Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)...

Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

Global Positioning System (GPS)

WMO/FAO Training Workshop on GIS and Remote Sensing Applications in Agricultural

Meteorology for the SADC

Thelma A. CincoThelma A. CincoSenior Weather Specialist ,PAGASA Senior Weather Specialist ,PAGASA

Resource Person Resource PersonPhilippinesPhilippines


Outline

• Introduction to Positioning

• Advantages of Space based system

• NavSTAR GPS

• GPS Segments:Space,Control & User Segments

• GPS receivers


Introduction to Positioning Determination of position is an essential

component in many disciplines. The position is particularly important to,• Navigators• GIS users• Military Forces• Surveyors• Explorers / Hikers


Introduction to Positioning As Earth is a three dimensional object, three

parameters are necessary to determine a location unambiguously on the earth. These three parameters can take different forms such as • North Co-ordinate, East Co-ordinate, Height• Geographical Latitude, Longitude, Height• Geodetic Latitude, Longitude, Height Surveyors• Geocentric X, Y and Z


In other words the positioning meansWhere am I on the Earth ?


Advantages of Space Based Systems

• Conventionally determination of position is done by astronomical observations.

• Astronomical observation procedures are tedious time consuming and subjected to interfered by weather atmospheric conditions

• Substantial amount of computations are also involved in the astronomical based positioning


The NavSTAR Global Positioning System (GPS)

• It is a brain child of United States Department of Defense and Joint Program Office (JPO).

• A reliable real time positioning system was very much needed for DoD for it’s military operations. The final product was the GPS system, originally designed only for the use of DoD.


The NavSTAR Global Positioning System (GPS)

• Navigation System with Time And Ranging

• The system is satellite based navigation system. Although it was designed to be a pure military tool at the beginning, the success achieved by the system forced it to allow for the civilian usage.

• With it’s remarkable capabilities it has been used for wide spectrum of applications


GPS SegmentsThe system can be divided in to three major segments.

1.Space Segment

2.Control Segment

3.User Segment


Space segment• The Space segment consist of

24 earth orbiting satellites in six different orbital planes.

• Each orbital plane is 55° inclined to equatorial plane of the Earth. All satellites are at the altitude of 20200km. above the Earth. This makes the period of the satellite exactly 12 sidereal hours. ( about 11hrs. 56 mins.)


Space Segments

The satellites were classified in to Blocks.

Satellites launched from Vandenberg Air Force Base from 1978 to 1985 are known as Block I satellites.• Weight 845 Kg• Powered by Ni-cd batteries with 7.25 sq.m.• Solar panels for recharging batteries.


Space Segments

Block II- represents production satellite The first block II satellite was launched from

Cape Canaveral on 14th February,1989.• Weight is about 1500kg.• Expected life time is 5 to 7 years• Can operate up to14 days independent

operation.

Block IIR – are replenishment/spare satellites


GPS Signal StructureSatellites Atomic Clock gives fundamental frequency 10.23 Mhz.

Two carrier frequencies L1 and L2L1= 10.23x154=1575.42 MHz (λ=19 cm.) L2= 10.23x124=1227.60 MHz (λ=24 cm.)

Two Code frequencies. Precise Code (P-Code) & Coarse Acquisition Code (C/A Code). The codes were encrypted on to the carrier waves by phase modulation.

L1 with P-Code & C/A Code

L2 with P-Code


Signal Structure

P-Code

L2 Carrier Data Message L1 Carrier

P-Code C/A Code


GPS signal structure• C/A Code frequency 1.023 million bits per second

(λ=300m) repeated in every Millie second. Each satellite broad casts unique C/A code on its L1 frequency.

• P Code frequency 10.23 M/S (λ=30 m) repeated in every 37 weeks. There fore the chirping rate of P code is ten times faster than C/A code.

• This also defines the satellite uniquely. Each satellite was given one week portion of P-Code and this week number is known as the satellites identity. (Space Vehicle Number or SV number)


Types of GPS ObservationThere are two main types of methods to determine

position in GPS. – Pseudo Range observations– Carrier beat phase

• Pseudo Range Observations:Pseudo range observations provide instantaneous positions and relatively low accuracy when compare with carrier phase measurements. Pseudo range is based on time measurements. The time difference dτ is the time taken for the electro magnetic wave to travel from the satellite to the receiver.


Control segment

• Control segment is the vital link in the GPS technology. The main functions of the control segment are:• Monitoring and controlling the satellite system

continuously• Determine GPS system time• Predict the satellite ephemeris and the behavior

of each satellite clock.• Update periodically the navigation message for

each particular satellite.


Control segment

• The master Control Station (MCS) is located at Consolidated Space Operation Center at Falcon Air Force Base in Colorado.

• There are five locations of controlling stations in the globe: – Ascension Island (MS & GA)– Diego Garcia (MS & GA) – Kwajalein (MS & GA) – Colorado Springs (MS) – Hawaii(MS)


How the GPS maintains its accuracy

Ephemeris

Ephemeris data is the corrected and exact satellite positional data transmitted by the master ground control station to the satellite. This data corrects a satellite’s own orbital data if in case it gets a little out of orbit.


User SegmentThe user community whom uses GPS as a tool for

wide spectrum of applications are known as user segment. – Navigators– Surveyors– GIS developers– Hikers

Appropriate GPS receivers are required to receive signal from GPS satellites for the purpose of navigation or positioning.


