Space systems — Calibration requirements for satellite-based passive microwave sensors

This document defines the requirements and verification methods from design to on-orbit operation for Satellite Based Passive Microwave Sensors. This document covers the requirements for, design, analysis, manufacturing, ground tests and on-orbit self-sensor calibration and validation. In addition, this document includes the conditions considered for on-orbit inter-comparison among sensors as preparation for cross-calibration. This document includes some examples on how to apply the development of passive microwave sensors as shown in Annex A through D.

Systèmes spatiaux — Exigences d'étalonnage des capteurs passifs d'hyperfréquences satellitaires

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Published
Publication Date
24-Jul-2018
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6060 - International Standard published
Start Date
25-Jul-2018
Completion Date
25-Jul-2018
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INTERNATIONAL ISO
STANDARD 20930
First edition
2018-08
Space systems — Calibration
requirements for satellite-based
passive microwave sensors
Systèmes spatiaux — Exigences d'étalonnage des capteurs passifs
d'hyperfréquences satellitaires
Reference number
ISO 20930:2018(E)
ISO 2018
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ISO 20930:2018(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2018

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

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Published in Switzerland
ii © ISO 2018 – All rights reserved
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ISO 20930:2018(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Abbreviated terms .............................................................................................................................................................................................. 2

5 Space based passive microwave sensor calibration overview .............................................................................. 2

5.1 Mission and system overview .................................................................................................................................................... 2

5.2 Types of passive microwave sensors ................................................................................................................................... 3

5.3 Concept of calibration and scope ............................................................................................................................................ 4

6 Requirements for pre-launch phase ................................................................................................................................................ 6

6.1 General ........................................................................................................................................................................................................... 6

6.2 Requirements for design and manufacturing ............................................................................................................... 6

6.2.1 Requirements for sensor instrument ............................................................................................................. 6

6.2.2 Requirements for spacecraft bus ....................................................................................................................... 8

6.2.3 Requirements for ground processing system ......................................................................................... 9

6.3 Ground test and requirements verification .................................................................................................................10

6.3.1 Requirements for the sensor instrument ................................................................................................10

6.3.2 Requirements for spacecraft bus ....................................................................................................................12

6.3.3 Requirements for ground processing system ......................................................................................12

7 Requirements for on-orbit phase ....................................................................................................................................................12

7.1 General ........................................................................................................................................................................................................12

7.2 Requirements for sensor instrument self-calibration .......................................................................................12

7.2.1 Resolution of antenna temperature .............................................................................................................12

7.2.2 Elimination of bias ......................................................................................................................................................12

7.2.3 Characterization ............................................................................................................................................................13

7.2.4 Deep space manoeuver ...........................................................................................................................................13

Annex A (informative) Nonlinearity correction (example)........................................................................................................14

Annex B (informative) Validation by SRT (Standard Reference Target) .......................................................................18

Annex C (informative) Ground-based information required for sensor calibration .......................................19

Annex D (informative) Examples of design requirements ..........................................................................................................23

Bibliography .............................................................................................................................................................................................................................25

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ISO 20930:2018(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso

.org/iso/foreword .html.

This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,

Subcommittee SC 14, Space systems and operations.
iv © ISO 2018 – All rights reserved
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ISO 20930:2018(E)
Introduction

Water is one of the key elements for survival of all creatures on earth. Water brings us the blessings of

food thanks to agriculture, but water is also responsible for providing forest products and fish, which

are essential for our lives. However, too much or too little water can lead to environmental disasters

such as hurricanes, heavy rains, floods, droughts and wild fires.

Many satellite sensors are launched through international cooperation ventures with the aim of

monitoring the behaviour of the water cycle and estimating some of the parameters related to it, e.g.

soil moisture, vegetation biomass, snow cover, sea ice, and so on. A systematic and timely monitoring

of land surface parameters that affect the hydrological cycle at local and global scales are of primary

importance in obtaining a better understanding of geophysical processes and in order to manage

environmental resources and mitigate for natural disasters.

