual time series
Source JPL
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TEC mapping
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PWV Results
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DORIS
Source DEOS
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Space Geodesy basic techniques, Quasar based

• Quasar
• A quasar is a very distant (5-15 billion
  light years) radio source which is not a star
  (but quasi-star) termed qausi-stellar radio
  sources (QSRs). Optical versions are called
  quasi-stellar objects (QSOs). A quasar has a huge
  red shift (speed 95 of light), possibly
  supermassive black holes in galactic nuclei,
  luminous intrinsically as 1000 galaxies combined.
• Very Long Baseline Interferometry (VLBI)
• Its unique and fundamental contribution to
  geodesy and astronomy is the realisation of the
  celestial reference system and the maintenance of
  the long-term stability of the transformation
  between the celestial and terrestrial reference
  frames.
• Maintenance of the ICRF
• The International Astronomical Union (IAU)
  has charged the International Earth Rotation and
  Reference Systems Service (IERS) with the
  responsibility of monitoring the International
  Celestial Reference System (ICRS) and maintaining
  its current realisation, the International
  Celestial Reference Frame (ICRF). Since 2001,
  these activities are run jointly by the ICRS
  Product Center (a collaboration between the
  l'Observatoire de Paris and the U.S. Naval
  Observatory) of the IERS and the International
  VLBI Service for Geodesy and Astrometry (IVS), in
  coordination with the IAU Working Group on
  Reference Systems.

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VLBI stations
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VLBI stations Matera
The Matera VLBI system includes a 20 meter
diameter antenna, designed and built in Italy by
Selenia Spazio (now Alenia Spazio)
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Haystack Westford antenna
Built in 1961, this very capable 18.3 meter)
radome-enclosed radio telescope has seen many
uses throughout its life. In the begining it was
used as an x-band radar to test the limits of
communications technologies. Since 1981 its
primany use has been in support of geodetic
VLBI operations. The Westford Antenna also acts
as a test bed for NASA in the development of new
equipment and techniques supporting the worldwide
geodetic VLBI program
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VLBI Data Acquisition System History
Data acquisition systems Mk I (1972 -1978),
single frequency, X-band, narrow bandwidth of 360
KHz Mk II (1973 -1978), dual frequency, S and X
band, 2 MHz bandwidth MK III (1978 1990s) 28
channels of 2 MHz, multi-track tape MK III A
(1990s) 28 channels of 2 MHz, multi-track tape,
upgrades to formatter MK IV (1990s) thin tape, 35
tracks MK V (2002- ) replaces tape units
developed at Haystack Observatory as the first
high-data-rate VLBI data system based on
magnetic-disc technology, PC-based components,
the Mark 5 system supports data rates up to 1024
Mbps, recording to an array of 8 inexpensive
removable IDE disks. A single Mark 5 system with
sixteen 700 GB disc drives will record
continuously 1024 Mbps for 24-hours unattended!
HartRAO currently has a MKIV terminal, MkIV
recorder (thin tape), Mk5A recorder
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HartRAO VLBI equipment
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VLBI technique
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ICRF
Distribution of defining sources on an Aitoff
equal-area projection of the celestial sphere.
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Applications of VLBI

• 1 January 1999 The reference point of the South
  African trigonometric survey system changes from
  the Cape Datum (Clarke 1880 reference system),
  based on traditional optical survey methods, to
  the Hartebeesthoek94 datum (World Geodetic System
  1984, WGS84), based on the position of the
  intersection of the axes of the Hartebeesthoek
  radio telescope.

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Space Geodetic Products

• Space Geodetic products can be divided into 2
  categories
• Pure scientific research
• Test Einsteins General Relativity (lunar laser
  ranging)
• Measuring earth rotation (VLBI)
• Satellite dynamics (SLR GPS)
• Tectonic motion and crustal dynamics (VLBI GPS)
• etc.
• Applied science
• Weather predictions (GPS-PWV)
• National defence and communication (GPS-TEC)
• Seismic hazard detection (VLBI GPS)
• Accurate surveying, mapping and navigation
  (differential GPS)
• Satellite orbit maintenance (SLR)
• etc.

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Finish
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