Topic 5: Global Positing

System

Aims

-Describe the development of the GPS and the impact it has on site

-Explain how GPS can be used to take code and phase measurements to

determine position and be able to explain the difference between these

-Identify the various sources of error in GPS and explain how each of these

affects the accuracy obtained

-Understanding the reasons why differential and relative methods are essential for

high precision surveying with GPS

-Outline the methods involved when performing static and kinematic surveys with

GPS

-Distinguish between the different types of GPS receivers and systems currently

available and be able to find further information to help choose one of these for

engineering surveys

-Identify the main applications for GPS in civil engineering and surveying

Components of GPS

GPS consists of three segments

-Space segment: satellites orbiting the Earth

-User segment: anybody that receives and uses a GPS signal

-Control segment: stations positioned around the Earth to control the satellites

GPS positioning methods

Code ranging: is the simplest form of GPS positioning and is carried out with a

single receiver. To determine the distance to each satellite by code ranging, a

receiver measures the time taken by the C/A code to travel from a satellite to the

receiver and then calculates the distance or range between the two.

Carrier phase measurements: are capable of determining highly accurate

distances, and therefore positions, with accuracies of between 10-50mm.

GPS coordinates have a degree of uncertainty associated with them and all

accuracies are quoted with a 95% probablity.

- GPS receivers that use codes only are not very expensive but not very accurate.

Receivers that use codes and carrier phase measurements are very expensive

but very accurate. Both are used in surveying but it depends on the accuracies

required.

-All measurements with GPS require that at least four satellites are tracked

simultaneously in order to be able to compute a 3D position

Errors in GPS

-Although GPS surveys do not require any observations or readings to be taken in

the same way as other surveys, the results they produce are still subject to errors

and it is important to know what these are, what their possible magnitudes are

and how to control them

-The worst sources of error in GPS are ionospheric and tropospheric delays, and

these are compensated for by mathematical modelling. This area of GPS

surveying is subject to much research at present.

-For most GPS surveys, it is recommended that no satellites with an elevation of

less than 10-15°above the horizon are used, because the signals from these

experience very larger ionospheric and tropospheric delays

- another serious source of error is poor satellite geometry resulting in a high

dilution of position. Unlike ionospheric and tropospheric delays, this is a site-

dependent error and can be controlled by choosing appropriate times to take

measurements.

Differential and relative GPS

Differential GPS (DGPS) is the name given to methods that are usually applied to

code measurements only. Compared to conventional point positioning, DGPS has

an accuracy of about 0.5-5m, depending on the receiver and antenna used and

how corrections are transmitted to each rover.

Surveying with GPS

Static

Although there are several methods that can be used for surveying on site with

GPS, the DGPS and augmentation systems are only used in engineering surveys

for small-scale mapping.

Static surveying was the first high precision method developed for GPS and is the

standard GPS method for determining the length of baselines that are longer

20km.

Using this method, the reference receiver is located at a known control point and

a rover that is set up at a point whose location us to be determined.

Both receivers are normally tri-pod mounted.

The receivers are placed at the control point, the duration that the GPS receiver is

left for 30mins it will result in an accuracy of 100mm, 1hour, 20mm

Rapid static

GPS is now the preferred method for control surveys on large construction sites.

For these surveys, the static method previously described but the GPS receiver

needs to be left for shorter occupation times of 10-30mins.

The reference receiver is located at a known point and the rover (or rovers)

occupy the unknown points. Because observation sessions are short, good

communication between the operators at the reference and rovers is required.

This method relies on a faster ambiguity resolution approach, and to achieve this

dual-frequency receivers must be used together with special post-processing

software. Baselines should not be longer than 10-20km, the shorter the baseline

the better.

Kinematic surveys

Kinematic GPS is used when a lot of points are to be surveyed in a relatively

small area and where the accuracy required is not as high for static surveys.

These include detail surveying (mapping) and construction measurements.

As for all high precision surveying with GPS, kinematic methods require a

reference receiver to be located at a known position, but in this case, instead of

remaining stationary, the rover is moved around the site recording position at

discrete points or whilst the rover is continuously moving.

At the start of a kinematic survey, the rover has to perform an initialisation in order

to resolve ambiguities. To do this a method known as kinematic on-the fly has

been developed and is performed while the rover is moving.

During a post-processed kinematic survey, all the data collected by the reference

and roving receivers is stored in a handheld computer, a controller or onboard

receiver and then transferred to a host computer after fieldwork has been

completed.

The problems with post-processed results not being available on site are

overcome by using a real-time kinematic (or simply RTK) GPS surveying system.

As with surveying methods, RTK surveying requires two receivers to operate at

the time and the reference receiver is again located at a known point.

Kinematic GPS can be carried out by walking with the antenna mounted on a

back-pack or by mounting it on a vehicle and with data collect

RFK surveying is the preferred method for all survey work with GPS on site,

mainly because all the results obtained with it are real-time and no post-

processing is required. This means that measurements taken for a detailed or

other dimensional survey can be checked, verified and edited on site, this means

it can be used for setting out.

GPS instrumentation

Mapping receivers

High-precision geodetic dual-frequency receivers

The category of GPS receiver that is required for nearly all work in surveying on

construction sites is a high-precision geodetic dual-frequency receiver (sometime

called survey grade receiver). These use precise relative methods for determining

position are capable of performing static and RTK surveys with accuracies at the

cm level.

