Explanation: Differential GPS (DGPS) works to significantly improve the accuracy of GPS positioning. A fixed ground station, whose exact location is precisely known, continuously monitors the GPS signals. This "base station" calculates the difference between its known position and the position it calculates based on the GPS satellite signals. This difference represents all the errors present in the GPS signals at that time and location, including atmospheric delays, satellite clock errors, and orbital inaccuracies. This correction data is then broadcast to mobile GPS receivers in the vicinity via a separate radio link. These links can use various frequencies, such as VHF, UHF, or even dedicated maritime radiobeacons. The mobile receivers apply these real-time corrections to their own calculated positions, effectively canceling out most of the common errors and providing much greater accuracy than a standalone GPS receiver. Option A is incorrect because comparing L1 and L2 signals is primarily used by multi-frequency receivers to mitigate ionospheric errors directly, not the core mechanism of DGPS transmitting this difference via the GPS signal. Option B is incorrect because DGPS corrects for *all* errors, not just time errors, and transmits these corrections via a *separate* radio signal, not the GPS signal itself. Option D is incorrect for similar reasons to A and B; while some radiobeacons are used, "phase errors between L1 and L2" isn't the sole or primary target, and it corrects for a broader range of errors.