Flight Tracking Tech

by Dan Streufert, President, ADS-B Exchange

If you’re curious about flight tracking, you might have come across many buzzwords and three-letter acronyms (TLAs) like ADS-B, MLAT, Mode C, Mode A, TIS-B, ADS-R, UAT, ADS-C, and ICAO. It can seem overwhelming, but don’t worry - we’ll help simplify these terms and concepts so they’re easy for anyone to understand. Let’s dive in and demystify some of these key terms in flight tracking.

The journey of aircraft tracking begins with a technology called “Primary Radar” (Radio Detecting And Ranging), a method that predates the modern digital age. Developed and first used during the Second World War, primary radar was a significant breakthrough in aviation and defense. Before technological advancements, the primary way to track aircraft was simply by sight - using the human eye. Primary radar changed this by allowing us to detect and track aircraft using radio waves, marking the start of a new era in air traffic monitoring and military defense.

Primary Radar operates independently of the aircraft, meaning it doesn’t require any response or signal from the airplane itself. This system works by sending out radio signals from large, spinning radar dishes, commonly seen at airports and various other locations. These signals are projected into the sky, and when they encounter an object like an aircraft, a portion of the signal reflects back to the radar antenna. By analyzing these reflected signals, the radar system can determine the position of the aircraft. This process, based on the simple principle of echo or reflection, allows us to track aircraft without needing any special equipment or response from them.

Since primary radar works without any devices or cooperation from the aircraft itself, this is what would be used for defense purposes to monitor for possible hostile aircraft, or aircraft attempting to “sneak through” air defenses. Stealth aircraft are built to reflect as little of the primary radar signal back to the antenna to evade detection. Since the aircraft is not “cooperating” with the ground radar, it can be difficult to determine the aircraft’s altitude, or whether a “blip” is an aircraft, a flock of birds, rain, or even a truck moving on the highway!

Primary also requires a sophisticated and expensive ground station. While it CAN be used for air traffic control purposes as a last resort, most air traffic control is done using what is known as “Secondary Radar”, which requires the plane to have a working “transponder”. From here on out, everything we’ll be talking about is secondary radar.

Secondary radar, like primary radar, also uses a “spinning radar dish” but functions quite differently. In this system, the ground-based radar sends out a signal that essentially “interrogates” or communicates with the transponder on each aircraft. When an aircraft’s transponder is interrogated by this radar signal, it responds by sending back specific information to the radar station. This interactive process between the ground radar and the aircraft allows for more detailed and accurate tracking of airplanes, as the information returned by the transponder includes essential data like the aircraft’s identity and altitude. This two-way communication is what sets secondary radar apart from the more basic primary radar system.

While the ground stations interrogate the aircraft using signals on 1030 Mhz, the aircraft reply on 1090mhz. 1090mhz is the international standard for secondary radar replies as well as Mode S and ADS-B transmissions.

Transponders, the communication devices on aircraft, have evolved significantly over time, becoming more sophisticated with advances in technology. An early and basic type of transponder is the “Mode C” transponder.

When a Mode C transponder is interrogated by ground radar, it provides two key pieces of information. First, it sends a 4-digit code, which ranges from 0 to 7 for each digit and is typically assigned by air traffic control. This code helps in identifying and managing the aircraft from the ground. The second piece of information it transmits is the aircraft’s pressure altitude.

Pressure altitude is determined using an altitude encoder, which could be either a separate unit installed in the aircraft or part of the transponder itself. This encoder calculates altitude based on a standard atmospheric pressure setting of 29.92 inches of mercury (inHg). However, the actual altitude readings, both on the aircraft’s altimeter and on the air traffic controller’s radar screen, are adjusted for local atmospheric pressure variations. This adjustment ensures that both the pilot and the controller have accurate, sea-level equivalent altitude data, crucial for safe navigation and airspace management.

Since the 4-digit “squawk code” utilized by Mode C can change based on assignment by ATC, or the aircraft’s flight rules (for example, all VFR aircraft squawk 1200 if not talking to ATC), Mode C cannot be used by flight tracking sites to specifically identify aircraft.

