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Satellite Image Geometry

Gene DialGIS in the Rockies

2012-09-21

Topics

› Background› Satellite Orbits› Satellite Image Geometry› Stereo Image Geometry› Satellite Image Products

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Niagara Falls

Background

Remote Sensing—Then & Now!

Mys Shmidta Air Field, Soviet Union

Collected August 18th, 1960GeoEye-1 Half Meter Imagery

Kutztown University – Collected Oct. 6, 2008

GeoEye-2 Technical Specs at a GlanceSystem Specification PerformanceSatellite Bus Size

Weight2.3 m x 5.3 m2100 kg dry, 2500 kg wet

Payload ApertureFocal LengthDynamic RangeGSDSwath

1.1 meter aperture16 meter focal length11 bit dynamic range with TDI34 cm Pan, 1.32m MSI14.5 km

Attitude Control System

Actuators

Sensors

Minimum Agility

Honeywell M-95 CMGs

Goodrich GR-1004 Star TrackersSIRU Inertial Reference UnitsMonarch GPS Receiver

Acceleration - 1.0 degree/sec2 Max Slew Rate – 2.7 degree/sec

Data Handling &Communications

Data Recorder

Wideband DLTlmy DLCommand UL

3.2 Terabit High Speed Storage Unit

800 Mbps Dual Pole, X band128 Kbps, X band64 Kbps, S band

X-band High

Gain AntennaTwo Axis Gimbal

Solar Array (5)Power

Control Unit

Battery (2)

Control Moment Gyros (4)

Data Storage Unit (2)Flight

Processor

CMG Electronics (4)

Focal Plane

Radiator

GPS Antenna (2)

Narrowband Antenna (2)

Sun Sensor (2)

Payload Electronics (2)

PL Electronics

Radiator (2)

Star Tracker (2)

See http://www.youtube.com/watch?v=lnv6cDiBF9o 5

Satellite Imagery

Satellite Orbits

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GeoEye-1 IKONOSGeoEye-2

GeoEye ConstellationGeoEye Constellation

Ground SwathsGE1 = Green GE1 = Green IK = Yellow IK = Yellow GE2 = BlueGE2 = Blue

Ground SwathsGE1 = Green GE1 = Green IK = Yellow IK = Yellow GE2 = BlueGE2 = Blue

› Frequent Access‒ Mean Time to Access < 1 day

› Long Duration Accesses‒ Average Access Time ~ 1 min/day

› High Resolution Access‒ GeoEye-1: 41 cm Nadir GSD‒ GeoEye-2: 34 cm Nadir GSD‒ True 50 cm products

› Huge Collection Capacity‒ IKONOS: 240,000 sqkm/day‒ GeoEye-1: 350,000 sqkm/day‒ GeoEye-2: 600,000 sqkm/day

GeoEye ConstellationHigh Resolution ImagesMove Beyond Mapping

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Satellite Image Geometry

Ground Sample Distance (GSD)

› Source image pixels are rectangular, W x H in size› GSD = sqrt(W x H)› A square pixel of GSD x GSD size has the same area as W x H› Product images may be resampled to a different GSD

W

H

GSD

GSD

Field of View

› Field of View (FOV) is angle from one edge of an image to the other.

› All rays of a high-resolution satellite image are at about the same angle.

Aerial camera

Satellite imaging at nadir

Satellite imaging off nadir

SatelliteField of View

Aerial CameraField of View

Camera FOV

Aerial 90°

IKONOS 0.95°

GeoEye-1 1.28°

GeoEye-2 1.22°

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Scan Azimuth› Scan Azimuth

‒ Describes scan direction or motion of aim point on ground

‒ North-to-South Scan Azimuth = 180°

‒ South-to-North Scan Azimuth = 0°

0° = North

90° = East

180° = South

270° = West

So

uth

to

No

rth

Sca

n A

zim

uth

0°

West to EastScan azimuth 90°

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East to WestScan azimuth 270°

Line of Sight (LOS)

› The Line of Sight (LOS) is the direction that the camera is imaging.

› A Line of Sight direction can be described by azimuth and elevation angles.

Lin

e of S

igh

t (LO

S)

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Satellite Collection GeometrySatellite Collection Geometry

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Azimuth

› Azimuth angle‒ Measured in the horizontal plane

at the target‒ Angle from north proceeding

clockwise to the projection of the line of sight into the horizontal plane.

‒ Example: 90° azimuth means the satellite is East of the target when the image is taken.

0° = North

90° = East

180° = South

270° = West

Collection Azimuth› North Up View

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Collection Azimuth› View from sensor perspective

GE1 Image acquired at 53.5° collection azimuth rotated180° - 53.5° CW on right to view from sensor perspective.20

Elevation

› Elevation angle‒ Measured at target‒ Angle from horizontal plane up to

line of sight.

› Alternatives‒ Incidence or Zenith angle‒ Off-Nadir or Obliquity angle

Elevation angle

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Computing height from shadows & layover

SensorElevation

Angle

Sensor

Layover

DV = DH * tan(EL)

DH

DV DV

DH

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Example: Republic Plaza (Singapore)Image collected at 67°

elevation angle Layover measured at 116

mHeight calculationH = 116 m * tan(67°) = 273 mActual height

280 m

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Elevation angle and terrain displacement

› EL = elevation angle› DV = vertical distance› DH = horizontal distance

EL

DH

DV

Zenith Sensor

Earth

DEM

DH = DV / tan(EL)

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Incidence, Elevation, & Off-Nadir Angles

› EL = Elevation = angle at target from horizontal to sensor.› IN = Incidence = angle at target from zenith to sensor.› OB = Obliquity = angle at sensor from nadir to target (off-nadir angle)› IN + EL = 90°› Obliquity is related to elevation by trig formula:

‒ Re = radius of earth ~ 6371 km

‒ Ho = orbit height ~ 681 km

)(

)(

eo

e

RH

ELCosRArcSinOB

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Incidence, Elevation, & Off-Nadir Angles

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Stereo Geometry Orbit Track

Ground Track

AOI

EL1EL2

Convergence Angle

About one minute of orbit timebetween left and right image ofa stereo pair.

