NIKKI MEADS TCInstCES. CREATED BY THE US DEPARTMENT OF DEFENCE FOR MILITARY PURPOSES IN 1978- 1985 CONSISTED INITIALLY OF 11 SATELLITES AND SHORTLY ROSE

NIKKI MEADS TCInstCES

GLOBAL POSITIONING SYSTEMS (GPS)

HISTORICAL USE• CREATED BY THE US DEPARTMENT OF DEFENCE FOR MILITARY PURPOSES IN

1978-1985

• CONSISTED INITIALLY OF 11 SATELLITES AND SHORTLY ROSE TO 18, THEN 24 IN 1979

• IN 1980 THE FIRST CARRYING SENSORS SATELLITE FOR ATOMIC EXPLOSIONS WAS LAUNCHED- THIS COULD DETECT ANY NUCLEAR TESTING ON EARTH, SEA AND SPACE!

• IN 1983 AFTER A PLANE WAS SHOT DOWN OVER SOVIET TERRITORY CIVILIAN USE OF THE GPS SYSTEM WAS MADE AVAILABLE AT A COST

• BETWEEN 1993 AND 2000 FREE CIVILIAN USE WAS APPROVED AND SELECTIVE DEACTIVATION MEANT ACCURACY WENT FROM 100m DOWN TO 20m.

NOWADAYS AROUND 30 ACTIVE SATELLITES ORBIT THE EARTH AT A DISTANCE OF 20200km


TYPES OF SYSTEMS

• NAVSTAR GPS – NAVIGATION SIGNAL TIMING AND RANGING GLOBAL POSITIONING SYSTEM, DESIGNED, CREATED AND CONTROLLED BY THE USA

• GLONASS GPS – GLOBAL’NAYA NAVIGATSIONNAYA SPUTNIKOVAYA SISTEMA. OR GLOBAL NAVIGATION SYSTEM, DESIGNED, CREATED AND CONTROLLED BY RUSSIA

• GALILEO GPS – EUROPEAN GPS SYSTEM, MULTI-OPERATIONAL SYSTEM WHICH IS FREE TO USERS AND CAN BE ACCURATE TO <1cm FOR FEE PAYERS



HOW GPS WORKS...

ORBITING SATELLITESBASE STATION RECEIVER

HOW GPS WORKS...

A SATELLITE TRANSMITS A CODED RADIO SIGNAL THAT INDICATES THEIR EXACT LOCATION IN SPACE AND TIME

THE LENGTH OF TIME IT TAKES A SIGNAL TO TRAVEL FROM A SATELLITE TO A RECEIVER ON THE GROUND

BASIC PRINCIPLES:

THE RECEIVER MEASURES HOW LONG IT TAKES FOR THAT CODED SIGNAL TO TRAVEL FROM THE SATELLITES

BY OBSERVING MEASUREMENTS FROM THREE OR MORE SATELLITES, THE LOCATION OF THE RECEIVER CAN BE DETERMINED BY TRIANGULATION

BY OBSERVING MEASUREMENTS FROM A FOURTH SATELLITE, AN ELEVATION CAN BE CALCULATED


CALCULATING POSITION

Position P can be defined by:

RSP = c (t – e)

Where

RSP = True range from satellitesc = Speed of light @ 299792458 meters per secondt = Transit timee = Clock error

POSITION ‘P’

RSP


CALCULATING POSITION

In terms of X, Y and Z coordinates this becomes:

[(XS – XP)2 + (YS – YP)2 + (ZS – ZP)2]1/2 = c (t – e)

In which the transit time t is called the observable and unknowns XP, YP and ZP (the position of P) and e exist. Observations to four satellites will give the four equations in four unknowns to give the receiver position and clock error.


ERROR DETERMINATION

SYSTEMATIC ERRORS

• SATELLITE AND RECOVER CLOCKS

• SATELLITE POSITION

• IONOSPHERE – ATMOSPHERIC

• MULTIPATH


ERROR DETERMINATION

RANDOM ERRORS/USER ERRORS

• MEASURING TO THE WRONG PLACE ON THE BASE STATION

• SETTING THE INSTRUMENT OVER THE WRONG GROUND MARKER

• NOT ALLOWING ENOUGH TIME FOR MEASUREMENTS

• TRANSFORMING TO THE WRONG COORDINATE SYSTEM


IN ORDER TO OBTAIN AN ACTUAL POSITION OF THE RECEIVER, ALL CLOCKS MUST BE ABSOLUTELY PRECISE

IF THE RECEIVER CLOCK IS FOR INSTANCE 0.5sEARLY THE THREE INTERSECTION POINTS ‘B’ ARE OBTAINED

IF THE TIME ON THE RECEIVER CLOCK IS NOW SHIFTED UNTIL THE THREE INTERSECTION POINTS ‘B’ MERGE TO ‘A’, THE CLOCK ERROR IS CORRECTED THEN ALL CLOCKS ARE THENSYNCHRONIZED

ACCURATE DISTANCES CAN THEN BEDETERMINED AND THUS AN ACCURATEPOSITION CALCULATED

AN ERROR OF 0.000001 SECOND IN THE RECEIVER CLOCK WOULD RESULT IN AN ERROR OF

300 METERS!!!

ERRORS IN SATELLITE AND RECEIVER CLOCKS



ERRORS IN SATELLITE POSITION

Determined position will be accurate due to a small quadratic field

Satellites appear in all quadrants and thus can be measured in four different directions.