The next level of transponder is called “Mode S”. In addition to including the 4-digit squawk code and pressure altitude, Mode S adds an ICAO 24-bit address (aka “ICAO hex code”). Ranges of hex codes are assigned by ICAO (the International Civil Aviation Organization) to each country. For example, the United States is assigned range A00000 - AFFFFF. Each country then assigns a unique hex code to each aircraft “tail number”. Through this method, the public can identify which aircraft is emitting any given Mode S signal. Since July 1992, the US FAA has required all newly installed transponders in aircraft flown commercially (charter or airlines) to be at least Mode S capable. Mode S is required for TCAS (Traffic Collision Alerting System), and TCAS is required for certain aircraft in the US and elsewhere.

While Mode S transponders do not broadcast the aircraft’s lat/lon GPS location, they do allow reasonable-cost aircraft tracking for the aviation enthusiast via multilateration (or MLAT).

MLAT involves 4 or more receivers listening to the aircraft’s transmissions. These receivers (using a centralized server) are able to coordinate and calculate the approximate origin of the aircraft’s signals based on the known locations of the receivers and the “Time Difference of Arrival” based on the speed of light.

These calculations require the receivers to each know their locations with reasonable accuracy. Typically, this information is entered during initial configuration of the receiver.

However, MLAT also requires the receivers to have extremely accurate synchronized timekeeping. This can be challenging since we are dealing with very short timeframes - light travels 1 mile in 5.3 microseconds (0.0000053 seconds). This means that if the clocks were inaccurate by even 1 millisecond, distance calculations could be off by 180 miles!

What is the fix for this? If all receivers participating in the calculation can see a specific ADS-B equipped aircraft, that aircraft will broadcast its position in the signal. So, we will know the aircraft’s position and the receiver’s position. Using these two positions, we can calculate the time it would have taken the signal to reach each receiver. The receivers can then use this ADS-B signal as a reference point to synchronize their timekeeping.

Geometry and number of receivers play a critical role in the accuracy of a position obtained via the MLAT calculations. In the example below from the ADSBexchange website, you can see a straight and accurate path with the system using approximately 16 receivers to calculate the aircraft’s position.

On the other hand, while we can tell generally where this aircraft is, it’s path is jagged because we barely have enough receivers to perform the calculation.

Often, we may be receiving a Mode S signal, yet not have enough receivers in the area to calculate a position. However, we still can deduce some information about the aircraft. For example, we may be able to tell it’s altitude, and that is “in the general area” (ie. within about 100 miles) of the receiver since we are receiving it’s signal.

The best aviation transponder, ADS-B, is essentially an “add on” to Mode S that includes more information, including the aircraft’s latitude, longitude, and GPS altitude (in addition to the barometric altitude). Of course, when the plane broadcasts it’s lat/lon coordinates, tracking becomes easy and only requires a single receiver.

While ADS-B is “Automatic Dependent Surveillance – Broadcast ADS-C stands for “Automatic Dependent Surveillance – Contract”. In this scenario, “contract” refers to a technical protocol - 2 way communication between the ground and the aircraft via satellite. ADS-C is used in areas without ground-based ADS-B receivers - typically oceanic and polar areas. Usually, these transmissions occur every 10-14 minutes, although recent changes are working to get these down to every 3.2 minutes. The satellite network typically used in these transmissions is Inmarsat, and consists of 4 geosynchronous satellites covering the areas shown below.

Signals from each of these can be picked up using specialized equipment, and the position reports uploaded to ADSBexchange. More details here.

All of the standards discussed thus far are in use worldwide. However the FAA also provides some US-specific technologies including TIS-B, UAT, ADS-R, etc…

Starting January 1, 2020, the FAA mandated ADS-B equippage for all US airspace above 10,000 ft., within 30 miles of Class B airports (major airports), and within 10 miles of Class C airports (medium airports). Consequently, the vast majority of aircraft in the country are ADS-B equipped. However, flights into ADSB-mandated airspace by non-equipped aircraft can be accomplished by asking for a waiver prior to the flight, which may or may not be granted depending on various factors.

