CSCDA Coordinated Data-access System (CDS) version 3

Customer: ESA Ref: CDS-TPZ-03-000077-TR

Contract No: 4000110565/14/I-LG Date: 08/01/2019

Title: Copernicus data Quality Control - Technical Note - Harmonisation of terms relevant to angles

Issue: 3.0

Document Title: Copernicus data Quality Control - Technical Note - Harmonisation of terms relevant to angles

CSCDA - Coordinated Data access System (CDS) version 3

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CSCDA – Coordinated Data-access System (CDS) version 3

Evolutions, Operations and Maintenance

Copernicus data Quality Control - Technical Note

Harmonisation of terms relevant to angles

Reference: CDS-TPZ-03-000077-TR

DIL ID: D-067

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Date: 08/01/2019




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This Page is Intentionally Blank




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Title:

Copernicus data Quality Control - Technical Note

Harmonisation of terms relevant to angles

Abstract: This technical note has been created to support ESA harmonisation activities towards the CCMEs in relation to the terminology used to identify various angles involved in the acquisition process.

Author: Fabio D. Vescovi, Charles Owen Verified: Lisa Haskell, Andrea Minchella

CDS V3 CQC Team

CDS V3 CQC Domain Leader

Approved: Daniel Smyth Authorised: Pierre Féménias, Clément Albinet, Valentina Boccia

CDS V3 EOM Operations Manager

ESA CDS V3 CQC Technical Officer

Accepted by ESA:

Filename:

D-067_CQC_T7_TN_09_v3.docx

Copyright © 2014 Telespazio S.p.A.

This document discloses subject matter in which Telespazio S.p.A. has proprietary rights. Recipient of the document shall not duplicate, use or disclose in whole or in part, information contained herein except for or on behalf of Telespazio to fulfil the purpose for which the document was delivered to him.

www.telespazio.com

Amendment Policy

This document shall be amended by releasing a new edition of the document in its entirety. The Amendment Record Sheet below records the history and issue status of this document.




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Amendment Status Sheet

Issue Date Reason for the Modification

1.0 04/04/2017 Document Creation

2.0 14/09/2018 Major internal review

3.0 08/01/2019 Final version, accepted by ESA

Document Change Record History

No Change in Issue

Description of Change Section Change Made By

1 1.0 Document Creation ALL ESA CQC Team

2 1.1 Document Update ALL Charles Owen

3 2.0 Document Update ALL ESA CQC Team

4 3.0 Document finalized ALL ESA CQC Team

Applicable and Reference Documents1

No Title Reference code

Issued by

[AD1] CSCDA Metadata and Filename Value Convention OSME-GSCDA-SEDA-TN-09-0201

ESA

[RD01] D2.8.II.3 Data Specification on Orthoimagery – Technical

Guidelines D2.8.II.3_v3.0 INSPIRE

[RD02] Pléiades Imagery User Guide http://www.cscrs.itu.edu.tr/assets/downloads/PleiadesUserGuide.pdf last seen 08/01/2019

- Airbus DS

[RD03] Guide to Magellan Image Interpretation

http://history.nasa.gov/JPL-93-24/p46.jpg last seen 08/01/2019 - NASA

[RD04] Data Products, Canada Centre for Remote Sensing, Natural Resources Canada. http://www.nrcan.gc.ca/node/9309 last seen 08/01/2019

- NRC

1 Further literature is quoted directly in text of tables 1 to 9. All websites last seen Sept. 2018.

http://www.cscrs.itu.edu.tr/assets/downloads/PleiadesUserGuide.pdf

http://history.nasa.gov/JPL-93-24/p46.jpg

http://www.nrcan.gc.ca/node/9309




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Table of Contents

TABLE OF CONTENTS .............................................................................................................................................................................. 5

LIST OF FIGURES ....................................................................................................................................................................................... 6

LIST OF TABLES ......................................................................................................................................................................................... 6

LIST OF ACRONYMS ................................................................................................................................................................................. 7

TECHNICAL NOTE ..................................................................................................................................................................................... 8

1. INTRODUCTION.................................................................................................................................................................................................. 8 2. CURRENT TERMS USED BY THE CCMES AND DEFINITIONS........................................................................................................................ 9 3. CQC PROPOSAL .............................................................................................................................................................................................. 18

3.1 Optical Terms ........................................................................................................................................................................................ 19 3.2 SAR Terms ............................................................................................................................................................................................... 22 3.3 Impact of the proposed terms on IPS Visualisation Tool, INSPIRE.xml and CR .......................................................... 23 3.4 Dissemination of the proposed terms ........................................................................................................................................... 26

4. APPENDIX I: BGDS ........................................................................................................................................................................................ 27




