GROUND-BASED NETWORKS OF CO-LOCATED SPACE GEODETIC TECHNIQUES (VLBI SLR GNSS AND DORIS) ARE THE BASIS FOR THE DEVELOPMENT AND MAINTENANCE OF THE INTERNATIONAL TERRESTRIAL REFERENCE FRAME (ITRF) WHICH IS OUR METRIC OF REFERENCE FOR MEASUREMENTS OF GLOBAL CHANGE. THOSE SITES WITH SLR VLBI GNSS ARE CONSIDERED CORE SITES; COLOCATION SITES ARE THOSE WITH EITHER VLBI OR SLR PLUS GNSS. THESE NETWORKS PROVIDE MEASUREMENTS OF STATIC AND TIME-VARYING COMPONENTS OF THE EARTH S GRAVITY FIELD; PRECISION ORBIT DETERMINATION CALIBRATION AND VALIDATION FOR ACTIVE SATELLITES SYSTEMS FOR ALTIMETRY (E.G. ALTIMETRY); AND TIME TRANSFER AND DETERMINATION OF FUNDAMENTAL CONSTANTS. DATA FROM THESE NETWORKS PROVIDE EARTH ORIENTATION PARAMETERS TIME HISTORY OF GROUND STATION POSITIONS AND BASELINE LENGTH STRAIN MODELS MEAN SEA LEVEL AND OCEAN SURFACE TOPOGRAPHY MARINE TIDE MODELS ATMOSPHERIC AND IONOSPHERIC PARAMETERS. THE GLOBAL GEODETIC OBSERVING SYSTEM (GGOS) OF THE INTERNATIONAL ASSOCIATION OF GEODESY (IAG) HAS ESTABLISHED A TASK TO DEVELOP A STRATEGY TO DESIGN INTEGRATE AND MAINTAIN THE FUNDAMENTAL GEODETIC NETWORK AND SUPPORTING INFRASTRUCTURE IN A SUSTAINABLE WAY TO SATISFY THE LONG-TERM REQUIREMENTS FOR THE REFERENCE FRAME. THE GGOS GOAL IS AN ORIGIN DEFINITION AT 1 MM OR BETTER AND A TEMPORAL STABILITY ON THE ORDER OF 0.1 MM/Y WITH SIMILAR NUMBERS FOR THE SCALE AND ORIENTATION COMPONENTS. THESE GOALS ARE BASED ON SCIENTIFIC REQUIREMENTS TO ADDRESS SEA LEVEL RISE WITH CONFIDENCE BUT OTHER APPLICATIONS ARE NOT FAR BEHIND. RECENT STUDIES INCLUDING ONE BY THE US NATIONAL RESEARCH COUNCIL (SEE FIGURE 1) HAVE STRONGLY STATED THE NEED AND THE URGENCY FOR THE FUNDAMENTAL SPACE GEODESY NETWORK. THE NEEDS ARE ARTICULATED IN MORE DETAIL ON THE GLOBAL GEODETIC OBSERVING SYSTEM: MEETING THE REQUIREMENTS OF A GLOBAL SOCIETY ON A CHANGING PLANET IN 2020 (PLAG H-P AND PEARLMAN M.R. 2009). THE REFERENCE FRAME IS DEFINED THROUGH A GLOBAL NETWORK OF CO-LOCATED VLBI SLR GNSS AND DORIS STATIONS AND A DENSER NETWORK OF GNSS AND DORIS GROUND STATIONS TO DISTRIBUTE THE REFERENCE FRAME GLOBALLY TO THE USERS SO THAT GEOPHYSICAL MEASUREMENTS ANYWHERE IN THE WORLD CAN BE POSITIONED IN THE FRAME ANY TIME OF DAY. THE FOUR TECHNIQUES MEASURE DIFFERENT QUANTITIES IN DIFFERENT WAYS AND EACH HAS ITS OWN SET OF SYSTEMATIC ERRORS. PROPER COMBINATION ALLOWS US TO TAKE ADVANTAGE OF THE STRENGTHS AND MITIGATE THE WEAKNESSES OF EACH. THE TECHNIQUES ARE CO-LOCATED SO THAT THE MEASUREMENTS AMONG THEM CAN BE RELATED TO SUB-MM ACCURACY. THESE SITES WILL ALSO HAVE ANCILLARY MEASUREMENT INCLUDING ABSOLUTE AND CRYOGENIC GRAVIMETERS TIDE GAUGES SEISMOMETERS ETC TO CONNECT OTHER GEOPHYSICAL MEASUREMENTS TO THE REFERENCE FRAME (SEE FIGURE 2). EARLY SIMULATIONS CONDUCTED BY E. PAVLIS EXAMINED ACCURACIES FOR ORIGIN SCALE AND ORIENTATION OF THE RESULTING ITRF BASED ON VARIOUS NETWORK DESIGNS AND SYSTEM PERFORMANCE TO DETERMINE THE OPTIMAL GLOBAL NETWORK TO ACHIEVE MM PERFORMANCE. THESE SIMULATIONS SHOW THAT ABOUT 30 CO-LOCATED STATIONS WITH MODERN TECHNOLOGY SLR VLBI GNSS AND DORIS (WHERE AVAILABLE) WOULD BE ADEQUATE TO DEFINE THE GGOS REQUIRED REFERENCE FRAME IF THE STATIONS WERE ON GEOLOGICALLY STABLE GROUND WITH GOOD WEATHER ESTABLISHED INFRASTRUCTURE AND LOCAL SUPPORT AND PERSONNEL. THIS IS A CERTAINLY A TALL ORDER AND IS A GOAL UNLIKELY TO BE ACHIEVED (AT LEAST IN MY LIFE TIME).