WEBVTT 00:00:00.000 --> 00:00:05.860 GPS can put you across the street when buildings distort satellite signals. 00:00:06.540 --> 00:00:11.820 Reflected signals take longer routes, which can shift the calculated position. 00:00:12.444 --> 00:00:14.020 You are standing here. 00:00:14.413 --> 00:00:16.480 But the map places you over there. 00:00:16.939 --> 00:00:21.980 A signal reaching your phone does not guarantee an accurate location. 00:00:22.418 --> 00:00:27.460 GPS estimates distance by timing radio signals from satellites. 00:00:27.856 --> 00:00:30.320 The signals include timing information 00:00:30.360 --> 00:00:34.740 and information about the satellites' positions. 00:00:35.390 --> 00:00:39.200 Your phone compares the signal's timing with its own clock. 00:00:39.760 --> 00:00:43.320 That timing helps estimate how far the signal travelled. 00:00:43.793 --> 00:00:48.460 But your phone's clock is not perfectly synchronized with satellite time. 00:00:48.930 --> 00:00:52.940 The calculation must account for that timing difference too. 00:00:53.360 --> 00:00:59.060 A standard position calculation normally needs at least four satellites. 00:00:59.760 --> 00:01:03.360 Together, their measurements help estimate your position 00:01:03.400 --> 00:01:06.160 and your clock's timing error. 00:01:06.637 --> 00:01:10.600 Tall buildings change which satellite signals reach your phone. 00:01:11.140 --> 00:01:14.400 Some direct paths disappear behind buildings. 00:01:14.876 --> 00:01:18.340 Other signals arrive after reflecting from a surface. 00:01:18.925 --> 00:01:22.540 Here, only the reflected signal reaches the phone. 00:01:23.069 --> 00:01:25.060 The direct signal is blocked. 00:01:26.280 --> 00:01:29.020 Engineers call this non-line-of-sight reception. 00:01:29.548 --> 00:01:33.860 The reflected signal travels farther than the direct route would. 00:01:34.382 --> 00:01:40.420 If that extra journey is unrecognized, the measured distance becomes too large. 00:01:40.956 --> 00:01:45.840 Imagine the reflection adds one hundred nanoseconds to the journey. 00:01:45.880 --> 00:01:49.320 That is one ten-millionth of a second. 00:01:49.758 --> 00:01:53.906 It adds about thirty metres, or roughly one hundred feet. 00:01:54.282 --> 00:01:57.920 That is an error in one satellite distance measurement. 00:01:58.395 --> 00:02:02.780 It does not mean your map position shifts by exactly thirty metres. 00:02:03.431 --> 00:02:07.760 Your position comes from combining several satellite measurements. 00:02:08.436 --> 00:02:13.207 One misleading distance can shift the result when the calculation accepts it. 00:02:13.630 --> 00:02:18.460 In the simplified example, one distance estimate becomes too large. 00:02:19.027 --> 00:02:24.040 The calculation shifts the estimated location to fit the measurements. 00:02:24.687 --> 00:02:26.407 The person has not moved. 00:02:26.907 --> 00:02:28.527 The blue marker has. 00:02:29.113 --> 00:02:33.796 The size and direction of that shift depend on several things. 00:02:34.310 --> 00:02:36.756 Which signals are affected matters. 00:02:37.257 --> 00:02:41.436 So do the satellite positions and the receiver's calculations. 00:02:42.080 --> 00:02:46.796 Reflections can also cause trouble when the direct signal still arrives. 00:02:47.396 --> 00:02:51.636 The direct signal and reflected copies combine at the receiver. 00:02:52.256 --> 00:02:54.876 That is called multipath interference. 00:02:55.502 --> 00:02:58.436 Buildings can also hide useful satellites. 00:02:59.116 --> 00:03:03.016 The remaining measurements may constrain some directions poorly. 00:03:03.056 --> 00:03:06.696 That can make the position estimate less reliable. 00:03:07.277 --> 00:03:12.396 A wrong map location is not always a satellite signal problem. 00:03:12.876 --> 00:03:16.916 The map itself might contain an incorrect address or road. 00:03:17.299 --> 00:03:21.016 That is different from calculating your position incorrectly. 00:03:21.724 --> 00:03:27.376 Your phone may also combine satellite information with other location clues. 00:03:27.990 --> 00:03:33.183 For example, Android location services can use GPS and Wi-Fi. 00:03:33.223 --> 00:03:38.563 The displayed blue marker may reflect that combined estimate. 00:03:39.013 --> 00:03:44.137 Some positioning systems use building maps to help interpret satellite signals. 00:03:44.734 --> 00:03:48.477 Those maps can help predict blocked or reflected paths. 00:03:49.150 --> 00:03:52.897 But the benefits depend on the system and its surroundings. 00:03:53.590 --> 00:03:58.897 Receiving more signals is useful only when their information is handled well. 00:03:58.937 --> 00:04:03.937 A stronger signal is not proof that it travelled directly. 00:04:04.439 --> 00:04:08.320 So the phone can receive a signal and still misplace you. 00:04:09.137 --> 00:04:13.517 Buildings can hide direct signals and send others along longer routes. 00:04:14.097 --> 00:04:18.597 Those measurements can shift the calculated position across the street. 00:04:19.035 --> 00:04:21.719 The blue marker is an estimate of your location. 00:04:21.759 --> 00:04:25.999 It is not a direct observation of where you stand.