accessible and which are only destined for a limited group 
of users, e.g., for the staff of certain organizations. 
Looking at the whole scene of automated information 
storage and retrieval, we may conclude that much effort 
worldwide has been invested in various kinds of databases. 
But, as noted in the Introduction, there is no single 
database for the whole field of aerospace. In each 
individual case of an on-line search it is up to the user to 
select the most appropriate database or combination of 
databases. One important advancement is that nowadays 
the best known hosts offer the possibility of cluster 
searching, i.e., when starting the retrieval procedure, you 
may combine some appropriate databases and search 
them as if they were one single database. 
We hope that this paper will help users to make the best 
choice of databases for an on-line search. 
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