At present, some applications to assist our human activities are provided, such as weather forecasts

and predictions of climate change. Nowadays, the observation data acquired by passive microwave

sensors are used for weather forecasts, fishery services, drought monitoring on a daily basis, and

for predicting climate change in the future. However, errors due to bias, gain, and sensitivity among

passive microwave sensors can degrade accuracy of applications and users could waste effort and time

for compensation by on-orbit operation.

This document standardizes calibration methods (requirements and verification methods) to minimize

errors of observation data among passive microwave sensors. It is expected that this document can

improve the accuracy of weather forecasts, sea surface temperatures for fishery services, soil moisture

monitoring to decrease water waste for farmers, snow cover and depth for water storage. Moreover,

these observations can provide useful information for climate change prediction that is relevant to our

daily lives.
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INTERNATIONAL STANDARD ISO 20930:2018(E)
Space systems — Calibration requirements for satellite-
based passive microwave sensors
1 Scope

This document defines the requirements and verification methods from design to on-orbit operation

for Satellite Based Passive Microwave Sensors.

This document covers the requirements for, design, analysis, manufacturing, ground tests and on-orbit

self-sensor calibration and validation. In addition, this document includes the conditions considered for

on-orbit inter-comparison among sensors as preparation for cross-calibration. This document includes

some examples on how to apply the development of passive microwave sensors as shown in Annex A

through D.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 10795:2011, Space systems — Programme management and quality — Vocabulary
ISO 14302, Space systems — Electromagnetic compatibility requirements
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 10795:2011 and the following

apply. ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
— IEC Electropedia: available at http: //www .electropedia .org/
— ISO Online browsing platform: available at https: //www .iso .org/obp
3.1
calibration

set of operations that establish, under specified conditions, the relationship between sets of values of

quantities indicated by a measuring instrument or measuring system and the corresponding values

realized by standards
3.2
validation

process of assessing by independent means the quality of the data products derived from the

system outputs
3.3
level one processing

type of processing where the antenna brightness temperature of the sensor instrument is calculated

and compensated radiometrically and geometrically based on evaluation results of ground test and on-

orbit calibration
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ISO 20930:2018(E)
4 Abbreviated terms
ANT Antenna sub-system
BTE Bench Test Equipment
CCT Cold Calibration Target
DT Data Processor
EIA Earth Incident Angle
FOV Field Of View
GCP Ground Control Point
GPS Global Positioning System
ICS Interface Control Specification
IF Intermediate Frequency
LO Local Oscillator
LCT Low Calibration Target
MR Main Reflector
NCS Noise Calibration Source
RE Radiated Emission
RF Radio Frequency
RS Radiated Susceptibility
RTT Radiative Transfer Theory
Rx Receiver
SRT Standard Reference Target
VTS Variable Temperature Source
WCT Warm Calibration Target
5 Space based passive microwave sensor calibration overview
5.1 Mission and system overview

Satellite based passive microwave sensors observe weak microwave energy emitted from various

components of the Earth’s surface (rain, water vapour, clouds, snow, sea ice, soil, vegetation, etc.) using

a variety of frequency bands from about 1 GHz to 200 GHz.

Each space-based passive microwave sensor design is tailored to meet its specific observation purpose.

Observation data are collected at defined intervals and over defined integration periods. These data

are downlinked through the spacecraft. Following Level One processing to perform radiometric and

geometric compensation, the data are delivered to information providers.
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ISO 20930:2018(E)

Figure 1 shows the general operational concept view of space-based passive microwave sensor system

integration. Space-based passive microwave sensors, to which this document applies, include the

following components:
a) Cold Calibration Target (CCT) to determine deep-space limit or equivalent;
b) Warm Calibration Target (WCT) to monitor ambient temperature;

c) Main Reflector (MR) to capture the power of microwave emission from the earth; and

d) Receiver (Rx) sub-system to amplify the tiny power of the microwave emission.

