Welcome to the Winter 2021 issue of NAVIGATION. In this issue, we feature articles on GNSS interference and monitoring, autonomous orbit determination and timekeeping using X-ray pulsars, and the improvement of GNSS clock corrections and phase biases. We are also featuring articles reporting new equipment technologies that highlight GNSS camera sensor fusion, improved high-precision GNSS with a passive hydrogen maser, and improved 3D mapping-aided GNSS using dual-frequency pseudorange measurements from smartphones.