http://blog.jgc.org/2010/06/1010-code.html

10.geo.js

var alphabet = 'ABCDEFGHJKMNPQRVWXY0123456789';
var base = alphabet.length;

function calculate_tt(lat, lon) {
  lat += 90.0;
  lon += 180.0;

  lat *= 10000.0;
  lon *= 10000.0;

  lat = Math.floor(lat);
  lon = Math.floor(lon);

  var p = lat * 3600000.0 + lon;

  var tt_num = p * base;
  var c = 0;

  for ( i = 1; i < 10; ++i ) {
    c += ( p % base ) * i;
    p = Math.floor( p / base );
  }

  c %= 29;

  tt_num += c;
  tt_num = Math.floor(tt_num);

  var tt = "";

  for ( i = 0; i < 10; ++i ) {
    d = tt_num % base;
    if ( ( i == 4 ) || ( i == 7 ) ) {
      tt = " " + tt;
    }
    tt = alphabet.charAt(d) + tt;
    tt_num = Math.floor( tt_num / base );
  }

  return tt;
}