TU Wien Course 122'018: Remote Sensing 2006 April 26 Satellites in Remote Sensing

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TU Wien Course 122.018 Remote Sensing2006 April
26Satellites in Remote Sensing

• Zoltan Bartalis, MSc
• Institute of Photogrammetry and Remote
  SensingVienna University of Technology
• zb_at_ipf.tuwien.ac.at
• www.ipf.tuwien.ac.at/zb

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Introduction

• In this presentation you will hear about
• Satellites as spacecraft
• Orbits of satellites in general
• Orbits of earth observation satellites in
  particular
• Examples of the relationship between spatial /
  temporal resolution and satellite orbit / sensor
  characteristics
• Using satellite orbit prediction and analysis
  software

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Satellites Background

• An (artificial) satellite a spacecraft that
  orbits the Earth or another celestial body.
• Project RAND, Preliminary Design of an
  Experimental World-Circling Spaceship (1946)
• "A satellite vehicle with appropriate
  instrumentation can be expected to be one of the
  most potent scientific tools of the Twentieth
  Century. The achievement of a satellite craft
  would produce repercussions comparable to the
  explosion of the atomic bomb..."
• First satellite Sputnik 1 (Soviet Union),
• launched on October 4, 1957

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Types of Satellites

• By purpose
• Earth Observation non-military environmental
  monitoring, meteorology, cartography, etc.
• Astronomical observing distant planets,
  galaxies, etc.
• Telecommunication acting as relays usually using
  radio at microwave frequencies.
• Navigation transmit radio time signals to allow
  mobile users on the ground to locate themselves.
• Reconnaissance military/intelligence versions of
  earth observation or telecommunication
  satellites.
• Space stations / space shuttle spacecrafts for
  long/short-term human activity in orbit.

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Structure of Satellites

• Like most spacecraft, satellites consist of the
  following typical subsystems
• Superstructure protective point of attachment
  for all other subsystems
• Attitude control positioning and orientational
  sensors, actuators and control electronics,
  thrusters with their fuel, tankage, valves,
  pipeage, etc.
• Communications the link between satellite and
  ground stations
• Tracking, Telemetry and Command (TTC) sensors
  for the internal and external state of the
  spacecraft and for diagnosing all other
  subsystems
• Power solar panels, batteries, etc.
• Thermal control heaters, insulators, radiators,
  etc.
• Propulsion Payload the actual scientific
  instrumentation
• Ground system mission operations facility,
  transmitting/receiving ground stations, data
  processing and storage facility, communications
  network
• Launch vehicle expendable or reusable transport
  into orbit

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International Satellite Launch Capabilities

• Countries with independent capability to place
  satellites in orbit
• Soviet Union/Russia 1957
• United States 1958
• France 1965
• Japan 1970
• China 1970
• United Kingdom 1971
• European Union 1979
• India 1980
• Israel 1988
• South Korea ?
• Pakistan ?
• Iran ?
• Brazil ?

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Orbital Elements 1

• Orbits are curved by the gravitational fields of
  all objects with mass
• Ellipses, parabolas hyperbolas conics
• 1618, Johannes Kepler
• High kinetic energy ?
• deflection hyperbola
• Not enough kinetic energy ?
• capture ellipse
• Transitional orbit ? parabola
• Law of gravity, 1687 Isaac Newton

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Orbital Elements 2

• Six parameters fully define
• the position of a satellite in orbit
• 1. 2. Semimajor axis a
• and eccentricity e
• OR
• Apogee (apoapsis) and
• perigee (periapsis) height
• 3. Inclination, i
• 4. Right ascension of
• ascending node, W
• 5. Argument of perigee
• (periapsis), w
• 6. True anomaly, n0

