GPS

a

guide

for users

Contents Foreword

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Section 1 What is GPS?

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Section 2 GPS applications

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Section 3 Choosing appropriate GPS equipment

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Section 4 Introductory GPS concepts and definitions

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Section 5 GPS and accuracy

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Section 6 Quality standards

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Further information

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Glossary

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Foreword

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s a modern society we all take advantage of information technologies undreamt of a few decades ago. The Global Positioning System (GPS) is one such advanced technology. GPS receivers can be bought for a relatively small investment. When placed in the hands of a user, they can instantly answer one of humankind's most fundamental questions where on Earth am I? Whether we are bushwalking in remote locations, angling in a new fishing hole, or four wheel driving in rugged locations it is important to be able to accurately and quickly determine our location.

the use of advanced satellite navigation systems. It is therefore important for new GPS users to understand the basics of this technology and how best to use it. I commend this booklet to you and hope it helps you keep pace with the rapid changes in technology that surround us all.

John Landy, AC, MBE Governor of Victoria

Modern mapping systems are now geared towards

Foreword

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1 GPS (Global Positioning System) is a means of determining location and navigation. Just as the Internet revolutionised the way we collect and distribute information, GPS has transformed the way we determine locations on Earth. Explained simply, GPS uses a network of satellites (often called the GPS constellation) in conjunction with groundbased technology to determine a precise location anywhere on Earth. GPS is available to everyone, is simple to learn, and for most purposes it’s free. GPS is divided into three key segments:

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What is GPS The space segment is the constellation of 24 satellites that orbit Earth twice a day emitting radio signals. They act a bit like artificial stars because you can use these signals as a reference point for finding locations on Earth. The user segment consists of a handheld GPS receiver and the person operating it. The type of GPS receiver and the techniques you use determine the accuracy and nature of the position it computes. Most handheld GPS receivers have a reliable accuracy of about 10 to 20 metres depending on operating conditions. Advanced GPS receivers and techniques can

provide real-time accuracy in centimetres. The control segment monitors GPS satellites from ground stations on Earth, uploading corrections as each satellite drifts from its original orbit due to solar winds and other small variables.

How GPS calculates a position on Earth GPS is based on the principle of trilateration, which allows you to calculate a position on Earth by knowing its distance from three other known locations. It’s a simple mathematical equation: Velocity x Travel Time = Distance. A GPS receiver knows the velocity of a satellite

What is GPS?

signal is the speed of light (299,792.458 kilometres per second). The GPS receiver then calculates how long it took for the signal to reach the Earth (travel time). By multiplying the time with the velocity, the receiver can calculate the distance to that satellite. The crucial part of this equation relies on a GPS receiver obtaining accurate time measurements from precise atomic clocks on board the satellites. The satellites’ orbits are arranged so a GPS receiver can lock on to at least four satellites (although it would be possible to use only three) no matter where you are located on Earth. Using the information sent from four satellites, the GPS receiver determines its position in three dimensions which allows it to calculate its position on Earth.

What is GPS?

A brief history of satellite positioning Many countries use satellites to determine positions on Earth. The generic term for all of these satellite systems is Global Navigation Satellite Systems or GNSS. Although the American Global Positioning System is the most popular, the Russian Federation operates a similar one called GLONASS and the European Union is developing a third called Galileo. The United States of America (USA) started developing GPS for military purposes in the 1970s and first allowed public access to the technology in the 1980s. However, it wasn’t until 1994 that all 24 GPS satellites were in place to provide the accuracy available today.

GPS—not ‘big brother’ Many people believe GPS can be used to monitor the whereabouts of people or objects. This is incorrect. Satellites have no way of telling when a GPS receiver is locked on to it. A GPS receiver simply receives information from GPS satellites, similar to tuning and receiving a signal from a radio station. Tracking technology is an additional capability that can be used in conjunction with GPS. For example, the latest transport security systems use transmitters fitted to vehicles. Mapping software is then used to locate the vehicle’s position while direction and speed information is transmitted to a monitoring centre.

