One question which comes up repeatedly in SPDS discussions is the extent to which SPDS is a redundant service, especially in view of recent improvements in services of NSSDC. Certainly any search for data should begin with a check of the NASA Master Directory (NMD). The advantages of the SPDS include oversight and support of the data by the expert team that took the data, plurality, and flexibility. The SPDS also provides ready access to information, such as nuclear interaction cross sections and neutron monitor data, that is not normally archived by the NSSDC. The basically volunteer nature of the SPDS keeps the cost to a minimum.
Another open issue in relations between SPDS and NSSDC is the search
for a techniques for guaranteeing that all SPDS data are eventually
migrated to NSSDC for `deep' archiving, since the institutional lifetime
of these volunteer efforts is not guaranteed.