between-epoch difference [GPS]—Difference between two complete carrier beat phase measurements made by the same receiver on the same signal (same satellite, same frequency) but at different time epochs.

between-frequency difference [GPS]—Instantaneous difference between (or, any other linear combination involving) the complete beat carrier phase measurement made by two receivers simultaneously observing the same received signal (same satellite, same frequency).

between-satellite difference [GPS]—Instantaneous difference in the complete carrier beat phase measurement made by the same receiver observing two satellite signals simultaneously (same frequency).

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

Part of LearnCST’s exam text bundle.