## 1 Introduction  
Vehicle sideslip angle (VSA) estimation has been a big challenge since the introduction of the very first on-board active systems controlling vehicle stability, such as the electronic stability control (ESC, also known as ESP) in the early 1990s [1]. Nowadays, vehicle control systems (VCS) such as rear wheel steering, active steering, direct yaw moment control (DYC) through active differentials or torque vectoring, advanced traction controls, and the above-mentioned ESC (in all its forms) are used in conjunction to extend the vehicle performance and stability envelopes. They enhance road-holding capabilities, cornering performance, drivability, and ultimately stability, especially assisting the driver during emergency manoeuvres and in poor \"grip\" conditions [2; 3; 4; 5]. All of these controls rely on vehicle state real-time assessments, and on VSA monitoring in particular. A fast and accurate estimation of the VSA can be considered a key to active safety, reducing the likelihood of dangerous events such as instability, loss of control, and roll-over, hence decreasing the number of accidents directly and abating the social impact of mobility. The VSA is strongly related with the dynamic behaviour of ground vehicles. In purely kinematic terms, the VSA amplitude depends upon lateral acceleration, vehicle mass and mass distribution,and rear axle slip angle; hence, it relies on properties such as rear type cornering stiffness, etc. [6]. Furthermore, it is well known that pneumatic types feature a typical, non-linear shape of side force versus slip angle characteristics due to saturation and load sensitivity. Consequently, a prompt action of VCS is imperative whenever their intervention is required. To put it simply, VCS should prevent the VSA from becoming large enough to digress into the type saturation range, otherwise, it might well be too late to \"catch\" the vehicle and restore driver control. This is even more important in low grip conditions, as tyre saturation might occur earlier, i.e., at very low slip angle values. The VSA amplitude and its rate of change also provide primary driver feedback in conjunction with the so-called steering feeling [7; 8], affecting perception and confidence in the vehicle, especially during turn-in and transients in general [4; 9; 10; 11]. Efficient VSA monitoring is also vital for novel ADAS (Advanced Driver Assistance System) devices, as well as for the success of oncoming semi-autonomous and fully autonomous driving technologies [12]. The VSA in itself is often used for objective chassis design evaluation as well [13]. Vehicle dynamics researchers generally agree that the VSA can't be measured directly for a series production vehicle. Unlike lateral acceleration or yaw rate, which are usually measured directly by means of low-cost sensors, the only way to accomplish a VSA measurement is to use an optical sensor such as the Corrsys-Datron [14]. Unfortunately, this sophisticated device is considered a research and development (R&D) tool rather than a sensor suitable for production vehicles, as it presents issues in terms of compatibility with vehicle packaging, cost, reliability, accuracy, and robustness to environmental conditions. With regard to motorsport, direct VSA measurement on racing cars is often forbidden by the sporting regulations issued by the governing body. Therefore, VSA cannot be measured on everyday cars, and a reliable estimation process is required to feed important active safety systems such as the ESC. For all these reasons, VSA estimation continues to attract considerable interest in the academic and industrial worlds. As a matter of fact, it is well known that research on active safety technologies in general can have a considerable impact on social life by preventing or reducing the potential severity of casualties.