fering some form of in- terference or multipath reflection. The survey process might struggle to complete because the re- ceiver never arrives at a solution for location that is consistent enough for accurate timing. Even worse, if the signal from the satellite becomes too weak or disappears, then the receiver will lose that signal and discard its data from the solution. As a rule of thumb, for a typical GPS receiver, acquisition is limited to signals above 35 dB-Hz, but it is feasible to continue tracking with a signal strength down to about 25 dB-Hz. This means that acquisition requires a higher signal strength compared to tracking. Below about 28 dB-Hz, the re- ceiver risks losing its ability to continue receiving the satellite signal. Table 12-3 is one example of a set of received C/N values from a GPS receiver owned by one of the authors. The pseudorandom noise (PRN) is the satellite's (GPS) code number, the signal is the C/N reading, the health bit indicates the signal data is valid, and the azimuth and elevation indicate the direction of the satellite and the angle above the horizon, respectively. Table 12-3 Example of Satellites Visible to GPS Receiver PRN No. Signal Healthy Azimuth Elevation The satellites in the table with a low signal strength (36-37) mostly have low elevation levels, which means they are both further away (up to 5000 km further) and their signals must pass through more atmosphere (because of the angle). For these reasons, those signals encounter another few decibels of power reduction caused by loss from the increased path length. You can see in Table 12- 3 that the satellites with the lowest three signal strengths are from those with elevations less than 20 degrees above the horizon. The low elevation means the signal has crossed through more of the atmosphere and is likely to suf- fer more from any propagation anomalies and errors introduced by the atmosphere. This is one reason why receivers tend to exclude satellites from the solution if they are low in the sky (say, less than 15 