GPS 101. An Introduction to Using a GPS Receiver

Transcription

1 GPS 101 An Introduction to Using a GPS Receiver The goal of this presentation if to provide a basic understanding of what is GPS, how it works, provide some basic terminology and to provide ideas on how a GPS can be used for hiking. There is no one correct way to use a GPS for hiking. The GPS can be used to determine the current location in relation to a paper map, to assist in navigation to a certain location, as an altimeter to assist with navigation, and to collect data about the trip. Any or all of these functions are possible using a GPS receiver along with other associated software. 1

2 What the Heck is GPS? Global Positioning System Air Force is Lead Agency Official Name is NAVSTAR Global Positioning System Navigation Satellite Timing and Ranging First Satellite Launched in Satellites in use by 1994 GPS is short for Global Positioning System. The GPS System is funded by the Department of Defense; and is operated and maintained by the U.S. Air Force. The official name is the NAVSTAR Global Positioning System. NAVSTAR is short of Navigation Satellite Timing and Ranging. The concept of GPS was conceived in The Air Force started developing the system. In 1974, when the first satellite was launched, the other branches of the military joined in on the project. By 1994, 24 satellites were in orbit. The system was fully operational in 1995 at a cost of ten billion dollars. 2

3 How GPS Works A Constellation of Satellites 21 Operational and 3 Spares Orbiting 11,000 to 12,000 Miles Above the Earth 12 Hour Orbit Pass over the Same Location Approximately Every 24 Hours Positioned to Provide at Least Six Signals to any Point on Earth Of the 24 satellites in orbit, 21 are in use and 3 are spares. The satellites orbit between 11,000 and 12,000 miles above the earth s surface. Each satellite takes 12 hours to complete its orbit and 24 hours to pass over the same spot. The satellites are positioned so that at least 6 satellites provide a signal to every point on earth. In addition, there are two types of signals broadcast by the satellites. One is for military use only and the second is for civilian use. 3

4 How GPS Works Determining Location Uses Triangulation Distance from at Least 3 Satellites is Calculated Each satellites signal includes data including the time the signal was broadcast. The GPS receiver receives the signal and by determining the difference between the time the signal was broadcast and the time the signal was received, the distance from the satellite is calculated. Once the receiver has acquired signals from three satellites, it can calculate a location. For example, say the first satellite is 11,000 miles away. As for as the GPS receiver is concerned, the current location is any point in space 11,000 miles from the satellite. Once a second satellite s distance is known, the number of current locations is reduced. If the second satellite is 12,000 miles away, the GPS receiver knows that it is somewhere in the common area where the two signals intersect. Enter a third satellite calculated to be 13,000 miles away. When the signal from this satellite is overlaid with the signal from the first two satellites, there are only two possible locations where all three signals intersect. One of these intersections is thrown out as being unreasonable. The means the other intersection is the current location. 4

5 How GPS Works 2d and 3d 3 Satellites = Two -Dimensional Location Fix Horizontal Coordinate 4 Satellites = Three -Dimensional Location Fix Horizontal Coordinate and Elevation When the GPS receiver has signals from three satellites, the horizontal location can be determined. When a fourth signal is obtained, elevation data can be calculated. 5

6 How GPS works A Note to Conspiracy Theorists Receiver Only Not a Tracking Device Does not Transmit Location A GPS receiver is just that, a receiver. It is not a tracking device and does not transmit information. However, there are other GPS-enabled devices available that do transmit GPS data (two-way radios, cell phones, etc.). 6

7 GPS Limitations Blocking the View Signal Doesn t Penetrate Solid Objects Dense Tree Coverage, Canyons, Tall Buildings, etc., may Block Signal Even Your Body may Block Signals GPS has limitations. GPS is dependent on satellite signals to function properly. The signals can be blocked by different solid objects. Without a good signal, a GPS receiver will not be accurate or may not function at all. 7

