Simply owning a scanner might be legal. However, it might be illegal if you plan to use a police scanner while committing or fleeing from crime. I think a phone app would necessarily be included in the definition of "mobile", but you should consult an attorney knowledgeable in this evolving area of the law about that and all other aspects of your question.
Some states appear to have outlawed mobile scanners. As far as I can tell, the New Jersey law isn't concerned with the scanner being mobile. Instead, the focus is on whether the device is used in the commission of a crime. I assume that mobility would be a feature useful in most crimes in which a scanner might be employed.
Title 2C of the New Jersey Code of Criminal Justice section 33-21, entitled "Interception or use of official communications", provides that, “Any person who intercepts any message or transmission made on or over any police, fire or emergency medical communications system, or any person who is the recipient of information so intercepted, and who uses the information obtained thereby to facilitate the commission of or the attempt to commit a crime or a violation of any law of this State, or uses the same in a manner which interferes with the discharge of police or firefighting operations or provision of medical services by first aid, rescue or ambulance squad personnel, shall be guilty of a crime of the fourth degree.”
The next section, 33-22, entitled "Possession of emergency communications receiver", prohibits, “Any person who, while in the course of committing or attempting to commit a crime, including the immediate flight therefrom, possesses or controls a radio capable of receiving any message or transmission made on or over any police, fire or emergency medical communications system, shall be guilty of a crime of the fourth degree.”
You should consult with a qualified criminal attorney before purchasing such a device or using an app like this. There could be serious criminal penalties depending on the laws in effect and how your activity with the device might fall under one of those laws.
I recommend spending the money ($1,000?) it costs to buy a scanner on Phillies tickets and having a peaceful, law-abiding springtime.
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