There are two levels of GPS services currently available – one for public use and another encrypted service for military use. The satellites used in the GPS program are known as Navstar (Navigation System by Timing And Ranging) space vehicles.

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2 GPS is rapidly being applied to a broad range of activities. GPS receivers are easy to use and the functions they offer are improving all the time. They are also becoming less expensive. Greater memory capacity and the ability to connect to the Internet and personal computers with mapping software are notably increasing business applications of GPS. The spatial information GPS technology can now provide is also improving the safety and efficiency of vital community services like firefighting and search and rescue. Around 85 per cent of all information used by government has a spatial

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GPS applications component. The Victorian Government uses spatial information for environmental and natural resource management, risk and asset management, land administration and land use planning, infrastructure planning and provision, and service planning and delivery.

driving, fishing and vehicular navigation. A new global adventure sport, geocaching, has even developed using GPS coordinates listed on the Internet at www. geocaching.com.

Industry uses GPS for agriculture, public safety, telecommunications and asset management. The scientific community applies it to archaeology, oceanography, weather research, geology, wildlife conservation and environmental research. Recreational and sporting users employ GPS for activities like bushwalking, four wheel

GPS applications

Precision farming One industry that surprises many people with its advanced use of GPS is farming. New farm machinery fitted with high accuracy equipment allows ‘hands free’ driving. Farmers can now precisely navigate around a paddock with ‘centimetre accuracy’ when ploughing, seeding and harvesting, which reduces overlaps and gaps. Farmers can also determine exactly what parts of a paddock need more fertiliser, apply the right amount, and measure harvests from those areas. Technology-savvy farmers can significantly save on seed, fertiliser, pesticide and fuel costs. Improvements in crop yields and profitability have also been reported. Precision farming also helps the environment because farmers don’t need to blanket-spread pesticide and fertilisers over entire paddocks.

GPS applications

Mapping Victoria’s forests Finding your way through Victoria’s forests and parks has become much easier thanks to GPS. The Department of Sustainability and Environment (DSE) is accurately recording every twist and turn and piece of infrastructure along the state’s many walking tracks. These maps are allowing better management of our forests and parks, are used by bushwalkers and are also vital for Victoria’s firefighting and prevention strategies.

Image above courtesy Victoria Police Search and Rescue Image below courtesy of Victorian Institute of Dryland Agriculture Walpeup

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3 When selecting a GPS receiver, your main consideration should be the activity you intend to use it for. This will help determine the complexity of receiver you need, its accuracy, size, weight and relevant accessories. For example, you may need a waterproof GPS receiver that fits in a pocket or one suited for mapping. There are hundreds of GPS receivers and accessories to choose from. Two main groups exist: Basic, low cost units for recreational use. Complex, more expensive units with greater accuracy, which also allow GPS receivers to be linked to personal computers and machinery for commercial use.

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Choosing appropriate GPS equipment It is advisable to request a demonstration from a retailer before purchasing a GPS receiver. It would also be preferable to test them in the field before buying. Don’t expect to rely on retailers for GPS advice and training as this is a specialised area. Talking to experienced GPS users will provide first-hand information about the best equipment for your activity. Recreational clubs (bushwalking, four wheel driving or boating) are usually happy to provide information and introductory training to new GPS users. For more advanced GPS activities and equipment, manufacturers often have specialised distributors who hire out GPS

receivers and provide training and ongoing support.

Key things to consider Accuracy The accuracy you need from your GPS receiver is a key factor in the choice you make. For example, if you are using GPS for recreational purposes, such as finding a good fishing spot or bushwalking, an inexpensive basic unit will probably be sufficient. Manufacturers and retailers of low cost recreational GPS receivers often generalise about the accuracy of their equipment – many state the ‘best case scenario’ accuracy level, which is rarely achievable in the field. Many GPS receivers

