1. Global Positioning System (GPS)
WMO/FAO Training Workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the SADC
Global Positioning System (GPS)
Thelma A. Cinco Senior Weather Specialist ,PAGASA Resource Person Philippines
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

2. Outline Introduction to Positioning Advantages of Space based system
NavSTAR GPS
GPS Segments:Space,Control & User Segments
GPS receivers
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

3. 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
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

4. 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
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

5. In other words the positioning means Where am I on the Earth ?
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

6. 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
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

7. 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

8. 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
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

9. GPS Segments The system can be divided in to three major segments.
Space Segment
Control Segment
User Segment
For better understanding of GPS, we normally consider three major segments
Space segment, Control segment and User segment. Space segment deals
with GPS satellites systems, Control segment describes ground based time and
orbit control prediction and in User segment various types of existing GPS
receiver and its application is dealt
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

10. 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.)
GPS Nominal Constellation
24 Satellites in 6 Orbital Planes
4 Satellites in each Plane
20,200 km Altitudes, 55 Degree Inclination
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

11. 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

12. 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
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

13. Two carrier frequencies L1 and L2
GPS Signal Structure
Satellites Atomic Clock gives fundamental frequency Mhz.
Two carrier frequencies L1 and L2
L1= 10.23x154= MHz (λ=19 cm.)
L2= 10.23x124= 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
Satellites transmit signals on two carrier frequencies,
the L1 and L2. Both signals carry status messages and pseudo-random codes for timing.
The L2 carrier is used for more precise military pseudo-random code.
There are two types of pseudo-random codes, the C/A (Coarse Acquisition) and the P (Precise) code.
The C/A modulates the L1 carrier and is the basis for civilian GPS use.
The P code modulates both the L1 and L2 carriers and is intended for military use.
Once it is encrypted it is called the Y code.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

14. Signal Structure L1 Carrier L2 Carrier Data Message P-Code C/A Code
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

15. GPS signal structure
C/A Code frequency 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 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)
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

16. Types of GPS Observation
There 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

17. 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

18. 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)
For continuous monitoring and controlling GPS satellites a master control
stations (MCS), several monitor stations (MS) and ground antennas (GA) are
located around the world .
The operational control segment (OCS) consists of MCS near Colorado springs (USA),
three Monitoring Station(MS) and Ground Antenna(GA) in Kwajaleian, Ascension and Diego Garcia
two more MS at Colorado Spring and Hawaii.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

19. 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

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

21. 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
Since, GPS is still in its development phase, many rapid advancements have completely eliminated bulky first generation user equipments and now miniature powerful models are frequently appearing in the market.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

22. Major components of GPS Receiver
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

23. GPS receivers can be classified on the purpose as :
Military receiver
Civilian receiver
Navigation receiver
Timing receiver
Geodetic receiver
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

24. Examples of GPS receivers
GPS receiver market is developing and expanding at a very high speed. Receivers are becoming powerful, cheap and smaller in size.
Classical Receivers
T GPS Navigator was manufactured by Texas Instrument in 1984.
Macrometer II, a dual frequency version was introduced in 1985.
Geodetic Receivers
Magnavox
TRIMBLE NAVIGATION
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

25. 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.
Since GPS is a military navigation system of US, a limited access to the
total system accuracy is made available to the civilian users. The service
available to the civilians is called Standard Positioning System (SPS) while the
service available to the authorized users is called the Precise Positioning Service
(PPS). Under current policy the accuracy available to SPS users is 100m, 2DRMS
and for PPS users it is 10 to 20 meters in 3D. Additional limitation
viz. Anti-Spoofing (AS), and Selective Availability (SA) was further imposed
for civilian users. Under AS, only authorized users will have the means to get
access to the P-code. By imposing SA condition, positional accuracy from
Block-II satellite was randomly offset for SPS users. Since May 1, 2000
according to declaration of US President, SA is switched off for all users.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

26. 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

27. How the GPS receivers maintains its accuracy
The Almanac Data
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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

28. How the GPS receivers maintains its accuracy
Atomic clocks
Satellites 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

29. How the GPS receivers maintains its accuracy
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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

30. Sources of errors Selective Availability (SA)
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, This made it possible for ordinary GPS receivers to get an accuracy of 6-12 meters.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

31. Sources of errors Ionosphere and troposphere delay
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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

32. Sources of errors Signal multi-path
This condition occurs when GPS signals are reflected off large objects such as tall buildings, rock or large surfaces before it reaches the receiver.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

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

34. Sources of errors Receiver clock errors Orbital 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

35. Sources of errors Number of visible satellites
The 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

36. Sources of errors Satellite geometry/shading
This 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.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

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

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

39. Civilian uses of GPS A. Transportation
1. Airplane, ships, rockets, etc .
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

40. Civilian uses of GPS B. Mapping Land use mapping Tax mapping
Cadastral mapping
Boundary surveying, etc.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

41. Civilian uses of GPS C. Sciences Mapping of sampling sites
Population dynamics and ecology
Tracking animal movements
Modeling, etc.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

42. Civilian uses of GPS D. Surveying Land surveying
Forest surveying, etc.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

43. Civilian uses of GPS E. Public Services Taxis and delivery services
Mobile phone services
Tracing carnapped vehicles
Inventory and mapping of Philippine hospitals and health centers, etc.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

44. Civilian uses of GPS F. Engineering Surveying Road construction
Building construction, etc.
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

45. Civilian uses of GPS G. Exploration Oil and gas exploration
Mineral exploration, etc.
H. Outdoor hobbies
Mountain trekking, biking, etc.
Road navigation
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

46. VARIOUS MAKES OF GPS TRIMBLE GPS AS A ROVER GARMIN 12 CHANNEL GPS
MAGELLAN GPS SERIES
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

47. References GPS Guide for Beginners Fundamentals of GPS by: P.L. Raju
Garmin GPS Website (www. garmin.com)
Trimble GPS Website (
Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)