Suggestion: Visit
http://www.dxdesktop.com/db_online/rdf.htm
http://www.directionfinder.com/rdf.htm
etc.
Essentially, it depends on nature of targets, what they transmit (if anything), etc. If it is a flying object without much of emission, then you need a radar to track and calculate its position. Some radars operate within the line-of-sight, others over the horizon. The latter one need to transmit and receive refletected waves. Visit
http://www.uksmg.org/tropo.htm
for more info on a troposcatter

You are right, it can seem as special radar and it just detect the direction only not the distance and range.

The operation theory is also not different than Radar. Most of the Radar transmits a narrow beam of energy to space and record the time when it bounces back. The direction information will be determined by the angle it transmit the energy. The DF may not have the transmitter or the transmitter is in the other unit or just passive receive.

Jbartos' web pages are very good to explain the passive receive DF. To work as a special application, some special technique can use for good performance DF than the traditional Radar design like use 4 antennas and compare the signals to get the direction information. You can find the antenna design in the web page from Jbartos as an example.

I just know the DF used in military like missile seeker, electronic warfare. May be you can use it in the commercial navigation system.

Suggestions:
1. One frequent commercial application of the DF is locating someone in distress, e.g. ship, person, etc.
2. Other is a guidance.