In addition to the international standard implementation of ADS-B on 1090mhz, the FAA offered a second method of compliance with the ADS-B mandate, on a separate frequency - 978mhz. This is also known as “UAT”, while traditional ADSB is also known as 1090ES.

For compliance with the FAA ADSB mandate, UAT is only allowed below 18,000 ft. UAT also does not meet ADSB compliance requirements in any countries outside of the US. Consequently, the only aircraft equipping with UAT are aircraft that do not fly internationally, and remain below 18,000 ft. Jets, or other pressurized/turbine will typically always equip with 1090ES instead.

However, the “owner flown”, small piston general aviation market in the US is the largest in the world. Given that UAT can sometimes be less expensive to install than 1090ES, as of October 2023, approximately 40,000 of the 160,000 US-registered civil aircraft are equipped with UAT.

Even though 25% of the US fleet is UAT equipped, jets and other commercially operated aircraft fly many more hours per year than personal piston aircraft, so far fewer than 25% of air traffic at any given time is broadcasting UAT. You will also notice that later at night, UAT traffic drops significantly, because there is less nighttime activity by UAT equipped aircraft.

Operating ADS-B on two distinct frequencies opens up a few complications. Aircraft use ADS-B to avoid collisions, but they may not always be using the same frequency. In this case, the FAA’s solution is something called ADSB-R - “Automatic Dependent Surveillance Rebroadcast”.

If an FAA ADSB ground station detects two ADS-B aircraft near each other but on different frequencies (< 15 nm apart with less than 5000 ft. of altitude difference), the ADS-B ground station “rebroadcasts” the 1090mhz traffic signal on UAT and vice versa. This allows the aircraft to see each other on any in-cockpit displays that may be in use, even though the planes are on different frequencies.

ADS-B receivers can pick up these broadcasts, and use them to fill in traffic data at times without needing line of site from your receiver to the aircraft. As long as your receiver can see the FAA tower, and the FAA tower can see the aircraft in question, you’ll get the data. An open-source map of FAA towers is available here: http://towers.stratux.me/

Aircraft being received via this method (or via direct UAT detection) are highlighted in green on the aircraft list on ADSBexchange as shown.

Similar to the way ADSB rebroadcast functions, the FAA also attempts to alert all ADSB equipped aircraft of other radar targets, even if those targets are not ADSB equipped (such as Mode C/Mode S aircraft detected on the FAA’s secondary radar systems).

This example shows one of these aircraft being picked up and rebroadcast by “TIS-B”. Unfortunately, in the case of a Mode C aircraft, we do not know the aircraft’s unique hex code, so its identity is unknown. For these types of aircraft, the FAA broadcasts a “pseudo-random” hex code, indicated by the code being prefixed with a “~” as shown.

Although we don’t have the aircraft’s hex code, we do know there is an aircraft there being picked up by FAA radar.

While police and military are able to turn off their ADS-B transmission for “operational requirements”, police helicopters operating in metro areas are NOT able to turn off their Mode C transponder entirely (which would be unsafe). Consequently, in some areas (such as the Los Angeles basin), many of these TIS-B targets can be seen scurrying around and circling, especially at night. They behave a lot like police helicopters!

This is also the reason why you might occasionally see a “ghost” target seemingly following another aircraft. For example, we may be receiving a Mode S only equipped aircraft via MLAT. At the same time, we also see the “anonymized” TIS-B target sent by the FAA. It’s the same aircraft, but coming from two different sources. Since the FAA does not pass along the hex code from Mode S only traffic, there is no way to tell for sure it’s the same plane, but it likely is.

In summary, the realm of flight tracking technology involves a variety of systems like ADS-B, MLAT, Mode S, and UAT. These technologies, from basic primary radar to more advanced ADS-B and MLAT systems, ensure that aircraft are tracked reliably and safely. These technologies, vital for air traffic control, defense, and aviation enthusiasts, demonstrate a significant blend of historical progress and modern innovation, playing a crucial role in maintaining the safety and organization of our skies.

yes i like it. thank you.