You can build a passive radar with two cheap rtl-sdr dongles. That's probably how the Serbs downed that F-117 back in 1999.
https://dopplerfish.com/passive-radar/
https://hackaday.com/2015/06/05/building-your-own-sdr-based-passive-radar-on-a-shoestring/
https://www.rtl-sdr.com/building-a-passive-radar-system-with-an-rtl-sdr/
https://www.rtl-sdr.com/passive-radar-dual-coherent-channel-rtl-sdr/
This modification essentially involves using the same clock source for two RTL-SDR dongles, which can be done by desoldering and removing the clock on one dongle, and then using a cable to connect the clock from another dongle to the clock input of the desoldered dongle.