Choosing appropriate GPS equipment

are also marketed as ‘mapping’ receivers yet only have enough memory to store calculated positions. Accurate mapping and navigation needs satellite range data which uses a lot of receiver memory. Extra memory adds to the cost of the receiver but provides a better result. Maps You may think you will not need a GPS receiver with built-in maps for your activity, but once you begin using GPS, chances are you’ll want to explore it more. Consider purchasing a GPS receiver that displays GPS positions, together with maps and other spatial data such as imagery and layered information, or one that can be linked to other handheld devices that provide these functions. A simple GPS dot on a blank screen or a set of coordinates doesn’t provide a great deal of useful information for inexperienced users.

evolving all the time. For more information about where to get quality Victorian data for your GPS, visit www.land.vic.gov.au. Display Most GPS receivers display between four and six navigation screen pages, and often sub-menu pages, which show things like satellite visibility and status, current location, waypoints (or ‘in between’ points) and landmarks. It’s a good idea to have most of the information you need for your particular activity displayed on a single screen – it eliminates the need to continually switch menus. While this may seem unimportant now, it will quickly

It is possible to have specific mapping data geo-referenced for use on your GPS receiver. The technology for georeferencing data is

Choosing appropriate GPS equipment

become annoying when using your GPS receiver if you can’t see all the information you need at a glance. The quality of the LCDs (liquid crystal displays) on GPS receivers also varies. Test the LCD in direct sunlight before purchase to be sure you can read it. Some GPS receivers also have screens that rotate from vertical (for holding in the hand) to horizontal (for mounting on a car dash). Information Storage GPS receivers can usually save from 100 to 1000 locations. Most GPS receivers allow you to name locations for easy identification later (eg ‘lookout’). This allows you to easily navigate to a point of interest (eg ‘camping ground’) or in a sequence (eg ‘camping ground 1’, ‘camping ground 2’ etc).

Image left courtesy Victoria Police Search and Rescue

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Basic GPS receiver checklist Is the receiver easy to use? What sort of accuracy will you need? Will your receiver need to be waterproof? How small does your receiver need to be? How many waypoints will your GPS receiver store? What sort of battery life will you need? Can the receiver be run off a vehicle cigarette lighter? Will you need to use your GPS receiver hands free?

Accessories and Software GPS receivers can now be linked to, or are directly integrated with, a wide range of other equipment. This allows positional information to be applied to a wide variety of activities. The most popular and widely available accessories are:

Will the receiver sound a signal when you are moving closer or further from your target? Do you need a digital compass? Can your receiver be connected to other handheld equipment that increases its abilities? How easy is the LCD screen to read?

- Autopilots - Laser ranger finders - Digital compass - Personal Digital Assistants (PDAs) - GPS enabled mobile phones - Boating instrumentation - Chartplotting devices

If you are buying GPS equipment for business purposes, the Victorian Government has produced a more detailed GPS technical standards publication, The Global Positioning System Handbook, available online at www.land.vic.gov.au

- Echosounders

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Choosing appropriate GPS equipment

4 The popularity of GPS receivers has highlighted the need for users to understand the limitations of their receivers. There are common misconceptions about what your GPS can actually do. By understanding some key GPS concepts, you will minimise the risk of errors, in both GPS positioning and in the use of associated mapping products. Some of the basic GPS concepts you should understand are:

Datum & Coordinates Maps created using GPS technology are based on a reference frame or datum. A datum looks a little like a local street

Introductory GPS concepts & definitions directory except with GPS it applies to a country or the entire surface of the Earth. There are many datums used around the world. The most recognised global datum is WGS84 (the World Geodetic System), established in 1984, which is widely used for marine and aeronautical activities. However, most countries have their own datum, which is specifically designed to better fit the shape of the earth in their location, and is updated to account for continental drift and other variations. In Australia, most maps created by government agencies use GDA (the Geocentric Datum of Australia).

Introductory GPS concepts and definitions

Maps using GDA carry this logo

Most Australian maps developed before 2000 use AGD (the Australian Geodetic Datum). This datum was superseded by GDA. There is, roughly, a 200 metre difference between these two datums.

