This document answers the following questions regarding Global Positioning System (GPS), Wi-Fi, and Cell Phone Jammers: "1. What are 'jammers'? 2. How do jammers work? 3. What laws prohibit the marketing, sale, and use of jammers? 4. Does the term 'marketing' have a specific meaning? 5. Why are cell phone jammers, Wi-Fi jammers, and GPS jammers illegal? 6. Some devices claim to block cell phone calls, text messages, and emails only inside a car. Are these illegal as well? 7. May I order and import a jammer into the U.S.? 8. What are the penalties for using a jammer? Can I go to prison? 9. What should I do if I already own a jammer? 10. Whom should I contact if I have additional questions? [...] 11. Is it illegal to sell a cell phone jammer, GPS jammer, or other jammer in the U.S. or its Territories? 12. I sell and ship jammers all over the world through a site on the Internet. What should I do to comply with U.S. law? 13. I sell signal jammers on my website but make certain to notify potential buyers that the jammers may only be purchased, leased, or operated in the U.S. by authorized federal agents. Can I be prosecuted if an unauthorized purchaser ignores this warning? 14. Can I legally manufacture a jammer in the U.S.? [...] 15. Can I operate a jammer in the U.S.?" Plus 8 more questions.
United States. Federal Communications Commission: http://www.fcc.gov/