A transponder is a subsystem that provides the connecting link between the transmission and reception antennas of a satellite or aircraft. It is among the most important subsystems within the aerospace and aviation segments. A transponder effectively performs the functions of both a transmitter and receiver/responder in a satellite. Therefore, the name ‘transponder’ comes from the combination of transmitter and responder.
Transponders perform two main functions. The first is to amplify a received input signal, and the second is to translate the frequency of the signal. Generally speaking, differing frequency values are chosen for both uplink and downlink in order to prevent interference between the transmitted and received signals. The process of a signal moving through a transponder looks like this:
Satellite/Antenna > Duplexer > Low Noise Amplifier > Carrier Processor > Power Amplifier > Duplexer > Satellite/Antenna
This is a basic explanation of a transponder’s operation, but let’s take a look at each part. The satellite or antenna receives a signal and sends it to the
duplexer. The duplexer is a two-way microwave gate that receives the uplink signal from the satellite antenna. The signal then moves to the low noise amplifier (LNA) which receives the weak signal and amplifies it. The carrier processor then performs the frequency down conversion of the received signal. The function of this block determines the type of transponder. The
power amplifier then amplifies the power of the frequency down-converted signal to the required level and sends it back to the duplexer. From here, the duplexer transmits the downlink satellite to the antenna, which finally responds by outputting a signal.
As stated, the function of the carrier processor determines the type of transponder. There are two types of transponders. These are bent pipe transponders and regenerative transponders. Bent pipe transponders receive microwave frequency signals. They convert the frequency of the input signal to RF frequency and subsequently amplify it. Bent pipe transponders are also referred to as repeaters and conventional transponders. They are suitable for both analog and digital signals.
Regenerative transponders perform the functions of frequency translation and amplification just as bent pipe transponders do. The difference is that, in addition to these two functions, regenerative transponders also perform the demodulation of the RF carrier to baseband, regeneration of signals, and modulation. Regenerative transponders are also known as processing transponders. They are only suitable for digital signals, but offer the advantages of improved signal to noise ratio (SNR) and improved flexibility of implementation.
Transponders first came into use to help military authorities identify friendly aircraft, which would transmit a coded signal when interrogated by military radar. This practice was known as IFF, or Identification Friend or Foe. Transponders have since become popular in civil aviation as well, where it is standard practice to allocate a specific transponder code to each aircraft in controlled airspace. This allows authorities to identify specific aircraft on crowded radar screens using secondary surveillance radar. Transponders are critical to
air traffic management, where they are used for many purposes including aircraft identification, enhancement of the controllers’ situational awareness, and development of air traffic control tools and safety nets.
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