Transition to ADS-B


The Story

Friend or Foe

During World War II, a scheme was enacted to identify friend or foe (IFF) using radio-frequency interrogation/response codes. The codes originally consisted of two octal digits (6 bits total), allowing 64 possible codes.

Mode A and Mode C

During the 1950's, the pulses became shorter so that more of them could fit within the 21 μs package, resulting in 12 bits of information represented as four octal digits for 4096 codes. This scheme became commonplace in international aviation, and is still used today—known as Mode A (temporary identity) and Mode C (altitude). These modes rely on pulse amplitude modulation: blasts of RF power at 1090 MHz are either present or not at the appropriate time for a bit. The overall format is shown below. Pulses are 0.45 μs in duration on a 1.45 μs cadence (1.0 μs gaps).

Mode A/C pulse patterns

Mode S

In the mid 2000's, Mode S transponders entered the scene, using 56 bits of coded information (including parity check) to communicate more versatile data, including permanent aircraft identification, altitude, assigned Mode A identity (squawk) code, as possible examples. Still at 1090 MHz, the transmission scheme switched to pulse position modulation: guaranteed transitions between zero power and full power in every one-microsecond symbol frame, but the direction of the transition within the frame conveys binary one or zero. Mode S is still interrogated; the format of the transmission (what information is sent) depending on the type requested by the interrogator. The figure below shows the beginning of an example Mode S pulse sequence. The blue scale bar defines 1 μs frames; note the data values correspond to the mid-frame transition (or alternatively, the value in the first half of each frame).

Mode S pulse patterns

ADS-B

More recently, Mode S type transmissions have been augmented to 112-bit formats that are spontaneously transmitted by the airplanes (e.g., once per second) independent of interrogation. This is still at 1090 MHz and called "extended squitter," or alternatively ADS-B (for punishment, look up what this stands for; you may be sorry). Information communicated may be similar to Mode S, but also may include latitude and longitude, or velocity, heading, and vertical rate, or a number of other data-rich transmissions.

Relation to TBAD

So far so good: TBAD is able to interpret all these forms of communication at 1090 MHz. ADS-B was added as a capability in late 2014, and the plot below shows the adoption rate (FAA target is full compliance by 2020).

Transition to ADS-B

The binning is quarterly. The numbers represent the total number of airplanes detected with some form of Mode S or ADS-B transmissions in that period. Shaded bars indicate the one-standard-deviation range given binomial sampling statistics. Overlapping shaded bars indicate statistically compatible levels. The overall trend is clear: adoption appears to be on track for full coverage in a few years.

The 978 MHz Wrinkle

So far the story is a happy one for TBAD. But it's about to take a turn. Part of the ADS-B effort involves offloading some of the radio traffic to another frequency, so that the 1090 MHz scene is not so crowded. General aviation airplanes (those remaining below 18,000 ft) are encouraged to operate at 978 MHz in what is called UAT (universal access transceiver). Transmissions are either 276 or 420 symbols long, consisting of a 36-bit sync word, 144 or 272 bits of information (payload), and 96 or 112 bits of Reed-Solomon error-correction code.

TBAD, as currently configured, is completely deaf to 978 MHz. Moreover, the modulation scheme is no longer simple RF pulses easily detected and decoded: it's a binary frequency shift keying scheme wherein the modulation is ±312.5 kHz and symbol duration is a short 0.96 μs (leaving less than 2 radians of phase development per symbol: hard). Power levels are somewhat lower than the 1090 MHz scheme.

But there is progress. We have an antenna design that is dual-band (narrow around both 978 MHz and 1090 MHz simultaneously), diplexers specifically built to split the two frequencies and eliminate all others, and can therefore add a parallel channel to the TBAD Discriminator unit to process threats from 978 MHz. A harder (but less essential) task is demodulating for data recovery, but the resulting latitude/longitude information allows a powerful post-analysis check on TBAD performance, so work is commencing on recovering this as well.


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