About two seconds of orbit timeto scan a 15 km by 15 km stereo scene. Longer scans are possible. A 100 km long stereo pair takes about 20 seconds to scan.

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Field of Regard (FOR)

› Field of Regard: Angle Range that Camera can Image by rotating› Satellite Field of Regard > 90°.› Field of View can be anywhere within the Field of Regard

90° FOR±45° FOR

0.95° FOV within Field of Regard

Field of Regard vs. Elevation Angle

› Wider Field of Regard at lower elevation angle› Wider Field of Regard from higher orbits

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0 100 200 300 400 500 600 700 800 900 10000.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Cross-Track Distance, km

GS

D,

m

GSD vs. Cross-Track Distance

IKONOS

GE1GE2

RunSatComparison

Field of Regard

60° Elevation Angle

60° Elevation Angle

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Revist Time (time between satellite accesses)

› Shorter revisit time at lower elevation angle & higher latitude

3-day revisit at 40° N at 60° elevation angle

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Revisit Time

› More frequent revisits at high latitudes because the orbits converge near the poles.

› Ground stations are located at high latitudes can contact the satellite nerly every orbital revolution.

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4

3

13

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15910

11

12

13

7

5

15

6

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Pan-MSI Alignment› Each MSI pixel covers 4x4 Pan pixels› 4 multispectral (MSI) bands› 1 panchromatic (PAN) band› Simultaneous PAN/MSI collection› 11-bit resolution

MSIGSD

Pan GSD

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CIR RGB

4-meter RGB Multispectral

4-meter CIR Multispectral 1-meter CIR Pan-Sharpened

1-meter RGB Pan-Sharpened

1-meter Panchromatic

Color enhances

interpretation for human

visual perception

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Camera Models

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Rational Polynomial Coefficient (RPC) Camera Models› RPC Camera Models

‒ Generic mathematical model mapping ground to image coordinates.

‒ Sensor software fits coefficients to physical camera model of image.

• Sensors‒ GeoEye, Ikonos, QB, WV, Cartosat …• Application Software‒ ERDAS, BAE, PCI, ZI, …

› Applications‒ Block adjust images with ground control

to improve accuracy.‒ Orthorectification‒ Stereo extraction‒ PhotogrammetryThe mathematics of satellite imagery is

complicated, but RPC models are simpleThe mathematics of satellite imagery is complicated, but RPC models are simple

Satellite Image Product Geometry

Product Geometry

Product Rectification Projection Image Model

Basic Synthetic Array Satellite Scan PathPhysical (attitude,

ephemeris & camera calibrations)

Geo Constant height Map RPC

Ortho DEM Map Ortho

Stereo Constant height Path, Map, or Epi-polar RPC or Physical

Elevationangle

Convergenceangle

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BASIC› Photogrammetric

Applications › Satellite Projected› Physical Camera Model

‒ High Accuracy

› RPC Camera Model‒ Rapid Positioning

IKONOS image of the moon (BASIC product)

GEO› Visual Interpretation

‒ Situational awareness‒ Intelligence‒ Media

› Photogrammetry‒ Block adjust with other imagery

or GCP to improve accuracy.‒ Orthorectify with DEM to

correct for terrain displacement

› Map Projected› RPC Camera Model

‒ High Accuracy

Tsangpo River Basin, Tibet

BASIC and GEO Products

Geo

East to West ScanNorth up Map Projected

N

E

S

W

N

E

S

W

BASIC GEO

RPC Model Physical Model

ProjectionSatellite Map

BASIC

East to West Scan Satellite Projected

Ortho› Applications

‒ Feature Extraction‒ GIS Map Base

› Terrain Corrected› Map Projected› Mosaics Available

Frankfurt Airport, Germany

Georectified or Orthorectified?› Georectified

‒ Terrain displace-ment errors

‒ Quick, Low cost

› Orthorectified‒ DEM corrects for

terrain displacement

‒ Accuracy for mapping

TopographicSurface

Constant Height Line of Sight

Topo-graphicSurface

Ortho-rectifiedImage

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March, 2009

Geospatial eXploitation Products™

What is an Orthophoto?

• An orthophoto is an image that has had all distortion due to camera obliquity, terrain relief, and features removed.

• The SOCET GXP Ortho Manager converts one or more original images into an orthophoto by transforming the pixels to their proper position according to the given sensor, terrain, and feature information.

• In the final product all points in the image appear as if the observer were looking down from nadir position.

DTM

Camera

Orthophoto

Original Image

Orthorectified

Georectified

Stereo› Attributes

‒ High resolution‒ Color‒ 3-dimensional

› Applications‒ DEM extraction‒ 3D feature extraction‒ Geomorphic

visualization

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Stereo

Projection

RPC

Model

Physical

Model

Satellite

Map Epi-Polar

Stereo Image Of Downtown Denver, Colorado

Big Bear Glacier, Alaska

Thank You!