Revelations that the Federal Bureau of Investigation was operating a secret fleet of small aircraft spying on the public below has prompted a backlash of sorts.

Lawmakers in the US Senate introduced legislation Wednesday that would require federal authorities to get a probable-cause warrant from a judge to surveil the public from above with manned aircraft or drones.

Further Reading Congress mulls law requiring warrant for e-mail data—yet again

"Americans' privacy rights don't stop at the treetops," Sen. Ron Wyden (D-Ore.) said of his proposal.

The Protecting Individuals from Mass Aerial Surveillance Act (PDF), also sponsored by Sen. Dean Heller (R-Nev.), comes two weeks after the Associated Press "traced at least 50 aircraft back to the FBI, and identified more than 100 flights in 11 states over a 30-day period since late April, orbiting both major cities and rural areas." What's more, the FBI obscured the planes' ownership through fake companies.

Ars' Sean Gallagher also described what he called the "FBI's secret surveillance air force—small planes with sensors perfected for battlefield intelligence in Iraq and Afghanistan that have quietly seen service all over the country." The planes are equipped with high-definition day and night surveillance systems.

Under Wednesday's proposed measure, information collected without a warrant during these surveillance flights would be inadmissible in court. Also, the US government could not contract with commercial or private plane operators to perform unauthorized surveillance.

Heller said the measure, which is being backed by civil rights groups, protects the public "from being trampled by the government's intrusion from above and provides much-needed clarity on what authority the federal government has related to aerial surveillance."

Further Reading How I tracked FBI aerial surveillance

The proposal, however, is riddled with loopholes.

For starters, it does not apply to local and state law enforcement agencies. Further, the measure does not apply to immigration patrol within 25 miles of a land border, and all bets are off during "exigent" or emergency situations. Wildlife management and searching for illegal marijuana-growing operations are also excluded.

Even with the loopholes, the measure has slim chances of landing on President Barack Obama's desk for signature, as lawmakers are loath to demand probable-cause warrants to protect Americans' privacy.

The most recent example was the passage of the USA Freedom Act two weeks ago. That measure Obama signed alters the bulk phone metadata spying program Edward Snowden disclosed two years ago. Here's how Ars described the act:

Under the new legislation, however, the bulk phone metadata stays with the telecoms and is removed from the hands of the NSA. It can still be accessed with the FISA Court's blessing as long as the government asserts that it has a reasonable suspicion that the phone data of a target is relevant to a terror investigation and that at least one party to the call is overseas. As we've repeatedly stated, the Constitution's Fourth Amendment standard of probable cause does not apply. The metadata includes phone numbers of all parties in a call, numbers of calling cards, time and length of calls, and the international mobile subscriber identity (ISMI) of mobile calls.

If that isn't enough evidence that the measure is likely doomed, consider that current law does not require the authorities to get a probable-cause warrant from a judge to demand ISPs fork over customers' e-mail if that e-mail has been stored on servers at least 180 days. Lawmakers often talk about changing that President Ronald Reagan-era law, but there's no real political will to actually get it done.