GPS: CONCEPT, TECHNOLOGY AND USE

IN SOIL SURVEY

Dr. M. Altaf Hossain

PSO

Survey Section

WHAT IS GPS? The Global Positioning System (GPS) is a

space-based satellite navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.

A global navigation satellite system consisting of positioning satellites and their associated ground stations.

The system provides critical capabilities to military, civil and commercial users around the world. It is maintained by the US government and is freely accessible to anyone with a GPS receiver.

DEVELOPMENT OF GPS The GPS project was developed in 1973 to overcome the limitations of previous

navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s.

GPS was created and realized by the U.S. Department of Defense (DoD) and was originally run with 24 satellites. It became fully operational in 1995. Bradford Parkinson, Roger L. Easton, and Ivan A. Getting are credited with inventing it.

Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS III satellites and Next Generation Operational Control System (OCX).

Announcements from Vice President Al Gore and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the modernization effort, GPS III.

In addition to GPS, other systems are in use or under development. The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s.

There are also the planned European Union Galileo positioning system, Chinese Compass navigation system, and Indian Regional Navigational Satellite System.

SUMMARY OF SATELLITES

BlockLaunchPeriod

Satellite launches

Currently in orbitand healthySuccess Failure In preparation Planned

I 1978–1985 10 1 0 0 0

II 1989–1990 9 0 0 0 0

IIA 1990–1997 19 0 0 0 9

IIR 1997–2004 12 1 0 0 12

IIR-M 2005–2009 8 0 0 0 7

IIF From 2010 4 0 10 0 4

IIIA From 2014 0 0 0 12 0

IIIB — 0 0 0 8 0

IIIC — 0 0 0 16 0

Total 61 2 10 36 31

(Last update: November 30, 2013)

BASICS OF GPS?

Satellites are placed in Medium Earth Orbit (MEO) at an altitude of 12,552 miles

Orbital periods of MEO satellites range from 2 - 12 hrs.

Orbital period of GPS satellites is 12 hours (2 rotations/day)

GPS Satellites travel at a speed of 7,000 mph

Orbits are arranged so that at any time, anywhere on Earth, at least four satellites are visible in the sky

ORBITING SATELLITE

A visual example of a 24 satellite GPS constellation in motion with the Earth rotating. About nine satellites are visible from any point on the ground at any one time, ensuring considerable redundancy over the minimum four satellites needed for a position.

BASIC CONCEPT OF GPSA GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include- the time the message was transmitted satellite position at time of message

transmission Differential time of arrival and

triangulation are the methods used to determine location in a GPS system.

Differential Time of Arrival: Differential time of arrival is the method used to determine how far each satellite is from a GPS device. Although each satellite transmits its position and the time it was at that position, it takes time for that signal to reach the Earth.

The receiver contains a very accurate clock, which can determine the difference in time between the current time and when the satellite sent the signal.  With this differential time and the speed of radio waves, the distance from each of the three satellites can be determined using the simple formula:

  Rate x Time = Distance

Trilateration Trilateration is a method that is used to determine

position on Earth in three dimensions. GPS deals with three-dimensions rather than two. Since the distance from the Earth to a satellite results in a sphere rather than a flat circle, the calculation is a bit complex. 

Using trilateration, rather than draw circles to determine position we need to draw spheres.

For example, if the first acquired satellite is 25,000 miles from position one cannot simply draw a circle around that satellite and determine a position 25,000 miles from it. A sphere must be plotted, extending toward Earth and away from Earth.

A second satellite is calculated to be 25,001 miles from position, resulting in another sphere. The two spheres intersect, creating a perfect circle. A circular plane now exists, extending down through the earth and out into space.  A large number of potential positions have now been eliminated, but there is not yet an exact location. Many potential positions still exist and a third satellite is needed to define a sphere that intersects with the two current spheres resulting in two points that define possible position. One point is in space and one is on earth. Since the world is roughly a sphere, the point in space can be eliminated and the approximate position of the GPS receiver is located on Earth.

A fourth satellite is necessary to account for altitude and provide an exact fix of the location. The plotting of a fourth sphere provides the exact location and altitude of the receiver at the time the four measurements were taken.

