nd a frequency/phase reference-they are often un- available at the remote location where the field engineer needs to test the equipment. So, devices like the Sentinel have two components that combine to provide a stable timing refer- ence. They are equipped with both a GPS receiver and a rubidium (Rb) oscillator in such a way that the GPS receiver can be used to condition and align the Rb oscillator. The engineer connects Sentinel to a portable GPS antenna and leaves it with a sky view for an extended period of learning (which could be up to 12 hours, depending how long since the Rb oscillator last received a valid GPS signal). To conduct field testing, the engineer disconnects the antenna, and the Rb oscillator goes into (bat- tery powered) holdover. When onsite, the engineer carries the device to the suspect network clock, plugs in the BITS and 1PPS ports, and measures the phase alignment and frequency wander using the Rb oscillator (that is in holdover). Obviously, the longer the Rb oscillator is in holdover, the less accurate the measurement is likely to be. Performing this test gives a definitive answer to the question of whether or not the network clock is properly phase aligned. One situation in which this might be valuable is where there is a large amount of asymmetry in the PTP link to the DUT that has remained undiscovered, and nobody knew that compensation was needed to correct it. 12.5.1 GNSS Receivers and Signal Strength GNSS receivers, no matter what the application, all have a method for indicating the signal strength of the different satellites they are tracking. Some receivers display the signal strength in visual form such as vertical bars, but most can express signal strength in terms of actual metrics such as carrier- to-noise-density ratio (C/No) or signal-to-noise ratio (SNR). These two terms are SO regularly used interchangeably that the differences between them are often overlooked. A full understanding of the differences is beyond the scope of this book, but you should know that they are 