Self-calibration is performed at specific time intervals using the WCT and the CCT.

Information providers process antenna brightness temperature to derive physical information and

deliver that information to end users.
5.2 Types of passive microwave sensors

Satellite based passive microwave sensors are generally categorized as follows. This document covers

the requirements for the following two types.
a) Microwave radiometer (or Microwave Imager)

To mainly measure energy emitted from water related substances at sub-millimetre-to-centimetre

wavelengths.
b) Microwave sounder

To mainly measure the vertical profile of atmospheric temperature and moisture for operational

weather and climate applications at sub-millimetre-to-centimetre wavelengths.
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ISO 20930:2018(E)
Figure 1 — Concept diagram of space based passive microwave sensor system
5.3 Concept of calibration and scope

Satellite based passive microwave sensors receive weak microwave emission from the earth. Errors

among the sensors would cause or degradation of observation data quality (e.g. 0,1 K or around for sea

surface temperature of climate change monitoring).

Therefore, it is necessary to understand the sensor characteristics in both pre-launch phase (design,

manufacturing and ground test before launch campaign) and on-orbit phase (On-orbit activity for

calibration).

This document covers not only requirements for the sensor itself but also those for the spacecraft bus

and ground system because interfaces between the spacecraft bus and the ground system affect sensor

performance.
Figure 2 shows the concept of calibration process and scope of this document.
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ISO 20930:2018(E)
Figure 2 — Concept of calibration process
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ISO 20930:2018(E)
6 Requirements for pre-launch phase
6.1 General

Before the launch campaign, the characteristics of sensor instruments shall be precisely specified and

verified, and those characteristics should be registered in the ground processing system. This sub

clause explains the requirements for designing, manufacturing and ground testing. The formula and

their parameters prepared through the ground test and/or analysis shall be submitted to the ground

processing system for accurate calibration and observation of data processing.
6.2 Requirements for design and manufacturing

This sub clause defines the requirements for the sensor instrument, the spacecraft bus and the ground

systems.

To satisfy the on-orbit performance, the following requirements shall be considered through design,

manufacturing and ground test phase.
6.2.1 Requirements for sensor instrument

The requirements for the sensor instrument, especially for the following equipment, are described in

this sub clause:
a) Cold Calibration Target (CCT);
b) Warm Calibration Target (WCT);
c) Receiver (Rx) sub-system;
d) Noise Calibration Source (NCS);
e) Antenna sub-system (ANT); and
f) Data Processor (DP).

Engineering formulas and parameters obtained during the development stage shall be submitted to the

ground processing system.
6.2.1.1 Cold Calibration Target (CCT)

The design of the Cold Calibration Target (CCT) shall meet the following requirements.

a) The CCT shall be designed (modelled or measured, or both) to meet the following:

i) reflectivity of the CCT shall be specified; and

ii) normal-incidence power reflection coefficient (Γ) shall be specified and included in the

uncertainty calculation for the antenna brightness temperature (Tb).

b) At the position of beam centre of the Rx front-end antenna corresponding to that of the CCT, an

analysis or measurement (or both) shall be conducted to obtain the distribution pattern of antenna

spill-over and its weighting parameters in order to accurately calculate the antenna brightness

temperature for CCT.

c) The specification of the CCT shall include either measuring or modelling the entire antenna power

pattern to tune the instrument performance precisely on-orbit.

d) For precise calibration, analysis, or measurement, or both shall be made to obtain data of emissivity

and antenna temperature of the interfered structure (or obstruction) out of −3 dB bore sight of the

CCT, and engineering formula and parameters based on the above data shall be prepared.