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List of Figures

FIGURE 1: A DIAGRAM INDICATING THE AZIMUTH ANGLE OF THE SUN. .................................................................................................................... 10

FIGURE 2 - DIAGRAM DEPICTING ELEVATION ANGLE .................................................................................................................................................... 11

FIGURE 3 – REPRESENTATION OF INCIDENCE AND VIEWING ANGLES. ....................................................................................................................... 12

FIGURE 4 - PITCH, ROLL AND YAW ANGLES. .................................................................................................................................................................... 14

FIGURE 5 – REPRESENTATION OF IMAGING MODES AND RELEVANT INCIDENCE ANGLES IN RADARSAT-1. (FROM [RD04]) .......................... 16

FIGURE 6 – ILLUMINATION AND COLLECTION AZIMUTH ANGLES, ILLUMINATION AND COLLECTION ELEVATION ANGLES. ............................. 19

FIGURE 7 - INCIDENCE AND VIEWING ANGLES (FROM [RD02]). ................................................................................................................................ 20

FIGURE 8 - PITCH, ROLL AND YAW ANGLES. .................................................................................................................................................................... 21

FIGURE 9 - NEAR AND FAR INCIDENCE ANGLES AND NEAR LOOK ANGLE. ................................................................................................................. 22

FIGURE 10 – CAMERA TILT IN THE IPS VISUALISATION TOOL, OPTICAL PART, IS REPORTED IN THE IVOS-29 BOX. ....................................... 23

FIGURE 11 - COVERAGE AND TEMPORAL CHARACTERISTICS IN THE IPS VISUALISATION TOOL, SAR-24 BOX. ................................................ 24

FIGURE 12 - LOCAL INCIDENCE ANGLE. ........................................................................................................................................................................... 29

List of Tables

TABLE 1 - OPTICAL TERM: AZIMUTH ANGLE ................................................................................................................................................................. 11

TABLE 2 - OPTICAL TERM: ELEVATION ANGLE .............................................................................................................................................................. 12

TABLE 3 - OPTICAL TERM: INCIDENCE ANGLE ............................................................................................................................................................... 13

TABLE 4 - OPTICAL TERM: OFF NADIR VIEW ANGLE .................................................................................................................................................... 13

TABLE 5 - OPTICAL TERM: VIEWING ANGLE .................................................................................................................................................................. 14

TABLE 6 - SAR TERM: ATTITUDE ANGLE ....................................................................................................................................................................... 14

TABLE 7 - SAR TERM: AZIMUTH STEERING ANGLE ...................................................................................................................................................... 15

TABLE 8 - SAR TERM: INCIDENCE ANGLE ...................................................................................................................................................................... 16

TABLE 9 - SAR TERM: LOOK ANGLE................................................................................................................................................................................ 17




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List of Acronyms

Acronym / abbreviation

Description

AOI Area Of Interest BRDF Bidirectional Reflectance Distribution Function BGD BackGround Document (a type of CQC QII) CCM Copernicus Contributing Mission CCME Entity responsible for one or more Copernicus Contributing Missions CDS Coordinated Data-access System (v3 in development, operations and evolution) CE90 Circular Error at 90th percentile CQC Copernicus data Quality Control CSCDA Copernicus Space Component Data Access CSP Copernicus Service Provider DG Digital Globe DEM Digital Elevation Model DMC Disaster Management Constellation of EO satellites DN Digital Number GCP Ground Control Point GRI Global Reference Image EO Earth Observation EUSI European Space Image JRC Joint Research Centre MODIS MODerate-resolution Imaging Spectroradiometer MS Multi Spectral QII Quality Information Item QL Quick Look RMSE Root Mean Square Error RPC Rational Polynomial Coefficients SAR Synthetic Aperture Radar SCI Services Customer Interface SNR Signal to Noise Ratio TOA Top of the Atmosphere




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Technical Note

1. Introduction

This Technical Note deals with the ambiguities related to angles involved during the satellite acquisition process. After reviewing various satellite image metadata files provided by the CCME’s and the relevant scientific literature on the subject, the CQC Team identified various ambiguities which are reported and discussed in this Technical Note.

The term ‘acquisition angle’ is often used as a collective term that encompasses the varying angles involved during the acquisition of satellite imagery. Typically, this term refers to at least two angles, the ‘Viewing angle’ and the ‘Incidence angle’. However, CCME’s refer to viewing angle and incidence angle in a wide variety of different ways and often state the angles as ‘across’/’along’ track or ‘near’/’far’. In addition, the minimum, maximum and mean viewing/incidence angles are sometimes provided. This indicates a lack of harmonisation between CCME’s in relation to defining viewing and incidence angles.