To avoid a possible 200 metre discrepancy, you must ensure that your GPS receiver and the maps you are using (whether in the receiver itself, or paper maps you refer to) have the same datum. For consistency, and to minimise the potential for errors, you should use GDA.

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Basic GPS receivers will default to WGS84. The positional differences calculated using WGS84 and GDA is minimal (in 2004 this was about 0.7 metres and increasing by 0.07 metres per year) and is not noticeable when using basic GPS receivers. However, you must use the GDA reference frame if planning to share business-critical information (like land surveys) with the Victorian Government and other organisations. More information on GDA and datums is available at www.icsm.gov.au

Positional Uncertainty and Local Uncertainty These are new terms adopted in Australia and are expressed in metres to describe a circle of uncertainty around a given point. They are used to quantify the reliability of GPS and other positions, which can be affected by satellite signals bouncing off buildings or vegetation (called multipath), varying atmospheric conditions and satellites moving slightly out of orbit.

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Positional Uncertainty is the uncertainty of GPS positions, relating to the horizontal and height components, in metres at the 95 per cent confidence level with respect to the datum you are using (eg GDA). Local Uncertainty is the average measure in metres at a 95 per cent confidence level with respect to the adjacent points in a datum (eg GDA). It is calculated between two points in question or from the point in question to adjacent points in the same area.

Dilution of Precision Dilution of Precision (DOP) is a measure of the quality of the geometry of the satellite constellation. A greater angle between the satellites lowers the DOP, and provides better positional accuracy than a higher DOP, which indicates poor satellite geometry.

Precision versus Accuracy Precision and accuracy are often assumed to mean the same thing but an important technical difference exists for GPS users. It is possible to have a GPS reading that is precise but inaccurate, or is accurate but imprecise. Precision refers to how closely several GPS readings taken from the same location match each other. If a reading is precise, it can be easily repeated. A precise reading will have the positions clustered close together (see figure on page 11). Accuracy relates to the correctness of the information collected using GPS. The more time you spend

Introductory GPS concepts and definitions

Maps and heights Precision vs Accuracy

Precision

High Precision

Low Precision Low Accuracy

High Accuracy

Accuracy

collecting data with your GPS receiver at one location, the more accurate the position measurements you take become. In the figure above, an accurate reading has the points clustered in or near the correct result centre target. Methods for obtaining higher accuracy are outlined in Section 5 GPS and accuracy.

People often wonder why the height determined by their GPS is different to the same height shown on a topographic map. The explanation is in the way GPS computes heights and the way maps are defined. A vertical datum is a point from which heights are measured. Australia’s vertical datum is the Australian Height Datum (AHD) which approximates mean sea level (or geoid) heights and is the basis of most published maps. GPS receivers generate (ellipsoid) heights using the WGS84 datum rather than mean sea level. These GPS heights need to be corrected to compute local AHD heights.

What does 95 per cent confidence mean? GPS accuracy levels are stated as having a 95 per cent confidence factor. This means, for example, that when using a basic GPS receiver, 95 per cent of the time it calculates your position within a 13 metre radius of your true position. The other 5 per cent of the time your GPS receiver could show your position outside a 13 metre radius from your true position. Experiments have shown these positions can be hundreds of metres from their true location.

The difference between the two datums is called the geoid-ellipsoid separation. In Australia the separation may range between -40 and +75 metres. Globally it can be in the range of -100 to +100 metres. AUSgeoid is a free height translation program available from the Geoscience Australia website www.ga.gov.au/nmd/geodesy /ausgeoid/.

