Originally Posted by pdlamb

Where multipath causes problems is (mainly) in cities, where you'll also get some signals blocked by the buildings in addition to buildings reflecting the signal. Say you've got 7 satellites visible, three pretty close to the direction of 7th St. (where you're riding), two to the north, and two to the south. The block-long tall building on the north side blocks the signals from the satellites to the north, and a shorter building blocks line of sight to the southern satellites. The three in your LOS let your GPS know you're between Walnut and Maple Streets. They can't triangulate well from north to south. But signals from the southern satellites bounce off the building on the north side of the street before getting to your GPS (multipath), indicating you're farther away from the satellites, and therefore farther north. It's possible your GPS will conclude you crossed Walnut, went up an alley to 6th or even to 5th St., and then sprinted back to 7th when you get to the end of the block. You speed demon cyclist, you just rode three blocks while your wheel sensor went one block!