Published on *GEOG 862: GPS and GNSS for Geospatial Professionals* (https://www.e-education.psu.edu/geog862)

Source: GPS for Land Surveyors

The time measurement devices used in both EDM and GPS measurements are clocks only in the most general sense. They are more correctly called oscillators, or frequency standards. In other words, rather than producing a steady series of audible ticks, they keep time by chopping a continuous beam of electromagnetic energy at extremely regular intervals. The result is a steady series of wavelengths and the foundation of the modulated carrier. 1 hertz is a full wavelength that takes 1 second to cycle through 360 degrees. As long as the rate of an oscillatorâ€™s operation is very stable, both the length and elapsed time between the beginning and end of every wavelength of the modulation will be the same.

Here is a little bit about phase angles. They are important to the modulation of the carrier by phase that is the method of attaching the codes to the GPS carriers.

Here's a sine wave: 0, 90, 180, 270, and 360 are known as phase angles in a single wavelength. The oscillators in the EDM or in the GPS satellite create very constant wavelengths, because like clocks or oscillators, they're known as frequency standards. They create electromagnetic energy that has a very constant wavelength. Therefore, the phase angles occur at definite distances.