Satellite Orbits

Satellite Satellite Meteorology/ClimatologyMeteorology/Climatology

Professor Menglin JinProfessor Menglin Jin

Satellite Orbits

At what location is the satellite looking?At what location is the satellite looking? When is the satellite looking at a given When is the satellite looking at a given

location?location? How often is the satellite looking at a How often is the satellite looking at a

given location?given location? At what angle is the satellite viewing a At what angle is the satellite viewing a

given location?given location?

Atmospheric Remote Sensing Sensors, Satellite Platforms, and Orbits

Atmospheric Remote Sensing Sensors, Satellite Platforms, and Orbits

Satellite orbits and platformsSatellite orbits and platforms

• Low Earth orbitLow Earth orbit

– Sunsynchronous and repeat coverageSunsynchronous and repeat coverage

– PrecessingPrecessing

• Geosynchronous orbitGeosynchronous orbit Sensor scanning modesSensor scanning modes

• Whiskbroom and pushbroom scannersWhiskbroom and pushbroom scanners

• Active and passive microwave radiometersActive and passive microwave radiometers

Orbital parameters can be tuned to produce particular, useful orbits

• Geostationary• Geosynchronous• Sunsynchronous• Altimetric

Special Orbits

Types of orbits

Sunsynchronous orbitsSunsynchronous orbits: An orbit in which the : An orbit in which the satellite passes every location at the same time satellite passes every location at the same time each dayeach day• Noon satellites: pass over near noon and midnightNoon satellites: pass over near noon and midnight

• Morning satellites: pass over near dawn and duskMorning satellites: pass over near dawn and dusk

• Often referred to as “polar orbiters” because of the Often referred to as “polar orbiters” because of the high latitudes they crosshigh latitudes they cross

• Usually orbit within several hundred to a few Usually orbit within several hundred to a few thousand km from Earththousand km from Earth

Types of orbits

Geostationary (geosynchronous) orbitsGeostationary (geosynchronous) orbits: : An An orbit which places the satellite above the orbit which places the satellite above the same location at all timessame location at all times• Must be orbiting approximately 36,000 km Must be orbiting approximately 36,000 km

above the Earthabove the Earth• Satellite can only “see” one hemisphereSatellite can only “see” one hemisphere

• Ascending and descending orbits should cross at 90º – Designed so that orthogonal components of surface slope will have equal accuracy

• Orbital inclination depends on location of altimetric needs

Altimetric Orbits

SectorSatellites in Orbit

(+mode) Operator LocationLaunch date Status

MTSAT-1R (Op) Japan 140°E 2/26/05 Fully functionalMTSAT-2 (B) Japan 145°E 2/18/06 Back-up to MTSAT-1RGOES-9 (B) USA/NOAA 160°E 5/99 Dissemination not

activatedEast-Pacific GOES-11 (Op) USA/NOAA 135°E 5/00 GOES-West

GOES-10 (B) USA/NOAA 60°W 4/97 South America coverageGOES-12 (Op) USA/NOAA 75°W 7/01 GOES-EastGOES-13 (P) USA/NOAA 89.5°W 5/06 In commissioning

Meteosat-6 (B)EUMETSAT 10°E 11/93 Rapid scan anomalyMeteosat-7 (B)EUMETSAT 0°E 2/97 To be relocated to 57.5°E

Meteosat-8 (Op)EUMETSAT 3.4°W 8/28/02 EUMETCASTMeteosat-9 (P)EUMETSAT 6.5°W 12/21/05 In commissioning

Meteosat-5 (Op)EUMETSAT 63°E 3/91 Functional but high inclination mode

GOMS-N1 (B) Russia 76°E 11/94 Standby since 9/98FY-2C (Op) China/CMA 105°E 10/19/04 Functional

Kalpana-1 (Op) India 74°E 9/12/02 DedicatedINSAT-3A (Op) India 93.5°E 4/10/03 Operational

West-Pacific

West-Atlantic

East-Atlantic

Indian Ocean

Geosynchronous Meteorological SatellitesWMO Member States

Low Earth Orbit Concepts

Equator

South Pole

Ground track

Ascending node

Inclination angle

Descending node

Orbit

Perigee

Apogee

Orbit

Sun-Synchronous Polar Orbit

Satellite

Orbit

Earth Revoluti

on

• Satellite orbit precesses (retrograde)– 360° in one year

• Maintains equatorial illumination angle constant throughout the year– ~10:30 AM in this example

Equatorial illuminatio

n angle

Sun-Synchronous Orbit of Terra

Spacing Between Adjacent Landsat 5 or 7 Orbit Tracks at the Equator

Timing of Adjacent Landsat 5 or 7 Coverage Tracks

Adjacent swaths are imaged 7 days apart

Polar-Orbiting Satellite in Low Earth Orbit (LEO)

Example from Aqua

A precessing low-A precessing low-inclination (35inclination (35°°), low-), low-altitude (350 km) orbit altitude (350 km) orbit to achieve high spatial to achieve high spatial resolution and capture resolution and capture the diurnal variation of the diurnal variation of tropical rainfalltropical rainfall– Raised to 402 km in Raised to 402 km in

