Download C-NAV PPP sub decimeter Rev2.1

C-Nav: Global SBAS for Sub-decimeter Precise Point
Positioning
C-Nav’s Global Satellite Based Augmentation System (SBAS) is a Precise Point Positioning
(PPP) capability with worldwide reach. Recent system modifications have resulted in
further improvements to absolute positioning accuracy at the decimeter level. Precise
Point Positioning performance, compared against long term International Terrestrial
Reference Frame (ITRF) absolute coordinates for the global monitoring network, confirms
the improvements.
The C-Nav service is managed out of two Central Processing facilities each employing an
advanced version of NASA’s Jet Propulsion Laboratory’s real-time GIPSY 1 (RTG) suite.
Originally developed for precise real-time orbit and clock determination of GPS for NASA’s
space program, C-Nav’s RTG forms the kernel of the latest improvements to C-Nav’s
positioning accuracy. Distribution of the RTG correction parameters is via highly optimized
data streams ensuring maximum signal strength across the L-Band communication
satellite footprints to the worldwide C-Nav user community.
This paper discusses recent advances using combined GPS and L-Band receiver hardware,
tropospheric and tidal modeling algorithms, the integration of inertial sensors and reducing
convergence time for the Precise Point Positioning algorithm to reach optimal dynamic
precision. Not discussed in this paper is RTK Extend™, the simultaneous use of Real-time
Kinematic (RTK) and orbit and clock data allowing the receiver to operate for up to 15
minutes without any gross impact on RTK positioning accuracy when terrestrial data links
are temporarily interrupted.
INTRODUCTION
Removing positioning uncertainties from the data acquisition process enhances resolution
while providing unprecedented levels of dynamic accuracy. For Dynamic Positioning (DP)
applications, the stability and precision of C-Nav provides the ideal dynamic position input,
optimizing thruster corrections and reducing fuel costs. C-Nav’s highly stable and precise
1
GIPSY - GPS-Inferred Positioning System
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height component means that accurate real-time height is also available, reducing the
need for costly tide gauges by providing a precise sea surface obs ervable for ocean
monitoring and co-tidal models.
Apart from the offshore and maritime community, C-Nav can be used for:
-
Land surveying, augmenting RTK projects
-
Military activities, including Mine Counter Measures
-
Airborne UAV positioning
-
Autonomous robotic vehicles and AUVs
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Aircraft positioning for geopotential surveys and LiDAR acquisition
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Naval charting surveys to IHO SP44 Special Order and Order 1
All these critical tasks benefit by real-time decimeter positioning solutions that avoid the
costly GNSS ground infrastructure commonly associa ted with such high performance.
The principle of GNSS augmentation is to provide additional data in real-time to refine the
observables used within the position solution. Unlike regional Satellite Based Augmentation
Systems such as WAAS and EGNOS and traditional DGPS, C-Nav’s SBAS network
generates a single set of GPS clock and orbit corrections with global validity. By taking an
approach at the fundamental level using the NASA / JPL proven RTG solution, C-Nav’s
GNSS augmentation provides for:
Correction
Satellite orbit
Satellite clock
Ionosphere
Troposphere
Multi-path
Receiver
Earth Tides
Implementation
1 Minute RTG
1-2 Second RTG
C-Nav L1/L2 Receiver Hardware
UNB WAAS Model
Multi-Path Mitigation Software and Antenna Technology
C-Nav L1/L2/L-Band Hardware
Sinko Model incorporated in the receiver
Table 1 GNSS fundamental augmentation
The C-Nav correction stream consists of the RTG generated GNSS precise orbit and clock
values for each SV differenced with respect to the GNSS broadcast ephemeris. These are
optimized for near-global distribution via L-Band communication satellites. The signal is
received by the C-Nav receivers through the same antenna as the GPS L1 and L2 signals.
C-Nav’s GNSS comprises of four main segments:
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−
Ground - Global Network
−
Control - Orbit and Clock Process
−
Space - Signal Distribution
−
User and System Monitors
Because C-Nav’s RTG solution operates at the fundamental GPS level, the inherent errors
associated with traditional DGPS reference stations are avoided. Instead, the ground
infrastructure comprises of a global network of reference stations and system monitors
providing a quality control feedback loop and completing the system cycle to ensure
performance metrics are maintained for the entire global C-Nav user family.
GROUND SEGMENT
GNSS
Current
C-Nav
66
GPS
12
GLONASS
8
Galileo
Beidou
Planned
70
18
29
29
3
Comments
Global
Global
Global
Global
PR China
Table 2 C- Nav and GNSS Ground Segme nt
Recent enhancements to the tracking network include an additional 12 reference / monitor
stations improving station visibility in the southern hemisphere. With the exception of
South Africa, each GPS satellite is tracked by a minimum of seven stations. C-Nav’s
proprietary SatVis tool provides common station visibility and maximum DOP values for
each satellite to optimize the ground tracking network locations.
