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2/4/2014
1
GNSS Precision – New RTKPositioning TechnologySteve Richter – Frontier Precision, Inc.
Outline
The Major Epochs of RTK Innovation
GNSS Positioning concepts
RTK methods, single base and VRS
HD-GNSS, precision based Surveying
xFill, Positioning without a Correction Stream
Summary & Questions
Introduction RTK has quickly become commonplace in Surveying
radio comms
correction streams
on-the-fly initialization
faster processors
Traditional GNSS Initialization techniques have been
effective, but these concepts remain from the 1990’s
No corrections – no precise positions
New technology has emerged for improving field
operation, field confidence, and overall
performance using Precision Based Algorithms
2/4/2014
2
Objectives
The following presentation aims to:
– Provide some theoretical background on high-
precision positioning
– Highlight the differences of HD-GNSS and
conventional RTK techniques
– Precision based RTK surveying without float/fixed
– Introduce the xFill technique
– Better understand how Precision Based RTK
improves productivity in the field
1988… Trimble introduced the 4000SD
When High Precision Surveying Began!
Ground-breaking technology (literally)
Dual-frequency, 5 L1, 5 L2 channels!
1 Mbyte Internal data storage!
Only 98 lbs (excluding battery), 66Watts!
Post-processing in the office
2.5 lbs (with battery)
... 2012 Trimble R10
Major Epochs of RTK Innovation4000SLD - First Dual-freq Backpackable (1988)
1990 1995 2000 2005 2010 20151985
Nine GPS Satellites in orbit Total Weight: 44 lbs (without car battery)
2/4/2014
3
Major Epochs of RTK Innovation4000SSE - First RTK with On-the-fly Initialization (1994)
GPS Declared Fully Operational
Total Weight: 15.4 lbs (with everything!)
1990 1995 2000 2005 2010 20151985
Major Epochs of RTK Innovation4800 - First RTK with No Strings Attached! (1997)
Total Weight: 8.5 lbs (full RTK rover)
1990 1995 2000 2005 2010 20151985
Major Epochs of RTK InnovationR8-GNSS, 2005, First GPS/GLONASS RTK
Selective Availability Switched Off
First Modernized GPS (IIR-M) Satellite Launched
Total Weight: 8.2 lbs (full RTK rover)
1990 1995 2000 2005 2010 20151985
2/4/2014
4
Major Epochs of RTK InnovationR10 – Technology for Productivity (2012)
Precision Based RTK – HD GNSS
31 GPS &24 GLONASS Satellites in orbit
Total Weight: 7.9 lbs (full RTK rover)
1990 1995 2000 2005 2010 20151985
Technical Background- GNSS Positioning Principles
Autonomous Positioning
GNSS Receiver measures
pseudorange (distance) to
each satellite in view
Location of each satellite is known
from orbital parameters
(‘navigation message’)
Position and time determined by
receiver using pseudoranges and
known satellite locations
(trilateration)
Technical Background- Errors affecting GNSS measurements
Satellite orbit and clock errors
directly affect the user-satellite
range measurements
Ionosphere (50-1000km altitude)
causes a frequency-dependent
error in GNSS measurements
Troposphere (< 50km) causes a
delay in the GNSS signals
Multipath is caused
by signal reflection
off nearby objects
Signal masking reduces
tracked satellites and
degrades range
measurement accuracy
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5
Autonomous Positioning- For Mapping and GIS applications
Autonomous position is normally
estimated using GNSS code
measurements with a typical
accuracy of around 10m
Satellite and atmospheric errors
all directly impact on the
accuracy of the computed user
position (single-frequency)
Differential Positioning- Example, using CMR / RTCM corrections
Satellite and atmospheric errors
are nearly identical for two
closely spaced GNSS receivers
With the differential technique, the
relative position of a rover is computed
with respect to a single reference station
Differential positioning accuracy
is superior to Autonomous
positioning (typically < 1m)Differential corrections are
generated by the reference
station and applied at the rover
Improving Precision- Carrier phase versus code
GNSS pseudorange
measurements are based
on PRN code data
Code measurements
have a precision of a
few decimetersCarrier phase
measurements have
mm-level precision
however they contain
an integer bias
Once the integer bias is resolved on
each satellite, carrier phase
measurements deliver precise range
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6
Multi-Frequency- Reducing the effects of ionospheric errors
Satellites broadcast on multiple frequency
bands
(e.g. GPS L1, L2 & L5; GALILEO E1, E5, E6)
Multi-frequency carrier phase and
code observations help correct for
ionospheric errors and enable
rapid ambiguity estimation
RTK Modes- Single-Base
Single GNSS base (reference)
station established near work site
and tracks all satellites in view
Satellite and atmospheric
errors are nearly identical for
closely spaced reference and
rover stations
Reference station corrections
delivered to rover via radio
datalink
Rover GNSS applies satellite
corrections supplied by reference
