Using GPS to find out exactly where we are is one of the more awesome powers of our mobile devices. It also poses some of the toughest challenges to developers, who must prevent excessive battery drain and deal with insufficient accuracy. In this talk we will learn how to face these challenges by answering questions such as: How does GPS even work? Where the heck did my battery go? Why does it take so long to get a good fix? And why does my phone think I’m in the middle of a lake? If you’ve ever wanted to understand how satellite navigation actually works, this is the talk for you. (It also features cold war intrigue, applied relativity theory, and a picture of a dinosaur. So you know it’s going to be good.)

2. Flight KAL007

4. Location Matters

5. Ronald Reagan issues a directive to make GPS available for civilian use

7. Richard Süselbeck
Principal Developer Evangelist
HERE Technologies
@sueselbeck @heredev
https://developer.here.com

8. The world leading location technology company

10. 100 million cars have our maps on board.
4 out 5 cars in EU and NA use our maps.
28 Terabyte of map data collected a day.
600.000+ km mapped in HD for autonomous driving.
Fleet of 400+ cars collecting map data.
700.000 3D data points collected per second, per car.

11. An Autonomous World

12. Routing API

13. CoreRouter router = new CoreRouter();
RoutePlan routePlan = new RoutePlan();
routePlan.addWaypoint(new RouteWaypoint(
new GeoCoordinate(40.728215,-73.958251)));
routePlan.addWaypoint(new RouteWaypoint(
new GeoCoordinate(40.64454, -73.79536)));
RouteOptions routeOptions = new RouteOptions();
routeOptions.setTransportMode(RouteOptions.TransportMode.CAR);
routeOptions.setRouteType(RouteOptions.Type.FASTEST);
routePlan.setRouteOptions(routeOptions);
router.calculateRoute(routePlan, new RouteListener());

14. Geocoder API

15. GeoCoordinate pickup = new GeoCoordinate(40.728215,-73.958251);
ReverseGeocodeRequest2 revGeocodeRequest = new
ReverseGeocodeRequest2(pickup);
revGecodeRequest.execute(new ResultListener<Location>() {
@Override
public void onCompleted(Location location, ErrorCode errorCode) {
updateTextView(location.getAddress().toString());
}
});

16. https://developer.here.com

18. We are currently at 40.72819, -73.95747. What does that mean?
Latitude & Longitude

19. Latitude & Longitude
Latitude Longitude
Equator
Prime Meridian
+60
-30
-90 (South)
+90 (North) 180
-120 (West)
+30 (East)
We are currently at 40.72819, -73.95747. What does that mean?

20. Where does the Latitude and Longitude come from?

21. PositioningManager positioningManager = PositioningManager.getInstance();
positioningManager.start(PositioningManager.LocationMethod.GPS_NETWORK);
GeoCoordinate coordinate = positioningManager.getPosition().getGeocoordinate();
String locationString = coordinate.getLatitude() + "," + coordinate.getLongitude();
Toast.makeText(getApplicationContext(), locationString, Toast.LENGTH_SHORT).show();

22. Where does the Latitude and Longitude come from?
GPS

24. 24 satellites at 20.000km, 4 visible at all times

25. 1. Find four satellites
2. Determine their position
3. Determine your distance to each
4. Trilateration!
How to determine your position using GPS

26. Step 4: Trilateration (in 2D)

27. 128 km from Magdeburg
Step 4: Trilateration (in 2D)

28. 128 km from Magdeburg
127 km from Szczecin
Step 4: Trilateration (in 2D)

29. 128 km from Magdeburg
127 km from Szczecin
107 km from Cottbus
Step 4: Trilateration (in 2D)

30. Step 4: Trilateration (in 3D)

31. Step 4: Trilateration (in 3D)
Yes, you only need three! Why?

32. 1. Find four satellites
2. Determine their position
3. Determine the distance to each
4. Trilateration! ✔️
How to determine your position using GPS

33. GPS satellites are permanently sending out a signal
Step 1: Find Four Satellites

34. 1. Find four satellites ✔️
2. Determine their position
3. Determine the distance to each
4. Trilateration! ✔️
How to determine your position using GPS

35. Step 2: Determine the Satellites' Positions

36. Step 2: Determine the Satellites' Positions
The GPS signal includes
• Date, Time, Health
• Ephemeris
• Almanac

37. Step 2: Determine the Satellites' Positions
The GPS signal includes
• Date, Time, Health
• Ephemeris (location of the satellite)
• Almanac (coarse location of all satellites, ionospheric model)

38. Step 2: Determine the Satellites' Positions
A-GPS (get this information via the Internet)

39. 1. Find four satellites ✔️
2. Determine their position ✔️
3. Determine the distance to each
4. Trilateration! ✔️
How to determine your position using GPS

40. Step 3: Determine the Distance to the Satellites
Satellite has an atomic clock
d

41. Step 3: Determine the Distance to the Satellites
Use travel time of signal to determine distance
d
𝑑 = 𝑡 × 𝐶
𝐶 ≅ 3 × 108
𝑚/𝑠

42. Step 3: Determine the Distance to the Satellites
Problem!
atomic clock
no atomic clock

43. Step 3: Determine the Distance to the Satellites
I can get the pseudoranges to the satellites

44. Step 3: Determine the Distance to the Satellites
If I can determine the offset, I'm good!

45. Step 3: Determine the Distance to the Satellites
This is where the fourth satellite comes in!

46. This is where the fourth satellite comes in!
Step 3: Determine the Distance to the Satellites

47. 1. Find four satellites ✔️
2. Determine their position ✔️
3. Determine the distance to each✔️
4. Trilateration! ✔️
How to determine your position using GPS

48. Where to?
Your driver will arrive in 4 mins
Confirm ride
Why is my location still wrong?

49. Where to?
Your driver will arrive in 4 mins
Confirm ride
GPS is only accurate to about 5 meters.
Why is my location still wrong?

50. Where to?
Your driver will arrive in 4 mins
Confirm ride
GPS is only accurate to about 5 meters.
(under ideal conditions)
Why is my location still wrong?

51. What affects GPS?

52. What affects GPS?
Relativity! (Special and General!)

53. What affects GPS?
The Ionosphere

54. Multipath errors due to reflections
What affects GPS?

55. GNSS: Galileo, GLONASS, BeiDou, IRNSS, QZSS

56. GNSS isn't the only way to get a location.
Radio signals:
Cell towers, Wifi, Bluetooth, etc
Can also work indoors.

57. All GPS calculation is baked into the chip

58. android.location.GnssMeasurement
Raw GNSS Measurements

59. Thank you!