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ISO 20930:2018(E)
6.2.1.2 Warm Calibration Target (WCT)

The design for the Warm Calibration Target (WCT) shall meet the following requirements.

a) The WCT shall be designed (modelled or measured, or both) to meet the following:

i) reflectivity of the WCT shall be specified;

ii) normal-incidence power reflection coefficient (Γ) shall be specified and included in the

uncertainty calculation for the antenna brightness temperature (Tb).

b) At the position of Beam Centre of the Rx front-end antenna corresponding to that of the WCT, an

analysis or measurement (or both) shall be conducted to obtain the distribution pattern of antenna

spill-over and its weighting parameters.

c) Temperature sensors shall be mounted in (or on) the WCT to estimate temperature gradation of the

effective area of the WCT. Temperature data measured by temperature sensors shall be delivered

to ground processing system at a specific interval to calibrate and maintain sensor instrument

performance.

The standard uncertainty of monitored temperature shall be specified not to degrade the accuracy

of measurement.

d) Analysis for temperature gradation of the effective area of the WCT shall be made to estimate

temperature distribution of the WCT on-orbit.

(The WCT should be designed to exclude (or minimize, at least) solar radiation intrusion, in order

to maintain the quality of the estimation about the WCT temperature distribution. Analysis shall

be performed to determine presence and extent of any solar radiation intrusion).
6.2.1.3 Receiver (Rx) sub-system
The design for Rx sub-system shall meet the following requirements.

a) Calibration targets, CCT and WCT, shall be mounted on the sensor instrument to minimize gain and

offset changes in the Rx sub-system due to radiation-induced thermal variations on orbit. Observed

data of the CCT and the WCT shall be delivered at specific intervals to calibrate and maintain sensor

instrument performance.

The Rx sub-system shall observe the CCT and the WCT in the specific interval that does not degrade

the calibration accuracy.

b) Monitoring points to evaluate nonlinearity of the Rx Sub-system shall be equipped on a module,

printed circuit, component, or the Rx sub-system shall be prepared to identify and evaluate

nonlinearity, dynamic range and uncertainty of the Rx sub-system.

In the design stage of the Rx sub-system, monitoring points on a module, printed circuit, component,

or Rx sub-system shall be prepared to identify and evaluate nonlinearity, dynamic range and

uncertainty of the Rx sub-system.

c) Electrical parts (e.g. MMIC, amplifier, diode, register capacitor) for the Rx sub-system shall be

evaluated by testing (radiation, temperature) or analysis, prior to mounting them on the module,

in order to guarantee the performance (including stability) of the Rx sub-system during on orbit

operation throughout its life-time.

The characteristics obtained by testing or analysis shall be input to level one processing to make

formulas and parameters for nonlinearity compensation or uncertainty estimation due to the

nonlinearity.

d) Temperature sensors shall be equipped inside or near receivers to monitor representative

temperature in order to compensate receiver gain and offset.
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ISO 20930:2018(E)

e) In case that an observation channel needs a narrow band based on mission requirements, sensor

instruments shall be designed to satisfy the stability requirements of the Local oscillator (LO) to

down-convert from Radio frequency (RF) to Intermediate Frequency (IF frequency). A LO with

minimum phase noise shall be selected and the total gain of the RX sub-system shall be adjusted to

bring the power up to an optimal level from the S/N point of view.
6.2.1.4 Noise Calibration Source (NCS)

The design of the NCS shall meet the following requirements. These requirements shall be applied in

cases where the NCS is an installed sensor instrument.

a) Stability of output power from the noise source (NCS) shall be specified based on an evaluation of

the operating temperature range and specified electrical voltage and/or current, in cases where

the NCS is required for three or four point self-calibration.

b) The temperature sensor shall be mounted in order to monitor the NCS temperature and accurately

estimate output power of NCS. Electrical current shall also be monitored to take additional

information, if required.