station and obtains cm-level results
RTK Modes- Virtual Reference Station
GNSS Reference stations
established over wide
geographic regions
(e.g. city, state, country)
Reference station spacing
typically 50-100km
Reference stations track
all GNSS satellites in view
Pivot Platform Server concentrates reference
station data and models satellite and
atmospheric errors over network
Network server generates a virtual reference
station next to rover and provides GNSS
corrections via wireless Internet connection
Rover obtains cm-level
accuracy within coverage
region
Internet
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Float / Fixed Solutions
Carrier phase ambiguities are by definition
integer quantities
Need to make use of the integer-nature of the
carrier phase ambiguities to gain utmost
accuracy from GNSS
0 +1 +2 +3-1-2-3
Traditional Ambiguity Resolution
Traditional approach to ambiguity resolution
involves first estimating the carrier phase
ambiguities as floating point (real-valued)
numbers
Hence the term float solution
Code measurements are used to refine the
float ambiguity estimates
0 +1 +2 +3-1-2-3
Traditional Ambiguity Resolution
Direction of
Satellite 2
Direction of
Satellite 1
Direction of
Satellite 3 Wavefront
for Sat 2
Wavefront
for Sat 3
Wavefront
for Sat 1
Integer candidate
locations
Correct integer
candidate
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Traditional Ambiguity Resolution The integer search picks one set of integer ambiguities and then
ignores information still available in the search space
‘Repeated searching’ is effective, but a band-aid
Large disparity between precision of the float solution and the
fixed solution
Incorrect fixing leads to position outliers with low reported
precisions
Po
sit
ion
Err
or
[m]
Time
Float Solution
Correctly
Fixed Solution
Incorrectly Fixed Solution
HD-GNSS involves state-of-the-art techniques for
processing GNSS carrier phase data, including:
– Generalized method for dealing with biases on
carrier phase data
– Using all the information within the search space
to provide statistically optimum ambiguity
resolution
– Improved apriori measurement noise models
– Rigorous generation of aposteriori position
precisions
Techniques have been made possible with the advent
of high performance microprocessors
HD-GNSS Basics
HD-GNSS BasicsAmbiguity Resolution in One Dimension (1D)
Probability Distribution of
estimated Float-Solution
(assumed Gaussian)
Flo
at-
am
big
uity
estim
ate
Most-Probable Integer
Ambiguity is closest to
float ambiguity estimate
Direction of
Satellite
Wa
ve
fro
nts
(In
teg
er
Am
big
uity
Ca
nd
ida
tes)
0 +1 +2 +3-1-2-3-4 +4-5
All integer candidates that
fall within the bounds of the
float solution are considered
in HD-GNSS solution
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9
Bia
se
d F
loa
t-A
mb
igu
ity
Estim
ate
HD-GNSS Basics Biased Float Solution 1D
Wa
ve
fro
nts
(In
teg
er
Am
big
uity
Ca
nd
ida
tes)
Direction of
Satellite0 +1 +2 +3-1-2-3-4-5 +4
Correct Integer Ambiguity
Candidate has Low
Probability based on
Biased Float-Solution
Must not discard!
Best integer solution based
on traditional ambiguity
resolution approach
Large bias in
Float Solution
HD-GNSS Basics Consideration of biased ambiguities (1D)
Wa
ve
fro
nts
(In
teg
er
Am
big
uity
Ca
nd
ida
tes)
Float-ambiguity
estimate
Bias & Probability of each integer
candidate is considered in the
formation of the HD-GNSS solution
Direction of
Satellite0 +1 +2 +3-1-2-3-4 +4-5
Require an over-determined
solution to be able to assess
the quality of each integer
ambiguity candidate
Precisions reported by
HD-GNSS solution
encapsulate distribution
of integer candidates
Direction of
Satellite 2
Wavefront
for Sat 2
Direction of
Satellite 3
Wavefront
for Sat 3
Direction of
Satellite 1
Wavefront
for Sat 1
HD-GNSS Basics Assessing integer candidate quality (2D)
Quality of each integer
candidate can be made
with an over-determined
number of satellites
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HD-GNSS -Precision Based Surveying
Reported precisions give a statistical measure of the
quality of a position solution
Need to understand that precisions are normally
given at a particular confidence level (e.g. 68%, 95%)
East
North
Up
3D Error Ellipsoid
East
North
2D Error Ellipse
Reported horizontal and vertical precisions are
a function of:
– Satellite geometry
(more satellites = improved precision);
– Measurement errors (primary multi-path)
(smaller measurement errors = improved precision)
– The term “initialized” is redefinedRTK started – precisions dependent on the environment
HD-GNSS -Precision Based Surveying
Excerpt from RTK Market Requirements Document Jan 2003:
“User-selectable accuracies:
The RTK customer should be able to specify a particular application, which
automatically drives different elements of the RTK engine and produces appropriately
accurate positions”
HD-GNSS -Precision Based Surveying HD-GNSS delivers seamless convergence to the same traditional
‘fixed’ precisions levels – fast!