A feature that is increasingly being incorporated into geodetic receivers is

Bluetooth technology. This enables portable equipment such as computers,

mobile and other electronic devices to communicate with each other without using

cables. When they are within range of each other (about 10 m), Bluetooth

devices will detect each other and establish contact — if they are programmed to

communicate then established between them for data transfer.

GPS in engineering surveying

Throughout the early years of GPS, it was only used in surveying for establishing

the positions of points in control networks covering large areas and even

continents, and often only for research purposes.

Today, the situation has changed completely and satellite positioning systems are

now used extensively in surveying and on site for an increasing number of

applications.

Where the different methods of surveying with GPS in a construction and

engineering environment are identified.

For all of these, the basic principles of surveying remain the same as for those

used with total stations and other equipment — a control network is put in place

first from which other measurements are taken.

The most important question to be asked by anyone proposing to use GPS is

whether it is the most appropriate method for their site. It may be useful to contact

some sites that are currently using GPS to ask how it compares to total stations

and other well-established methods of surveying.

It is worth noting that GPS is not as accurate as total stations, theodolites and

levels for measurements that are taken over relatively short distances and when

features such as structural grids and lines are set out.

This, plus the fact that GPS equipment is more expensive than conventional

surveying equipment, may put GPS at a disadvantage for work requiring a high

accuracy of less then 10 mm on construction sites.

However, compared to total stations, GPS has the advantages that it only requires

a single operator to use it although some total stations are single-operator.

Having decided to use GPS, it is recommended that each of the following are

considered when choosing the equipment that will be used:

-For all engineering and construction work, carrier phase geodetic receivers are

essential, as these will provide the best possible accuracy for measuring and

setting out. These are the most expensive type of GPS receiver and some of their

specifications have already been discussed in the previous section.

-Precise relative positioning must be used, again to provide the best possible

accuracy. This requires base stations to be established, possibly with repeaters

on those sites where radio reception is difficult. Access to a mobile phone network

may also be required.

- When results are post-processed, a suitable computer must be available that

can run suitable software and that can also store the large amounts of data

generated by GPS surveys.

-For some sites where the use of GPS is only occasional, it maybe more cost-

effective to hire the equipment required rather than purchase it or even contract

out the GPS surveying altogether to a specialist company.

-The issue of training with GPS is very important. If staff who are expected use to

the equipment have little or no experience of GPS, training will have to be

provided. If any results are to be post-processed, tuition in the use of the software

for this will also have to be provided.

-As well as training, it is advisable to find out what technical support is offered by

a manufacturer to assist with any problems that may be encountered after making

a purchase.

- Finally in this section, it is obvious that GPS technology is developing at a fast

pace— will the equipment being purchased become out of date quickly or will it

capable of using the extra signals proposed for GPS and the satellite coverage

planned for GLONASS and Galileo?

GPS combined with a total station

A logical progression in the development of GPS and total stations is to integrate

these into one instrument. The Leica SmartStation shown in below combines their

GPS SmartAntenna (an antenna and receiver mounted in a single housing) with

their TPS 1200 total station.

When the SmartStation is used for GPS measurements, the SmartAntenna is

connected to the top of the TPS 1200 together with an RTK communications

device.

When detail surveying with the SmartStation, it is not necessary to set it up over a

control point; it can simply be set up anywhere convenient for taking

measurements.

The position of the point occupied is then determined to centimetre accuracy

using GPS with reference to a base station.

Having determined the coordinates of the SmartStation, a second unknown point

is sighted. The coordinates of this second point are not determined at this time but

the total station is orientated to it and measurements are taken to the required

points of detail.

Following this, the SmartStation is set up at the second point and its position is

now determined once again using the integrated GPS SmartAntenna.

Since the coordinates of both points are now known, the correct bearing between

them is determined by the SmartStation, which then re-computes the coordinates

of all the detail surveyed from the first point using the correct bearing.

To continue measurements, the SmartStation now works as a total station and

is orientated by sighting the first point occupied.

When a detail survey is carried out conventionally, traversing or GPS might be

used to determine the positions of a series of control points on site from which

further observations would be taken to locate detail.

This requires two separate surveys but by integrating GPS with a total station,

a survey could be completed in one operation with one instrument resulting in a

saving in time.

In addition, given that GPS must be used to fix the position of at least two

points, the technique allows a total station to be used where GPS signal

reception can be unreliable (for example near to vegetation and in built-up

areas).

Ideally, a check should be carried out on any detail survey carried out with the

SmartStation by coordinating a known position with the TPS 1200 or by

measuring the distance between the two points with the TPS 1200 and by

comparing this with that obtained from their known coordinates.

When setting out with the SmartStation, it is again set up wherever convenient

and coordinates are determined at this point using RTK GPS.

The instrument is then moved to a second unknown point and the coordinates

of this are also determined. By using the first point for orientation, setting out

can then be performed with the TPS 1200.

In this case, fixing the position of the SmartStation with GPS is rather like using

a free station point, but without the need for any control to be sighted or

occupied.

This can have advantages when site control points become damaged or

obstructed and cannot be used. Again, if GPS signal reception is intermittent or

unreliable on site but can be used to fix the two set up points, it allows setting

out to continue with a total station, without the need for control points