Plane of intersection of possible positions is considerably larger and elongated

Satellites appear in one or two quadrants and thus can only be measured in a couple of directions


ERRORS CAUSED BY MULTIPATH

Multipath is caused by the reflexions of the radio signal sent from the satellites on objects. It is the same sort of effect that ‘ghosting’ you would have had on your old television sets with antennae.

For GPS this mainly only happens when the position to be fixed is close to tall buildings or in built up areas.

Not only does the signal reflect onto objects in this case, it also gives bad satellite geometry and so has reduced the chances of accurate positioning.

If control is required in built up areas it would be far more practical to establish GPS primary control away from any error influences and then use traditional survey techniques to acquire secondary control.


TIME AS A MODE OF ACCURACY

Depending on accuracy requirements, it is common practice to record readings for a number of hours or even days in some cases.

• The more readings that are taken the more accurate the ‘meaning’ process is, and thus the more accurate the position will be.

•The longer the base station is receiving the higher the probability of good satellite geometry.

•Readings taken in the morning often differ from those taken in the afternoon.


THE COORDINATE SYSTEMETRS89 (European Terrestrial Reference System 1989) This is our national coordinate system for 3D GPS

positioning. It is a much more exacting definition of the GPS coordinate system than the better known WGS84 standard. Consequently, ETRS89 coordinates are also WGS84 coordinates, but be aware that general WGS84 coordinates do not necessarily meet the ETRS89 standard. ETRS89 is the GPS coordinate system standard used for high-quality GPS surveys throughout Europe.

OSGB36 National Grid (Ordnance Survey Great Britain 1936) This is our national coordinate system for

topographic mapping. It is used for Ordnance Survey mapping at all scales, and for many private topographic surveys.

The OSGB36 part of the name refers to the geodetic datum (system of latitude and longitude) used, and the National Grid part refers to the map projection and grid referencing convention for Eastings and Northings.

ODN (Ordnance Datum Newlyn)

This is our national coordinate system for heights above mean sea level (orthometric heights). It was originally based on tide gauge readings at Newlyn, Cornwall. ODN is the usual definition of height above mean sea level in mainland Britain and some islands.

WGS84 - WORLD GEODETIC SYSTEM 1984

MOST GPS SURVEYS IN GREAT BRITAIN ARE TO OSGB36 SINCE IT IS A MUCH BETTER REPRESENTATION OF THE LONGITUDES AND LATITUDES IN THIS LOCATION ON THE GLOBE

IT IS NORMAL PRACTICE TO HEIGHT THE POSITIONS FROM AN ORDNANCE SURVEY BENCH MARK AS THE HEIGHTS GIVEN FROM THE GPS ARE NOTORIOUSLY INACCURATE


ADVANTAGES OF GPS• MORE FLEXIBLE AND LESS TIME CONSUMING

• IT CAN BE USED AROUND THE CLOCK

• INTERVISIBILITY BETWEEN STATIONS IS NOT REQUIRED

• IT IS INDEPENDENT OF WEATHER CONDITIONS

• POSITIONAL ACCURACY IS A FUNCTION OF INTERSTATION DISTANCE AND NOT OF NETWORK GEOMETRY

• PROVIDES THREE DIMENSIONAL INFORMATION

• HIGH ACCURACIES CAN BE ACHIEVED WITH RELATIVELY LITTLE EFFORT

• THE STATION POSITIONS CAN BE PLACED WHERE THEY ACTUALLY NEED TO BE AND THUS MORE EFFICIENT

DISADVANTAGES OF GPS• HIGH EFFICIENCY HAS IT’S PRICE – YOU MAY ONLY NEED TO ESTABLISH A HANDFUL OF POSITIONS, BUT YOU WILL BE THERE A LONG TIME

• IT CANNOT BE USED UNDERGROUND, IN FORESTS, IN AREAS OF DENSE UNDERGROWTH, HIGH RISE BUILDINGS OR IN PLACES SUCH AS QUARRIES

• WGS84 COORDINATES WILL NEED TO BE TRANSFORMED INTO THE LOCAL GEODETIC SYSTEM BEFORE IT COULD BE INTEGRATED WITH THE ANY CONVENTIONAL SURVEY TECHNIQUES

• GPS VERTICAL INFORMATION IS NOT PROVIDED IN THE HEIGHT SYSTEM GENERALLY USED AND SO IT WOULD ALSO NEED A TRANSFORMATION TO OSN

• GPS RECEIVERS ARE CONSIDERABLY COSTLY COMPARED TO THE TOTAL STATION (AND YOU WOULD NEED TWO OF THEM)

• BASE STATION WOULD NEED TO BE EITHER MANNED OR LOCKED SECURELY – IT IS NOT ADVISABLE TO LEAVE THE EQUIPMENT ON IT’S OWN!


