unused spectrum. Another proposed solution to the spectrum scarcity problem is to enable communications systems to occupy spectrum that was previously allocated for radar use and to cooperatively share spectrum. This approach has received increased attention recently with several research programs, including DARPA projects, investigating several methods of cooperative radar-communications spectrum sharing. More alternatives are underway such as spectrum sharing in cellular networks. Spectrum scarcity has emerged as a primary problem encountered when trying to launch new wireless services. The effects of this scarcity are most noticeable in spectrum auctions where the operators often need to invest billions of dollars to secure access to specified bands in the available spectrum. In spite of this scarcity, recent spectrum utilization measurements have shown that the available spectrum opportunities are severely underutilized, i.e. left unused. This artificial "access limitation"-based scarcity is often considered to result from the static and rigid nature of the command and control governance regime. Interested parties have started to consider possible improvements in the governance regime by relaxing the constraints on spectrum access. Two prevailing models are the "spectrum commons" and the "spectrum property rights" approaches. Spectrum commons theory Under US law, the spectrum is not considered to be the property of the private sector nor of the government except insofar as the term "government" is used to be synonymous with "the people". The original use of the term "the commons" was the practice by which the public at large had limited rights to use the commons; each person then had an interest in their own usage rights, but the commons themselves were not property, nor were the rights "property" since they could not be traded. The term "tragedy of the commons" was popularized by Garrett Hardin in a 1968 article which appeared in Science. The tragedy of the commons illustrates the philosophy that destructive use of public reservations ("the commons") by private interests can result when the best strategy for individuals conflicts with the "common good". In such a scenario, it asserts that even though the contribution of each "bad actor" may be minute, when the results of these actions are combined the resource could be degraded to the point of uselessness. This concern has led to the regulation of the spectrum. Spectrum property rights model The spectrum property rights model advocates that the spectrum resources should be treated like land, i.e. private ownership of spectrum portions should