11Terms to KnowAntenna – 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.
12TermsChannel – 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
13TermsDifferential 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
14Terms 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.
15TermsGeographic 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.
16TermsLatitude – The angular distance north or south of the equator measured by lines circling the earth parallel to the equatorDegrees from 0 to 90Longitude – The angular distance east or west of the prime meridianMeasured by lines perpendicular to the parallels and converging at the poles0 to 180 degrees
20TermsMinute – 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 yards.
21TermsMulti-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.
30Terms Select Availability (SA) – Government-induced signal error. SNR – Signal to noise ratio. Measure of signal quality.
31TermsSpatial 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.
32TermsYield 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.
42What is GPS? GPS (Global Positioning System) Created by US Dept. of DefenseOperates 24 hrs/dayStarted in early 1980’sFully operational April 27, 199524 NAVSTAR satellites (Navigation with Satellite timing and ranging)
43What is GPS? Orbit 10,900 miles above earth surface Each orbits every 12 hours6 orbital paths, 4 satellites per pathAt least 4 satellites always visibleTransmit radio waves which travel at a rate of 186,300 miles/sec. (speed of light)
44What are the Satellites Like? Weight: 1900 lbs.Size: 17 feetPower: 80 watts – 480 wattsSignal:Send two signalsCivilian useMilitary use
45How are the Satellites Controlled? The US Air Force controls SatellitesMain tracking station: Falcon Air Force BaseColorado Springs, CO5 tracking stations around the EarthSatellites are trackedNew orbits predictedClock errors modeledThis data is uploaded to satellites to send to user’s receiver. This info is called the almanac
48How do we Communicate with the Satellites Basic Components RequiredAntenna – Receives signal from satellitesTwo required for certain differentially corrected systems (may be housed in one unit)Receiver – Processes information contained in signalThis information is refereed to as almanacPDA – Personal Decision Aid (ex. computer)Includes software needed to visualize informationPower Source
50Factors Affecting GPS Accuracy Selective availabilityTechnique used by the DoD to reduce accuracySatellite clocksEach satellite has an atomic clock but they are not completely accurateAn error of one millisecond would produce an error or 186 milesSatellite orbitsOrbit changes due to gravitation from sun and moon. Known as Ephemeris
51Continued… Earth’s atmosphere Slows radio signals Occurs mostly in ionosphere(50 to 250 miles up) blanket of charged particlesAlso in troposphere – from water vapor
55Improving Accuracy Differential Correction Uses a stationary station of a known locationThe base station calculates the pseudo range and determines the amount of error of each measurement and then calculates the true distance