Based on the NCEP HALF-RAOB study in fall 1996, experiments for specific hypothese which is testing the redundancy of ACARs and selected rawinsondes near busy airports have been formulated by NAOS ( Northern American Observation System ). Experiments using the NCEP/GDAS ( Global Data Assimilation System ) and MRF ( Medium-Range Forecast ) model with T126 ( horizontal ) and L28 ( vertical ) resolutions and a 32km Regional Eta System have been made. The experiments period covers almost the full 1997-1998 winter season ( December 20 1997 - February 28 1998 ) with TOVS- 1b data including in GDAS. Five-day forecasts have been made for 00Z and three-day forecasts for 12Z. The experiments include 1). Control run ( CTL ) which using new implemented GDAS/MRF model ( February 12 1998 ). 2) Experiment run ( EX1 ) which using the same model as CTL run, but excluding rawinsonde reports at 14 RAOB stations in close proximity to airport. 3) Experiment run ( EX2 ) which excluding ACAR reports around 14 RAOS stations ( radius is 150 km and height is 7620 meters (=25000 feet) ). Results of these experiments will be presented and are expected to provide useful information on designing a future observing system over north American.