Other angles that are sporadically used by the CCME’s include ‘nominal collection Azimuth’, ‘nominal collection elevation’, ‘off nadir view angle’, ‘Azimuth angle’, ‘illumination Azimuth angle’, ‘illumination elevation angle’, ‘look angle’, attitude angles’, ‘Azimuth steering angle’ and ‘heading angle’. Due to the sparsity of definitions for these terms in scientific literature it is advisable that some of these should be discounted as correct terms for scientific literature. The used terms not only vary between CCMEs but in some cases are also erroneously used interchangeably which causes confusion and misunderstandings in the generic literature. Especially when working at any project which requires overlapping of multi-source imagery on the same map, this misunderstanding may lead to unacceptable image discrepancies.

Harmonisation of the use and definition of these terms is required. This harmonisation can be achieved by setting out clear definitions of the various terms and identifying which terms best describe the angles involved in the acquisition of satellite imagery, in the relevant context.

This Technical Note is structured as follows:

Review of the CCME technical publications and scientific literature to identify the terms currently in use in their literature (chapter 2);

CQC proposal of recommended terms: some relevant terms are proposed and defined in their context. Furthermore, their use is clarified and their impact (changes) on the current Copernicus documentation is proposed (chapter 3);

Proposed Background Documentation (BGD): in order to set clear definitions, disambiguation and use of the terms, a number of relevant BGDs are created and proposed. These will be disseminated to the users through the online IPS Visualisation Tool and a planned Glossary (Appendix I).




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2. Current terms used by the CCMEs and definitions

After reviewing the CCME literature, various tutorials, scientific and commercial literature, the CQC Team identified the most common terms used when referring to acquisition angle. Their varying definitions and relevant sources are reported in the Tables 1 to 9 in chapter 2.1.

For the sake of clarity, the header of these tables will note if a term is used predominately when discussing optical or SAR imagery. The considered terms are the following (in alphabetic order):

Optical terms:

Azimuth angle (collection/illumination)

Elevation angle (collection/illumination)

Incidence angle (along/across track)

Off nadir view angle

Viewing angle (along/across track)

SAR terms:

Attitude angle

Azimuth steering angle

Incidence angle (near/far)

Look angle (near/far)

As demonstrated in the various definitions reported from Tables 1 to 9, numerous variations of definitions can be found for each of the angle terms used by CCMs, various tutorials, scientific literature and end users. While in general the usages are broadly the correct in the various sources of these definitions, discrepancies and ambiguities are present.

For example, the difference between ‘illumination Azimuth angle’ (from the Sun) and ‘collection Azimuth angle’ (from the sensor) is essential but both of these terms are interchangeably described as ‘Azimuth angle’ in some literature. The same applies for elevation angles. Additionally, the term ‘incidence angle’ is differently defined in optical and SAR and, within SAR, it is ambiguous across CCMEs and tutorials (Table 8).




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Optical Term: Azimuth angle

Source Definition CCME documentation

Airbus FR

http://www.cscrs.itu.edu.tr/assets/downloads/PleiadesUserGuide.pdf

Azimuth: The arc of the horizon measured clockwise from the north point to the point referenced, expressed in degrees. Azimuth indicates direction, and not location.

BlackBridge

Satellite Imagery Product Specifications, Version 6.1

Illumination Azimuth Angle: Sun Azimuth angle at centre of product, in degrees from North (clockwise) at the time of the first image line.

Education tutorials

http://whatis.techtarget.com/definition/Azimuth -and-elevation

The Azimuth angle is the compass bearing, relative to true (geographic) north, of a point on the horizon directly beneath an observed object (see figure 1).

The horizon is defined as a huge, imaginary circle centred on the observer, equidistant from the zenith (point straight overhead) and the nadir (point exactly opposite the zenith).

As seen from above the observer, compass bearings are measured clockwise in degrees from north. Azimuth angles can thus range from 0 degrees (north) through 90 (east), 180 (south), 270 (west), and up to 360 (north again).

http://pveducation.org/pvcdrom/2-properties-sunlight/Azimuth -angle

The Azimuth angle is the compass direction from which the sunlight is coming. At solar noon, the sun is always directly south in the northern hemisphere and directly north in the southern hemisphere.

The Azimuth angle varies throughout the day as shown in the animation below. At the equinoxes, the sun rises directly east and sets directly west regardless of the latitude, thus

making the Azimuth angles 90° at sunrise and 270° at sunset (see Figure 1).