Basic concept of GPS Receiver and its components

The main components of a GPS receiver are • Antenna with pre-amplifier• RF section with signal identification and signal

processing• Micro-processor for receiver control, data

sampling and data processing• Precision oscillator• Power supply• User interface, command and display panel• Memory, data storage


Major components of GPS Receiver


GPS receivers can be classified on the purpose as :

– Military receiver

– Civilian receiver

– Navigation receiver

– Timing receiver

– Geodetic receiver


Examples of GPS receiversGPS receiver market is developing and expanding at a

very high speed. Receivers are becoming powerful, cheap and smaller in size.

Classical Receivers– T1 4100 GPS Navigator was manufactured by Texas

Instrument in 1984.

– Macrometer II, a dual frequency version was introduced in 1985.

Geodetic Receivers– Magnavox

– TRIMBLE NAVIGATION


International Limitation to System accuracy

• Standard Positioning Service (SPS)- C/A code is designed for most civilian applications. Which is low in terms of accuracy. ( ± 100 m.)

• Precise Positioning Service (PPS)- P code is designed to get higher degree of accuracy in positioning and is restricted to users authorized by the DoD

• Selective Availability (SA)-Selective availability is the intentional degradation of codes.


• In general, an SPS receiver can provide position information with an error of less than 25 meter and velocity information with an error less than 5 meters per second. Up to 2 May 2000 U.S Government has activated Selective Availability (SA) to maintain optimum military effectiveness.

• For many applications, 100-meter accuracy is more than acceptable. For applications that require much greater accuracy, the effects of SA and environmentally produced errors can be overcome by using a technique called Differential GPS (DGPS), which increases overall accuracy.


The Almanac Data

How the GPS receivers maintains its accuracy

An almanac data contains the approximate location of the satellites in space at any given time. This data is continuously transmitted to the GPS and is stored in its memory so that it will know where to look for the satellites. The almanac data is regularly updated on locations of the orbiting satellites.


Atomic clocksSatellites are equipped with very precise atomic clocks. Perfect timing is essential for it to compute for an accurate location and distance.

Consider the formula (velocity x travel time = distance), where velocity = 186,000mi/sec of the speed of light.

A little time discrepancy in the clocks of two satellites is translated into a wider distance.



More satellite locks produce less positional error


For a GPS receiver to output a position in two-dimensional mode it should be able to lock on three satellites with healthy signals and four or more for a three-dimensional mode. Two-dimensional mode will only display your latitude and longitude while a three-dimensional mode will also display your altitude or height.


Selective Availability (SA)

Sources of errors

Selective availability refers to the intentional degradation made by the US Department of Defense to the accuracy of GPS receivers. This was done to prevent their adversaries from receiving accurate GPS readings. The US DoD turned off selective availability on May 2, 2000. This made it possible for ordinary GPS receivers to get an accuracy of 6-12 meters.


Ionosphere and troposphere delay

Sources of errors

Satellite signal slows down when it passes through the atmosphere. The GPS uses a model to correct for the error introduced by the delay. But it is possible that the model may not work accurately on some atmospheric conditions.


Signal multi-path

Sources of errors

This condition occurs when GPS signals are reflected off large objects such as tall buildings, rock or large surfaces before it reaches the receiver.


Sources of errors


Sources of errors

Receiver clock errors

Because GPS receivers do not use atomic clocks, errors can sometimes happen in the receiver’s clock.

Orbital errors

This refers to the inaccuracies on the satellite’s reported location.


Sources of errors

Number of visible satellitesThe number of visible satellites is highly affected by the surrounding conditions. Thick forest foliage, tall buildings, tall mountains etc. will make it difficult for the receiver to receive signals from satellites.


Sources of errors

Satellite geometry/shadingThis refers to the relative position of satellites at any given time. Ideal satellite geometry exists when your satellites are located at wide angles relative to each other. Poor geometry results when the satellites form a line or are grouped together.


Correcting errorsDifferential GPS (DGPS)

In a differential GPS environment another GPS receiver is added to the system and will serve as the reference station. The location and the elevation of the reference station is always known. Thus, it can compute for the error of the satellite signals and can transmit differential corrections to DGPS receivers. Differential corrections are transmitted over FM radio frequencies by satellites or by the beacon transmitters of the US Coast Guard. With a DGPS, an accuracy of 1-5m is guaranteed even if “selective availability” is turned on.


Differential GPS (DGPS)


Civilian uses of GPS

A. Transportation

1. Airplane, ships, rockets, etc .



B. Mapping

1. Land use mapping2. Tax mapping3. Cadastral mapping 4. Boundary surveying, etc.


Civilian uses of GPSC. Sciences

1. Mapping of sampling sites2. Population dynamics and ecology3. Tracking animal movements 4. Modeling, etc.



D. Surveying

1. Land surveying2. Forest surveying, etc.



E. Public Services

1. Taxis and delivery services2. Mobile phone services3. Tracing carnapped vehicles4. Inventory and mapping of Philippine hospitals

and health centers, etc.



F. Engineering

1. Surveying2. Road construction3. Building construction, etc.



G. Exploration

1. Oil and gas exploration2. Mineral exploration, etc.

H. Outdoor hobbies

1. Mountain trekking, biking, etc.2. Road navigation


GARMIN 12 CHANNEL GPS

TRIMBLE GPS AS A ROVER

VARIOUS MAKES OF GPS

MAGELLAN GPS - 315 SERIES


References

GPS Guide for Beginners Fundamentals of GPS by: P.L. RajuGarmin GPS Website (www. garmin.com)Trimble GPS Website (www.trimble.com)

Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)...

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