There are a few things I can suggest off the top of my head: 1) Run the orthorectification process using just RPCs (no ground control) to see if you can get a reasonable result. Ir's rare, but sometimes the geographic grids stored inside of Level 1B files are really inaccurate and the resulting RPCs can conflict with the more accurate user-provided GCPs. 2) Make sure that your GCPs have accurate X, Y, and Z (elevation) values. 3) Try providing a DEM as well (if you are not doing so already). Even a 90m SRTM data set should help, 4) Limit your GCPs to approximately three extraordinarily well-known locations. Because of the large spatial discrepancy between 15-90m ASTER data and 1m NAIP data, it is possible to introduce a lot of error during the GCP definition process. It is very important to make sure that your GCPs are as accurate as possible. ENVI mainly uses the GCPs to update the RPC model, so providing a large number of GCPs can actually do more harm than good. 5) If you provide a ground control point and notice that the original map coordinates (e.g., UTM) are converted to Geographic Lat/Lon, resist the temptation to change the coordinates back to their original values, Orthorectification takes place in Geographic Lat/Lon space, so changing the coordinates back will put the GCPs well outside of the expected range.