As noted here:
The source terrain data is only available between 60 North and 56 South latitudes.
From USGS: "Endeavour orbited Earth 16 times each day during the 11-day mission, completing 176 orbits. SRTM successfully collected radar data over 80% of the Earth's land surface between 60° north and 56° south latitude with data points posted every 1 arc-second (approximately 30 meters)."
If you can point us towards a public and royalty free 1 arc-sec data set that covers the upper latitudes we could look at trying to fill it in. But we need the data from somewhere to be able to serve it...