The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite using the speed of light.

Each of these distances and satellites' locations defines a sphere.

The receiver is on the surface of each of these spheres when the distances and the satellites' locations are correct.

These distances and satellites' locations are used to compute the location of the receiver using the navigation equations.

This location is then displayed, perhaps with a moving map display or latitude and longitude; elevation or altitude information may be included, based on height above the geoid (e.g. EGM96).

In typical GPS operation, four or more satellites must be visible to obtain an accurate result.

HOW IT WORKS?

Global Positioning System Operation - GPS Diagram

STRUCTURE OF GPSThe current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.

THREE SEGMENTS OF GPS The space segment is composed of 24 to 32

satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit.

The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations.

The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service.

CONTROL SEGMENT: Ground monitor station used from 1984 to 2007

SPACE SEGMENT: Satellite

USER SEGMENT: GPS

Figure: Structure of GPS

COMPOSITION OF RECEIVERS

GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator).

They may also include a display for providing location and speed information to the user.

A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously.

Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels.

APPLICATION OF GPS While originally a military project, GPS is

considered a dual-use technology, meaning it has significant military and civilian applications.

GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking, and surveillance.

GPS's accurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids by allowing well synchronized hand-off switching.

CIVILIAN APPLICATIONS Astronomy: both positional and clock synchronization data is used in Astrometry and

Celestial mechanics calculations. It is also used in amateur astronomy using small telescopes to professionals observatories, for example, while finding extrasolar planets.

Automated vehicle: applying location and routes for cars and trucks to function without a human driver.

Cartography: both civilian and military cartographers use GPS extensively.

Cellular telephony: clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. The U.S. Federal Communications Commission (FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.

Clock synchronization: the accuracy of GPS time signals (±10 ns) is second only to the atomic clocks upon which they are based.

CONTD

Disaster relief/emergency services: depend upon GPS for location and timing capabilities.

Meteorology-Upper Airs: measure and calculate the atmospheric pressure, wind speed and direction up to 27 km from the earth's surface

Fleet Tracking: the use of GPS technology to identify, locate and maintain contact reports with one or more fleet vehicles in real-time.

Geofencing: vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate a vehicle, person, or pet. These devices are attached to the vehicle, person, or the pet collar. The application provides continuous tracking and mobile or Internet updates should the target leave a designated area.[72]

Geotagging: applying location coordinates to digital objects such as photographs (in exif data) and other documents for purposes such as creating map overlays with devices like Nikon GP-1

CONTD GPS Aircraft Tracking GPS for Mining: the use of RTK GPS has significantly improved

several mining operations such as drilling, shoveling, vehicle tracking, and surveying. RTK GPS provides centimeter-level positioning accuracy.

GPS tours: location determines what content to display; for instance, information about an approaching point of interest.

Navigation: navigators value digitally precise velocity and orientation measurements.

Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to compute phasors.

Recreation: for example, geocaching, geodashing, GPS drawing and waymarking.

Robotics: self-navigating, autonomous robots using a GPS sensors, which calculate latitude, longitude, time, speed, and heading.

Surveying: surveyors use absolute locations to make maps and determine property boundaries.

Tectonics: GPS enables direct fault motion measurement in earthquakes.

Telematics: GPS technology integrated with computers and mobile communications technology in automotive navigation systems

USING A GPS RECEIVER FIND YOUR LOCATION

Proceed to an outdoor location with a clear view of the sky from horizon to horizon. You should stand well away from the building, trees, etc., so that you have an unobstructed view of the sky.

Hold the receiver at arm's length from your body so the built-in antenna (the flat area above the display) is parallel to the ground. Power-on the GPS receiver by pressing the red key. After the Welcome Page, by default the receiver displays the Satellite Status Page (sky view) and begins searching for satellite signals. GPS receivers get their information from a system of 24 orbiting satellites located approximately 18,300 kilometers (11,000 miles) above the Earth's surface. To provide accurate position information, the receiver must be able to "see" three or four satellites.