Introductory GPS concepts and definitions

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5 GPS is a very efficient and effective means of locating positions on Earth. However, even under ideal conditions it is unlikely to be ‘100 per cent’ error free. This is because the position your GPS receiver calculates contains some degree of uncertainty (or bias). Sometimes the effect of this uncertainty can mean positions are tens or even hundreds of metres from your true position on Earth. These positional inaccuracies probably won’t bother recreational users. However, if you are using GPS for surveying, mapping or businessrelated activities or intend to share data with other organisations, you need to understand why these inaccuracies occur and how to minimise them. 12

GPS and accuracy Errors Errors exist in GPS positions for a variety of reasons. They can be categorised into two main groups: Environmentally induced errors, which include atmospheric errors and multipath. These errors are generally inconsistent and can occur at any time. Technical errors, including satellite errors and receiver noise. Environmental and technical errors can both be minimised using the right techniques and equipment. Remember, your GPS receiver knows where each satellite in the GPS network is at any given moment. It calculates its distance from these

satellites by measuring the time it takes for signals to travel down to it. Using this distance information, the receiver works out your position on Earth. Your GPS receiver assumes satellite signals are always travelling at the speed of light in a direct path when making these calculations. If the signal is slowed down or doesn’t travel in a completely straight path to your receiver, the time and ranging measurements can be thrown out. Timing calculations can also be affected by the atomic clocks in the GPS satellites when compared to the time kept by your GPS receiver. Errors can also occur when delays in

GPS and accuracy

processing signals occur inside your receiver. If you make a mistake in setting the configuration of your GPS receiver, the result may also be inaccurate. Accuracy can also be affected if a user neglects significant details or fails to gather enough information to adequately define a ground feature.

Don’t forget to collect enough information GPS receivers are often used to collect location information for mapping, however not collecting enough information can result in an inaccurate picture. For example, if the edge of a carpark pavement was curved but you only collected three points of the arc shape, you would not have an accurate shape in your map file. Curves require more positions to define the shape of the arc.

GPS and accuracy

Environmental errors Atmospheric errors are one of the main causes of GPS errors and occur because charged particles and moisture in the atmosphere can slow satellite signals down. You can minimise atmospheric errors in a standard GPS receiver by predicting what a typical delay might be on a typical day, but atmospheric conditions are rarely consistent. The best way to minimise atmospheric errors is using Differential GPS (DGPS) (explained later in this section). Multipath occurs when satellite signals bounce off objects such as cars, buildings, fences, vegetation, water or even thick smoke, as they travel to your GPS receiver. Multipath is difficult to detect and is sometimes hard to avoid. It can be corrected to some extent using sophisticated receivers, special antennas, careful planning and good field procedures (like averaging positions over time or repeating observations with a different set of GPS satellites).

Don’t expect inexpensive GPS receivers to effectively deal with multipath errors.

Technical errors Satellite orbit (or ‘ephemeris’) and timing errors occur when the satellites in the GPS constellation drift from their predicted orbits, causing errors in the distance calculations your GPS receiver makes. Ephemeris errors are usually small but must be taken into account if you’re seeking greater accuracy. Although satellite signals are synchronised to your receiver with highaccuracy atomic clocks, your GPS receiver maintains less accurate time. As a result your GPS needs to receive data from four or more satellites to eliminate this effect. Receiver noise refers to how well a GPS receiver measures and calculates information received from satellites. The level of receiver noise depends on things like the quality of antenna, electronics and signal processing. Sometimes a receiver will round off numbers or electromagnetic interference will distort

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the satellite signal. Lower cost receivers usually have a greater receiver noise than higher cost alternatives.

Low DOP 8 = less accurate position

While DOP is a good way to quickly assess the accuracy of your position, it is only one of many error assessments you need to consider, particularly when applying data to business purposes.

Differential GPS Differential GPS (DGPS) reduces errors and improves the reliability of positions a GPS receiver calculates. The DGPS method is based on the concept that atmospheric and satellite errors affecting the accuracy of your GPS receiver are very similar to errors calculated by other receivers located in the surrounding area. This is because the signals obtained by all of these receivers have travelled through virtually the same slice of atmosphere. DGPS uses a fixed receiver (called a base station or reference station) which has had its position on Earth accurately surveyed. Victoria’s Department of Sustainability and Environment operates a base station network called GPSnet (more information about GPSnet is available at www.land.vic. gov.au/gpsnet). By tracking at least four common satellites simultaneously,

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GPS and accuracy

DGPS uses the known position of the base station to calculate and correct the GPS-derived position. The base station then transmits these corrections to GPS receivers in the surrounding area with DGPS capability. DGPS significantly improves GPS accuracy by providing correctional information for atmospheric and satellite errors. However, the distance between the base station and your receiver should not exceed 300km.

