tively defining the system response to known, controlled signal 
inputs") and validation ("... the process of assessing by inde- 
pendent means the quality of the data products derived from 
the system outputs") of Earth observations for the benefit of 
CEOS members and the international user community. 
The WGCV has four subgroups. In the order of their creation, 
these are 
e the Synthetic Aperture Radar (SAR) Calibration Subgroup, 
e the Infrared and Optical Sensors Subgroup (IVOS), 
e the (passive) Microwave Sensors Subgroup (MSSG), 
e the Terrain Mapping Subgroup (TM). 
4.2.1 SAR Cal: This very active group has established proce- 
dures for SAR performance assessment techniques, (internal 
and external) calibration techniques, formats and definitions. 
It has co-ordinated multinational calibration campaign to 
develop expertise for intra-sensor and cross-sensor calibration, 
has participated in ERS-1, RADARSAT and SIR-C/X-SAR 
calibration activities and has held SAR calibration workshops 
at regular intervals. The group is about to change the scope of 
its activities. 
4.2.2 IVOS: This group promotes international and national 
co-operation in the calibration and validation of all (satellite, 
airborne and ground-based) IVOS, with an emphasis on sys- 
tems providing global coverage, to assist in an improved appli- 
cation of data from all sensors. As part of identifying calibra- 
tion and validation requirements, the group attempts to 
identify test sites to facilitate intercomparisons of the data 
provided by different sensors. 
4.0.5 MSSG: This group is concerned with all microwave 
sensors excluding SAR and promotes a.o. accurate sensor cali- 
bration and validation through the establishment of laboratory- 
type standard calibration targets, including electromagnetic 
characteristics of reference targets, describing methods for 
reference temperatures, developing recommendations for field- 
use portable standard calibration targets and recommending 
requirements for antenna characterization. Other objectives 
are the establishment of general characteristics for in-flight 
(spaceborne) calibration targets, the investigation of 
calibration reference target designs and the establishment of 
recommendations for end-to-end analysis of in-flight calibra- 
tion techniques. 
4.2.4 TM: This group is for the ISPRS community without 
doubt the most interesting one. The Terrain mapping Sub- 
group has the following terms of reference: 
e to promote wider awareness and use of remote sensing satel- 
lite data for terrain height determination; 
e to promote an intercomparison of the state-of-the-art in the 
different techniques for determining terrain heights from 
space and their validation, to work towards common 
terminology and to determine terrain height accuracies 
including the following techniques: optical stereo, SAR 
stereo, SAR interferometry, shape-from-shading, laser 
altimetry, radar altimetry and others; 
e to identify and/or establish relatively stable test sites acces- 
sible by a variety of sensors. (Reference data sets accurate 
enough to allow validation of various space methods are 
being assembled in suitable aggregated form to meet differ- 
ent resolution requirements; data from different sensors and 
reference data sets are freely made available to members of 
the subgroup) 
The subgroup has in the past few years been a strong supporter 
of sensor requirements needed to enable SAR interferometry, 
and has succeeded in obtaining operational parameters of 
ESA’s ERS-1/ERS-2 tandem mission suitable for SAR 
interferometry. 
One of the activities of all subgroups is the preparation of a test 
site dossier which gives for selected sites the following infor- 
mation, if available: 
e site name, 
e contact name and address, 
e site characteristics (location, size, relief, climate, vegetation, 
geology, previous use), 
e DEM characteristics (spacing, source, extent, accuracy), 
e map coverage, 
e ground control point availability, 
e available image data, 
e references. 
Access to these test data would undoutedly be of interest to the 
ISPRS community. This requires, however, affiliate status 
with the CEOS. 
The TM subgroup has also looked into the question of evaluat- 
ing DEM's derived from spacebome data and has to prepare a 
guide to the methodology of evaluation. 
4.3 CEOS WG on Information Systems and Services 
The WGINS has four subgroups: 
e the Format Subgroup (FS) 
deals a.o. with format system requirements, CEOS super- 
structure, CEOS SAR formats, data compression, data de- 
scription languages, data objects and format translations, 
e the Auxiliary Data Subgroup (ADS) 
is surveying current auxiliary data usage for product gen- 
eration, preparing guidelines and recommendations, and 
has begun (in co-operation with FS) a task force on global 
mapping aiming at establishing a Global Land One-kilome- 
tre Base Elevation (GLOBE) elevation model, 
e the Catalogue Subgroup (CS) 
carries out its activities by several task teams 
(International Directory Network, CEOS Inventory 
INTeroperability ^ EXperiment, Protocol Definition, 
Browse, World Wide Web, Guidelines), 
e the Network Subgroup (NS) 
has task teams for Browse, IDN, CINTEX, Performance 
Measurement, 1-km AVHRR and Network Register and is 
also works on network security, networking among CEOS 
agencies and new technologies. 
4.4 CEOS Participants 
In addition to the member Governmental organizations identi- 
fied under 4.1 the CEOS has two additional classes of mem- 
bership, observers and affiliates. Observers are governmental 
organizations that are international or national in nature and 
currently have a civil space-segment activity in Phase-A(pre- 
liminary analysis to determine whether a candidate mission is needed, 
feasible and compatible with agency goals and technology state-of-the- 
art)/pre-Phase-A(advanced study to determine new and promising 
missions deserving further study - not yet a part of the project cycle life) 
or equivalent of system development, or a significant ground- 
segment activity that supports CEOS objectives. Observer 
status is granted by invitation of the CEOS members. There 
are at present four observers. 
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