This is relevant. Whether you need GNSS for boot-straping a node or not the OSS and Hardware should be adaptable to your requirements. I could see a Zone flipping to self accuracy once the nodes are harmonized.
Time Card — a PCI Express (PCIe) card that can turn almost any commodity server into a time appliance. […] The Time Card allows any x86 machine with a NIC capable of hardware time-stamping to be turned into a time appliance. This system is agnostic to whether it runs for NTP, PTP, SyncE, or any other time synchronization protocol, since the accuracy and stability provided by the Time Card is sufficient for almost any system.