Viewshed tool quit working

[MichaelWeber2]

Why?  I am teaching myself Viewshed analysis (in ArcGIS 10.2) to expand my skill set.  I had made one test MXD with the raster and point files projected to UTM. I was able to run the tool successfully a couple times.  I tried to do other tests in a different parts of the world, but the process doesn't work.  All I get is the generic ERROR 999999: Error executing function; "Failed to open raster dataset"; "Failed to execute (viewshed)."  I have set up both MXDs the same.  THe only difference is I have been using different elevation models.  The original MXD was done using SRTM and the others have been with different levels of DTED.  The point has been set the same too (LATDD, LONGDD, OFFSETA, OFFSETB, RADIUS2).

What am I doing wrong?  I do not want to offer this as something I can do until I can consistantly preform the analysis.  Thanks for any help you can provide.

[JakeSkinner]

What is the coordinate system of the DTED?  Would you be able to upload a sample of your data?

[DanPatterson_Retired]

You should be using projected data since all inputs are assumed to be the  same , ie...don't mix decimal degrees with meters

[MichaelWeber2]

All data is being projected to the appropriate UTM hemisphere and zone.

The only Spatial Analyst, Surface tool that works with the DTED is the Contour tool.  That doesn't need the Input Point, but it is the same point file I used when the SRTM process worked.  If it was a problem reading the DTED, i would expect none of the tools to work.

I am really stumped now.

[DanPatterson_Retired]

then I am confused by these fields ... LATDD, LONGDD,