Global Positioning Systems                                            

OnStar Navigation System

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.

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

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

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.

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.

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

Lines of Latitude

Lines of Longitude

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.

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.

Precision Farming Yield Map

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.

Garmin GPS Handheld Receiver

GPS receivers

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

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

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.

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.

Examples of Variable Rate Technology

Harvesting with VRT

Variable Rate Anhydrous Ammonia

Parallel Tracking

2004 Corn Yield Map

2003 Soybean Yield Map

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)

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)

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

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

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

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

Continued… Earth’s atmosphere Slows radio signals Occurs mostly in ionosphere (50 to 250 miles up) blanket of charged particles Also in troposphere – from water vapor

Continued…. Multi-path errors Signal bounce off near objects

Improving Accuracy Differential Correction Uses a stationary station of a known location The base station calculates the pseudo range and determines the amount of error of each measurement and then calculates the true distance

Applications In Ag Soil sampling Yield mapping Variable rate planting/fertilizer application Weather data Production costs Variable rate chemicals Field scouting Others???