Blocking Aircraft Still Possible

Aviation International News » September 2014
September 3, 2014, 4:45 AM

As more aircraft equip with ADS-B out capability, which broadcasts position, velocity, altitude and other information in unencrypted formats on easily received frequencies, business aircraft operators are concerned about whether they can continue blocking their aircraft from display on flight-tracking websites.

In fact, there currently is no physical way to block reception of mode-S transponder or ADS-B signals by a simple receiver. The FAA, however, still offers a way for operators to request blocking of particular aircraft from FAA radar data feeds, and this also can help prevent the particular aircraft from showing up on most flight-tracking websites.

Formerly known as the Block Aircraft Registration Request (Barr) program and administered by NBAA, the program now appears to have no official name other than being referred to as “ASDI block” by the FAA. ASDI is the Aircraft Situation Display to Industry data feed of all IFR traffic receiving radar services from the FAA in the National Airspace System, which presumably now includes ADS-B information that is showing up on controllers’ radar screens.

Any aircraft owner or operator can make a simple request to the FAA to be on the block list, and NBAA has a useful summary of the process on its website. Updates are made on the first Thursday of each month, and the block remains active until cancelled by the requestor.

There are two types of blocking request: source blocking or subscriber-level blocking.

With source blocking, no ASDI data is sent to the ASDI (flight-tracking) vendors. Until ADS-B came along, this pretty much kept blocked aircraft from being visible on flight-tracking websites (with the exception of those with multilateration capability; more about that later). A disadvantage of source blocking is that the operator making the request cannot take advantage of flight-tracking services to see his own aircraft.

Subscriber-level blocking means that the ASDI data feed still gets delivered to the vendor, but the vendor agrees (by formal memorandum of agreement with the FAA) not to display that aircraft publicly on its flight-tracking website. However, the operator can arrange with the vendor to view his own aircraft (which usually entails a fee). Note that ASDI data is filtered to remove sensitive operations and military aircraft.

High-tech Planespotting Network

The value of blocking is being questioned, however, because of ADS-B out. Mandatory in the U.S. after midnight Dec. 31, 2019, ADS-B out transmits information about the equipped aircraft every second to about 630 ground stations, which route the information to controllers. ADS-B signals are unencrypted and can be “seen” by simple ADS-B receivers. Hobbyists are placing these receivers on roofs all over the world, connecting them to the Internet and sharing the resulting data feed, creating what in effect is an enhanced planespotter system.

Flight-tracking provider FlightRadar24 provides receivers to people with a clear view of the sky and a good Internet connection then adds the data gathered by the receiver to its traffic display. But the receivers–there are about 3,000 around the world–aren’t the sole source of information; FlightRadar24 also obtains information from the FAA ASDI data feed and multilateration and adds airline and airport schedule and flight status data.

Multilateration takes raw mode-S transponder signals from aircraft without ADS-B out, and according to FlightRadar24 “we also calculate positions of aircraft with the help of multilateration by using a method known as time difference of arrival. By measuring the difference in time to receive the signal from aircraft with an older mode-S transponder, it’s possible to calculate the position of these aircraft.” This requires at least four FlightRadar24 receivers and usually sees aircraft flying above 10,000 feet, “as the probability that the signal can be received by four or more receivers increases with altitude.”

No matter how it obtains its data, the company told AIN, “We adhere to the [FAA] block list.”

FlightAware says it, too, complies with the block list: “Although we have broad ADS-B coverage in more than 70 countries, we choose to exclude aircraft on the [FAA block] list or aircraft that are on any block list we operate for other countries.” This doesn’t change as more aircraft upgrade to ADS-B out. And large companies that have a reputation to protect will continue to abide by block requests because it’s good for business. “Organizations such as FlightAware want to have products and services that are well regarded in aviation,” said CEO Daniel Baker.

The one missing link in blocking of airborne traffic is that there is no central organization that manages requests from operators. For countries other than the U.S., FlightAware will honor a block request. “We’re happy to oblige,” he said, and there is no charge unless the operator wants access to tracking information for its own fleet. “We see ourselves as a really good player in this space and we work well with all interests and try and behave well. We’re happy to be part of any industry effort to make it simpler for foreign operators to participate,” Baker added.

While the big players such as FlightAware, FlightRadar24 and others work with the industry and comply with block requests, Baker isn’t sure that this will always keep blocked aircraft from being tracked by those who want to see the aircraft’s movements. “There have always been other ways of obtaining this information, [such as] multilateration, and the way things are going I think this will be a mechanism by which people will be able to pick up data from [aircraft] that are blocked.”

Anyone can download free PlanePlotter software (PC only) and view an enormous amount of data about aircraft movements all over the world. There is a vibrant PlanePlotter community developing applications that use PlanePlotter data for various purposes, such as traffic overlays on maps, individual flight cockpit views, capturing and reading Acars messages and even multilateration. This works because PlanePlotter gets its data from hobbyists who are plugging in simple ADS-B receivers and capturing data to share on PlanePlotter. The program works with a variety of receivers; a simple antenna/receiver that plugs into a computer’s USB port costs about $250.

Ultimately, while some blocking is possible and will keep an aircraft from being tracked on commercial flight-tracking websites, there is no way to eliminate completely the possibility of being tracked other than turning off the transponder, which is neither legal nor a good idea.

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