8 GPS Limitations Flying with the Crows Distances are Shortest Possible Route GPS Receiver does not Account for Obstacles or Terrain Vertically Challenged Elevation Readings are Generally 50% less Accurate than Horizontal Readings When a GPS calculates the distance between two points, it uses a straight line. It does not account for obstacles between the two locations. So, even though the GPS may say it is a mile to the next destination, the actual route may be further. The route to the next location may involve detouring around water bodies, mountains, trees, etc. GPS receivers also have a margin of error when calculating a location. Elevation readings are even less accurate. For example, if the current location of the GPS receiver is off by 20 feet, then the elevation will be off by 30 feet. 8

9 GPS Limitations Keep it Moving Velocity = Distance / Time Time = Distance / Velocity GPS Receiver Needs Movement to: Determine Direction Time to Reach Destination GPS receivers can calculate both speed and estimated time of arrival (ETA). But, to do this, the GPS receiver needs movement. Without movement, neither speed nor ETA can be calculated. 9

10 GPS Limitations Battery Dependence Carry Spares Adds Weight Backlight Consumes Energy No Power = No GPS GPS receivers are electronic devices. Therefore, they are dependent on an electrical source, or batteries. Therefore, extra batteries must be carried. Of course, carrying spare batteries results in additional weight. Many individuals have had good luck using Lithium-Ion batteries. Generally, these batteries seem to last longer, especially at lower temperatures; and are not as heavy as alkaline batteries. Turning off or reducing the backlight function of a GPS receiver will reduce the energy consumed. And for those GPS receivers with a built-in electronic compass, the compass consumes additional energy. Since a GPS receiver is an electronic device, it can fail. In addition, GPS receivers are not a replacement for other navigation aids (paper maps, compass, altimeter, etc). These other aids still need to be carried and it is important to know how to use each, 10

11 Antennas Internal Antennas Most GPS Receivers have One of Two Types of Antenna Under Ideal Conditions, there is Very Little Difference Most GPS receivers have a built in antenna. There are two types of internal antennas. Both are equally sensitive but both have pluses and minuses. 11

12 Antennas Internal Antennas Patch Antennas Good at Picking Up Signals Directly Overhead Hold Parallel to the Sky for Best Reception Not as Good at Picking Up Signals from Satellites on the Horizon More Streamlined and Smaller The first type of internal antenna is a patch antenna. A patch antenna is a small, flat antenna usually located just under the front of the GPS receiver. A patch antenna is very good at picking up signals directly overhead but may have difficulty with signals on the horizon. In addition, the GPS receiver needs to be held so that the patch antenna is parallel to the sky for best reception. 12

13 Antennas Internal Antennas Quadrifilar Helix (Quad Helix) Good at Picking Up Signals Lower on the Horizon Hold with Antenna Towards the Sky for Best Reception Not as Good as Picking Up Signals from Satellites Directly Overhead Usually Bulkier The second type of internal antenna is a quad-helix antenna. The quad-helix antenna is bulkier than a patch antenna. But, a quad-helix antenna is better at picking up satellite signals on the horizon. 13

14 Antennas External Antennas Connect via Cable Additional Bulk and Weight More Sensitive than Internal Antennas Many GPS receivers allow the additional of an external antenna. An external antenna connects to the GPS receiver by a cable. An external antenna also adds additional bulk and weight. But, an external antenna is more sensitive than an internal antenna. 14

15 GPS Accuracy Selective Availability Original Theory was 100 Meters Accuracy Actually Turned Out to be 20 to 30 Meters Selective Availability (SA) Developed to Provide Less Accuracy by Adding Errors to the Data Accuracy of 100 Meters Originally, it was thought that the civilian side of GPS would be accurate to around 100 meters. It turned out the actual accuracy was between 20 and 30 meters. The military was afraid this was too accurate for civilian and enemy use. So, Selective Availability was developed. Selective Availability intentionally added errors to the GPS signal so that the accuracy was no less than 100 meters. 15

