Remote Sensing and Active Remote Sensing and Active TectonicsTectonics

Barry Parsons and Richard WalkerBarry Parsons and Richard Walker

Michaelmas Term 2013Michaelmas Term 2013

Lecture 2Lecture 2

Satellite Orbits (1)Satellite Orbits (1)

1. Satellite orbit is an ellipse, with the centre of mass of the Earth at one focus of the ellipse.

2. In practice, remote sensing satellites are in almost circular orbits, i.e. the eccentricity e is small (e << 1).

Newton’s law of motion:

Newton’s law of gravitation:

Period of orbit:Altitude (km) Period

200 88 mins

700 99 mins

36000 1447 mins

= 24.11 hours

M

FRv

m

Satellite Orbits (2)Satellite Orbits (2)

Athens

Istanbul

What was the Time of Day?

Mount Parnassos

Delphi

Effect of Time of Day on the ImageEffect of Time of Day on the Image

1. Time of day affects length of shadow. It is this contrast that reveals reveals topographic features. Want some shadowing, i.e. not midday when Sun is overhead, but not too much, i.e. not early morning. Time of day also affects illumination.

2. If the orbital plane remained fixed in space, which is what would happen if the earth really did act like a point mass, then the time the image was acquired would vary throughout the year as the Earth rotates about the Sun.

3. It is convenient if the images at a given location are always acquired at the same solar time.

Orbital PrecessionOrbital Precession

1. The satellite moving round its orbit has angular momentum like a top.

2. The mass in the equatorial bulge produces a net torque on the satellite.

3. The torque causes the orbital plane to precess like the earth’s gravity causes a top to precess.

Sun-synchronous OrbitsSun-synchronous Orbits

Rate of orbital precession:

For sun-synchronous orbits: km

(J2 is the coefficient of the P20 term in the expansion for the gravitational

potential that describes the gravitational effect of the equatorial bulge.)

Choosing the Orbital ParametersChoosing the Orbital Parameters

1. The period of orbital motion is determined by the orbital radius R and inclination i.

2. For any R can choose i such that orbit is sun-synchronous.

3. Also choose R and i such that the period of orbital motion satisfies the condition

where Ωe is the rate of rotation of the Earth, and no and nr are integers. This condition means that, after no orbits and nr rotations of the Earth relative to the orbital plane, the orbit retraces its ground track. This means that any location can be re-imaged on a regular basis. For Landsat 7 coverage repeats after 233 orbits and 16 days.

Orbital Parameters for Landsat 7Orbital Parameters for Landsat 7

Ground Coverage for Landsat 7Ground Coverage for Landsat 7

Imaging by Line ScanningImaging by Line Scanning

Spectral Bands For Landsat 7 Enhanced Spectral Bands For Landsat 7 Enhanced Thematic Mapper Thematic Mapper

Line Scanning System for Landsat 7Line Scanning System for Landsat 7

Arrangement of Detectors for Landsat 7 Arrangement of Detectors for Landsat 7 ETMETM

Location of 1999 Izmit EarthquakeLocation of 1999 Izmit Earthquake

Landsat TM image (542) acquired 1 day after the 1999 Izmit earthquake

Landsat Image Scanning Revealed by Saturated Landsat Image Scanning Revealed by Saturated BandBand

SPOT Earth Observation SatellitesSPOT Earth Observation Satellites

Dual Instruments Pushbroom Recording

BandSpectral Range (μm)

Resolution (m) (Spot 1-3)

Resolution (m) (Spot 4)

Resolution (m) (Spot 5)

1 0.50-0.59 20 20 10

2 0.61-0.68 20 20 10

3 0.79-0.89 20 20 10

SWIR 1.58-1.75 20 20

Panchromatic 0.51-0.73 10

Panchromatic 0.61-0.68 10

Panchromatic 0.49-0.69 2.5/5

Spectral Bands for Spot SatellitesSpectral Bands for Spot Satellites

Spot Landsat

Altitude 822 km 705 km

Inclination 98° 98.2°

Period 101.4 mins 98.8 mins

W track drift 2823 km 2752 km

Revs/day 14+5/26 14+9/16

Cycle duration 26 days 16 days

Orbits per cycle 369 233

Swath width 60 km 185 km

Stereo Imaging by the Spot SatellitesStereo Imaging by the Spot Satellites