August 2001 August 2001

Tropical Rainfall Measuring Mission Orbit (Precessing)

TRMM Coverage

1 day coverage 2 day coverage

The orbital period of a satellite around a The orbital period of a satellite around a planet is given byplanet is given by

where where 00 = orbital period (sec)= orbital period (sec)

RRpp == planet radius (6380 km for Earth)planet radius (6380 km for Earth)

HH == orbit altitude above planet’s surface (km)orbit altitude above planet’s surface (km)

ggss == acceleration due to gravity (0.00981 km sacceleration due to gravity (0.00981 km s-2-2 for for Earth)Earth)

Definition of Orbital Period of a Satellite

T0 2(Rp H )Rp H

gs Rp2

SatelliteAltitude

(km)Inclination

(°)Orbital Period

(min)Repeat

Coverage Orbits/dayJason-1 1336 66 112.3 10 12.8Meteor-3M/SAGE III 1020 99.5 105.5 13.7Landsat 1-3 907-915 99.2 103.1 18 14.0SPOT 832 98.7 101.5 26 14.2NOAA 850 98-99 102-104 11 14.0QuikScat 803 98.6 100.9 14.3ACRIMSAT 720 98.1 99.1 14.5Landsat 4-7 705 98.2 98.8 16 14.6Terra, Aqua, Aura 705 98.2 98.8 16 14.6

ICESat 600 94 96.6 – 14.9UARS 585 57 96.3 – 14.9ERBS 610 57 96.8 – 14.9SORCE 640 40 97.5 – 14.8TRMM 402 35 92.6 – 15.6TRMM 350 35 91.5 – 15.7

Orbital Characteristics of Selected MissionsLow Earth Orbit & Precessing Missions

Ellipse An ellipse is defined as follows: For two given points, the An ellipse is defined as follows: For two given points, the focifoci, ,

an ellipse is the an ellipse is the locuslocus of points such that the of points such that the sumsum of the of the distance to each focus is distance to each focus is constantconstant..

BTW, Locus-A word for a BTW, Locus-A word for a setset of of pointspoints that forms a that forms a geometric figuregeometric figure or or graphgraph

Kepler’s laws

1. Satellites follow an elliptical orbit with the Earth as one 1. Satellites follow an elliptical orbit with the Earth as one focusfocus

PerigeeApogee

Foci

Period of orbit

Valid only for circular orbits (but a good Valid only for circular orbits (but a good approximation for most satellites)approximation for most satellites)

Radius is measured from the center of the Earth Radius is measured from the center of the Earth (satellite altitude+Earth’s radius)(satellite altitude+Earth’s radius)

Accurate periods of elliptical orbits can be Accurate periods of elliptical orbits can be determined with Kepler’s Equation determined with Kepler’s Equation

T2= r342

Gme

Period of orbit

Gravitational constant Mass of the Earth

Radius of the orbit

Sunsynchronous image (SMMR)

Geostationary Image (GOES-8)

•Meteosat•GOES

•Operational•Specific wave bands•Specific use•High temporal frequency

Geostationary satellites

Space-time sampling

GeostationaryGeostationary• Fixed (relatively) field of viewFixed (relatively) field of view• View area of about 42% of Earth’s surfaceView area of about 42% of Earth’s surface

SunsynchronousSunsynchronous• Overlapping viewsOverlapping views• See each point at several viewing anglesSee each point at several viewing angles

Other orbits (“walking orbits”)Other orbits (“walking orbits”)• Passes each location at a different time of dayPasses each location at a different time of day• Earth Radiation Budget SatelliteEarth Radiation Budget Satellite• Useful when dirunal information is neededUseful when dirunal information is needed

• Cross-track scanner• Whiskbroom scanner• Pushbroom sensor

Imaging Systems

Cross-track Scanner

• “back and forth” motion of the foreoptics• scans each ground resolution cell one-by-one• Instantaneous Field of View (IFOV) ofinstrument determines pixel size• Image is built up by movement of satellitealong the orbital track and scanning acrosstrack

Along-track scanner(“Pushbroom)

• Linear array of detectors (aligned cross-track)– reflected radiance passes through a lens and onto a line of detectors

• Image is built up by movement of the satellite along its orbital track (no scanning mirror)

• Area array can also be used for multi-spectral remote sensing– dispersion used to split light into narrow spectralbands and individual detectors

Scanning techniques VidicomVidicom

• Like television camera; “sees” everything at onceLike television camera; “sees” everything at once SwingingSwinging

• Results in a zig-zag pattern of scanningResults in a zig-zag pattern of scanning SpinningSpinning

• Satellite spins in order to create imageSatellite spins in order to create image PushbroomPushbroom

• Multiple scanning elements, relies on forward motion of satelliteMultiple scanning elements, relies on forward motion of satellite

• On-board recording and pre-processing

• Direct telemetry to ground stations– receive data transmissions from satellites– transmit commands to satellites (pointing,turning maneuvers, software updating

• Indirect transmission through Trackingand Data Relay Satellites (TDRS)

Getting the Data to the Ground