An important aspect of the scheme is that it delivers
corresponding converging precisions
The polarized switch from ‘float to fixed’ is gone (and these terms!)
An environment that caused a ‘bad init’ reports higher precisions
Po
sit
ion
Err
or
[m]
Time
Float Solution
Incorrectly Fixed Solution
HD-GNSS
Solution
Correctly
Fixed Solution
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11
HD-GNSS -Positioning advantage example, 11km
HD-GNSS SolutionFloat Solution
HD-GNSS Solution produced
where conventional fixed
solution would have failed44 sec
HD-GNSS -Positioning advantage example, 11km
HD-GNSS Solution
Float Solution
HD-GNSS Solution produced
where conventional fixed
solution would have failed
44 sec
HD-GNSS - Summary
Precision-based measurements
Start measuring sooner
Shorter occupation times
Measure with confidence in
more challenging
environments
(Trimble R8/R7/R6/R5/R4)
(Trimble R10)
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12
xFill Extension Service- Basics of field operation
There are times when reference
GNSS corrections are blocked
Rover positioning normally
ceases soon after reference
GNSS correction interruption
The Trimble RTX technology
delivers precise GNSS orbit and
clock data to users via satellite
xFill helps to maintain precise
rover positioning while normal
GNSS corrections are blocked
xFill- RTX Network Concepts
Trimble has established a
global GNSS receiver tracking
network ~ 100 stations
GNSS data from the tracking station network are
processed at a central RTX control center (server)
to produce cm-level satellite orbits and clocks
RTX – Satellite
Orbits and
Clocks
xFill Extension Service
RTX Control
Center
RTX L-band
Satellite
Virtual Reference
Station (VRS) correction
R10 Rover
Physical Reference
Station correction
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RTX L-Band Delivery Services
RTX Service L-Band Frequency [MHz]
Bandwidth [bits / second]
Coverage
RTXWN 1557.8615
600
Western North America
RTXEN 1557.8590
600
Eastern North America
RTXEA 1539.9525
600
Europe, portions of Russia, Africa and
the Commonwealth Independent
States
RTXAP 1539.8325
600
Russia, Asia and Australasia
RTX services provided via Inmarsat satellites in
equatorial, geosynchronous orbits
GNSS errors
for the rover
Differencing Errors
Usual method when primary stream available
Errors common to single base / rover cancel
VRS stream errors similar to rover using
interpolation – appears as a short baseline
GNSS errors for the
reference station / VRS
Residual errors in
difference GNSS
Modelling Errors
Rover must operate autonomously
RTX service provides satellite clock & orbit
Modeled errors removed at rover
Local atmospheric effects estimated
GNSS errors
for the rover
GNSS errors
modeled in RTK
stream
GNSS atmosphere
algorithms used in
RTX
Residual errors
in Trimble RTX
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SBL/VRS Position SBL/VRS PositionPosition Outage
Primary Stream Blocked
Radio or Cellular Modem signal unavailable
Physical Reference
Station Correction
SBL/VRS Position SBL/VRS PositionxFill
xFill using RTX L-band stream
Rover generate positions autonomously
Physical Reference
Station Correction
RTX L-band
Satellite
Field Considerations
xFill starts seamlessly without convergence
Effect of loss of L-band RTX stream
– xFill continues for 20-secs (current time-out)
– Resumes without delay for up to 5-minutes
Effect of loss of GNSS satellites
– Loss and Gain OK if 4 satellite maintained
– Drop to 3 satellites requires radio-link to re-start
– Precisions affected by satellite geometry (PDOP)
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xFill Positioning Example
xFill Positioning Example
Summary
Shift from traditional Initialization Algorithms
HD-GNSS sets a new industry benchmark for
RTK technology and performance
Precision Based GNSS allows you to measure
more confidently and be more productive
xFill seamlessly bridges correction-stream
drop outs (radio or GPRS modem)
2/4/2014
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Questions?