I use the files available from the US dept of defence as well, for use with OziExplorer. The elevation sample that is available for free is on a scale of (about)1:250 000 or 30 sec arc.
From the OziExplorer web site:
Level 1 and level 2 Data
The data available to the public is called Level 0 and has a 30 arc second spacing.
Other higher resolution data called Level 1 and Level 2 is not available to public but can be used in OziExplorer if you have it by making sure the DTED path name ends in a 1 or a 2 ( example dted1 ) depending on the level you are using. OziExplorer uses the last character in the path name to determine the files to search for ( .dt1 or .dt2 ). Note the use of Level 0 data does not require a special folder name.
Here are some instruction to follow when you are on the page.
Downloading
When you open the web page you will see a map of the World with many options.
Just below the map of the World select the "Recenter and Zoom In" option
Click on the map of the World to select the area you want to download or use the Find or Define AOI tab options in the lower Frame to search or specify the Latitude/Longitude.
There is a limit on the amount of data you can download at one time.
When you have the area selected click on the NIMA Coverages tab, a new frame will be shown.
Make sure just the DTED Level 0 box is ticked.
Click on the Download/Order tab, a new frame will be shown.
More than 1 dataset will be shown, only use the "Digital Terrain Elevation Data (DTED®)" table
Level will be DTED Level 0
Format must be Military Specification (any other format will not work).
Platform should be Windows (this will produce a .zip file, if you select Unix you will get a .tar file)
Press the Download button immediately to the right of this table.
You will be presented with instructions in a new browser window, follow these.
It is suggested you download the files to a folder called Elevation DataDTED located under the folder where OziExplorer is installed.
Good Luck!!