ation tracking. A whole industry has grown up supplying low-cost GNSS jammers that can be used, for example, to prevent tracking of one work truck but also blocking the use of GNSS for other uses in the same area as an unintentional consequence. Unfortunately, the sale and possession of such jammers is in general not illegal. Of course, attacks on GNSS can also have more sinister motives and actors. For example, spoofing of GNSS with the intention of offsetting GNSS-recovered time by few microseconds could be a very powerful technique for bringing down communication networks, with an inevitable destabilizing impact on societies. General availability-For any given GNSS service, there are typically only a small number of ground stations that uplink configuration and timing data to the satellites in the constellation, and it may, therefore, be a considerable time-several hours-before any given satellite can be contacted by a ground station. This means that, for example, any misconfiguration of satellites can take an extended time to correct. Just such an incident occurred a few years ago when an accidental misconfiguration of a number of GPS satellites resulted in digital radio broadcasts in Europe being disrupted for several hours as they lost the ability to synchronize via GNSS. Alternatives to GNSS for providing synchronization do exist, such as other satellite sys- tems using low-Earth orbit constellations. Low earth orbiting satillite based systems help mit- igate against attack by virtue of having a stronger signal. Alternatively, calibrated fiber based solutions are a great source of very precise time (<1 ns with proper calibration) but unlikely to be affordable outside a small number of institutions. Therefore despite its risks and limita- tions, GNSS is likely to remain the main player in terms of time distribution on a global scale for many years to come. Of course, you cannot just stick a GNSS antenna in the air and pull something represent- ing time out of it. Instead, a GNSS receiver is needed which in 