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Orbital Perturbations

• Apart from the main gravitational force
  (point-like mass in centre of gravity),
  influences from other forces
• Perturbation due to non-sphericity (irregular
  weight distribution of the Earth)
• Atmospheric drag
• Gravitational pull from other bodies Sun, Moon,
  etc (direct 10-6 m/s2, indirect 10-9 m/s2)
• Photon pressure on satellite surface (direct
  indirect 10-7 m/s2)
• Solar wind (particles)
• Relativistic effects, 10-10 m/s2
• Magnetic field effects
• Outgassing (slow release of gas trapped inside
  materials)
• Complications for eclipse periods
• Orbit not a conic anymore
• Orbit predictions by
• including some of the perturbations analytically
• numerical methods (differential equation solving)
  for most precise models

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Earth Observation Satellites 1

• Although expensive, satellite remote sensing
  offers cost-effectiveness over airborne or
  ground-based remote sensing if we are interested
  in
• Long term data acquisition
• Periodical data acquisition
• Wide regional coverage
• Consistency in the quality of the data acquisition

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Earth Observation Satellites 2

• The spatial and temporal resolution and coverage
  of a product delivered from a satellite sensor
  depends on the
• sensor characteristics
• satellite orbit geometry
• sensor and spacecraft
• orientational flexibility
• limitations in the
• operational time (e.g. when
• more sensors share the same
• platform) and data
• transmission

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EOS Orbits 1

• Earth Observation Satellites (EOS) usually have
  circular orbits (the six parameters reduce to
  four)
• Ellipsoidal orbits would yield varying altitude ?
  varying size of the image elements
• Newtons law of universal gravitation gives the
  centripetal force
• where
• G 6,674?10-11 m3 kg-1 s-2 (gravitational
  constant)
• ES satellite Earth centre distance
• ME mass of the Earth
• MS mass of the satellite
• GME µ (geocentric gravitational constant
  3,986005?1014 m3 s-2

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EOS Orbits 2

• In the case of uniform circular motion, the
  centrifugal force is
• where vS satellite tangential velocity
• Equating the centripetal and centrifugal forces
  yields
• If the satellite angular velocity is ?S then
  and

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EOS Orbits 3

• Further, the orbit period TS will be then given
  by the so-calledThird Kepler Law applied to
  circular motion
• The orbit period does NOT depend on the satellite
  mass, but only on the satellite altitude H ES
  RE (RE Earth radius).
• If the angular velocity of the satellite equals
  the angular velocity of the Earth (the case of
  the so-called geosynchronous orbit)
• (Notice the usage of the sidereal day rather
  than the solar day.)

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EOS Orbits 4

• If we consider
• the satellite ground track longitude ?t and
  latitude ?t
• t0 at the so-called ascending node K and
• the zero meridian of ?t passing through K at t0
  and
• the Earth rotating by the angle ?E t
• then the spherical sine and cosine theorems will
  yield

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EOS Orbits 5

• The special geosynchronous case (?S ?E) with
  i0 will yield the geostationary orbit case, with
  even ?t 0. For an observer on the ground, the
  satellite seems to stand still in the sky.
• If ?S is a factor of ?E , i.e. ?S n?E and
  n?N , the satellite will revisit the same point
  on the Earth surface after one day.
• The theoretical upper limit of ?S is set by the
  fact that the orbit height H gt 0.

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EOS Orbits 6

• In practice, the orbit altitude H is chosen so
  that
• where
• nT is an integer representing the number of
  days after which the orbit will repeat itself
  and
• nu is a non-integer representing the number of
  orbits per day.
• The fact that nu is a non-integer ensures that
  the orbits are drifting within the repeat cycle,
  making it possible to observe different swaths of
  the Earth surface with every orbit.

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EOS Orbits 7

• Often it is desired that the incidence angle of
  the solar rays be as constant as possible during
  each orbit passage.
• Compensation for the seasonal variation of the
  solar incidence angle is in practice impossible
• Compensation for the daily variation of the solar
  incidence angle is possible if the satellite
  constantly passes over the same point at the same
  Local Mean Time ? the orbit plane rotates 360
  during one year (so-called sun-synchronous orbit).

