Phase Modulated Waveforms for Transponder-Based Radar Sensing: Signal Optimization and Experiments

Both the theoretical and engineering aspects of using transponder-based signal waveforms, such as pulse-position-modulated 1090-MHz Automatic Dependent Surveillance-Broadcast, for noncooperative target sensing are discussed in this article. The random binary phase modulation coding technique is applied to the original transponder signal waveforms to improve the autocorrelation and sidelobe performance. A detailed study of the characterizations of this waveform and its potential for target detection is presented. The impact of random phase coding is analyzed from both signal spectrum (based on stochastic signal model) and radar ambiguity functions (based on deterministic signal model). The combination of pulse-position modulation and maximum length binary random phase coding sequence is shown to have satisfactory performance. In terms of optimal receiver, the traditional correlation receiver (or matched filter) and mismatched filters with different criteria are analyzed. The tradeoffs of receiver design for transponder-based radars are discussed. The experimental implementation of a transponder-based radar sensor with arbitrary waveform generator and commercial, off-the-shelf components is introduced and discussed. The experiment results show that noncooperative target detection using such a radar sensor is feasible with appropriate design of waveforms, system architecture and signal processors.

I Introduction

In the aviation safety domain, collision avoidance is one of the most critical issues involving detection, localization, and alerting of external traffic. External traffic can be cooperative (with transponder response) or noncooperative. There are many types of noncooperative targets, for example, small general aviation (GA) aircraft without mode-A/C transponders, military aircraft equipped with Automatic Dependent Surveillance-Broadcast (ADS-B), or unmanned aerial vehicles, as well as aircraft with hostile intentions. Noncooperative targets can become hazards when they intrude into an aircraft's airspace. Detection or “to-see” of those targets is a basic requirement to avoid a collision with them. The traditional ground-based primary radars used for such purpose have significant limitations, including the following: 1) As a centralized information processor, ground radar systems have limited effective range, resolution, and target handling capabilities. 2) There are time delays from ground system detection to information delivery to aircraft, and this communication link is subject to weather conditions and interferences. 3) The number of ground-based radars is going to be gradually reduced in the future in accordance with the next-generation air transportation system [1] architecture. Moreover, the advanced airborne primary radars are expensive and not available to GA aircraft [2]. As an airborne secondary surveillance sensor, the Traffic Alert and Collision Avoidance System (TCAS) [3], [4] system is fully functional, but it is accurate only when it encounters similarly equipped aircraft with mode-S transponders. It is not effective for many small aircraft with only mode-A/C transponders. The sensing and navigation system based on a global positioning system (GPS), such as the ADS-B, will deliver benefits such as enhanced situation awareness, reduced costs, and increased automatic traffic control capacity [5]–[7]. However, ADS-B is only effective for transponder-equipped cooperative targets, and the GPS signal can be jammed or compromised. This challenge has led to data-fusion solutions in combined ADS-B and radar environments [8], [9].