will tend to be much less accurate than conventional surveillance radars, whilst multistatic radars are capable of achieving somewhat greater accuracies. Most passive radars are two-dimensional, but height measurements are possible when the deployment is such there is significant variation in the altitudes of the transmitters, receiver and target, reducing the effects of geometrical dilution of precision (GDOP). Advantages and disadvantages Advocates of the technology cite the following advantages: Lower procurement cost Lower costs of operation and maintenance, due to the lack of transmitter and moving parts Covert operation, including no need for frequency allocations Physically small and hence easily deployed in places where conventional radars cannot be Rapid updates, typically once a second Difficulty of jamming Resilience to anti-radiation missiles. Opponents of the technology cite the following disadvantages: Immaturity Reliance on third-party illuminators Complexity of deployment 1D/2D operation, but possible use 2 different systems for 3D (height + range). Commercial and academic systems Passive radar systems are currently under development in several commercial organizations. Of these, the systems that have been publicly announced include: Lockheed-Martin Silent Sentry - exploiting FM radio stations BAE Systems CELLDAR - exploiting GSM base stations Selex ES Aulos passive radar Thales Air Systems Homeland Alerter - FM radio-based system Hensoldt multiband passive radar Silentium Defence Maverick-M passive radar surveillance ERA VERA-NG Passive Surveillance ESM Tracker Daronmont Technologies SECAR-P Passive Radar for Australia Several academic passive radar systems exist as well: Manastash Ridge Radar Alim radar system Silentium Defence Oculus space domain awareness observatory (Maverick-S passive radar) Current research Research on passive radar systems is of growing interest throughout the world, with various open-source publications showing active research and development in the United States (including work at the Air Force Research Labs, Lockheed-Martin Mission Systems, Raytheon, University of Washington, Georgia Tech/Georgia Tech Research Institute and the University of Illinois), in the NATO C3 Agency in The Netherlands, in the United Kingdom (at Roke Manor Research, QinetiQ, University of Birmingham, University College London and BAE Systems), France (including the government labs of ONERA), Germany (including the labs at Fraunhofer-FHR), Poland (including Warsaw University of Technology). There is also active research on this technology in several governments or university laboratories in China, Iran, Russia and South Africa. The low-cost nature of the system makes the technology particularly attractive to university laboratories and other agencies with limited budgets, as the key requirements are less hardware and more algorithmic