Radar

Doppler
http://www.ebay.com/itm/HB100-Microwave-Sensor-Module-10-525GHz-Doppler-Radar-Motion-Detector-Arduino-/171297069092

https://www.youtube.com/watch?v=2RXSVBzEs14 RCWL-0516 RCWL 0516 Microwave Radar Sensor, human. On aliexpress for $1. Range about 5m, which compares well with pet safe passives. Avoid radar signature interference when installing multiple units. See OutdoorBox

Radar Sensor Module Wireless HB100 Microwave Doppler Radar Detector Probe Wireless Sensor Module 10.525GHz from Aliexpress.com shop  ModuleFans  $2.65. attach to satellite dish 10m range

Surveillance radar

 * https://en.wikipedia.org/wiki/Compact_surveillance_radar hack these Patents held by five companies.
 * https://spotterrf.com/ https://www.google.com/patents/US20150204969 A method for determining location information for a target may include generating a cyclic transmit signal having a fixed frequency slope over a substantial portion of each cycle and an operational frequency that is selected to penetrate foliage, transmitting the cyclic transmit signal without up-conversion with at least one transmit antenna, down-converting two or more return signals received from corresponding return antennas with the cyclic transmit signal to provide a corresponding set of baseband signals, extracting magnitude and phase information for each return signal as a function of range from the plurality of baseband signals, and determining a location of a target from the magnitude and phase information. A corresponding apparatus, system, and computer readable medium are also disclosed herein.
 * http://www.magosys.com/
 * http://www.geolux.co.za/Products/