16 GPS Accuracy Selective Availability Temporarily Turned Off in 1990 During the Persian Gulf War Allowed Military to use Civilian GPS Receivers Turned Off Permanently in 2000 Accuracy of 15 Meters Signal can be Degraded on a Regional Basis During the 1990 Persian Gulf Ware, there was a shortage of military GPS receivers. So, Selective Availability was disabled so the military could use civilian GPS receivers. After the war, SA was re-enabled. In 2000, SA was permanently disabled. This improved the accuracy of civilian GPS receivers to about 15 meters. To prevent enemies from using GPS against the military, the civilian GPS signal can be degraded on a regional basis, for example over Afghanistan or Iraq. 16

17 GPS Accuracy Differential GPS (DGPS) Increases Accuracy Uses a Stationary Receiver Measures GPS Timing Errors and Broadcasts Correction Information Freely Broadcast by a Series of Coast Guard Stations Receiver Must be Able to Receive DGPS Signals Requires a Separate Beacon that Connects to the GPS Unit. While a GPS receiver may be accurate to within 15 meters, sometimes there is a need for additional accuracy. So, Differential GPS was developed. DGPS uses a stationary receiver that measures GPS timing then broadcasts correction information. In order to receive and use DGPS signals, additional hardware that connects to the GPS may be required. DGPS is designed for maritime use and is operated by the Coast Guard. 17

18 GPS Accuracy Wide Area Augmentation System (WAAS) Operated by Federal Aviation Administration (FAA) Developed so GPS can be Used for Airplane Flight Approaches To provide additional accuracy for non-maritime use, the Wide Area Augmentation System (WAAS) was developed. WAAS was developed for airplane flight approaches and is operated by the FAA. However, WAAS also improves the accuracy for non-flight related activities. 18

19 GPS Accuracy Wide Area Augmentation System (WAAS) Uses Ground Reference Stations Master Stations Created GPS Message that Includes Corrections for Satellite Drift and Atmospheric Conditions WAAS uses 25 reference stations. The reference stations send data to two master stations. The master stations then make a correction to the signal and re-broadcast the signal to one of the WAAS satellites. 19

20 GPS Accuracy Wide Area Augmentation System (WAAS) Corrected Data Sent to non -NAVSTAR Satellites 3 Satellites Stationary Orbit over Equator WAAS Satellites Broadcast to WAAS -Enabled GPS Receivers As of late 2006, there were four WAAS satellites in stationary orbit over the equator. Three of the satellites were in actual use and the fourth was undergoing testing. Each WAAS satellites receives information from the master stations on earth then re-broadcast the WAAS signal back to earth. It is important to note the WAAS satellites are not NAVSTAR satellites and are completely separate. In order to use a WAAS signal, a GPS receiver must be WAAS enabled. 20

21 GPS Accuracy Distance in Feet Distance in Meters GPS without SA GPS with DGPS GPS with WAAS 10 3 The standard GPS signal provides accuracy to within 15 meters, or approximately 49 feet. Using Differential GPS improves the accuracy to between three and five meters. WAAS increase the accuracy to approximately 10 feet. 21

22 Map Projections Map Datums Mathematical Model Describes Shape of An Ellipsoid In this Case, Earth Many Different Datums are Used Throughout the World To understand GPS, it is helpful to have a basic understanding of Map Projections. Maps are drawn so that every point is a known distance and height from a standard reference point called a datum. There is many different datums used throughout the world. 22

23 Map Projections U.S. Datums NAD-27 Most Popular for USGS Quads NAD-83 Mostly same as WGS -84 Default for Most GPS Receivers Mismatched Datums can Result in Location Errors of 200 Meters In the United States, both the NAD-27 and the NAD-83 datum are used. The NAD-27 datums is based upon a different map projection than the projection used by NAD-83. Most people who hike are familiar with the NAD-27 datum. This datum is used by the USGS topo quads. WGS-84 is used by most aviation charts. In addition, it is the default datum for most GPS receivers. Describing a location using two different datums will result in a mismatch of the locations. For example, if NAD-27 is used to describe a location to someone else using NAD-83, the actual location can be off 200 meters. 23

