They transmit on multiple frequencies. The signals on every frequency will have its own phase offset between the sender and receiver. Think of it as constructiv

They transmit on multiple frequencies. The signals on every frequency will have its own phase offset between the sender and receiver. Think of it as constructive / destructive interference patterns which varies across space, except they have a method where they measure across time multiple times. At certain multiples of a distance, with synchronized clocks, you'll have offset X between channel Y and Z (on channel Y you may be perfectly phase synchronized, but Z is half the phase off). Measuring the offsets for multiple sets of frequencies allows you to calculate multiple sets of possible distances, to then figure out which exact distance it is that repeats among all of the calculated distance multiples (like 1, 2, 3; 0.5, 2, 4; 1.5, 2, 3 - the number 2 occurs everywhere). If for example you're perfectly in phase on 10 frequencies, the distance between you must be a must be a specific shared multiple of the wavelengths of all the frequencies. Least common denominator.