A Tri Axial Square Helmholtz Coil system for the study of palaeomagnetic studies was successfully commissioned at the Alibag magnetic observatory in the year 1985. This system was used for few years after which the system encountered technical problems with the control unit. Rectification of the same could not be undertaken as the information document related to this system was not available and as a result the said system had been lying in an un-used state for a long time until 2015 when the system was re-commissioned and upgraded as a test facility for calibrating the magnetometer sensors. We have upgraded the system with a constant current source and a data logging unit. Both these units have been designed and developed in the institute laboratory. Also re-measurements of the existing system have been made thoroughly. The upgraded system is semi automatic, enabling non-specialists to operate it after a brief period of instruction. This facility is now in broad use for the parent institute and external institutions to calibrate their magnetometers and also serves as a national facility. Here the design of this system with the calibration results for the space borne fluxgate magnetometers is presented.