24 Map Projections Datum Should be Listed in Legend of Paper Map When to be Concerned about Datums: Location is Plotted on Two Different Paper Maps A Paper Map and a GPS Receiver are Used Together to Plot a Location Two GPS Receivers are Used Together to Plot a Location Each paper should list the datum the map is based upon. Generally, there are times when it is important to be concerned with datums: When a location is plotted on two different maps. When a paper map and a GPS are used together. When two GPS receivers are used together. 24

25 Map Coordinate Systems Latitude / Longitude Oldest Coordinate System for Plotting Locations Created as a Way of Representing the Earth on a Flat Piece of Paper Every location on earth can be described by its coordinates. However, there are numerous coordinate systems in use. For GPS, the first coordinate system commonly used is latitude/longitude. It is the oldest of the map coordinate systems and was developed as a way of representing the Earth on a flat piece of paper. 25

26 Map Coordinate Systems Latitude North from the Equator North Latitude Increases to 90 Degrees at the North Pole Expressed in Degrees North South from the Equator South Latitude Increases to 90 Degrees at the South Pole Expressed in Degrees South Latitude is the distance, measured in degrees, from the equator. In the Northern Hemisphere, latitude is expressed in Degrees North and in the Southern Hemisphere as Degrees South. 0 degrees represents the equator, 90 degrees north is the North Pole, and 90 degrees south is the South Pole. 26

27 Map Coordinate Systems Longitude Measured East and West from the Prime Meridian 0 Degrees Longitude Passes Through Greenwich, England Longitude is similar to Latitude except that is measured east/west. 0 degrees represents the Prime Meridian, which passes through Greenwich, England. 27

28 Map Coordinate Systems Longitude East from Prime Meridian Longitude Increases to 180 Degrees Expressed in Degrees East West from Prime Meridian Longitude Increases to 180 Degrees Expressed in Degrees West Where to two 180 -Degree Longitudes Meet is the International Date Line East from the Prime Meridian is expressed as Degrees East and West is expressed as Degrees West. Longitude increases to 180 degrees. Where the two 180 degree longitudes meet is commonly referred to as the International Date Line. 28

29 Map Coordinate Systems Here is a map showing both Latitude (north-south) and Longitude (east-west). 29

30 Map Coordinate Systems Latitude / Longitude Degrees Composed of Smaller, Fractional Units 1 Degree = 60 Minutes 1 Minutes is about 1.2 Miles 1 Minute = 60 Seconds 1 Second is about.02 Miles One degree of Latitude or Longitude is around 70 miles. So, each degrees can be broken down into smaller units to more accurately describe a location. Each degree consists of 60 minutes. Each minute is about 1.2 miles in distance. Each minute can be broken down into 60 seconds. Each second is about two one-hundredths of a mile in distance. 30

31 Map Coordinate Systems Coordinate Locations Degrees, Minutes, Seconds 43 o N 121 o W Degrees and Decimal Minutes Seconds Dropped Decimal Versions of Minutes Used 43 o N 121 o W Decimal Degrees Minutes and Seconds Dropped Decimal Representation of Degrees Used o N o W To make matters even more confusing, Latitude and Longitude can be expressed in different ways. For example, coordinates can be expressed as: Degrees, Minutes, Seconds Degrees and Decimal Minutes or, Decimal Degrees Even though each of the examples displayed use different notations, they all describe the same location. So, it s important when discussing latitude and longitude with someone, that both parties use the same notation. 31

32 Map Coordinate Systems Coordinate Notations Coordinate Conversion Calculators Available latlong.asp There are websites available that provide coordinate conversion calculators. 32