Figure 1: A diagram indicating the Azimuth angle of the sun.

http://whatis.techtarget.com/definition/azimuth-and-elevation

http://whatis.techtarget.com/definition/azimuth-and-elevation




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Table 1 - Optical Term: Azimuth angle

Optical Term: Elevation angle


DigitalGlobe

QuickBird Imagery Products – Product Guide

The spacecraft elevation angle measured from nadir to the product centre-line as seen from the spacecraft

Planet Labs (former BlackBridge)

Satellite Imagery Product Specifications, Version 6.1

Sun elevation angle at centre of product, in degrees. (definition of the term “illuminationElevationAngle” as found in the product metadata)

Education tutorials

http://pveducation.org/pvcdrom/2-properties-sunlight/elevation-angle

The elevation angle (used interchangeably with altitude angle) is the angular height of the sun in the sky measured from the horizontal. Confusingly, both altitude and elevation are also used to describe the height in meters above sea level. The elevation is 0° at sunrise and 90° when the sun is directly overhead (which occurs for example at the equator on the spring and fall equinoxes).

The elevation angle varies throughout the day. It also depends on the latitude of a particular location and the day of the year.

Figure 2 - Diagram depicting elevation angle

http://whatis.techtarget.com/definition/Azimuth -and-elevation

The elevation (el) angle, also called the altitude, of an observed object is determined by first finding the compass bearing on the horizon relative to true north, and then measuring the angle between that point and the object, from the reference frame of the observer. Elevation angles for objects above the horizon range from 0 (on the horizon) up to 90 degrees (at the zenith). Sometimes the range of the elevation coordinate is extended downward from the horizon to -90 degrees (the nadir). This is useful when the observer is located at some distance above the surface, such as in an aircraft.

Commercial literature







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http://www.landinfo.com/buying-optical-satellite-imagery-2.html

Sun elevation is the angle of the sun above the horizon. Imagery collected with low sun elevation angles may contain data that are too dark to be usable. A typical minimum sun elevation angle is 30 degrees, but adhering to this requirement means that northern latitudes above 35 degrees will have black-out periods during the winter months when imagery with an acceptable sun elevation angle can’t be collected.

Decreasing the minimum required sun elevation angle to 15 degrees means that only northern latitudes above 50 degrees will have a black-out period; even a 30-degree sun angle is low for many applications. For example, increased shadow areas are problematic for classification and stereo projects. This will be more pronounced in high-relief areas and areas with taller objects, such as trees and buildings, where low sun elevation angles mean longer shadows will be cast.

Table 2 - Optical Term: Elevation angle

Optical Term: Incidence angle


Airbus FR

http://www.intelligence-airbusds.com/en/6201-angle-conversion

The incidence angle is the angle from the target point of view. It represents the angle between the ground normal and look direction from the satellite, combining the pitch and roll angles.

Figure 3 – Representation of incidence and viewing angles.

Airbus FR

Pleiades Imagery User Guide, October 2012

Incidence angle β (or global incidence) is the angle between the ground normal and look direction from satellite.

Incidence angle and Azimuth of satellite track: The look direction from the satellite on the ground may be projected on two planes: (normal to the ground, Scan Axis) and (normal to the ground, OrthoScan axis). Thus, incidence may be measured in both planes:

Incidence Angle Along Track: Incidence in the Scan axis direction (image line axis on the ground)

Incidence Angle Across Track: Incidence in the OrthoScan axis direction







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Planet Labs (former BlackBridge)

https://earth.esa.int/documents/10174/896711/RE_Product_Specifications_ENG.pdf

The angle between the view direction of the satellite and a line perpendicular to the image or tile centre.

Table 3 - Optical Term: Incidence angle

Optical Term: Off nadir view angle


Planet Labs (former BlackBridge) https://earth.esa.int/documents/10174/896711/RE_Product_Specifications_ENG.pdf

The angle between nadir and the point on the ground that the satellite is pointing to.

Digital Globe QuickBird Imagery Products, Product Guide Revision 4.7.3

The angle between nadir and the point on the ground that the sensor is pointing. Off-nadir angle can be measured in the along-track (forward) direction or across-track (sideways) direction.

Tutorials

CloudEO GIS Dictionary

http://www.cloudeo-ag.com/definition-terms

The off-nadir angle is the angle between the acquired scene's centre, the satellite and the nadir (the normal to the Earth from the satellite's centre). Each image operation is defined by its starting and ending off-nadir angle, which changes along the imaging process (as the satellite moves in its orbit). As a rule, the smaller the off-nadir angle is, the better the resolution (GSD) is.

Table 4 - Optical Term: Off nadir view angle

Optical Term: Viewing angle


Airbus FR http://www.intelligence-airbusds.com/en/6201-angle-conversion

The viewing angle is the angle from the instrument point of view. It represents the angle between the look direction from the satellite and nadir, combining the pitch and roll angles

(see Figure 3 in Table 3).