Real-time positioning has the advantage of providing improved accuracy while you are using your GPS receiver in the field. However, things like signal coverage, the length of time corrections take to get to your receiver, and your distance from base stations can affect accuracy. Post-processing allows you to concentrate on collecting data and organising information into files. GPSnet provides for both realtime and post-processing operations.

DGPS does not correct errors caused by multipath or receiver noise. The known coordinates at the base station also provide a convenient means to allow the user’s calculated position to be on the correct datum (GDA) - it also complies with Victoria’s digital mapping data, Vicmap. Corrections from base stations can be obtained after fieldwork is finished (called post-processing), or in real-time.

GPS and accuracy

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6 GPS is used to collect and process spatial information for a wide range of government, business and recreational applications. The Victorian Government uses GPS technology to collect information for geological mapping, surveying, resource mapping and management, emergency response and environmental conservation. This information is collated into a set of spatially related data products called Vicmap. Vicmap provides spatial data on things like our road and rail networks, city and town layouts, forests and parks.

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Quality standards If you intend collecting information that is accurate enough to be integrated into Vicmap data or you intend to use GPS for business purposes, you must adhere to minimum quality standards and procedures.

You should refer to this when collecting spatial information you wish to share with others.

For example, you must use DGPS if you wish to integrate mapping information into the Victorian Government’s mapping database. You must also ensure that the datum is GDA94. The Victorian Government has produced a more detailed technical standards publication, The Global Positioning System Handbook, which outlines these minimum requirements.

Quality standards

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Further information

For more information about GPS, the following websites are recommended. General GPS information GPSnet, www.land.vic.gov.au/gpsnet GPS, www.navcen.uscg.gov/ NAVSTAR, https://gps.losangeles.af.mil/ To find more information about GPS technology and equipment suppliers, type GPS into your preferred Internet search engine. Datums and Coordinates Geocentric Datum of Australia, www.icsm.gov.au/icsm/gda Geoscience Australia, www.ga.gov.au Geocaching www.geocaching.com Professional GPS Technical Standards The Global Positioning System Handbook, www.land.vic.gov.au Image above courtesy Victoria Police Search and Rescue

Further information

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8 95 per cent confidence Used to describe the GPS accuracy of a position. accuracy Refers to the closeness of GPS positions to their true location on Earth. AGD Australian Geodetic Datum. Used to develop maps in Australia from the 1960s up to 2000. It has been superseded by GDA94 since 1 January 2000. AGD66 The 1966 adjustment of the Australian Geodetic Datum adopted in Victoria that has been superseded by GDA. GDA and AGD coordinates vary by around 200m in the north easterly direction. AMG66 Universal Transverse Mercator grid

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Glossary coordinates (east, north and zone) generated from AGD66 latitudes and longitudes.

conjunction with DGPS and refers to the cancellation of errors that affect GPS positions.

atmospheric errors Errors introduced into GPS positions when charged particles and moisture in the atmosphere slow satellite signals travelling down to a GPS receiver.

datum A mathematical surface that best fits the shape of the earth allowing you to describe geographic positions. Also known as a reference frame.

base station A fixed station on Earth that is accurately surveyed and sends signals to help reduce errors in GPS calculations. Essential for DGPS. Also known as a reference station.

DGPS Differential GPS. Increases the accuracy of GPS positions by reducing atmospheric and satellite errors.

control segment Ground stations on Earth that upload corrections to GPS satellites. corrected The term generally used in

DOP Dilution of Precision. A measure of the quality of the angle of a GPS satellite in relation to a GPS receiver. The greater the angle, the lower the DOP and the higher the accuracy.

