Definition Problem History Benefits What is NAVSTAR What

Definition

Problem

History

Benefits

What is NAVSTAR?

What makes a GPS? • The constellation of satellites which transmits positional information using the speed of signal to determine relative positions. Each Satellite has very precise atomic clocks to make these measurements. • Five control stations around the world: - Colorado (master station), Ascension, Kwajalein, Diego Garcia and Hawaii which calibrate the satellite’s clocks. • Passive receiving handunits - which have low cost quartz clocks and receive and intrepret signals. Often have software to help collect, manage and interpret locational information.

How does a GPS work?

Introducing the second Satellite

Three satellites are manadatory We really need Four Satellites to determine which of the two positions!

How doe these satellites provide positional information? • Each satellite broadcasts its orbital position in “pseudo code” • The receiver on the ground calculates the time the signal (pseudo code) took to get from the satellite to ground and turns these time units into distance based on the speed the light travels at (“pseudorange”) • Using information from 3 to 4 satellites allows triangulation to the GPS receivers position.

What is the Accuracy of a GPS? • A typical receiver that you purchased and used prior to the selective availability being switched off was accurate from 50 to 100 m. • The same receiver with select availability switched off is now accurate to 10 -20 m. • Using differential correction sub-meter accuracy is possible.

What influences the Accuracy of a GPS? • Paradoxically errors in the clocks produce an error of no more than 1 m • The position and number of satellites that are viewable will increase accuracy – PDOP values and make big differences • The atmospheric conditions also significantly impact on your accuracy of up to 4 meters • Selective availability imposes an error of approximately 50 m

What is Selective Availability? • the US Department of Defence who operate the satellites deliberately generated a random error of about 50 metres. • This error was referred to as selective availablity and was applied to all civilian and non-USA defence force activities but was switched off in May 2000. • The primary purpose of selective availability was to ensure than any USA hostile agents could not use their technology to delivery missiles etc with any sort of real precision. It would probably be switched on again if the USA were to become involved in a military war.

How can you determine the accuracy of your GPS? • Most GPS receivers will provide a PDOP reading • PDOP stands for Positional Dilution of Precision and is assessed on a scale of 1 to infinity • Your usual values for PDOP should be between 2 and 8 m as the range of best to worst that you can expect. • The PDOP is a scale that measures the geometry of all of the observed satellites. The more satellite that are viewable and the higher their position above the horizon the lower the PDOP value.

Are there differences between GPS receivers? • Eight channels receivers were the normal, but are increasingly being replaced by more accurate 12 channel units. • While the basic GPS provides positional accuracy in the order of 50 -100 metres this may not be accurate enough. • More expensive receivers should allow various methods of differential correction to overcome selective availability. • The most expensive receivers also provide realtime differential calculations and you will get to submeter accuracy in the field and can transfer the data as GIS coverages.

What is Differential Correction? • Differential GPS works on the principle of having at least two receivers, each obtaining information from at least four respective satellites. • These receivers are used such that one is stationary and the second moves (rover) around collecting information. • The stationary one being in a fixed position will know the precise errors occurring at that particular time in the signal and these can be applied to the rover unit.

How does a Differential GPS work? • A typical you would setup a stationary GPS referred to as the base station at a known location such as survey beacon. • The deviations from the known position are then transmitted to the roving GPS and can “correct” the signal. • This is referred to as real time differential GPS mapping and no further corrections need to be applied. • More recently companies have launched satellites of there own which remain in a fixed position and can be used as the base station and will transmit the signals to the roving GPS to correct their positional accuracies in real time.

GPS receivers are programmable. • Modern mapping GPS are basically small hand held computers with some form of information storage either as a mini-hard disk or more commonly as Read Only Memory ROM units. • By pre-programming your GPS you can select one of several features, e. g. a coastal line, a river edge, pathway or road, or locational spots such as a survey beacon, a marine buoy etc and these can be saved as separate features to be stored within your GIS. • A second advantage of the modern GPS is that you prestore positions and you can use to navigate to.

What can you do if you do not have a differential GPS signal? • To obtain a real-time differential GPS signals either requires two receives or a secondary signal based on a subscription service. • It is, however, possible to obtain differential locational information in South Africa with which to correct nondifferentially corrected GPS data. • TELKOM loads down GPS information at various stations. • Using this information on their website that matches where you collected your data and the time frame when you collected your data you can use the two sets of data to post-process you data into differentially correct readings.

References All About GPS http: //www. trimble. com/gps/howgps/gpsfram 1. htm A GPS Tutorial – Basics of high precision Global Positioning Systems Javad Positioning Systems. Copyright for pictures used The three drawings were taken from the www. trimble. com website: No portion of this service may be reproduced in any form, or by any means, except as authorized below, without prior written permission from Trimble Navigation Limited.