SURVEYING APPLICATIONS•

MONITORING

CONTROL NETWORKING

ARCHAEOLOGY

REMOTE AREAS



ENGINEERING APPLICATIONS

CHECKING DESIGN

EXCAVATING

GRADING

SETTING OUT


SURVEYS AND SCALES

• LARGE SCALE IS USED TO DESCRIBE MAPS THAT SHOW FEATURES ON THE GROUND AS THEY ACTUALLY APPEAR – YOU CAN SCALE ACCURATELY THE WIDTH OF A FOOTPATH FOR INSTANCE . EXAMPLE SCALES THAT WOULD BE CLASSIFIED AS LARGE SCALE ARE:

1:50 1:1001:200 1:5001:1000

IT IS ALWAYS BEST PRACTICE TO CHOOSE A GENERIC SCALE AS THIS CAN ALWAYS BE MEASURED WITH A SCALED RULE

• SMALL SCALE IS USED TO DESCRIBE MAPS THAT COVER A LARGER AREA BUT FEATURES ARE NOT REPRESENTATIVE OF THAT ON THE GROUND – IT WOULD NOT BE POSSIBLE TO ACCURATELY SCALE FROM. TYPICAL SMALL SCALES ARE:

1:1250 1:25001:10 000 1: 25 000

THESE TYPES OF MAPS ARE USUALLY USED FOR REPRESENTING A GENERAL LOCATION AND MORE COMMONLY ORDNANCE SURVEY TOWN PLANS

THE DECIDED SCALE WILL INDICATE HOW ACCURATE A SURVEY SHOULD BE AND VICE VERSA! IT WOULD BE POINTLESS HAVING AN ACCURACY OF +/- 3mm IF THE SCALE REQUIRED IS 1:10 000 AS THIS WOULD BE NEAR IMPOSSIBLE FOR THE NAKED EYE TO SEE!

MAP SCALES


SUMMARY

GPS IS A REVOLUTIONARY WAY OF FIXING A POSITION, IT IS EFFICIENT AND CAN BE VERY ACCURATE WHEN USED IN THE CORRECT MANNER

IT IS MORE SUITABLE FOR LARGER PROJECTS AND EARTHWORKS PROJECTS DUE TO COST IMPLICATIONS AND THE RELIANCE OF LARGER BASELINES

IT CAN BE EASILY INTEGRATED WITH TRADITIONAL SURVEY TECHNIQUES AND WORKS WELL WITH PROJECTS THAT CONTAIN LARGE AREAS OF LAND AND DENSELY POPULATED AREAS

IS IDEAL FOR ESTABLISHING PRIMARY CONTROL OVER LONG DISTANCES

MUST BE LEFT SECURELY AS THEY ARE EXPENSIVE ITEMS AND VERY ATTRACTIVE TO THIEVES

CONSTRUCTION PLANT CAN USE IT FOR EXCAVATIONS, SLOPES AND EMBANKMENTS AND SAVES A LOT OF TIME AND MAN HOURS

CAN BE USED AS QUICK REFERENCE (REAL TIME GPS) AS TO THE PROJECTS PROGRESSION

IT IS A FLEXIBLE PIECE OF EQUIPMENT AND CAN MOUNTED ONTO A TRIPOD, CARRIED IN A BACKPACK, MOUNTED ON MACHINERY OR VEHICLE


EXAMPLE: GPS INTEGRATION

PRIMARY STATIONS GPS

SECONDARY STATIONS

FLY STATION


THINGS TO CONSIDER...AS A CIVIL ENGINEERING CONTRACTOR

• WHO PROVIDED THE DATA?

• HOW ACCURATE IS IT?

• WHAT METHOD WAS USED?

• DO I NEED TO CHECK THE DATA AND IF SO, HOW?

• IS THE DESIGN UP-TO-DATE AND WAS IT PRODUCED FROM A SURVEY?

• HAS ANYTHING CHANGED SINCE THE DATA WAS PRODUCED?

• AM I USING THE DATA IN THE INTENDED WAY?

•DO I HAVE ALL THE AVAILABLE INFORMATION?

THINGS TO CONSIDER...


AS A CIVIL ENGINEERING DESIGNER/PLANNER• DO I REALLY NEED A SURVEY UNDERTAKING?

• HOW ACCURATE DOES IT NEED IT TO BE?

• HOW BIG DOES THE AREA THAT NEEDS SURVEYING HAVE TO BE?

• WHAT SHOULD I INCLUDE ON THE SURVEY?

• WHAT METHODS SHOULD I SPECIFY?

• HOW AM I GOING TO USE THE DATA?

• HOW IS THE DATA GOING TO BE PRESENTED?

• DO I HAVE ALL AVAILABLE INFORMATION FOR ME TO UNDERTAKE MY JOB?

Documents

NIKKI MEADS TCInstCES. CREATED BY THE US DEPARTMENT OF DEFENCE FOR MILITARY PURPOSES IN 1978- 1985 CONSISTED INITIALLY OF 11 SATELLITES AND SHORTLY ROSE