GPS Receivers University of Manitoba Recreation Services GPS Workshop.

What is GPS? GPS is the acronym for Global Positioning System. GPS consists of a series of satellites (24+) that transmit data that allow receivers on Earth to determine their location with great accuracy. Also included in this system are ground stations (5) that are used for detecting and updating errors. Generally we only think about the receivers and call those a GPS or GPS units. 24 (+ spare) earth-orbiting satellites – at least 6 in the sky at any time, up to 12 at any one time (thus 12 channel receivers) 5 ground-based monitoring stations Individual GPS receivers The satellite orbits are roughly 25,000 kilometers from the earth's centre, or 20,000 kms above the earth's surface. The orbital paths of these satellites take them between roughly 60 degrees North and 60 degrees South latitudes. What this means is you can receive satellite signals anywhere in the world, at any time. As you move closer to the poles (on your next North Pole or Antarctic expedition!), you will still pick up the GPS satellites. They just won't be directly overhead anymore. This may affect the satellite geometry and accuracy but only slightly. The satellites are constantly moving, making two complete orbits around the Earth in just under 24 hours. If you do the math, that's about 2.6 kilometers per second.

The Short Story! GPS Receiver (Hand Held Unit) Triangulation with at least 3 satellites & a good view of the sky (4 satellites needed for X, Y and Z coordinates) How accurate? ±10-20m (elevation ~ ±2x horizontal error) 0.5cm with additional equipment Basically the 10-20m is dependent on many things including atmospheric conditions, satellite arrangement (ephemeris), Multi-path bounces, and clock accuracy. All commercial civilian GPS’s from the 100$ variety to the 500$ variety and more give you the same accuracy and point location.

Where You Are - Triangulation 1 satellite 2 satellites 3 satellites (Lat/Long 2D) 4 satellites gives altitude In receivers that can use more than 4 parallel signals at one time the best four are typically used. Some GPS units probably will use more but not most of the hand held units.

Enhancements to Accuracy (WAAS) Wide Area Augmentation System Geo-stationary Satellites ~25 ground stations of known location (North America) corrects for GPS signal errors caused by ionospheric disturbances, timing and satellite orbit errors Improves accuracy to 3-5m Limits Must have good view of sky Antenna/receiver must be powered all of the time.

Enhancements to Accuracy (DGPS) Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements. The stationary receiver is the key.  It ties all the satellite measurements into a true known position. Base stations for small area corrections cost ~500-1000$. Additional antenna for use with the base station is another ~400$. Working with CDGPS requires a receiver ~1500$ There are several services available for post processing if necessary. You can also create your own post processing with a stationary GPS unit connected to a computer or data logging unit.

DGPS Accuracy 1-3 meter with U.S. Coast Guard signal 0.5 cm with local base station (used extensively in Westside farming)

Limitations for Hiking Need to see the sky! Trees, buildings, mountains, some car windshields, etc... Only indicates place not movement or direction So how does it measure speed and direction? Minimum speed ~1.5km/hour (some up to 4km/hour) Power Bring extra batteries Cold/Winter Temperatures LCD Screen Note on batteries – make sure the batteries are field replaceable, some will not take replaceable batteries. Cold temperature use is something to consider in Canada (temp range typically -15C - +70C. There are two issues. Power – use lithium batteries. LCD screen can freeze and then your unit can be trashed. Under cold conditions you might want to consider an external antenna and keep the unit next to you. Don’t keep unit in your pack but an inside pocket.

Sources of Error (~10m) Measures (some receivers display) DOP (dilution of precision) EPE (Estimate Position Error) Minimize Error Averaging Better Satellite Config Move to get better sky view Re-orient receiver (patch/helix) Wait  Error Value Ionosphere 4.0 meters Clock 2.1 meters Ephemeris Troposphere 0.7 meters Receiver 0.5 meters Multipath 1.0 meter Total 10.4 meters DOP – Dilution of precision – mostly the arrangements of satellites. Look for lower values. EPE – Estimate Position Error based on DOP, Satellite signal quality, etc… Look for lower values. Depending on the maker this is roughly the point where you are 50% likely to be within this range. errors are especially bad near dawn and dusk when the ionospheric changes are greatest, and often receivers will report repeated loss of lock at these times. Multipath errors can cause very large errors as well especially with some older GPS receivers where they will seem to spring several hundred or more meters – even up to 1or2km, some funny reports of errors over 1000km. A receiver with a lot of internal noise can have more error than listed here.

Of Datums and Coordinates NAD27 (Canada), NAD83, WGS84 Co-ordinate Lat/Long (dd.dddd, dd°hh’mm.ss”) N49°52'10.11" W097°13'45.95" UTM/UPS (Zone Easting Northing) 14U 627231 5525622 MGRS (Zone GridRef Easting Northing) 14U PA 27231 25622 GPS system is based on WGS (World Geodetic System – 1984). Geodetic datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth. Hundreds of different datums have been used to frame position descriptions since the first estimates of the earth's size were made by Aristotle. Datums have evolved from those describing a spherical earth to ellipsoidal models derived from years of satellite measurements. Modern geodetic datums range from flat-earth models used for plane surveying to complex models used for international applications which completely describe the size, shape, orientation, gravity field, and angular velocity of the earth. While cartography, surveying, navigation, and astronomy all make use of geodetic datums, the science of geodesy is the central discipline for the topic. Referencing geodetic coordinates to the wrong datum can result in position errors of hundreds of meters. Different nations and agencies use different datums as the basis for coordinate systems used to identify positions in geographic information systems, precise positioning systems, and navigation systems. The diversity of datums in use today and the technological advancements that have made possible global positioning measurements with sub-meter accuracies requires careful datum selection and careful conversion between coordinates in different datums.

The earth is not a perfect sphere and bulges somewhat around the equator.  What many people are not aware is that different ellipsoids are used in projecting.  North American Datum of 1927 (NAD27) is a datum based on the Clarke ellipsoid of 1866.  The reference or base station is located at Meades Ranch in Kansas. There are over 50,000 surveying monuments throughout the US and these have served as starting points for more local surveying and mapping efforts.  Use of this datum is gradually being replaced by the North American Datum of 1983. North American Datum of 1983 (NAD83) is an earth-centered datum based on the Geodetic Reference System of 1980. The size and shape of the earth was determined through measurements made by satellites and other sophisticated electronic equipment; the measurements accurately represent the earth to within two meters.  There are differences in the two ellipsoids ranging from 200-300 feet in the western US to several tens of feet in the central and eastern US as show below:

Some Terms WayPoint Bearing Track (log) Route Waypoint Marking where you are currently Marking a known point Recorded internally in WGS84 but entered as particular datum Navigate to a waypoint MOB Bearing Direction from current location to waypoint when navigating Related to this is the course and track which are the directions currently being traveled Track Direction or path currently being taken Most GPS will drop a line of bread crumbs as you are walking These can usually be saved and/or traced back Some GPSr will ‘average’ lines to minimize memory use Route Two or more waypoints strung together in a route that you want to take.

Cost: $100-50,000 Based mostly on the need for accuracy

GPS Applications in Agriculture Determine field boundaries Crop scouting (maturation, weeds, pests) Soil testing Tillage Spraying (pesticides, herbicides, fertilizers) Yield monitoring