The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
The authors acknowledge the hospitality of JIVE where part of this work was carried out. SF and ZP acknowledge financial support received from the Hungarian Space Office, the Netherlands Organization for Scientific Research (NWO) and the Hungarian Scientific Research Fund (OTKA, grant no. N31721 & T031723). LIG acknowledges partial support from the European Commission's TRM Programme, "Access to Large-Scale Facilities", under contract ERBFMGECT950012 and the European Commission's TRM programme, Research Network Contract ERBFMRXCT 96-0034 "CERES". WKS and ART acknowledge financial support from the Canadian Space Agency.
This research has made use of