Airbus FR Pleiades Imagery User Guide, October 2012

Look direction from the satellite may be projected onto two planes defined in the local orbital frame: (yaw axis, pitch axis) and (yaw axis, roll axis) Thus, viewing may be measured in both planes: Viewing angle across track: viewing angle in the across-track axis direction (roll)

Viewing angle along track: viewing angle in the along-track axis direction (pitch)















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As this angle combines the platform pitch and roll angles, it is sometimes also referred to as the Viewing angle. The pitch, roll and yaw rotations of the spacecraft are illustrated in

Figure 4 .

Figure 4 - Pitch, roll and yaw angles.

Table 5 - Optical Term: Viewing angle

SAR Term: Attitude angle

Source Definition Tutorials

The Unplanned Roll, Earth Observatory (NASA)

http://earthobservatory.nasa.gov/Features/LearningToFly/fly_3.php

Attitude manoeuvres (that) involve changing the orientation of the spacecraft in any of three possible axes (called roll, pitch, and yaw). Spinning on its X-axis is called a “roll”; spinning on its Y-axis is a “pitch”; and spinning on its Z-axis is a “yaw” manoeuvre (see

Figure 4 in Table 5).

Attitude Determination and Control (Olivier L. de Weck, MIT)

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.208.3894&rep=rep1&type=pdf and: http://www.mpoweruk.com/satellites.htm

Orientation of a defined spacecraft body coordinate system with respect to a defined external frame.

Satellites often change orientation due to torque produced by the environment (solar radiation pressure etc...) or by itself (movement of parts) therefore the angular orientation has to be actively controlled.

Table 6 - SAR Term: Attitude angle



http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.208.3894&rep=rep1&type=pdf



http://www.mpoweruk.com/satellites.htm

http://www.mpoweruk.com/satellites.htm




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SAR Term: Azimuth steering angle


eGEOS COSMO-SkyMed SAR Products Handbook (Italian Space Agency)

Azimuth Steering: Array of the Azimuth angles of the antenna beam set at the Azimuth Ramp Code Change Lines. While for the ScanSAR and StripMap case such value should be constant within the strip/burst, in the Spotlight case the array including the Azimuth direction of the antenna beam due to the repointing implied by the instrument mode should be given. The array dimension corresponds to the number of occurrences of angle's changes.

Table 7 - SAR Term: Azimuth steering angle

SAR Term: Incidence angle


EUSI

Kompsat-5 Product Attributes, Product Attributes Specifications (Korea Aerospace Research Institute)

http://www.si-imaging.com/lfile/KOMPSAT-5%20Products%20Attributes_v1.2.pdf

Far Incidence Angle: Absolute value of the incidence angle measured at the far range on the ellipsoid in zero-doppler geometry as derived by the sampling window times represented in data.

MDA Radarsat Illuminated (Radarsat International)http://spatialnews.geocomm.com/whitepapers/radarsat1.pdf

Defined as the viewing angle (line of sight between the radar and the object) of the radar beam from the vertical. The local incidence angle takes into account the slope of the terrain at the object’s location. Incidence angle can have an important influence on the radar





http://spatialnews.geocomm.com/whitepapers/radarsat1.pdf






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backscatter.

Figure 5 – representation of imaging modes and relevant incidence angles in Radarsat-1.

(from [RD04])

eGEOS COSMO-SKYMED System Description and User Guide (Agenzia Spaziale Italiana)

Incidence angle: It is the angle measured between the slant range direction and the normal to the tangent plane to the Earth surface in the specified point on ground.

Local incidence angle: The angle between the radar beam centre and the normal to the local topography. The difference between the global incidence angle and the terrain slope.

Tutorials

Synthetic Aperture Radar (SAR) Equations in the ASF User Tools, Orion Sky Lawlor

https://www.cs.uaf.edu/~olawlor/ref/asf/sar_equations_2006_08_17.pdf

Angle from straight up over to satellite, as measured from the target point

Table 8 - SAR Term: Incidence angle







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SAR Term: Look angle


eGEOS COSMO-SkyMed SAR Products Handbook (Italian Space Agency)

http://www.cosmo-skymed.it/docs/ASI-CSM-ENG-RS-092-A-CSKSARProductsHandbook.pdf

Look angle of a SAR: the angle from the nadir at which the radar beam is pointed.

Look angle of a target: the angle between the SAR-nadir and SAR-target lines.

Near Look Angle: Look angle measured at the near range on the ellipsoid in zero-doppler geometry as derived by the sampling window times represented in data.

