SUBSIDENCE MONITORING USING

THE GLOBAL POSITIONING

SYSTEM(GPS)

Aabhas Singh Thakur Mohd ManeebMasood

M.Tech (Part-I)

IIT(BHU), Varanasi

INTRODUCTION

Official name of GPS is Navigational Satellite Timing And Ranging Global Positioning System (NAVSTAR GPS)

Global Positioning Systems (GPS) is a form of Global Navigation Satellite System (GNSS) First developed by the United States Department of Defense

Consists of two dozen GPS satellites in medium Earth orbit (The region of space between 2000km and 35,786 km)

INTRODUCTION-CONT.

GPS provides specially coded satellite signals that can be

processed with a GPS receiver, enabling the receiver to

compute position, velocity and time.

A minimum of four GPS satellite signals are required to

compute positions in three dimensions and the time

offset in the receiver clock.

Accuracy and precision of data increases with more

satellites.

THREE PARTS

Space segment

Control segment

User segment

SPACE SEGMENT

SPACE SEGMENT--INFORMATION

The GPS uses a constellation of 24 satellites that orbit the earth at about 11,000 nautical miles, once every 12 hours.

The orbital position is constantly monitored and updated by the ground stations.

Each satellite is identified by number and broadcasts a unique signal.

The signal travels at the speed of light.

Each satellite has a very accurate clock, 0.000000003 seconds

SPACE SEGMENT--SATELLITE SIGNALS

Because the GPS receiver calculates its location by trilateration, the task of the receiver is to determine its distance from multiple satellites.

The GPS system uses two types of signals to calculate distance. Code-phase ranging

Carrier-phase ranging

CODE-PHASING RANGING

Each satellite has a unique signal.

It continuously broadcasts its signal and also sends out a time stamp every time it starts.

The receiver has a copy of each satellite signal and determines the distance by recording the time between when the satellite says it starts its signal and when the signal reaches the receiver.

DISTANCE EXAMPLE : CODE PHASE RANGING

The signals from the GPS satellites travel at the speed of light-186,000 miles/second.

How far apart are the sender and the receiver if the signal travel time was 0.23 seconds?

Distance (ft)= Velocity (mi/sec) x Time (sec)

= 186,000 mi

sec x 5208

ft

mi 0.23 sec= 2,257,8400 ft

• It should be clear that this system requires very accurate measurement of time and synchronization of clocks.

• These time errors limit the precision of this system.

CARRIER-PHASE RANGING

Surveying quality receivers use the underlying carrier frequency.

Easy to determine number of cycles.

• The proportion of a partial cycle is difficult to determine.

• This is called phase ambiguity.

• Phase ambiguity error is resolved by comparing multiple signals from multiple receivers.

• More precise system.

RECEIVER SEGMENT

RECEIVER

The receiver collects, decodes and processes the satellite signals.

The basic receiver does not include a transmitter.

Different levels of precision are available.

The receiver determines its location by trilateration.

GROUND SEGMENT

GPS TRILATERATION

With signals from two satellites, the receiver can narrow down its location to just two points on the earths surface.

Were the two circles intersect.

GPS TRILATERATION--CONT.

Knowing its distance from three satellites, the receiver can determine its location because there is only two possible combinations and one of them is out in space.

In this example, the receiver is located at b.

The more satellite that are used, the greater the potential accuracy of the position location.

FACTORS INFLUENCING POSITION ACCURACYThe number of satellites (channels) the receiver can track. The number of channels a receiver has is part of it’s design.

The higher the number of channels-the greater the potential accuracy.

The higher the number of channels-the greater the cost.

The number of satellites that are available at the time. Because of the way the satellites orbit, the same number are not available at

all times.

When planning precise GPS measurements it is important to check for satellite availability for the location and time of measurement.

If a larger number of channels are required (6-10), and at the time of measurement the number available was less than that, the data will be less accurate.

The number of different systems that the receiver can track. WAAS [Wide Area Augmentation System] FAA & DOT

GLONASS [GLObal'naya NAvigatsionnaya Sputnikovaya Sistema] Russian

IRNSS

BEIDOU

FACTORS INFLUENCING POSITION ACCURACY--CONT.

The system errors that are occurring during the time the receiver is operating.

The GPS system has several errors that have the potential to reduce the accuracy.

To achieve high levels of precision, differential GPS must be used.

Differential GPS uses one unit at a known location and a rover.

The stationary unit compares its calculated GPS location with the actual location and computes the error.

The rover data is adjusted for the error.

Real Time Kinematic (RTK)

Post processing

GPS ERRORS

(a) those originating at the satellites:ephemeris, or orbital, errors, satellite clock errors, and the effect of selective availability.

(b) those originating at the receiver:receiver clock errors, multipath error, receiver noise, and antenna phase center variations.

(c) those that are due to signal propagation (atmospheric refraction)delays of the GPS signal as it passes through the ionospheric and tropospheric layers of the atmosphere

INTRODUCTION TO GPS IN SUBSIDENCE

•The Global Positioning System (GPS) can be used asan alternative surveying tool.

•Current GPS positioning techniques for monitoring applications typically include Precision and Real Time Kinematics(RTK) monitoring.

•Each of these techniques has associated trade-offs between system installation and maintenance costs, and the quality of the resulting coordinate time-series.

HOW DOES IT WORK?

•GPS positioning is based on measuring the transittime of radio signals emitted by orbiting satellites.

•The satellites send radio signals covering a certain area(view).

•For a receiver to compute its stand-alone position, it must be in view of at least four satellites.

GPS TECHNIQUES TO MEASURE SUBSIDENCE

•Precise Positioning GPS

•Kinematic GPS(RTK)

PRECISE POSITIONING GPS

•The technique requires specific locations to be observed repeatedly.

•The antennae must be placed in exactly the same location each time the point is measured(fixed mounting points).

•The place to which the antennae are fixed must move with the surface of the ground so that any change in position of the antenna reflects and can be interpreted as a change in surface height.

PRECISE POSITIONING GPS

KINEMATIC GPS(RTK)

•RTK technique requires at least two GPS receivers.

•One is fixed and operates at a known location while the “rover” receiver moves across the region of interest, profiling the ground in a series of transects.

•Depending on the size of the region to be monitored, the roving antenna can be hand-carried or mounted on a vehicle.

•The height of the roving antenna relative to the fixed system is estimated and, if the transects are measured repeatedly over a period of time, changes in surface height can be determined.

KINEMATIC GPS(RTK)

SUBSIDENCE MONITORING BY GPS, PERTH.

SPECIAL CONSIDERATIONS FOR MONITORING

•The selected targets must reflect the mean behaviour of the surrounding area.

•Tree canopy, buildings and other obstacles thatcould restrict the reception of the satellite signalshould be avoided.

•To mark the station points, it is advisable to use sturdy stations.

•It is convenient to include within the control network several fixed points outside the subsidence area.

•To strengthen the results, it will also be helpful to use different methods and repeat the observations.

MAIN STRENGTHS OF GPS

•The ability to detect mm-scale movements.

•GPS can easily cover larger areas than surveying.

•The results between consecutive surveys are coherent and precise.

•Can be assimilated into Inrerferometric Synthetic Aperture Radar (InSAR) analysis to improve the accuracy of the results.

•Can provide continuous temporal(of or relating to time) monitoring.

•After a due a training period, the instruments are easy to use.

MAIN LIMITATIONS OF GPS

•Requires significant fieldwork and human resources if repeat kinematic GPS surveys are to be conducted.

•Provides information at only discrete locations.

•Relatively expensive to install and operate a continuous network of GPS sites.