Glossary

DSE Department of Sustainability and Environment. ellipsoid Mathematically the ellipsoid is a rotated ellipse, an oval that revolves about its shortest dimension. It is a mathematical approximation of the geoid. It is used for exact measurements over long distances, across continents or oceans. ephemeris errors Errors introduced into GPS positions caused by GPS satellites moving slightly out of their original orbits by solar winds and other variables. Galileo The GPS network being built by the European Union. GDA Geocentric Datum of Australia. For most practical purposes, GDA is compatible with positions generated by autonomous GPS handheld receivers. GDA94 Geodetic coordinates (latitude and longitude) computed in terms of the GDA at 1 January 1994. geo-reference Assign coordinates from a realworld reference system, such as latitude/

Glossary

longitude, or map grid to the page coordinates of a raster (image) map. Georeferencing raster data allows it to be viewed, queried and analysed with other geographic data and used by a GPS receiver. geocaching An adventure sport involving finding hiding places using GPS. geoid A simplification of the Earth’s surface using mean sea level of the ocean with all continents removed. Due to variations in the earth’s mass distribution (oceans and land), the geoid has an irregular shape that is described as “undulating”. GIS Geographic Information System. A computer-based system that collects, manages and analyses geographic spatial information. GLONASS Global’nava Navigatsionnaya Sputnikovanva Sistema. The GPS network operated by the Russian Federation. GNSS Global Navigation Satellite Systems, which encompasses GPS, GLONASS, Galileo and other satellite based navigation systems.

GPS Global Positioning System. GPS constellation GPS satellites orbit the earth every 12 hours, emitting continuous navigation signals. With the proper equipment, users can receive these signals to calculate time, location and velocity. GPSnet The network of permanent GPS base stations and supporting infrastructure operated by DSE that records, distributes and archives GPS satellite correction data for DGPS purposes. www.land.vic.gov.au/ gpsnet local uncertainty The average measure in metres at a 95 per cent confidence level with respect to the adjacent points in a datum. MGA94 Universal Transverse Mercator grid coordinates (east, north and zone) generated from GDA94 latitudes and longitudes. multipath When satellite signals bounce off vegetation or buildings as they travel from satellites to a GPS receiver. It introduces errors into GPS positions.

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positional uncertainty Expressed in metres, it describes a circle of uncertainty around a GPS position. post-processing Correcting errors in GPS positions by using data from base or reference stations some time after the moment of position determination. precision Refers to how closely several GPS positions taken from the same location match each other. Precision is a measure of repeatability. receiver noise Refers to how well a GPS receiver measures and calculates information received from GPS satellites. The higher the noise, the greater the risk of error. reference frame A mathematical surface that best fits the shape of the Earth allowing you to describe geographic positions. Also known as a datum.

describing the physical location of an object. trilateration The principle which allows you to calculate a position on Earth using distance measurements. user segment A GPS receiver and the person using it. Vicmap A set of spatially related data products produced by DSE, which underpin Victoria’s mapping and geographic information systems. waypoints Points in between the start and end points of a particular sequence of points of interest. WGS84 World Geodetic System 1984. The globally recognised reference frame or datum used by the GPS.

space segment The constellation of 24 satellites used by GPS receivers to calculate positions on Earth. spatial information Spatial information is any information recording or

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Glossary

Published by Spatial Information Infrastructure Division Department of Sustainability and Environment PO Box 500 East Melbourne Victoria 3002 The Department of Sustainability and Environment is responsible for managing Victoria's GPS base station network, GPSnet, and spatial information infrastructure. Find more information about the Department of Sustainability and Environment at www.dse.vic.gov.au or call 136 186.

ISBN 1 74152 013 4 © The State of Victoria, Department of Sustainability and Environment, 2004 This publication may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication. Printed by Chillipress Printed on Hanno Art Silk - Managed Plantation Fibre - Totally Chlorine Free - Acid Free - Environmentally Accredited

Department of Sustainability and Environment Spatial Information Infrastructure Strategic Policy and Projects PO Box 500 East Melbourne VIC 3002 Tel (03) 8636 2333 Fax (03) 8636 2813 www.land.vic.gov.au