17 Passive Radar
By using two directional antennas one pointing directly at existing RF source* and the other pointing in the opposite direction. As long as both captured IQ streams are phase coherent, which with the LimeSDR they should be since the same clock was used, then after initial calibration, the distance to a plane (or meteor trails) can be calculated. And the Doppler shift up or down would correspond to the approximate speed towards or away from the transmitter. It is a two dimensional Radar, you know the rough distance and approximate speed, but not the height or the angle.
*Terrestrial television stations (analogue/digital), Radio stations (FM, DAB, DRM+, HD-Radio), Mobile phone base stations, Satellites (GPS). The only real requirement is to use a wavelength at least an order of magnitude less than the size of the object.
To simplify, D1+D2 could be calculated by counting the number of samples that were required to shift back in time the signal at RX1 so that it was aligned with the weaker reflected signal at RX2. And dividing that distance by 2 would give the approximate distance to the plane (or a meteor trail).
If you were sampling at 20 MSPS (IQ) then each sample would be 50 nanoseconds in duration. And the distance than light in air can travel in 50 nanoseconds is ~14.9851 meters. So each sample in the offset would correspond to ~15 meters (~49 feet).
The above description is close to a Pulse-Doppler radar. But the configuration is probably closer to a monostatic radar in which the TX bandwidth is critical, for calculating the distance. If a single 100kHz bandwidth FM station was being used ( http://www.radartutorial.eu/01.basics/Range%20Resolution.en.html ) then minimum distance resolution would be 1.5km. The problem is that in the frequency domain you can not extract more information from a system than is in the system to begin with. So if there is only 100kHz of information you can not extract 20MHz of useful details.
One interesting side note is that the returned RF will be re-modulated by the vibrations on the surfaces of the aircraft, so at least in theory it should be possible to hear and distinguish, after proper processing, the downconverted audio returned by a helicopter, a propeller aircraft and a jet engine aircraft.
(ref: http://kaira.sgo.fi/2013/08/passive-radar.html )
(ref: http://hackaday.com/2015/06/05/building-your-own-sdr-based-passive-radar-on-a-shoestring/ )
(ref: https://en.wikipedia.org/wiki/Bistatic_radar )
(ref: https://en.wikipedia.org/wiki/Doppler_effect )