The Global Precipitation Measurement (GPM) is an international effort led by the National Aeronautics and Space Administration (NASA) of the USA and the National Space Development Agency of Japan (NASDA) for the purpose of improving climate, weather, and hydrological forecasts through more frequent and more accurate satellite measurement of precipitation globally. GPM will incorporate and assimilate data streams from many constellation spacecraft with varied orbital characteristics and instrument capabilities. A core satellite, the heart of GPM, will carry a Dual-frequency Precipitation Radar (DPR) and a conical scanning microwave radiometer, the GPM Microwave Imager (GMI). The GMI has many channels similar to the current Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and the Special Sensor Microwave Imagers (SSMI). The passive microwave channels and the two radar frequencies of the core are carefully chosen for investigating the varying character of precipitation over ocean and land, and from the tropics to the high-latitudes. With its unique instrument capabilities and inclined orbit, the core spacecraft will serve as a calibration 'transfer standard' to the remainder of the GPM constellation. In preparation, inter-satellite comparisons using the Tropical Rainfall Measuring Mission (TRMM) with the sun-synchronous SSMI radiometers have been conducted. We demonstrate a possible strategy for an inter-satellite calibration and transfer technique of the GPM constellation using realtime data from the TMI (serving as the core radiometer) and several of the SSMI instruments. Observational intersections of the core with the constellation spacecraft are essential in applying this technique to the member satellites. Information from core spacecraft retrievals during space-time orbital intersection events will be transferred to the constellation radiometer instruments in the form of improved calibration and, with experience, improved radiometric algorithms. Ongoing research involves study of critical variables in the inter-comparison, such as correlation with spatial-temporal separation of intersection events, frequency of intersection events, variable azimuth look angles, and variable resolution cells for the various sensors.