Modular Test Approaches for SSR Signal Analysis in IFF Applications: Part II

Posted on Monday, September 17, 2018

Military radar applications call for highly specialized test equipment. For these applications custom RF test racks are assembled for the specialty testing performed on this equipment. The second part of our two part blog series on modular test approaches explores interrogation modes, highly specialized RF test panels and USB-driven RF components.


Interrogation Modes

The military interrogation modes have dramatically evolved over the decades with newer stochastic processes and waveform generation techniques. While air traffic control has learned to adopt some legacy modes, the military primarily leverages more modern techniques to ensure secure messaging for IFF applications. 


The Mode 1 interrogation signal can elicit 64 various reply codes and is often used in military air traffic control towards determining the type of aircraft flying and what type of mission it is on. Mode 2 includes an identification request with the “tail number” of an aircraft, there are 4096 possible replies for mode 2 interrogations. Mode 3/C replies include information of the altitude of an aircraft by increments of 100 feet is often mixed with mode 3/A to provide a 4-digit octal identification code, this interlacing of modes is known as mode 3 A/C, both modes have 4096 replies. Mode 4 requires both the interrogator and transponder to work with a cryptographic unit to ensure communication with a friendly aircraft. The crypto encodes and decodes signals with long chains of pulses including a preamble to ensure a secure message, then an ISLS pulse, and finally a message. The crypto itself must be calibrated on a daily basis with several key codes that are valid for only 12 hours. Similar to mode 4, mode 5 is also secured with a crypto to encode an interrogation and reply. Mode 5 leverages modern modulation, coding, and cryptographic techniques to overcome the security limitation of the legacy modes. 


Mode S, or mode ‘select’, employs addressed interrogations to reduce redundant replies. For mode S, each target has a unique address, ‘all call’ interrogations elicit replies from all aircraft in the beam while ‘roll call’ interrogations select specific mode S equipped aircraft with a 24 bit address.With 16 million unique permutations, the ground station can call upon a specific aircraft. Under an ‘all-call’ interrogation, ‘lockout’ can be enabled to lock out particular mode S equipped aircrafts so they do not receive the interrogation. These unique identifications for mode S equipped aircraft are used to eliminate false replies unsynchronized in time, or FRUIT. FRUIT often occurs in environments when an aircraft is in the proximity of beams from two or more different interrogators, in this scenario an aircraft can receive transmission from two or more different ground stations causing asynchronous replies, or overlapping replies in time. As the population of air traffic increases, the number of ground stations and transponders increase and this directly correlates with an increases in FRUIT. SSR techniques such as mode S equipped aircraft and ISLS pulses are implemented to minimize this phenomena. 


Highly Specialized RF Test Panels

IFF radar signal analysis often requires customized RF test racks for test and measurement of the interrogators and transponders. Along with the updates in fighter aircraft technology come the updates and revisions of their respective RF test systems just to optimize test capabilities. From production test to specialized engineering test, components such as phase shifters, digital attenuators and RF switches that are modular enough for specialized test applications to automated test equipment (ATE) applications can add value to the testing and development process of radar signal analysis. 


USB-Driven RF Components

Vaunix, a massachusetts-based test and measurement company, offers USB-driven ‘lab bricks’, or RF test components, to mitigate restrictions due to complexity of generating automated test equipment. With USB-driven phase shifters, digital attenuators, RF switches, and signal generators their small aluminum lab bricks can readily serve as a replacement for components in legacy RF panels. 


The LPS-202 model of phase shifters are programmable 0° to 360° with a phase resolution of 1° from 1-2 GHz, with the ability to operate in 1030 MHz and 1090 MHz transmission frequencies. The ready-to-use GUI can track and control several connected phase shifters simplifying multiple device set ups, this way the phase can be changed from a computer without opening the up a test panel and manually adjusting a knob to change the phase for sum/difference channel adjustment. 


The benefit that comes with USB-controlled RF switches is the elimination of the external DC bias and switching paths from the input to a particular output port by the click of a button as opposed to manually adjusting the TTL control or creating biasing circuitry. Vaunix offers both SPDT as well as SP4T for cascaded switch setups as well as a power handling of 10W for high powered transmissions. These RF switches allow for automated switching between the sum and difference channels in a test setup.  


The USB-control can provide a very similar benefit for programmable digital attenuators that are normally TTL controlled with micro-D connectors or by the manual biasing of discrete bits. Vaunix digital attenuators provide step sizes as small at 0.1 dB with a range as large as 120 dB and can be easily programmed for a fixed attenuation value or swept attenuation for dynamic level control.  


USB-driven devices can be an effective alternative to traditional methods of biasing TTL controlled components particularly for ATE production test applications where fast throughput and ease-of-test is key. In military systems where information is highly proprietary and even testing systems are specialized,  it is essential to have accessibility to modular components that enable automated test so that internal engineers can generate test racks.  Vaunix lab bricks provide a strong alternative to current RF test components due to the their small size, cost-effectiveness, and turn-around time—a very good option to join the approved vendor list for the next revision of a test rack.


To learn more, download our tech brief "Modular Test Approaches for SSR Signal Analysis in IFF Applications."

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