33 Map Coordinate Systems Universal Transverse Mercator (UTM) Developed in the 1940s Based on Meters Widely Used by GPS Receivers Another popular map coordinate system is Universal Transverse Mercator (UTM). UTM was developed in the 40s and is based on meters. Although the U.S. doesn t use the metric system, UTM is widely used by GPS receivers. In addition, UTM is easier to use when plotting a location on a map. 33

34 Map Coordinate Systems Universal Transverse Mercator (UTM) World is Divided into Zones Sixty Primary Zones Run North -South Numbers Identify North -South Zones Twenty Optional Zones Run East -West Letters Identify East -West Zones UTM divides the Earth into zones. There are sixty primary zones running North-South. These zones are identified by numbers. There are also twenty optional zones, running East-West, that are identified by letters. 34

35 Map Coordinate Systems Universal Transverse Mercator (UTM) Uses Two Units to Provide Location Easting Distance in Meters to the East from the Start of the UTM Zone Line The Letter E Follows Easting Values Northing Distance in Meters from the Equator The Letter N Follows Northing Values 10T E N UTM uses two units to describe a location. The fist unit is called Easting. Easting is the distance, in meters, to the east of the start of the UTM zone line. The second unit is called Northing. Northing is the distance in meters from the equator. There is no such thing as a Southing. In the Southern Hemisphere, the distance south from the equator is also called a Northing. In the example shown, this location is about 6.7 million meters east from the start of zone 10T and 4.8 million meters north of the equator. 35

36 Map Coordinate Systems Universal Transverse Mercator (UTM) This diagram shows the various UTM zones 36

37 GPS Terminology Waypoints A Place on Earth Location can be Defined Using either Latitude / Longitude or UTM Also Referred to as Landmarks Can be given a Unique Name Entered into GPS Manually or via Computer There are a few terms to become familiar with when discussing GPS. The first is a waypoint. A waypoint is a place on Earth. It can be defined using Latitude/Longitude, UTM or any other map coordinate system. Waypoints are also referred to as Landmarks and can be given a unique name. Waypoints can be entered into a GPS manually, which can be both time consuming and difficult. Or, waypoints can be entered into a GPS via a connected computer. 37

38 GPS Terminology Routes A Series of Waypoints Entered into GPS Manually or via Computer A route is just a series of waypoints. For example, Waypoint 1 to Waypoint 2 to Waypoint 3 is a route. Like waypoints, routes can be entered into a GPS directly or via a computer. 38

39 GPS Terminology Tracks A Digital Breadcrumb A GPS receiver can keep a record of the route taken. This is called a track. The easiest way to describe a track is as a digital breadcrumb. In theory, if the GPS receiver has a track of the route, it should be impossible to get lost. If the track is available, then the GPS can help the user follow the track back to its starting location. 39

40 Digital Maps Basemap Also Called a Built -In Map Permanently Loaded into the Memory of the GPS Receiver Cannot be Modified Only Shows General Detail Highways Towns Major Water Bodies In the GPS world, there are three types of electronic maps to be considered with. The first is called a basemap. A basemap is built into the GPS receiver and cannot be modified. To save memory, many basemaps only show major highways, towns and major water bodies. Some GPS receivers contain basemaps and some do not. 40

41 Digital Maps Proprietary Maps Only Work with that Company s GPS Receiver 1:100,000 Scale Most Common Features may be Eliminated by the Manufacturer Maps can be Uploaded to GPS Receiver In order for the GPS receiver to display additional map information, many GPS manufacturers provide proprietary maps that can be uploaded, at a cost, to the GPS. These proprietary maps only work with that particular manufacturer s GPS units. For example, it is not possible to use Magellan s maps with a Garmin GPS. Most of the proprietary maps are based on the USGS 1:100,000 maps. These maps may not provide quite the detail needed for off-trail travel but do provide enough information to that one can determine a location by glancing at the GPS screen. 41