Far Look Angle: Look angle measured at the far range on the ellipsoid in zero-doppler geometry as derived by the sampling window times represented in data.

Tutorials

Basic Synthetic Aperture Radar (SAR) Concepts and Terminology, Alaska Satellite Facility.

https://www.asf.alaska.edu/about-sar/sar-basics/

The angle between the direction the antenna is pointing and the nadir is the look angle.

Synthetic Aperture Radar (SAR) Equations in the ASF User Tools, Orion Sky Lawlor.


Look angle: angle from straight down over to target, as measured from the satellite.

Table 9 - SAR Term: Look angle













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3. CQC Proposal

Clearly clarification is required to ensure the true meaning of a term is clear and consistent across CCMs. This will greatly benefit product end users by allowing them to easily understand the imagery products that they use - particularly when more than one CCM is involved in a project. To this end the following terminology is proposed and explained in the following sections.




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3.1 Optical Terms

The proposed simplified and consolidated definitions for optical terms are given below. Where contradictory or duplicated terms have been suggested for removal, this is also stated.

Azimuth angle: This term will be subdivided into two parts; Illumination Azimuth angle and Collection Azimuth angle. As these terms refer to different subjects, they will be separated and treated as individual terms. Illumination Azimuth angle: The angle (from the observer) between the sun and North, measured on the

horizontal plane clockwise from North. This is separate from Collection Azimuth angle (Figure 6). Collection Azimuth angle: The angle (from the observer) between the sensor’s line of sight and North,

measured on the horizontal plane clockwise from North. This is separate from Illumination Azimuth angle (Figure 6).

Elevation angle: This term will be subdivided into two parts; Illumination elevation angle and Collection elevation angle, and treated as separate terms. Illumination elevation angle: The vertical angle measured from the horizontal plane up to the sun. When

the sun is directly overhead the Illumination elevation angle will be 90°. This is separate from collection elevation angle (Figure 6).

Collection elevation angle: The vertical angle measured from the horizontal plane up to the line of sight of the sensor. When the sensor is directly overhead the collection elevation angle will be 90°. This is separate from Illumination elevation angle (Figure 6).

Figure 6 – Illumination and Collection Azimuth angles, Illumination and Collection elevation angles.




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Incidence angle: The vertical angle measured from the line of sight of the sensor to a line perpendicular to the target surface (see Figure 7). This can be measured on two planes; Incidence angle along track: The Incidence angle in the direction of satellite track. Incidence angle across track: The Incidence angle across the direction of satellite track.

Figure 7 - Incidence and viewing angles (from [RD02]).

Off nadir view angle: This term will be removed as it is a duplication. See collection elevation angle.

Viewing angle: The vertical angle from the point of view of the sensor, measured between the satellite nadir and the line of sight of the sensor (see Figure 7). This is the equivalent of the look angle for SAR imagery. This can be measured on two planes; Viewing angle along track: The Viewing angle in the direction of satellite track (pitch). Viewing angle across track: The Viewing angle across the direction of satellite track (roll).

Combined angle2: This angle represents the angle between the line of sight of the sensor and the satellite nadir. It combines the platform pitch (along track) and roll (across track) angles (see Figure 8). The definition of Viewing angle as synonymous of Combined angle is accepted.

2 Combined angle is a term particularly used in the literature of Airbus FR constellation.




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Figure 8 - Pitch, roll and yaw angles.




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3.2 SAR Terms

Azimuth steering angle: This term will be removed as it is a duplication. See Collection Azimuth angle in Optical section. Incidence angle: The vertical angle measured from the line of sight of the sensor to a line perpendicular to the target surface. This is the average of the near and far incidence angles (see following). Near incidence angle: The incidence angle measured from the near edge of the target. Far incidence angle: The incidence angle measured from the far edge of the target.

Look angle: The vertical angle from the point of view of the sensor, measured between the satellite nadir and the line of sight of the sensor. This is the equivalent of the viewing angle for optical imagery. This is the average of the near and far look angles (see Figure 9). Near look angle: The look angle measured from the near edge of the target. Far look angle: The look angle measured from the far edge of the target.

Figure 9 - Near and Far incidence angles and Near look angle.




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3.3 Impact of the proposed terms on IPS Visualisation Tool, INSPIRE.xml and CR

The CQC proposed terms may impact on the Copernicus metadata (i.e. IPS, INSPIRE.xml and CR) with the following modifications.

Proposed modifications in the IPS Visualisation Tool – Optical component

The IPS Visualisation Tool in its optical component currently reports the definitions relevant to satellite acquisition angles in IVOS-29 for Camera tilt (Figure 10).

Figure 10 – Camera tilt in the IPS visualisation Tool, optical part, is reported in the IVOS-29 box.

