Accurately counting airport operations is a challenging endeavor for public use airports that do not have a continuously staffed control tower (e.g. the vast majority!). The various count techniques employed by these airports are insufficient to generate sample sizes for accurate operational counts. Additionally, the opportunity to correlate additional data sets that may be available regarding operations offering great potential benefit to airport operators and transportation agencies, such as via the aircraft registry database, are missed.
Courtesy of Wikipedia, Automatic Dependent Surveillance – Broadcast (ADS-B) is a surveillance technology in which an aircraft determines its position via precision GPS and periodically broadcasts it, enabling the aircraft to be tracked. ADS–B is “automatic” in that the broadcasts require no pilot or external input. Aircraft do not need to be interrogated by Secondary Surveillance Radar, transponder data is automatically broadcast. It is “dependent” in that it depends on data from the aircraft’s navigation system. In one form it is a modified Mode S transponder, or Mode S Extended Squitter, which is compatible with ADS-B.
ADS-B consists of two different services, ADS-B Out and ADS-B In. ADS-B Out periodically broadcasts information about each aircraft, such as identification, current position, altitude (both barometric and geometric), and velocity, through an onboard transmitter. ADS-B Out provides air traffic controllers with real-time position information that is, in most cases, more accurate than the information available with current radar-based systems. ADS-B In is the reception by aircraft of Flight Information Services-Broadcast (FIS-B) data, such as weather text and weather graphics, and Traffic Information Services-Broadcast (TIS-B) data, for ground station broadcast of other local air traffic, as well as other ADS-B data such as air-to-air direct communication from nearby aircraft.
By January 1, 2020, aircraft in the United States that fly in most of the controlled airspace must be equipped with ADS-B Out. Which means most, but not all, aircraft will be employing ADS-B Out. ADS-B In is optional.
Mode C is part of a set of transponder signals that are trigged by interrogation received from ground-based Secondary Surveillance Radar, which also triggers Mode A and in some form Mode S transponders.
When interrogated, Mode A broadcasts a pilot configured transponder code, AKA a squawk code, used by ATC. Mode C adds to Mode A by also including a pressure altitude reading in the response. Mode S builds on Mode A and Mode C by enabling selective interrogations by ground radar to improve the quality of ATC compared to Mode A and Mode C. Part of Mode S systems are squitters, which are unsolicited transmissions from the aircraft transponders akin to ADS-B. In fact, Mode S Extended Squitter is compatible with ADS-B. Mode S Short Squitter, for aircraft without GPS, are akin to Mode C and are treated here the same as Mode C transmissions.
ADS-B Out makes it conceivable to effectively log each airport operation by tracking the GPS coordinates and altitude of aircraft. While commercial services already exist for airports to attempt to do this, none of them are focused specifically on operations around one particular airport. Instead these reports, available for considerable fees, are an offshoot from the core business of providing flight information for use by consumers and aficionados. Additionally, coverage specific to an airport is not guaranteed as these systems are crowdsourced. That said an airport can obtain one of these sensor devices to ensure their data is fed into a crowdsourced system, but again it only covers ADS-B aircraft and the output is not optimized for use by airport operators or transportation agencies.
An ADS-B satellite-based tracking system is coming online, but the goal of such a system is worldwide tracking of commercial aircraft filling in the coverage holes that have existing with ground radar systems. It has specific requirements for aircraft ADS-B antenna placement and power output that are beyond the typical ADS-B installation expected for GA aircraft. Pricing and packaging for such a service are not expected to be a strong fit for the needs of non-towered airports.
As ADS-B includes GPS-based position and geometric altitude, an algorithm to determine airport operations, both take-off and landing, while important programming work, is a straight-forward implementation to put in place around a particular airport using ADS-B.
What is missed through this approach are the operations of aircraft that are not using ADS-B, which for GA aircraft is a significant percentage of the total actual operations.
This is where Mode C comes into play. Technically a Blueavion receives all the Mode A, Mode C, and non-ADS-B Mode S transmissions in addition to ADS-B. But because Mode C contains pressure altitude data and a squawk code, with the proper software model it can be used to determine if a plane has either taken off or landed noting the raw barometric altitude information by itself is insufficient to precisely identify airport operations.
Mode C does not provide location. Here is where Blueavion uses the blended data approach applying the technology licensed from Purdue Research Foundation via data that is supplied by the Blueavion receiver, the relative signal strength indicator (RSSI). By logging the RSSI of all ADS-B aircraft with known positions that are classified as take-off or landing operations combined with altitude readings, Blueavion can build a model of appropriate changes in RSSI for a location that correlate to both take-offs and landings. The Mode C altitude data alone is insufficient to precisely identify an operation at an airport, but when combined with ADS-B data using Blueavion’s algorithm it becomes useful and very precise. There is a learning period where the software model is self-calibrated, and Blueavion applies the model to use Mode C data to determine take-off and landing operations for Mode C aircraft. For more information on this approach, please review the research paper posted on IEEE Estimation of Aircraft Operations at Airports Using Mode-C Signal Strength Information by Dr. John Mott and Dr. Darcy Bullock (membership required): https://ieeexplore.ieee.org/document/7932966/
Combining the logged Mode C and ADS-B operations together represents a more comprehensive approach for capturing all airport operations, which is further supplemented by a Bayesian estimation algorithm developed by and also licensed from Purdue University. As an example, as part of the IEEE paper data was gathered over a 30-day period at KLAF. Relying on ADS-B Mode S data alone resulted in approximately 300 operations counted. Adding Mode C along with the Bayesian estimation algorithm resulted in about 550 operations, which was very close to the observed operations during the study, thus proving to be highly accurate.
In closing, the Blueavion system combines ADS-B with Mode C transponder signals and applies a Bayesian estimation algorithm to precisely estimate aircraft operations for an airport. Available supplemental information regarding individual aircraft when aircraft identifiers are obtained (as is the case with ADS-B) is also provided for use by aircraft operators and transportation agencies.