Starting with AO4, VSOP Announcements of Opportunity will occur three times per year, coordinated with the calls for proposals by the ground VLBI networks. HALCA's orbital elements precess with periods of the order of a year, so AO4 proposals are particularly encouraged for sources in those regions of sky allowing good (u,v) coverage during the first half of 2001. All-sky (u,v) coverages are available as images and as a text version; as a result proposers no longer have the option of including one page of (u,v) coverages per source with their proposals.
It was intended that the trimester proposal system would be adopted from AO3, however the loss of one of HALCA's four reaction wheels and the associated recovery period resulted in the adoption of the trimester system being delayed. HALCA is now operating with three reaction wheels and, as a result, a new set of sun-angle constraints compared with previous AOs --- these are described in section 2.1. AO4 proposals will be scheduled for up to a year after they are accepted, unless a loss of observing time affects the scheduling of the proposal. If an AO4 observation is not scheduled within one year, a new proposal will need to be submitted in response to a later Announcement of Opportunity.
AO4 proposers will be expected to take responsibility for preparing the schedule files for the ground radio telescopes assigned by the VSOP Science Operations Group for their observation. Proposers with little experience in preparing these schedule files may request assistance or consultation from the VSOP Science Operations Group on the proposal cover sheets. Finally, as with AO3, the scientific justification may be submitted either by e-mail in postscript form or by surface mail (see section 4.3.2).