There are several methods for implementing direction finder.
1. One of the method is by using directional antenna pointing to target device, and reading the received signal strength from the target device. This method also need the device to keep transmitting, The following "direction finder" use such simple directional antenna and is described here: http://www.pki-electronic.com/products/interception-and-monitoring-systems/gsm-direction-finder/
Yet, if this is the method that is used with the pki device, I wander how it can discern the device among other devices, because as I understand, the uplink frequency is shared among several devices nearby.
2. There are other method too, using Pseudo-doppler DF method, such as done by Balint Seeber: http://wiki.spench.net/wiki/SDRDF But I see that the demonstration is done on VHF (not GSM/LTE)
I would like to ask:
- How does the first method (directional antenna) can find a device among other with same frequency ?
- Can the 2nd method (SDRDF) be used to locate GSM-frequency mobile device ? and discern a device among several with same frequency ?