Over the air calibration of transmitter distortion for active antenna arrays

Licentiate thesis, 2024

Radar systems for air surveillance have long utilised large antenna arrays to generate high enough gain for a narrow enough transmitted beam. In most modern systems this is accomplished through the use of large active electronically steered antenna arrays.

Calibrating a large active antenna array is a challenging endeavour. Distorting effects stemming from the amplifying circuitry can have a non-negligible effect on the transmitted signal, and the effects can vary across the array. In modern and future transmitter arrays, individually controlled antenna elements will be a feature. With such systems, distortion that varies over an array can be combatted. One challenge is how to collect data about what signal is being transmitted and in what way the actual signal differs from the desired transmitted signal.

The work presented in this thesis studies aspects of characterising a model describing the distortion generated in an active antenna array using an over the air channel. Specifically, the first included article presents a method where signals collected using an over the air channel generated by targets of opportunity are used to estimate the parameters of a non-linearly parametrised model for a radar transmitter. In this article, the radar is assumed to transmit different signals on each antenna element. The second included article studies conditions for identifiability for the setup in the first article as well as for other similar setups. These works constitute the second part of this thesis.

To put this work into context, the first part of this thesis presents an overview of the problems caused by distortion in a radar system. This is investigated under different modelling assumptions and for radars operating in a number of different modes.