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Here's a sine wave illustrating a single wavelength of 1 Hertz, that is a wavelength that takes 1 second to cycle through 360 degrees. The 0°, 90°, 180°, 270° and 360° are known as phase angles. They are important to the modulation of the carrier by phase and that is the method of attaching the codes to the GPS carriers.
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 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. As long as the rate of an oscillator’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. Therefore, the phase angles will also occur at definite and constant distances.