The Camera tilt refers to the ‘Maximum off Nadir pointing angle’, a term which is not recommended by this document as it is a duplication of other terms. For this reason, the CQC Team proposes the following modifications of IVOS-29 in the IPS Visualisation Tool:

Instead of:

Camera tilt [deg] Maximum off Nadir pointing angle in across/along track directions.

The following should read (changes marked in yellow):

Camera tilt [deg] Maximum viewing angle in across/along track directions.

Proposed modifications in the IPS Visualisation Tool – SAR component

The IPS Visualisation Tool in its SAR component currently reports the definitions relevant to satellite acquisition angles in SAR-24 for Coverage and Temporal Characteristics: (Figure 11Error! Reference source not found.).




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Figure 11 - Coverage and Temporal Characteristics in the IPS visualisation Tool, SAR-24 box.

The Swath refers to minimum off nadir angle - this term has been suggested for removal by this document as it is a duplication of other terms. For the reason above, the CQC Team proposes the following modifications of SAR-24 in the IPS Visualisation Tool:

instead of:

Swath [m] Description of the swath characteristics. The swath is the width of the imaged area, computed at minimum off-nadir angle, and considering the mean altitude of the spacecraft.

The following should read (changes marked in yellow):

Swath [m] Description of the swath characteristics. The swath is the width of the imaged area, computed at minimum look angle, and considering the mean altitude of the spacecraft.

Proposed modifications in the INSPIRE.xml file

Instead of the current script for Pleiades and SPOT imagery, relevant to orthorectification:

<gmd:lineage> - <gmd:LI_Lineage> - <gmd:statement> <gco:CharacterString>Pleiades 1A data processed to Level 3 (ortho). This Ortho product is a georeferenced image in Earth geometry,

corrected from acquisition and terrain off-nadir effects. As a standard, the Ortho is produced with fully automatic processing and proprietary algorithms. The Standard Ortho product is an image that has been corrected (viewing angle and ground effects) so that it may be superimposed on a map. On top of radiometric and geometric adjustments, a geometric process using a relief model (known as orthorectification) eliminates the perspective effect on the ground (not on buildings), restoring the geometry of a vertical shot. The ortho process inherits geometric and radiometric corrections performed at previous steps on the raw image acquired by the sensor: - geometric corrections (if applicable to the sensor or spectral mode): combination of all sub-swaths across in the field of view, correction of instrument and optical distortions, co-registration of all spectral bands, attitudes and ephemeris refining; - radiometric corrections (if applicable to the sensor or spectral mode): inter-detector equalization, aberrant detectors correction if any, band restoration. At ortho level the main additional adjustments are: - planimetric reset: on request, if ground reference data is available, the location is reset on Ground Control Points; - altimetric reset: correction of the panoramic effects induced by the off-nadir incidence angles over the relief thanks to a Digital Elevation Model (DEM). By default, the Reference3D DEM layer is used where available, otherwise SRTM is used; - Map projection or geographic projection. For more information please refer to our user guides available on our web site (http://www.geo-airbusds.com)</gco:CharacterString>

</gmd:statement> </gmd:LI_Lineage>




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</gmd:lineage> </gmd:DQ_DataQuality> </gmd:dataQualityInfo> </gmd:MD_Metadata>

The following modification should read (changes marked in yellow):

<gmd:lineage> - <gmd:LI_Lineage> - <gmd:statement> <gco:CharacterString>Pleiades 1A data processed to Level 3 (ortho). This Ortho product is a georeferenced image in Earth geometry,

corrected from acquisition and terrain off-nadir effects. As a standard, the Ortho is produced with fully automatic processing and proprietary algorithms. The Standard Ortho product is an image that has been corrected (viewing angle and ground effects) so that it may be superimposed on a map. On top of radiometric and geometric adjustments, a geometric process using a relief model (known as orthorectification) eliminates the perspective effect on the ground (not on buildings), restoring the geometry of a vertical shot. The ortho process inherits geometric and radiometric corrections performed at previous steps on the raw image acquired by the sensor: - geometric corrections (if applicable to the sensor or spectral mode): combination of all sub-swaths across in the field of view, correction of instrument and optical distortions, co-registration of all spectral bands, attitudes and ephemeris refining; - radiometric corrections (if applicable to the sensor or spectral mode): inter-detector equalization, aberrant detectors correction if any, band restoration. At ortho level the main additional adjustments are: - planimetric reset: on request, if ground reference data is available, the location is reset on Ground Control Points; - altimetric reset: correction of the panoramic effects induced by the incidence angles over the relief thanks to a Digital Elevation Model (DEM). By default, the Reference3D DEM layer is used where available, otherwise SRTM is used; - Map projection or geographic projection. For more information please refer to our user guides available on our web site (http://www.geo-airbusds.com)</gco:CharacterString>

</gmd:statement> </gmd:LI_Lineage> </gmd:lineage> </gmd:DQ_DataQuality> </gmd:dataQualityInfo> </gmd:MD_Metadata>

Proposed modifications in the CR

In the current versions of the Circulation Report all angles are correctly mentioned. No modifications are currently required.




