racy may be obtained by Rubidium atomic oscillators. A primary time reference clock (PRTC) according to ITU-T G.8272 has its internal fre- quency locked to a PRC and is required to keep time to within 100 ns (for a PRTC-A) or 40 ns (for a PRTC-B) of the desired time standard (e.g. UTC). Recently, the ITU-T has specified even more stringent clock types (ITU-T G.8272.1). In order to prevent gNB transmissions deviating from their allotted frequency alloca- tions, frequency accuracies of 50 parts per billion (ppb) for macrocells and 100 ppb for small cells (observed over a 1 ms window) are required (3GPP TS 38.104). These accura- cies seem lenient as compared with PRC/ePRC levels, but are actually not trivial to obtain at the cell site. LTE TDD macrocells have a requirement for +5 us absolute time error, while the accuracy requirements for TDD small cells is +1.5 us and certain LTE-A features may require +500 ns accuracies (ITU-T G.8271). With 5G's scalable sub-carrier spacing, the basic requirement becomes stricter, for example, +780 ns for 30 kHz and +390 ns for 60 kHz. CA may put even more stringent demands on the relative time error (i.e. the error between neighbor- ing base stations) (3GPP 36.104), requiring 260 ns relative time error for the inter-band non-contiguous case (half that for the intra-band contiguous case, but this requirement does not impact the transport network). MIMO may drive down the relative time error to 65 ns, and CPRI interfaces require very strict transport delay accuracy of 16 ns. Highly accurate location-based services will require even stricter time accuracy targets. Aiming for 1 m accuracy (3GPP 22.862) will necessitate less than 3 ns of relative time differ- ence between participating base stations. While the latest time distribution standards (IEEE 1588v3) address sub-nanosecond accuracies, such tolerances do not come without a cost. In some areas of the world macro cells have traditionally relied on non-network sources of timing, for example, via a Global Navigation Satellite Syste