return time and percentage of longitude locations imaged were re-analysed for a particular maximum return time 
(i.e. 0.5, 1, 3, 7, 14 and 28 days). An assessment was then made, for each key parameter within each 
application area, of the synergistic temporal sampling requirements in terms of the degree of latitudinal 
coverage, longitudinal coverage and frequency of return times for various orbit and system parameter 
combinations. The results are summarised in Section 5. 
5 - SYNTHESIS OF RESULTS AND CONCLUSIONS 
The main results in terms of the temporal sampling requirements derived for each application area are given in 
Table 1. These are specified for two approaches of satellite data use : (1) general temporal requirements without 
specific consideration of synergistic use; (2) temporal requirements if satellite data are used synergistically. The 
general sampling requirements represent the maximum time interval required to observe and monitor underlying 
change in a particular parameter, whilst the synergistic temporal sampling requirements represent essentially the 
time interval over which there is no underlying change in the fundamental dynamics of features. For agriculture, 
the temporal sampling requirements reflect the changes in parameter that may be seen at the time of critical 
growth, and hence may be relaxed somewhat at other times. 
For all of the parameters investigated within this study, there is only the ‘weak’ requirement 
for synergy (case ‘D’) referred to in Section 1, i.e. although a given parameter can be derived from SAR or 
optical data only, it is still possible to derive a synergistic temporal sampling requirement in terms of the 
improvement in the parameter derivation when the SAR and optical image acquisition is within this time 
interval. A good example of a quantifiable improvement in parameter determination arising from microwave and 
optical synergy is in the classification of snow / ice areas for Mountain Glaciers referred to in Section 3.3. 
Comparing the results obtained between the different application areas in Table 1, it can be 
seen that the smallest maximum synergistic sampling interval for any parameter is 1 day (for the areal extent of 
seasonal snow cover in lowland regions, and soil moisture and watercourse run-off after rain). Temporal 
dynamics of snow are particularly high for regions with little relief variation, the change from full snow cover 
to completely snow-free areas occurring within as little as a few days. The change of soil moisture after heavy 
tropical rainfall is an important indicator of the water-holding capacity of the soil. 
Comparing the temporal sampling requirements obtained for LAI at the Feltwell and Vihti 
test sites there is a good correspondence. However, for crop height, no correlation was observed in the Vihti test 
site data between canopy height and optical data, suggesting the need for crop growth models in combination 
with the SAR and optical data. From the results on soil moisture obtained at the Vihti and Mali test sites, 
again no correlation was determined with SAR backscatter at the Vihti test site. However, as soil moisture was 
being determined at a site with vegetation cover, it is possible that other parameters may be masking the 
sensitivity to soil moisture. 
The results of the assessment as to whether the synergistic temporal sampling requirements 
derived for each of the four main application areas can be met for various orbit and system parameter 
combinations and both one- and two-platform mission scenarios are given in Tables 2 and 3, from which it can 
be seen that a significant improvement in latitude and longitude coverage is attained by consideration of aM 
optical instrument with steerable swath. However, even with a scanning capability in the optical instrument of 
± 30° and a 400 kms SAR swath, there are an inadequate number of return times for 14 and 35 day repeat periods 
in order to meet a 1 day synergistic sampling requirement, indicating a medium resolution sensor (e.g. AVHRR) 
may be more useful. No significant difference was found between the one- and two-platform mission scenarios 
in terms of latitude and longitude coverage or number of return times, suggesting that single-platform satellites 
combining SAR and optical sensors would be feasible for future missions. 
6 - REFERENCES 
Sephton A.J., Cross M., Green I., Häme T„ Komp K.-U., Meadows P.J., Rott H„ Wielogorska A. and Rast 
M., 1993. Simultaneous Implementation of a Synthetic Aperture Radar and a High-Resolution Optical Imager. 
Draft Final Report for ESTEC contract 10063/92/NL/SF, ESTEC, 2200 AG Noordwijk, The Netherlands. 
Perez P., Hoekman D., devers J., Taconet O., van den Broek B. and Bouman S., 1993. Synergy in the 
Modelling of Microwave with Optical Remote Sensing Data. Draft Final Report for ESTEC contract 
9837/92/NL/GS, ESTEC, 2200 AG Noordwijk, The Netherlands.