If you're a U.S. Air Marshal patrolling the friendly skies, you'll want to communicate discreetly with fellow on-board marshals, airport ground crew, cockpit crew and flight attendants if you need to thwart an attack.

You might also want to tap into the plane's digital system to know where you are at any time, how far the nearest airport is and how much fuel you have left on the plane.

To do this, you'll want something like the Federal Air Marshal Service Communication System (FAMSCOM), an application that runs on any off-the-shelf wireless PDA. Here's how the web site of Honeywell, maker of the FAMSCOM, describes it:

These integrated technologies allow the officer to roam anywhere in the aircraft cabin, communicating securely and covertly with FAMS ground operations, cockpit crew, other onboard air marshals and airline cabin crew as well as digital aircraft systems. While the value of communicating with crew or ground personnel may be obvious, the ability to access onboard aircraft systems is also a significant advantage. For example, knowing the distance and time to the nearest airport or time-based fuel remaining could be invaluable in planning a response to an onboard attack With FAMSCOM, an officer can coordinate a response and it’s timing with other officers or cabin crewmembers, greatly increasing the probability of success. . . . To meet these needs for global and onboard connectivity, the cabin wireless network is interconnected with the existing Communications Management Unit and ACARS digital air-ground datalink. . . . The Honeywell Secure ACARS technology protects the privacy and integrity of ACARS message exchanges and authenticates the identities of the communicating parties.

Now let's take a look at what kind of communication runs through ACARS. Again, from Honeywell:

The Aircraft Communications Addressing and Reporting System (ACARS) is the air-ground data link necessary for global communication of vital information, like AOC and ATC messages for commercial airlines and Global Air Traffic Management (GATM) /Mobility 21 (M21) information for military applications. ACARS messages are exchanged via VHF, HF, or SATCOM links with Data link Service Providers (DSP) operating terrestrial communication networks, which uplink and downlink ACARS messages to and from aircraft and provide routing to and from terrestrial Command and Control (C2) systems.

And, finally, let's see who is allowed to become an air marshal. From an investigation done last year by the nonprofit journalism group ProPublica:

Shawn Nguyen bragged that he could sneak anything past airport security using his top-secret clearance as a federal air marshal. And for months, he smuggled cocaine and drug money onto flights across the country, boasting to an FBI informant that he was "the man with the golden badge." Michael McGowan used his position as an air marshal to lure a young boy to his hotel room, where he showed him child porn, took pictures of him naked and sexually abused him. And when Brian "Cooter" Phelps wanted his ex-wife to disappear, he called a fellow air marshal and tried to hire a hit man nicknamed "the Crucifixer." Since 9/11, more than three dozen federal air marshals have been charged with crimes, and hundreds more have been accused of misconduct, an investigation by ProPublica has found. Cases range from drunken driving and domestic violence to aiding a human trafficking ring and trying to smuggle explosives from Afghanistan. The Federal Air Marshal Service presents the image of an elite undercover force charged with making split-second decisions that could mean the difference between stopping a terrorist and shooting an innocent passenger. But an examination of police reports, court records, government reports, memos and e-mails shows that 18 air marshals have been charged with felonies, including at least three who were hired despite prior criminal records or being fired from law enforcement jobs. A fourth air marshal was hired while under FBI investigation. Another stayed on the job despite alarming a flight attendant with his behavior.

UPDATE: A spokesman for the U.S. Air Marshal program says that air marshals are not using this Honeywell product, and they've asked Honeywell to remove the web page. Spokesman Nelson Minerly said that Honeywell developed and pitched the product to the marshals program, but the program rejected it.

Photo montage: ProPublica