As satellites are acquired, you will see bars appear on the graph at the bottom of the display; these bars indicate the strength of the satellite signal. Once enough satellites have been acquired, the Satellite Status Page will disappear automatically and be replaced with the Position Page (graphic compass).

FINDING THE MYSTERY LOCATION Your GPS receiver has been pre-programmed

(by your instructor) with a mystery location. Now let's explore how the GPS receiver can be used to navigate to an unknown location.

Randomly choose three-to-five different

locations on the grounds. These locations should be fairly distant from each other (at least 500 feet apart). Remember to choose locations where the GPS receiver will have a good view of the sky.

Proceed to Point No. 1. Record the following information in the data table below:

Use the Position Page (graphic compass) to acquire your current position. Record your latitude and longitude.

Press the GOTO key. The Navigation Page (graphic highway) will appear with the waypoint field highlighted. Press the up or down arrow keys to scroll through the available waypoints until "MYSLOC" (short for "mystery location") is displayed.

Press the ENTER key to confirm that you want to navigate to "MYSLOC". Record the bearing (in degrees) and distance (in kilometers) to the mystery location.

Briefly describe the location.

Repeat Steps 1-3 until you have visited at least three different locations on the grounds. Do not actually go to the mystery location!

Field Data

PointNo.

Latitude(deg. N)

Longitude(deg. W)

Bearing(deg.)

Distance(km)

BriefDescriptio

n

1

2

3

4

5

STEPS TO FIND MYSTERY LOCATION Using your Field Data for Point No. 1

(latitude, longitude, and distance), draw Circle 1. Technique Hint: Use latitude and longitude to locate Point 1 on the map; use the map scale to measure the radius of Circle 1; draw the circle.

Using your Field Data for Point No. 2, draw Circle 2.

Using your Field Data for Point No. 3, draw Circle 3.

You would discover that there is one and only one point where all three circles intersect.

Yield Monitoring Systems Yield monitoring systems typically utilize a mass flow

sensor for continuous measuring of the harvested weight of the crop. The sensor is normally located at the top of the clean grain elevator. As the grain is conveyed into the grain tank, it strikes the sensor and the amount of force applied to the sensor represents the recorded yield. While this is happening, the grain is being tested for moisture to adjust the yield value accordingly.

At the same time, a sensor is detecting header position to determine whether yield data should be recorded. Header width is normally entered manually into the monitor and a GPS, radar, or a wheel rotation sensor is used to determine travel speed. The data is displayed on a monitor located in the combine cab and stored on a computer card for transfer to an office computer for analysis.

Yield monitors require regular calibration to account for varying conditions, crops, and test weights. Yield monitoring systems cost approximately $3,000 to $4,000, excluding the cost of the GPS unit.

HOW IS GPS USED IN FARMING?

FIELD MAPPING WITH GPS AND GIS GPS technology is used to locate and map

regions of fields, such as high weed, disease, and pest infestations. Rocks, potholes, power lines, tree rows, broken drain tile, poorly drained regions, and other landmarks can also be recorded for future reference.

GPS is used to locate and map soil-sampling locations, allowing growers to develop contour maps showing fertility variations throughout fields.

The various datasets are added as map layers in geographic information system (GIS) computer programs. GIS programs are used to analyze and correlate information between GIS layers.

GPS technology is used to vary crop inputs throughout a field based on GIS maps or real-time sensing of crop conditions. Variable rate technology requires a GPS receiver, a computer controller, and a regulated drive mechanism mounted on the applicator. Crop input equipment, such as planters or chemical applicators, can be equipped to vary one or several products simultaneously.

Variable rate technology (VRT) is used to vary fertilizer, seed, herbicide, fungicide, and insecticide rates and for adjusting irrigation applications. The cost of all of the components necessary for variable rate application of several products is approximately $15,000, not including the cost of the GPS receiver. Technology capable of varying just one product costs approximately $4,000.

PRECISION CROP INPUT APPLICATIONS

DRAWBACK OF GPS

The drawback to current GPS units is that they cannot track positions inside of buildings or other places that shield signals coming from satellites.

Thanks for patience hearing