 limitations, which need to be appreciated when relying on it for a source of synchronization: Installation challenges and costs-To guarantee continuous visibility of the required minimum four satellites, a GNSS receiver really needs to have a properly mounted antenna, positioned so as to have a near-360 degree view of the sky. For a macrocell that is typically located in an open space on a high tower, this is largely a nonissue. However, for an in-building O-RU, it might involve provisioning of a roof-mounted antenna that can be both costly in terms of initial capex and ongoing opex related to access leases. For even harsher environments, such as an in-house femtocell, achieving reliable antenna positioning might be close to impossible. Urban canyons-Even with optimal antenna placement, for example, on the roof of a building-the nearby terrain and topology might be such that a guaranteed view of the necessary number of satellites is not always possible. This is often the case in the so- called urban canyons, where the sky view from even a relatively tall building is blocked by surrounding taller buildings. This is a case where sometimes the single-satellite mode can be useful-if the location of the antenna can be carefully measured at installation, only a single satellite needs to be in view to recover time from GNSS. 272 Open Radio Access Network (O-RAN) Systems Architecture and Design Jamming and spoofing-This is when the GNSS signals are either blocked, rendering the service unusable, or deliberately spoofed to provide a plausible, but wrong signal. Since the GNSS signal is coming from several hundred miles in space, it is very easy to interfere with the signal by means of a nearby transmitter operating on the same frequency. Such interference can be accidental, such as the time that a United States navy ship caused a GPS disruption in large parts of southern California, or can be deliberate. Sadly, such deliberate attacks are becoming more common, largely driven by the increased use of GNSS for workforce loc