degrees). But whatever the elevations, if most of the signal strengths are too low, then the place to begin investigation is with the antenna and cabling for the receiver. The next aspect to consider is the number of satellites that the receiver can receive with good signal strength. It is necessary to have at least four satellite signals to be able to determine an accurate loc- ation and timing solution. It is also advantageous to have the visible satellites widely dispersed around the receiver (in azimuth and elevation) to improve the geometry of the data reference points. Once the survey is completed, receivers can recover a satisfactory timing solution even with only one satellite visible. For that mode to be feasible, the antenna must be fixed in a location known by the receiver (this can come from the survey mode). This is basic mathematics; the receiver needs four satellites to solve the equations for four unknowns (x, y, Z, and t), and if the location (x, y, z) is known, then a single satellite can solve for a single unknown, t. Once you are satisfied that the receiver is successfully receiving good signals, the next steps are to ensure that the receiver is showing the expected location and height and to ensure that there is a valid value for the cable compensation. If the location is wrong or incompletely surveyed, then the antenna installation and view of the sky is the logical place to start investigating. In summary, when investigating problems with a timing distribution network, do not forget to first check the sources of time to ensure they are accurately receiving a good timing signal and are trace- able to UTC. For further information on GNSS systems in general, and their use as sources of time for telecom- munications networks, a good reference source is the ITU-T technical report on GNSS systems, "GSTR-GNSS Considerations on the use of GNSS as a primary time reference in telecommunications." 12.5.1 GPS Rollover The last issue to be aware of is the problem with GPS week rollovers and how that is 