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Frequency

Overview

The old ATCRBS system had operated on two frequencies: 1030 and 1090 MHz. Ground stations used the 1030 MHz to interrogate nearby planes, and planes responded on 1090 MHz. In order for Mode S to interoperate, it had to use these same frequencies.

Cost also played a major role in the use of ATCRBS frequencies of 1030 and 1090 MHz for the interrogation and reply channels. In 1973, the researchers asked several manufacturers Bendix Avionics, Hazeltine, Collins, and Bendix Communications to evaluate the cost of developing transponders using the 970 and 1153 MHz channels. They found the cost about 20-30% higher, which also helped justify continuing using the 1030/1090 MHz frequencies.

Electromagnetic compatability

A major part of the Mode S link design was spent investigating electromagnetic compatability issues. The Lincoln Lab researchers needed to investigate the effects of Mode S transmissions on ATCRBS, and the link performance degradation caused by radio frequency interference generated by ATCRBS. Interference also played a major role in the select of an appropriate modulation for Mode S.

Even though both systems used the same frequency for communication, the Mode S signal needed to be transparent to existing ATCRBS equipment. In other words, the Mode S signal had to be designed so that ATCRBS equipment would never attempt to interpret the signals. This problem was challenging on many levels, but ultimately, the Lincoln Labs researchers found a clever way to exploit an existing feature of the ATCRBS system and ensure transparency.

Transponder Testing

When Lincoln Labs chose to use the same 1030/1090 MHz frequencies, the Mode S design problem became much more challenging, because Mode S would have to share the communication channel with the existing ATCRBS system. The most obvious solution would be to create a Mode S waveform that would be ignored or go unnoticed by ATCRBS transponders.

This initial idea had the added benefit of being cost-efficient, because no modifications would be needed to existing transponders. The researchers had experimented with different modulating schemes including frequency shift keying (FSK) and phase shift keying (PSK) hoping that the ATCRBS transponders would simply ignore these signals. In addition, they attempted to vary pulse widths and the time intervals between pulses in search of a waveform that would not elicit a response.

A general field test of transponders used at the time revealed that no such universally ``invisible'' waveform could be found. Not one signal went unnoticed by all of the available transponders. One of the problems was that at this time, there were more than five hundred transponders available on the market for use in commercial jets, military aircraft, and private planes. The companies that manufactured these transponders, which included Collins, Bendix, and Wilcox all claimed to adhere to the specifications outlined by the FAA's National Aviation Standard (March 8, 1971). For the most part, they were compliant, but still responded to a wide range of additional signals.

The experimentation had revealed a fundamental flaw in the FAA's National Aviation Standard itself. The standard had "said lots of stuff about what the ATCRBS transponder should do and nothing about what they shouldn't do," recalled Lincoln Laboratory researcher Thomas Goblick. For instance, several transponders would even respond to sinusoidal waveforms even though these signals were extremely simple and not mentioned in the specification. Because transponders exhibited such a diverse range of behavior, finding and proving the applicability of a universally ``invisible'' signal was impossible. The Lincoln Laboratory researchers needed to find a workaround. Ironically, their solution led full circle back to an existing ATCRBS feature known as sidelobe suppression.