COHOWeb data development starts with hourly-resolution data files provided to NSSDC by Principal Investigators on the relevant spacecraft missions. These files were typically, but not necessarily, in ASCII format. Some contained magnetic field data and others plasma data. Most had spacecraft position data. There were multiple coordinate systems used to specify magnetic field, flow velocity and spacecraft position vectors.

At NSSDC a new set of spacecraft-specific hourly ASCII records ("COHOWeb-input records" in the terminology of Section 1 above) was created wherein magnetic field, plasma and spacecraft position data were merged. Each such record contained a common set of parameters, including magnetic field Cartesian components in RTN coordinates and spacecraft position data in Heliographic Inertial (HGI) coordinates. These coordinate systems are defined below. Coordinate transformations, when needed, were performed by NSSDC. Some spacecraft-specific records had a few additional spacecraft-specific parameters.