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EOS Orbits 8

• The sun-synchronous orbit is achieved by taking
  advantage of the disturbance due to the
  non-sphericity of the Earth.
• At certain orbit heights and inclinations, the
  ellipsoid/geoid shape of the Earth yields exactly
  the desired satellite orbit precession movement.
• In practice, since the orbit height is already
  locked by the desired repeat cycle, the orbit
  inclination is used to control the precession.
• Most Low Earth Orbit (LEO) satellites have
  therefore an inclination of around 100
  (retrograde motion)
• In addition, choosing local mean times
• of passage at around 6 AM / PM will
• ensure that the satellite is never eclipsed

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EOS Orbits 9

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LEO (Low Earth Orbit) 1

• Typically between 200-1200 km altitude
• Main perturbations non-sphericity of earth,
  atmospheric drag
• Advantages
• Low launch energy requirements
• Communication distances short
• Shuttle access
• Disadvantages
• Short ground contact periods (e.g. 10 min)
• Usually frequent eclipses
• Lots of drag
• Typical missions
• Earth observation ERS, Radarsat, Envisat
• Science Spacelab
• Microgravity Eureca

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LEO (Low Earth Orbit) 2
MIR
Landsat-7
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GEO (Geostationary Earth Orbit) 1

• 35 786 km altitude
• Advantages
• Requires only one ground station for continuous
  coverage
• Continuous view of a large part of the Earth
  surface
• Antennae do not have to move (commercial TV
  satellite dishes)
• Disadvantages
• High launch energy requirement
• Complicated injection into equatorial plane
• Needs frequent orbit control manoeuvres
• Typical missions (not only EO)
• Applications OTS, ECS, Marecs, Olympus, Artemis
• Meteorology MSG-1
• Science GEOS

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GEO (Geostationary Earth Orbit) 2
Gorizont 23 (10 days drift)
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GEO (Geostationary Earth Orbit) 3

• Reaching GEO involves a change in altitude and
  usually plane
• Geosynchronous Transfer Orbits (GTO) Hohmann
  transfer
• Sometimes supersynchronous transfer orbits
  (60000-70000 km altitude) are used
• GEO rocket boosters stay in orbit for a long time
  as debris or eventually reentering the atmosphere
  to burn up

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GEO (Geostationary Earth Orbit) 4
Intelsat 4-2 rocket (Atlas Centaur booster)
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Other Orbit Types 1

• Molniya orbits
• High inclination, eccentric, usually 12 h period
  (repeater)
• Used at high latitudes, where GEO satellites
  would be too low on horizon

Molniya 3-47
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Other Orbit Types 2

• MEO (Mid-Earth Orbit)
• Mid-inclination, usually 12h period (repeater),
  circular, approx 20000 km altitude
• e.g. navigation satellite constellations (GPS)
  4 satellites always visible

GPS2-12
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Other Orbit Types 3

• HEO (Highly Eccentric Orbit)
• low perigee height ( 1000 km) and high apogee
  height (several 10000 km)
• Advantages
• Very high resolution achievable around perigee
  (reconaissance)
• Relatively low launch energy requirements
• Most of the time outside of immediate Earth
  environment, i.e. atmosphere, radiation belt
  (suitable for astronomy)
• Rare eclipses
• Long ground coverage periods
• Disadvantages
• Long eclipse duration
• Drag
• Prone to tidal effects
• Typical missions
• Earth science HEOS, Cluster
• Astronomy COS-B, Exosat, ISO, XMM-Newton,
  INTEGRAL

Chandra
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One Day Swath Coverages 1

• Landsat ETM

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One Day Swath Coverages 2

• ASCAT onboard MetOp (double swath)

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One Day Swath Coverages 3

• Modis

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One Day Swath Coverages 4

• SeaWinds onboard QuikSCAT

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One Day Swath Coverages 5

• SRTM-C onboard the Shuttle

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Some Web Links

• To download current spacecraft orbit element
  sets
• http//www.celestrak.com/NORAD/elements/
• Satellite ToolKit from AGI An advanced software
  for space mission and orbit analysis
• http//www.agi.com/products/desktopApp/stkFamily/
• NASA J-Track 3D A useful applet to track down
  satellites
• http//science.nasa.gov/Realtime/JTrack/3d/JTrack3
  D.html