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3.4 Dissemination of the proposed terms

The CQC team proposes the creation of a number of short technical BackGround Documents (BGDs) for the above proposed definitions. Some of these may need explanation for the different context they may be used and/or disambiguation as respect to other similar terms. The BGDs proposed by CQC Team are reported in Appendix I. The BGDs are the following (in alphabetic order):

Optical terms: Azimuth angle (collection/illumination) Elevation angle (collection/illumination) Incidence angle (along/across track) Viewing angle (along/across track)

SAR terms: Incidence angle (near/far) Look angle (near/far)

In order to help to harmonise the IPS Visualisation Tool the relevant terms will be linked to their BGDs.

In addition, a more comprehensive Glossary is being written by the CQC Team. This will be consistent with the relevant BGDs. Eventually the Glossary will be uploaded in the CDS portal for online consultation of the Copernicus users.




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4. Appendix I: BGDs

Azimuth angle This term will be subdivided into two parts; Illumination Azimuth angle and Collection Azimuth angle. As these terms refer to different subjects, they will be separated and treated as individual terms. Illumination Azimuth angle: The angle (from the observer) between the sun and North, measured on the horizontal plane clockwise from North. This is separate from Collection Azimuth angle (Figure 6). Collection Azimuth angle: The angle (from the observer) between the sensor’s line of sight and North, measured on the horizontal plane clockwise from North. This is separate from Illumination Azimuth angle (Figure 6).




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Elevation angle This term will be subdivided into two parts; Illumination elevation angle and Collection elevation angle, and treated as separate terms. Illumination elevation angle: The vertical angle measured from the horizontal plane up to the sun. When the sun is directly overhead the Illumination elevation angle will be 90°. This is separate from collection elevation angle (Figure 6). Collection elevation angle: The vertical angle measured from the horizontal plane up to the line of sight of the sensor. When the sensor is directly overhead the collection elevation angle will be 90°. This is separate from Illumination elevation angle (Figure 6).




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Incidence Angle In optical context the incidence angle is the vertical angle measured from the line of sight of the sensor to a line perpendicular to the target surface (see Figure 7). This can be measured on two planes; Incidence angle along track: The Incidence angle in the direction of satellite track. Incidence angle across track: The Incidence angle across the direction of satellite track. In SAR context the incidence angle is the vertical angle measured from the line of sight of the sensor to a line perpendicular to the target surface. This is the average of the near and far incidence angles (see Figure 9). Near incidence angle: The incidence angle measured from the near edge of the target. Far incidence angle: The incidence angle measured from the far edge of the target.

When the imaged area is characterized by terrain relief, an incidence angle and a local incidence angle are distinguished as in the example in (Figure 12). The local incidence angle is the angle between the radar beam and a line perpendicular to the slope at the point of incidence (Figure 12).

Figure 12 - Local incidence angle.




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Viewing angle

The vertical angle from the point of view of the sensor, measured between the satellite nadir and the line of sight of the sensor (see Figure 7). This is the equivalent of the look angle for SAR imagery. This can be measured on two planes;

Viewing angle along track: The Viewing angle in the direction of satellite track.

Viewing angle across track: The Viewing angle across the direction of satellite track.

Combined angle: This angle represents the angle between the line of sight of the sensor and the satellite nadir. It combines the platform pitch (along track) and roll (across track) angles (see Figure 8). The definition of Viewing angle as synonymous of Combined angle is accepted.




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Look Angle The angle between the satellites nadir and the look direction from the satellite. This term is used more in SAR than in optical context. Look angle: The vertical angle from the point of view of the sensor, measured between the satellite nadir and the line of sight of the sensor. This is the equivalent of the viewing angle for optical imagery. This is the average of the near and far look angles (see Figure 9). The look angle can be distinguished in “Near” or “Far” look angles. These are the look angles taken from the near and far edges of the image collection swath, respectively, the near edge being the part closest to the satellites nadir and the far edge being furthest away (see Figure 9). Near look angle: The look angle measured from the near edge of the target. Far look angle: The look angle measured from the far edge of the target. Also used “Right Looking” being “+“ and “Left Looking” being “-” with Right and Left to be seen from the spacecraft in relation to satellite flight track.