42 Digital Maps Third-Party Maps Digital Picture of Paper Map Map is Georeferenced 1:24,000 Scale is Available Maps Cannot be Uploaded Waypoints, Routes and Tracks can be Uploaded and Downloaded Third-Party mapping software is available. These maps are scanned versions of the USGS quads and are georeferenced in that every location can be expressed using coordinates. While the actual maps cannot be uploaded to the GPS receiver, waypoints, routes and tracks can be exchanged between the software and a GPS. 42

43 Digital Maps Third-Party Maps National Geographic Topo! Maptech Terrain Navigator Maptech Terrain Navigator Pro DeLorme TopoUSA Some of the more popular Third-Party mapping products are listed. 43

44 Uploading Data to the GPS Many Different Ways to Perform this Task One way is to use Third -Party Maps to create waypoints and routes After waypoint and route creation, print paper copy of map Upload waypoints and routes to GPS Receiver Each user needs to determine if they want to exchange data between a computer and the GPS. In addition, there are various types of data that can be exchanged. One way of adding waypoints and routes to a GPS is to first create the information using one of the available third-party mapping products. Once the waypoints and routes have been defined, the data can be uploaded to a GPS and is ready for use. 44

45 Using the GPS in the Field Start with a fresh Track Log Reset the Odometer, Speed and Timers (if desired) Activate the Route to Follow Use the GPS to follow the Route Many Times, Terrain will Dictate the Path Don t get Caught Up Trying to Follow the GPS Route Mark Waypoints of Interest There are many different ways to use a GPS in the field. The GPS can be used for navigation, or provide location information, or to collect information about the trip. It is up to each user to determine how they want to use the GPS. When using a GPS for hiking, it is helpful to start with a fresh track log at the beginning of the trip. This way, the track log will only contain information from the particular trip. In addition, it may be helpful to reset other GPS functions (odometer, speed, timers, etc). If a route will be used for navigation, don t forget to activate the route on the GPS. However, if following a route on the GPS, let the terrain dictate the path, not the GPS. If desired, use the GPS to mark waypoints that might be of interest later on. 45

46 Using the GPS in the Field Save and Clear Track Logs as Necessary Use the GPS to Determine Elevation and Distance to Destination Deactivate and Activate Routes as Necessary Track Logs collect a finite amount of information. So, it is important to save and clear track logs as necessary. If multiple routes are used, don t forget to activate new routes as necessary. 46

47 Downloading Data from the GPS Many Different Ways to Perform this Task One way is to use Third -Party Maps to Download Waypoints and Track Logs Turn Track Logs into Routes Change Waypoints into Meaningful Symbols Turn GPS Data into a Map Actual Trip Including Actual Route Taken, Waypoints of Interest (Campsites, Water, Etc), Distance, and Elevations Can Print Map for Future Reference At the end of the trip, a GPS may contain information that can be used for future reference. For example, it is possible to produce a map showing the exact path taken along with waypoints of interest that were identified. Again, how a user uses the information collected by the GPS is an individual preference. There is no right or wrong way. 47

48 Useful Websites GPS Overview and Primer tes/gps/gps_f.html gps/index.html GPS Tutorials d=18 d=279 48

49 Useful Websites GPS Buying Guide learn/camp/gpsf.jsp Third-Party Mapping Products

50 Useful Websites GPS Manufacturers

51 Useful Websites Principles and Practice of GPS Surveying survey/principles_gps.htm Exchange Waypoints, Routes and Tracks Between Dissimilar GPS Receivers 51

52 Useful Websites Manufacturer Specific Instructions GPSGuideforBeginners_Manual.pdf UsingaGarminGPSwithPaperLandMaps _ Manual.pdf 52

53 Useful Books GPS for Dummies by Joel McNamara GPS Made Easy: Using Global Positioning Systems in the Outdoors by Lawrence Letham Outdoor Guide to Using Your GPS by Steve Featherstone 53

54 Useful Books Wilderness Navigation: Finding Your Way Using Map, Compass, Altimeter & GPS (Mountaineers Outdoor Basics) by Bob Burns and Mike Burns 54