Surveying or land surveying is the technique and science of accurately determining the terrestrial or three-dimensional space position of points and the distances and angles between them. These points are usually on the surface of the Earth, and are often used to establish land maps and boundaries for ownership or governmental purposes. In order to accomplish their objective, surveyors use elements of geometry, engineering, trigonometry, mathematics, physics and law. Leica Geosystems What is surveying?
Transcript
Surveying or land surveying is
the technique and science of
accurately determining the
terrestrial or three-dimensional
space position of points and the
distances and angles between
them. These points are usually
on the surface of the Earth, and
are often used to establish land
maps and boundaries for
ownership or governmental
purposes. In order to accomplish
their objective, surveyors use
elements of geometry,
engineering, trigonometry,
mathematics, physics and law.
Leica Geosystems
What is surveying?
The measurement of position and
associated information of items of
economic value that are owned by an
individual or organisation
Leica Geosystems Market Segment
Asset Collection & Management
Leica Geosystems GNSS
Before GNSS
Leica Geosystems GNSS
What is GNSS?
Global Navigation Satellite
Systems
Global positioning
Not restricted by terrestrial lines of sight
‘Always on’ positioning
Mandated method of obtaining height in GB
Your benefits:
Work in new environments
No levelling to OSBMs
Operate solo
Value
Diversify work types
More productive
Less overheads
GNSS Surveying Not just US satellites…
The 4 Global Navigation Satellite Systems are:
GPS (USA)
GLONASS (Russia)
GALILEO (European Union)
BEIDOU (China)
GPS
GPS: Global Positioning
System
NAVSTAR Constellation
Designed and run by US Dept
of Defense
Freely available to civil users
(at reduced accuracy)
Worldwide Navigation System
All weather, 24 hours per day
Three Segments
SPACE-SEGMENT
L1, L2, Time
Emphemeris
Add Info
Three Segments
SPACE-SEGMENT
L1, L2, Time
Emphemeris
Add Info
CONTROL-SEGMENT
Central
Time Synchronisation
Tracking Stations
Three Segments
USER-SEGMENT
Receive Satellite Signal
SPACE-SEGMENT
L1, L2, Time
Emphemeris
Add Info
CONTROL-SEGMENT
Central
Time Synchronisation
Tracking Stations
Satellite Constellation
Satellite Constellation
4 Satellites per
orbital plane
55º Inclination to the Equator
20,200 km above Earth
12 hour orbits
Satellite Constellation
24 Satellites
6 Orbital Planes
55º Inclination to the Equator
20,200 km above Earth
12 hour orbits
Outline Principles
Satellites are merely radio transmitters (L1 and L2 frequency)
Part of message is satellite position
At any moment in time, satellite position is known
Each satellite broadcasts almanac of whole constellation
GPS receiver stores almanac, to find visible satellites
Satellite transmissions very weak- need line of sight
Outline Principle - Range
Outline Principle - Range
Outline Principle - Range
Outline Principle - Range
Time taken x speed of light = range
Outline Principle - Range
Outline Principle - Position
We are somewhere on a Sphere, Radius R1
R1
Outline Principle - Position
2 Spheres intersect as a circle ...
R1
R2
3 Spheres intersect at a point
Outline Principle - Position
R1
R2
R3
3 ranges to resolve - latitude, longitude, and height
Outline Principle - Position
3 ranges to resolve - latitude, longitude, and height
4th range to resolve - receiver clock offset
Outline Principle - Position
4 ranges to solve for latitude, longitude, height and time.
The broadcast orbits are only good to a few metres
The satellite signals are delayed in the atmosphere on their
way to the GPS receiver
So accuracy with a single GPS receiver will be 2m-5m
GPS Error Sources
Orbit and atmospheric error
Pseudorange
Re
ce
ive
r N
ois
e
Multip
ath
Sa
telli
te C
lock
Tro
po
sp
he
ric
Ep
he
me
ris
Ionospheric 0
100
200
300
400
User Equivalent Range Errors M
etr
es
Pseudorange
Re
ce
ive
r N
ois
e
Multip
ath
Sa
telli
te C
lock
Tro
po
sp
he
ric
Ep
he
me
ris
Ionospheric 0
100
200
300
400
User Equivalent Range Errors M
etr
es
Re
ce
ive
r C
lock
NAVIGATION SOLUTION
P
Z
X
Y
Absolute Accuracy
Accuracy
P, wrt XYZ = <30m
P
Q Z
X
Y
Relative Accuracy
DIFFERENTIAL SOLUTION
P
Z
X
Y
Absolute Accuracy
Accuracy
Q, wrt P = <1m ?