C-Nav’s reference station GNSS receiver infrastructure has been upgraded to include two
NCT-2100D GPS L1/L2 engines operating from a single IGS style Choke Ring antenna
supplemented where necessary by C-Nav iTC user receivers to monitor the L-Band signal
strength and system performance. The number of GPS receivers at each ground station,
compounded by the number of simultaneous observations to each satellite from the
station network, provides a unique level of redundancy and unprecedented system
robustness. Each station is equipped with stable, minimized multi-path antenna
installations and located within a secure facility with secure communication data links to
the control segment and with power backup. Historical analysis demonstrates that station
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downtime is minimal and the quantity of stations available within the network ensures
continued system performance even with multiple simultaneous failures.
CONTROL SEGMENT
The Control segment consists of two independently operating Control Centers; one in
Torrance, California, the other in Moline, Illinois. Each Control Center receives the full
complement of reference station data with a typical latency of less than 2 seconds. High
speed two-way communications provide continuous high capacity feeds between the
Control Centers and JPL’s own hub that acts as a further backup to C-Nav.
There are two production layers - Primary and Secondary. Each Control Center handle s the
data completely independently of the other producing two sets of correction values per
center.
Tertiary and Quaternary layers (not shown) within the infrastructure allow testing and
approval of software updates and workflow scenarios prior to release into the higher
layers production layers. For system robustness and control, all computers use the Linux
operating system with full encryption of data to preserve security.
Figure 1 Control System Data Flow
Referring to Figure 1:
Collectors manage the data and communication links to the global ground network,
collating the GNSS observations for input into the Processors.
Processors are dual Pentium units running the RTG code. Production layers operate the
same code version and state space. A six second buffer is used to collate the raw data for
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the 1Hz clock process, a highly optimized rapid calculation process using a subset of the
global network augmented with atomic frequency standards.
The once-per-minute orbit process uses the full ground segment station network. The
resulting orbit and clock parameters are differenced with respect to the global GPS
broadcast ephemeris, optimized for GNSS receiver hardware and L-Band distribution.
Distribution decides which production layer output will be uploaded and manages the
communication links to the six Land Earth Station uplinks (Net-1 and Net-2).
Monitors check the ‘heartbeat’ of all system hardware components throughout the
worldwide network infrastructure providing constant system performance metrics with
respect to geodetic truth and expected signal strength levels.
Interface provides the control, archive and performance metrics. Out-of-tolerance
conditions generate system alerts online and via pager with StarMon providing network
operator oversight and control.
The Real Time Gipsy (RTG) code used within the system is the latest state-of-the-art
implementation from NASA's Jet Propulsion Laboratory and uses a number of different
models given in Table 1.
Coordinates for the reference stations are updated at least four times a year to account
for tectonic plate motion and for any local variations. These coordinates, accurate to ∼1cm
are determined from a periodic global network adjustment constrained to the International
Terrestrial Reference Frame. The C-Nav system time is steered to GPS system time via a
GNSS receiver co-located at the United States Naval Observatory (USNO). Additional
reference stations with atomic frequency standards are available within the ground
network to act as control backups and for quality control of the clock process.
CONTROL PERFORMANCE
In an assessment for the last two weeks of July 2006, real-time calculated orbit and clock
values were compared with post-processed IGS Final Precise orbit and clock. Figure 2 and
Figure 3 (24 July 2006) show typical daily comparisons for this period. The User Range
Error (URE) is a composite of the radial and clock. The high correlation of radial and clock
absolute values tend to compensate for each other resulting in relatively low URE figures.
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Figure 2 orbit and clock performance vs. IGS Final, 24 July 2006
Figure 3 orbit and clock mean RMS performance vs. IGS Final, 24 July 2006
In analyzing the data, it was noted that there was a partial correlation between satellites
experiencing eclipse events and larger RMS values. Further, there appeared to be a
difference between Block II/IIA and IIR/IIRM SVs. The comparison between the real-time
orbits and clocks versus IGS post-processed Final orbits and clocks disclosed that Block
II/IIA Eclipsing to be an outlier event - during the period, only one II/IIA (PRN10) was in
eclipse. With the exception of PRN10, URE agreed at the 6-8cm level. Given that IGS Final
has an orbit precision of 5cm and a clock of 0.1ns (3cm), this was an unexpectedly close
comparison particularly as C-Nav is a real-time solution while IGS is a post-processed
combination from multiple analysis centers.
