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Conference Presentations - Assessment of a New Rover Enhanced Network Based RTK GNSS Data Processing Strategy
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Assessment of a New Rover Enhanced
Network Based RTK GNSS Data Processing
Strategy
Nicholas Zinas
Ph.D. Researcher
Department of Civil, Environmental and Geomatic Engineering
UCL, UK
ION GNSS 2009, Savannah-Georgia
Overview
• A very short Introduction to Network RTK
• Sensor Networks and Multiple Rover Concepts
• The Multiple Rover Positioning Algorithm
• Sample Test Network
• Results
• Conclusions and Recommendations
2
Evolution of GPS Networks
3
2. Single Baseline RTK positioning using the Pseudoranges
(DGPS)
1. Active GPS reference Stations
3. Single Baseline RTK positioning using the Carrier Phase
and Pseudorange observables (OTF RTK)
4. RTK clusters broadcasting corrections from the closest
reference station
RTK Clusters Vs RTK Networks
• Early RTK Networks appear around 2003.
RTK Clusters RTK Networks
Vs
RTK Corrections provided
based on one reference
station
RTK Corrections provided
based on most or all
reference stations
One Way Communication
Links
One Way Or Two Way
Communication Links
Processing at User site Processing either at the
user site or at a CPF4
Network RTK Processes
1. Resolution of the the Ambiguities between the Reference
Stations
2. Generation of Corrections
3. Transmission of Corrections
• Errors = Carrier Phase Range Observations – Geometric Range
• Errors = As derived from Linear Combinations
• Errors = Estimated as Unknown Parameters
•FKP, VRS, MAC…5
Transmission of Corrections
6
FKP - Reference stations estimate errors and broadcast
correction parameters to users
VRS - Estimates network errors and interpolates to
approximate position of rover
MAC – Broadcasts raw observations from Master
Station and correction differences from the Auxiliary
Stations
Sensor Networks & Multiple Rover
Algorithms (1)
A wireless sensor network (WSN) is a wireless network
consisting of spatially distributed autonomous devices using
sensors to cooperatively monitor physical or environmental
conditions ,…., at different locations [Kay et al, 2004].
A Real Time GNSS Network consists
of spatially distributed sensors of
GNSS signals.
Users of the network Additional Sensors
8
Sensor Networks & Multiple Rover
Algorithms (2)
Lachapelle (1993), introduces a quadruple receiver
configuration approach to reliably resolve ambiguities on the fly
using the integer ambiguities closure constraints.
Luo (2003), presents a method to realise the relative precise
positioning of multiple moving platforms (MultiKin)
Alves (2004), proposes a tightly coupled approach where the
precise rover positions are estimated with network ambiguities
Luo (1999), shows that the time to fix can be reduced by 50%
in the case of three moving platforms.
9
The Rover Enhanced Network Based
GNSS RTK Algorithm*
…requires a centralized
network architecture with two
way communication means
established and that all
available data is collected at
the Central Processing Facility
(CPF).
*Algorithm implemented in the UCL GNSS software suite10
Step 1: Multiple Epoch Reference Station
Ambiguity Resolution
Helmert-Wolf
Method
ambiguities resolved?no yes
Collect
Next
epochBack
substitutionLAMBDA
Global
ParametersDD Ambiguities
Local
ParametersRelative Zenith
Ionospheric
Estimates
Float
estimates
Collect data for a number
of epochs
11
Step 2: Filtering & Interpolation of the
Ionospheric Estimates
The moving window is defined by the number of epochs
(n) needed for ambiguity resolution
An Exponential Moving Average is applied on the moving
window time series
A Distance Weighted Linear Interpolation is then
performed (Gao et al 1997)*12
A) Reference
Station
System &
User System
C) Increase
redundancy using
information from all
Ref Stations
Step 3: Multiple Rover Positioning: Method (1/2)
B CA
B) Selection of:
Rover ‘Shortest Walk’
Primary Rover
Primary Ref Station
A
1
13
Unknown Parameters
Rover position vectors
Between Rover ‘Shortest Walk’
Primary Rover to Primary Ref Station
Single Epoch Weighted
Least Squares Estimator
Step 3: Multiple Rover Positioning: Method (2/2)
N
N
14
South California Integrated GNSS Test Sub
Network (2/2)December 2007
Starting:00.00 UTC
Period: 2hrs 8 mins
3 Reference
Stations
4 Rovers
Distances between the
Reference Stations
Ambiguities for the
shortest RS baselines
resolved after 41 mins. 16
106km 72km
69km
Double Difference Residuals
0102030405060708090
-0.1
-0.05
0
0.05
172860 174710 176560 178410 180260
E L
E V
A T
I O
N
(de
gre
es)
DD
re
sid
ual
s (m
ete
rs)
G P S T I M E (sec)
L1 L2
PRN 21
PRN 18
0102030405060708090
-0.1
-0.05
0
0.05
172860 174710 176560 178410 180260
E L
E V
A T
I O
N
(de
gre
es)
DD
re
sid
ual
s (m
ete
rs)
GPS T I M E (sec)
L1 L2
PRN 21
PRN 18
(1σ) (rms)
L1 = 1.2cm 1.35cm
L2 = 2.2cm 2.39cm
(1σ ) (rms)
L1 = 1. 7cm 1.48cm
L2 = 2.7cm 2.49cm
106km
72km
17
Relative Zenith Ionospheric Estimates
-0.1
-0.05
0
0.05
0.1
172860 174710 176560 178410 180260met
ers
G P S T I M E (sec)
-0.1
-0.05
0
0.05
0.1
172860 174710 176560 178410 180260met
ers
G P S T I M E (sec)
106km
72km (rms)
L1 = 1.81cm
(rms)
L1 = 2.92cm
18
Multiple Rover Positioning (1)
Rover Baselines
Primary Rover – Primary RS
Multiple Rover
Approach
Vs
• Single Baseline
Solution (SBS)
• Single Rover Network
Solution (SNS)
RTK processing simulated by
Single Epoch Positioning
19
31km
13km
19km
25km
Multiple Rover Positioning (2)
Station SBS SNS Multiple
Rover
BGIS
RHCL
OXYC
WRHS
94.1%
85.6%
75.8%
87.5%
99.3%
99.3%
96.0%
79.5%
99.3%
99.3%
99.3%
97.4%
Ambiguity success rates for each rover applying the
relative zenith ionospheric estimates
20
Multiple Rover Positioning (3)
-0.05
-0.03
-0.01
0.01
0.03
0.05
-0.04 -0.02 0 0.02 0.04
No
rth
ings
(m)
Eastings (m) -0.05
-0.03
-0.01
0.01
0.03
0.05
-0.04 -0.02 0 0.02 0.04
No
rth
ings
(m)
Eastings (m)-0.05
-0.03
-0.01
0.01
0.03
0.05
-0.04 -0.02 0 0.02 0.04N
ort
hin
g s
(m)
Eastings (m)
RHCL
(MultiRov)
OXYC
(MultiRov)
WRHS
(MultiRov)
21
Conclusions & Recommendations (1/2)
Multiple Rover Approach Improved Ambiguity
Success Rates
Greater impact for rovers
operating outside the
network
Geometry of Solution
affects the precision
22
Performs better than
both SBS and SNS
solutions
Conclusions & Recommendations (2/2)
23
Multiple Rover Approach Beneficial for Operators
that want to expand their
sphere of influence
May prove advantageous
for coastal or near shore
surveys
Improved atmospheric
modeling
Single Frequency users
could also benefit