Global Positioning Systems

1. Global Positioning Systems

7. OnStar Navigation System

11. Terms to Know • Antenna – Receives signal from satellites and differential source. • Attribute – A characteristic that describes a Feature. They can be thought of as questions which are asked about the Feature.

12. Terms • Channel – Consists of the circuitry necessary to track the signal from a single GPS satellite. • Degree – Unit of angular measurement used to measure latitude and longitude. • Department of Defense (DoD) – Created Global Positioning System

13. Terms • Differential Correction (DGPS) – Comparing GPS data collected in the field to data at a known point. By collecting data at a known point a correction factor can be determined and applied. • Dilution of Precision (DOP) – Positions tagged with a higher DOP value have poorer accuracy than those with a lower DOP

14. Terms • Ephemeris – The predicted changes in the orbit of a satellite • Ephemeris Errors – Errors that originate in the data transmitted by a GPS satellite. Ephemeris errors are removed by differential correction. • Feature – The object which is being mapped for the use in a GIS system. Features may be points, lines, or areas.

15. Terms • Geographic Information System (GIS) – A mapping system that combines positional data with descriptive information to form a layered map. • Global Positioning System (GPS) – A system providing precise location which is based on data transmitted from a constellation of 24 satellites. • Raw GPS signal is only accurate to 100 meters.

16. Terms • Latitude – The angular distance north or south of the equator measured by lines circling the earth parallel to the equator • Degrees from 0 to 90 • Longitude – The angular distance east or west of the prime meridian • Measured by lines perpendicular to the parallels and converging at the poles • 0 to 180 degrees

17. Lines of Latitude

18. Lines of Longitude

20. Terms • Minute – Unit of angular measurement subdivision of Degrees. Each degree is divided into 60’ (minutes). • One minute of latitude anywhere on the earth and one minute or longitude at the equator equals 1.15 miles or 1 nautical mile. • Second – Unit of angular measurement subdivision of minutes. Each minute is divided into 60” (seconds). • One second of latitude anywhere on the earth and one second of longitude at the equator is equal to 33.82 yards.

21. Terms • Multi-path – The interference to a signal that has reached the receiver antenna by multiple paths. • Usually caused by the signal being bounced or reflected. Signals from satellites low on the horizon will have high multi-path error. • Precision Farming – Carefully tailoring soil and crop management to fit the different conditions found in each field.

22. Precision Farming Yield Map

23. Terms • Pseudo range – An uncorrected measurement of the distance between a GPS satellite and a GPS receiver • Receiver – Hardware component needed to process signals received by antenna.

24. Garmin GPS Handheld Receiver

25. GPS receivers

26. Satellite Constellation – The group of 24 GPS satellites from which data is used to determine a position.

30. Terms • Select Availability (SA) – Government-induced signal error. • SNR – Signal to noise ratio. Measure of signal quality.

31. Terms • Spatial Data – Data pertaining to the location, shape, and relationship among geographical features. • Value – Descriptive information about a Feature. • Variable Rate Technology (VRT) – Instrument used for varying the rates of application of fertilizer, pesticides, and seed as one travels across a field.

32. Terms • Yield Monitoring – Regular intervals where a harvested weight has been obtained along with a GPS reading. A display of the weights translated to bushels/acre or yield provides a yield map.

33. Examples of Variable Rate Technology

36. Harvesting with VRT

38. Variable Rate Anhydrous Ammonia

39. Parallel Tracking

40. 2004 Corn Yield Map

41. 2003 Soybean Yield Map

42. What is GPS? • GPS (Global Positioning System) • Created by US Dept. of Defense • Operates 24 hrs/day • Started in early 1980’s • Fully operational April 27, 1995 • 24 NAVSTAR satellites (Navigation with Satellite timing and ranging)

43. What is GPS? • Orbit 10,900 miles above earth surface • Each orbits every 12 hours • 6 orbital paths, 4 satellites per path • At least 4 satellites always visible • Transmit radio waves which travel at a rate of 186,300 miles/sec. (speed of light)

44. What are the Satellites Like? • Weight: 1900 lbs. • Size: 17 feet • Power: 80 watts – 480 watts • Signal: • Send two signals • Civilian use • Military use

45. How are the Satellites Controlled? The US Air Force controls Satellites • Main tracking station: Falcon Air Force Base Colorado Springs, CO • 5 tracking stations around the Earth • Satellites are tracked • New orbits predicted • Clock errors modeled • This data is uploaded to satellites to send to user’s receiver. This info is called the almanac

48. How do we Communicate with the Satellites • Basic Components Required • Antenna – Receives signal from satellites • Two required for certain differentially corrected systems (may be housed in one unit) • Receiver – Processes information contained in signal • This information is refereed to as almanac • PDA – Personal Decision Aid (ex. computer) • Includes software needed to visualize information • Power Source

50. Factors Affecting GPS Accuracy • Selective availability • Technique used by the DoD to reduce accuracy • Satellite clocks • Each satellite has an atomic clock but they are not completely accurate • An error of one millisecond would produce an error or 186 miles • Satellite orbits • Orbit changes due to gravitation from sun and moon. Known as Ephemeris