Model
Geopotential
RTG
JGM3
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Solid Tide (Dynamic)
Solid Tide
(Kinematic)
Pole Tide
Ocean Tide
Ocean Loading
Troposphere
Solar Pressure
Relativity (Dynamic)
Relativity
(Kinematic)
Phase Windup
Wahr 3x3 - 47 constituents
Included
Included
CSR + TEG2B 20x10 - 767
constituents
Included
Neill
T20
Included
Included
Included
Table 3 RTG Models in current use
GNSS MONITORING
GPS performance has improved significantly with enhancements in the GPS control
segment and the addition of more ground stations. However, the response time by the
GPS Master Control Station to a failed satellite component is not immediate. This was
demonstrated last year with a clock failure in PRN 30 that took 12 minutes to identify. The
C-Nav Control Centers identified the same problem within 36 seconds of the event and
flagged PRN 30 unhealthy. Analysis of the nearest monitor (Johannesburg) showed no
position degradation during this incident demonstrating the value of the system's robust
global monitoring capability, fast clock updates and low latency delivery rate.
SPACE SEGMENT
The C-Nav space segment consists of six geostationary communication satellites providing
global hi-power L-Band distribution between approximately 75° north and south latitudes.
The communication satellite constellation is uplinked through six Land Earth Stations
configured as NET-1 or NET -2. Each LES is equipped with Primary and Secondary layers of
equipment. Each layer receives corrections from both Control Centers with the Primary
layer comparing the two correction data sets for integrity and then, independently,
selecting the data set to be uplinked.
Data flow between the Control Centers and the Land Earth Stations is via secure highspeed cable and VSAT with ISDN backups.
Land Earth Station
Satellite
Satellite
longitude
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NET-1
Laurentides, Canada
Burum, NL
Perth, WA
Inmarsat High Power
Inmarsat High Power
Inmarsat High Power
98°W
25°E
109°E
NET-2
Santa Paula, Ca, USA
Southbury, Cn, USA
Auckland, NZ
Inmarsat High Power
Inmarsat High Power
Inmarsat High Power
142°W
15.5°W
143.5°E
The six communication satellites are constantly monitored by the Control Centers to
ensure service continuity, with backup channel capacity available on adjacent satellites
over the same regions. In addition, the network of ground stations monitors the received
L-Band signal strength to confirm satellite transmission signal levels, and the veracity and
precision of received data, in a continuous baseline comparison process.
USER SEGMENT RECEIVER TECHNOLOGY
The C-Nav stable of receiver technology benefits from an extraordinary user community of
more 40,000 units. At the heart of every dual frequency receiver is the Touchstone 4 ASIC
precise GPS L1/L2 technology, proprietary L-Band receiver and tri-band antenna for
L1/L2/L-Band reception.
C-Nav’s positioning algorithm uses a Kalman filter to solve for satellite and receiver
channel biases plus the code-phase floating ambiguities. A least squares solution is then
applied to calculate the position based on phase-smoothed refraction and bias-corrected
code observables. Unlike terrestrial augmentation methods, which suffer from distorting
Earth tides, C-Nav’s space-based solution has Earth tides removed through an algorithm
accessing the proprietary Sinko Earth tide model.
USER PERFORMANCE
An added benefit of the C-Nav receiver algorithm has been the enhancement of
positioning performance with WAAS in North America. Dynamic performance is amply
illustrated by the flight dataset in Figure 4 showing the difference between real-time
positions and post-processed kinematic during an airborne LiDAR program. Some GNSS
receivers, when experiencing large height changes, can introduce a height bias due to the
way that the troposphere is modeled. Note the stability of height during the rapid climb
indicating the consis tency of the receiver’s tropospheric model.
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Figure 4 Flight Real-Time StarFire minus Post Processed Kinematic L1/L2
To assess the performance of the positioning algorithm, IGS Final orbits and clocks were
used to constrain orbits and clocks via an 11th Order interpolation. The resulting correction
stream was then used to re-run the datasets and compare the resulting positions against
the standard C-Nav determined positions. This was done for six geographically diverse
sites as shown in Figure 5.
Figure 5 Positioning results versus IGS Final
The convergence times in minutes, and sigma values in meters, show little difference
between the two sets as indicated by the number of amber colored cells with differences
less than 1 minute or 1cm. Convergence times are given for accuracies less than 10cm in
each axis.
GEODESY
C-Nav’s geodesy is the latest implementation of ITRF 2005. C-Nav can be used as-is, but
to relate it to existing databases or data constrained to a local geodetic reference frame, a
4D transformation is required. This takes into account the relationship between the local
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geodetic reference frame and ITRF at a fixed epoch, plus the vector change that has
occurred between the fix ed epoch and the current epoch.
