The incorporation of crossover stations into the cruise plans is an important measure of intercalibration.
A crossover station is a location where the track of one cruise overlaps with that of another cruise (as illustrated in the photo below) although the research vessels do not have to be simultaneously at the same place. It is to the benefit of each cruise to collect and analyse samples at that location. Concentrations of most dissolved trace elements and Isotopes (TEIs) in the deep ocean are not expected to change significantly on the time scale between cruises, so comparing results from crossover stations provides a measure of internal consistency for all parameters.
Recommended procedures for sampling at crossover stations are described in a document here.
Alternative methods for intercalibration
For those cases when a crossover station is not possible, GEOTRACES has defined alternative methods for intercalibration. The procedure to intercalibrate for cruises without crossover station is described here.
For further information see page 6 of the Cookbook.