The primary aim of this study is to investigate the combination of SAR and optical data to 
determine, for a number of application areas, the so-called ‘synergistic’ temporal sampling requirements, i.e. the 
time interval within which both optical and SAR data must be recorded in order to derive a particular ground 
parameter or give a specified improvement. Four main types of synergy can be defined : A. No synergy (must 
use data from one particular sensor type); B. Data from either type of sensor can be used; C. Strong requirement 
for data from both sensor types; D. Weak requirement for data from both sensor types (one data-set supports the 
other, but is not essential). 
The present study (performed under contract to ESA, and described in detail in Sephton et al., 
1993) is essentially an empirical approach, complementing a parallel ESA study (Perez et al, 1993) on the 
modelling aspects of SAR - optical data synergy. Section 2 outlines the data survey performed by each of the 
four Sub-Contractors involved in the study. The four main application areas of interest within the study and 
associated Sub-Contractors are Vegetation (Hunting Technical Services Ltd.), Hydrology (VTT), Cryosphere 
(Institute of Meteorology and Geophysics, University of Innsbruck) and Geology / Pedology (EFTAS GmbH). 
Parallel to the data surveys, satellite and airborne instrument surveys were also performed (by Matra Marconi 
Space U.K. and GEC-Marconi Research Centre respectively) in order to derive the main system and orbit 
parameters required for the Mission Analysis stage of the study. In Section 3, an assessment is made of the 
temporal variations inherent in each of the four main application areas for a selected number of ground 
parameters for which validation data is available, whilst in Section 4 the temporal sampling requirements are 
analysed in order to assess whether single-platform satellites combining SAR and optical sensors would be 
feasible in future missions for each of the application areas considered. Finally, in Section 5, a synthesis of the 
results obtained for the different application areas is presented. 
2 - DATA SURVEY 
A survey was performed by each of the four Sub-Contractors in order to establish suitable test sites for which 
both SAR and optical, high-resolution, image data are available, as well as supporting ground-truth data for 
validation. Key parameters were also selected in each application area, depending on factors such as their 
importance to the particular sub-application under consideration, their sensitivity to determination by remote 
sensing data, and whether their determination is likely to be enhanced by a combination of microwave and 
optical data. 
For the vegetation application, data were obtained from the Feltwell test site for both of the 
selected sub-applications of agriculture and forestry. AVHRR, Landsat TM, SPOT XS and ATM optical data 
were available in combination with ERS-1 AMI, NASA/JPL-AIRSAR (‘Maestro’ 1989 and 1991 campaigns) 
and Agriscatt 1987 (airborne scatterometer campaign) microwave data. Ground data comprised crop and tree 
stand maps, together with measurements of the key parameters (c.f. Section 3.1). The site contains both areas 
of woodland (the main body of Thetford Forest contains a wide range of tree species of varying age and 
condition, with conifer species predominating) and agriculture (the five main crops are sugar beet, potatoes, 
wheat, carrots and barley). 
For the hydrology application, the selected sub-application is evapotranspiration (the 
evaporation of water from soil and plant surfaces), a key factor in the global hydrological cycle. A test site was 
chosen in Southern Finland near the village of Vihti which includes five different crop types (wheat, barley, 
turnip rape, rye and oats) covering an area of approximately 1 km x 1 km, and surrounded by forestry land. 
Auxiliary data were available in the form of a numerical soil map, a digital elevation model (‘DEM’), 
atmospheric data (measured at 20 minute intervals), and ground-truth data in the form of 35 parameters measured 
at weekly intervals. The satellite data consisted of 5 rectified ERS-1 SAR images, 1 rectified Landsat TM 
image, and 15 cloud-free NOAA AVHRR images, all recorded in the Summer of 1992 and covering the period of 
time from initial growth until after harvest. From the AVHRR data, Normalised Differential Vegetation Indices 
(‘NDVT values) were derived. However, because the resolution of the AVHRR data is comparable with the size 
of the test area, the precise method of determining the NDVI values was made by comparison with a reference 
area 50 kms west of the test site. 
For the cryosphere application, two test sites were chosen : the high altitude ótztal region of 
the Austrian Alps for the seasonal snow cover and mountain glacier sub-applications, and the Antarctic region of 
Heimefrontfjella / Amundsen Ice / EkstrOm Ice Shelf in Dronning Maud Land (with complementary data from 
the Ronne Ice Shelf) for the polar ice sheets sub-application. For both the seasonal snow cover and mountain 
glacier investigations, 11 ERS-1 SAR images of the ótztal test site were acquired between April and October 
1992, covering a wide range of snow cover conditions at various altitude zones. Landsat 5 TM quarter-scenes