National geodetic infrastructure is increasingly being defined with respect to ITRF at a
specific epoch with an associated vector change field to account for tectonic plate motion.
These parameters permit a precise relationship between C-Nav positioning and local
mapping and can usually be found within GNSS Survey Controller devices.
A common misconception is to consider all WGS84 positions to be equivalent to ITRF. The
original definition of WGS84 was accurate to ∼2m. The current implementation of WGS84
by the GPS is closely aligned to ITRF to within a few centimeters hence it is essential to
factor into any comparison between C-Nav positions and an existing WGS84 position,
when and how that position was defined.
LOOKING FORWARD
As the GPS develops with new frequencies and new generation satellites, so the C-Nav
Ground Segment will develop to keep pace. Where the traditional DGPS vendors will
require even more transmission bandwidth to cater for the technological developments of
the future, C-Nav will only need to supplement the slight increase for the additional orbit
and clock data for each GPS satellite. As other GNSS systems such as GLONASS, Galileo,
QZSS, Compass and IRNSS develop, so these also will require a further increase in
bandwidth, but only a fraction of that demanded by the dated ground based augmentation
systems.
C-Nav is a global data stream with updates required less frequently than traditional DGPS
solutions demanding updates each second for a smaller number of visible satellites while
offering a lower accuracy. Table 4 illustrates the relative quantity of observables for each
augmentation type. The percentage differences in bandwidth between orbits+clocks
versus GBAS are considerably greater when the actual bits per observable are taken into
consideration with optimization.
GNSS
GPS L1/L2
C-Nav
Total (Obs*Sats*Hz)
30.5
(Orbit*30*1/60 + Clock*30*1)
GPS L5
GLONASS L1/L2
0
24.4
Local GBAS Station
Total (Obs*Sats*Hz)
40
(4*10*1)
20
(2*10*1)
32
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(Orbit*24*1/60 + Clock*24*1)
GLONASS L3
0
GALILEO E1, E5A, E5B, E6
Compass C1, C5, C6
Total Now
Total Future
30.5
(Orbit*30*1/60 + Clock*30*1)
30.5
(Orbit*30*1/60 + Clock*30*1)
30.5
(100%)
115.9
(+280%)
(4*8*1)
16
(2*8*1)
80
(8*10*1)
60
(6*10*1)
40
(+31%)
248
(+713%)
Table 4 Augmentation bandwidth requirements
CONCLUSIONS
•
C-Nav’s extensive 66 station global monitoring network, backed up by the 100
stations of the JPL network, has a very fast response time to anomalous GPS
behavior and uses a tighter tolerance for GPS satellite signal performance than the
GPS Master Control Station.
•
C-Nav’s clock and orbit corrections for the GPS satellites provide a User Range
Error <8cm RMS compared with IGS Final clocks and orbits.
•
The C-Nav positioning algorithm has a standard deviation of <10cm horizontal and
<20cm vertical and is capable of centimetric performance after convergence.
Convergence times depend upon the ambient satellite conditio ns during the
convergence period and can vary from 9 to 24 minutes for an accuracy less than
10cm in each axis.
•
In the unlikely event C-Nav corrections are interrupted, the system can maintain
decimeter horizontal accuracy for a period in excess of 20 minutes.
•
C-Nav’s technique of using precise orbits and clocks requires less observables and
hence less costly bandwidth than traditional GBAS. This difference will become
greater in the future as GNSS systems are enhanced and deployed.
•
The C-Nav positioning algorithm using IGS Final post-processed orbits and clocks
show no noticeable positioning improvement compared to the use of real-time
orbits and clocks.
REFERENCES
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The full version of this paper first appeared as StarFire™: A Global SBAS for Sub-Decimeter Precise
Point Positioning, authored by Kevin Dixon, MRICS BSc PGDip MBA, Product Manager at NavCom
Technology, Inc., the partner company of C&C Technologies, Inc.
Hatch R, Sharpe T and Galyean P. A Global, High-Accuracy, Differential GPS System, ION NTM
2003, Institute of Navigation National Technical Meeting, Anaheim, 22-24 Jan 2003.
Mendes V de Brito. Modeling the Neutral-atmosphere Propagation Delay in Radiometric Space
Techniques, a PhD dissertation published as Department of Geodesy and Geomatics Engineering
Technical Report No. 199, University of New Brunswick, Fredericton, New Brunswick, Canada, April
1999.
Roscoe-Hudson J, Sharpe T. Globally Corrected GPS (GcGPS): C-Nav GPS System, Dynamic
Positioning Conference, 18-19 Sep 2001.
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