Accuracy
Absolute Position… Poor
Accuracy
Absolute Position… Poor
Accuracy
P
Q
Z
X Y
Differential GPS (DGPS)
Relative Position… Good
Differential Position
Post Processing
Differential Position
Real Time
With 1 GPS receiver and no "corrections“
we‘ll typically get a position accuracy of a
few metres
We need to use the data from 2 (or more)
GPS receivers to get centimetre accuracy
This could either be the Reference (Base) &
Rover method or the Network RTK method
Real Time Kinematics GPS
Accuracy corrections
Real Time Kinematics GPS
Reference & Rover
We use 2 GPS receivers to
“cancel out” most of these
accuracy errors
The Reference stays on the
tripod tracking the satellites
and broadcasting that data to
the Rover over a radio link
The Rover tracks the satellites
as well as getting the data
from the Reference
Both receivers track the same satellites at the same
time with the same orbit and atmospheric errors
Real Time Kinematics GPS
Base & Rover
The size of these errors is almost identical at the Base
and Rover so the errors cancel out in the sums
Centimetre relative accuracy
Real Time Kinematics GPS
Base & Rover
Instead of having your own reference receiver on a
tripod we use a Network of reference stations
We get “corrections” sent to us over the internet
So you need a mobile phone signal
Real Time Kinematics GPS
Network RTK
The Ordnance Survey operates a
network of 110 permanent GPS
receivers across Britain
The OS streams the raw GPS data
straight to Leica
Leica’s servers receive the data and
calculate parameters for the GPS
error sources we looked at earlier
Real Time Kinematic GPS
Leica SmartNet infrastructure
When you switch on your GPS kit,
your “uncorrected” GPS position
is sent to the SmartNet server
Raw GPS data from the nearest 5
or 6 OS GPS stations is streamed
to your Rover over the internet
The error parameters are also
streamed to you
So your Rover can solve these
long baselines in real time to
provide down to centimetre
accuracy
Real Time Kinematics GPS
Leica SmartNet MAX method
Questions?
Standalone or differential?
Differential is the calculation of a roving receiver with
respect to a Reference Station(s)
DGPS Realtime code solution - measuring range
to satellites (L1 Frequency only)
RTK Realtime phase solution - using differential
corrections to obtain accuracy of mm
(L1 and L2 Frequency)
Accuracy
Navigation : 2m to 10m
Differential Code: to 0.2m
Differential Phase: to 0.01m
Health Warning - Height is NOT as good as plan !
Depicting the Earth
Europe N. America
S. America Africa
Topography
The Real Earth
Depicting the Earth
O1
Europe N. America
S. America Africa
N Topography
The Real Earth, The Ellipsoid
Depicting the Earth
O1
O2
Europe N. America
S. America Africa
N Topography
N
The Real Earth, The Ellipsoid, but which one?
Depicting the Earth
The Best Mean Fit - World Geodetic System 1984
(WGS84)
Europe N. America
S. America Africa
N
Topography
Height
Ellipsoid
h
P
Topography
h Ellipsoidal height
Height
Ellipsoid
Geoid N
H h
P
Topography
h Ellipsoidal height
H Height above Geoid
(~Orthometric height)
N Geoid separation
Geoid = equipotential surface – in GB this is accurately defined
GNSS Error Sources
Obstructions and multipath
Neither Reference & Rover or SmartNet solves these
problems!
Obstructions in the Northern sky will obstruct fewer GPS
satellites (there are less satellites in that part of the sky)
Multipath means you receive both the direct signal and the
reflected signal
Global Navigation Satellite Systems
4 global constellations
the Global Navigation Satellite Systems are:
GPS (USA) Currently 32 satellites - new launches only when old sats. are
decommissioned. New satellites triple frequency.
Glonass (Russia) Currently 24 satellites.
New satellites triple frequency.
Gallileo (European Union) Currently 4 operational satellites – many launches planned over the
next 24-36 months. Signals interoperable with future GPS sats.
Beidou (China) Currently 4 mid earth orbit satellite – currently less useful outside
Asia/Pacific region
average no. satellites in 2017 with 15° elevation mask
GPS only full GNSS
+ Quicker initialisation times
+ Better position reliability
+ Better productivity in urban canyons (where view of sky is reduced)
Global Navigation Satellite Systems
Why are the other constellations interesting?
Global Navigation Satellite Systems
Why are the other constellations interesting?
Single freq.
GPS only
20 min init.
Down-time
Dual frequency
GPS only
On-the-fly init.
Dual frequency
GPS & Glonass
+ productivity
Triple frequency
GPS & Galileo &
Glonass
+ Speed
+ reliability
+ productivity
1985 1990 1995 2000 2005 2010 2015 2020
Global Navigation Satellite Systems
Triple frequency GNSS
The most recent GPS satellites already
broadcast a 3rd frequency (called L5)
All future GPS and Galileo satellites will
broadcast an interoperable signal on this
frequency
Triple frequency GNSS will give:
faster intialisations
more reliable intialisations
Improved multipath resistance
Galileo is launching lots of satellites
Enough satellites (16) to be useful by 2017?
Full constellation by 2018?
GPS is replacing old satellites
New satellites will be triple frequency
New satellites will be fully interoperable with Galileo