The monitored data shall be delivered to the ground processing system at a specific interval to calibrate

and maintain sensor instrument performance.
6.2.1.5 Antenna sub-system
The design of the antenna sub-system shall meet the following requirements.

a) The Main Reflector (MR) shall be designed (modelled or measured, or both) to meet the following.

i) Reflectivity of the MR shall be specified.

ii) Γ shall be specified and included in the uncertainty calculation for the antenna brightness

temperature (Tb).

iii) The MR shall include temperature monitoring if Γ < 0,999 9 (e.g. exact number is open for

discussion).

b) The specification of the antenna sub-system shall include either measuring or modelling the entire

antenna power pattern to tune the instrument performance precisely on orbit.

c) Spill-over and cross-polarization of the MR shall be determined either by analysis or by

measurements to calibrate unnecessary power.to calculate how much of the brightness temperature

is reduced for the calculation of own calibration.

d) Antenna efficiency shall be specified. The physical temperature of the antenna shall be monitored

over time to correct the added noise temperature of the antenna due to its loss.
6.2.1.6 Data processor

The data processor of the sensor instrument shall be designed to output its observation and calibration

data with the information of the sensor instrument status and the data required for high level data

processing.

The information shall be specified in accordance with the design of the ground processing system.

6.2.2 Requirements for spacecraft bus

In this sub clause, the interfaces between spacecraft bus and sensor instrument are specified. The

spacecraft bus shall be designed and manufactured carefully, especially considering the following

interface items with the sensor instrument:
a) RF Interface;
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ISO 20930:2018(E)
b) Field of view interference;
c) Spacecraft attitude; and
d) Observation and calibration data downlink.
6.2.2.1 RF Interference
The design for RF interface shall meet the following requirements.

a) RF Interface design specifications with the spacecraft bus (including active instruments mounted

on the spacecraft bus) shall be specified in the ICS to avoid excessive RF radiative emission which

interferes with sensor performance and damages the sensor instrument. ISO 14302, or its equivalent,

shall be referred to in order to obtain the design criteria for interference and damage level.

b) Interface design requirements with the spacecraft bus shall be specified to avoid RF multi-path

interference due to Tele-communication/tele-command devices and direct data transmission

devices, in order to not degrade the performance of sensor instruments.
6.2.2.2 Field of View (FOV) Interference

The Field of View (FOV) interface design specification shall be specified in the ICS to eliminate

unnecessary power input brought through the MR (or the CCT) by the spacecraft bus, specifically

regarding the variation of spacecraft bus configurations on orbit, such as during the use of a solar paddle.

6.2.2.3 Spacecraft attitude

Interface design requirements with the spacecraft bus shall be specified to meet the following

requirements.

a) Information of spacecraft attitude shall be delivered to sensor instruments to determine the Earth

Incident Angle (EIA) when the sensor instrument observes the earth.

b) Errors of spacecraft attitude shall be specified in the ICS, considering EIA requirements for sensor

instruments.
6.2.2.4 Observation and calibration data downlink

The observation and calibration data with the information specified in 6.2.1.6 shall be delivered to the

ground processing system at the specific interval determined in the ICS between the spacecraft bus,

the sensor instruments and the ground processing system.
6.2.3 Requirements for ground processing system

The design of the ground processing system to calculate antenna brightness temperature shall meet

the following requirements.

a) Processing errors of the ground processing system shall be designed in a way that they do not to

degrade the requirement for the antenna brightness temperature.

b) The ground processing system shall receive the observation and calibration data with the

information specified in 6.2.1.6 at the specific interval determined in 6.2.2.4.

c) The ground processing system shall process both calibration and observation data to eliminate

unnecessary power input from the structure (or obstruction) of the spacecraft bus such as, the

structure of the solar paddle of the spacecraft bus.

In addition, the ground processing system shall eliminate the unnecessary bias or effect on RF

interference, regarding the angle and direction of the solar cell panels, the elevation angle and trajectory

of the sun, the location of radiation sources on earth, and the location (GPS) of spacecraft on orbit.

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ISO 20930:2018(E)
6.3 Ground test and requirements verification
6.3.1 Requirements for the sensor instrument
6.3.1.1 Cold Calibration Target (CCT)

The following requirements for the CCT shall be proved by analysis or test or both.

a) The CCT characteristics specified in 6.2.1.1 a) shall be verified by test and/or analysis.

NOTE 1 If measured f
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