Transcript Mitigating Ionospheric Threat Using a Dense Monitoring Network

ION GNSS 2007
Fort Worth, TX
Sept. 25-28, 2007
Mitigating Ionospheric Threat
Using a Dense Monitoring Network
T. Sakai, K. Matsunaga, K. Hoshinoo, K. Ito, ENRI
T. Walter, Stanford University
ION GNSS 25-28 Sept. 2007 - ENRI
Introduction
SLIDE 1
• The ionospheric effect is a major error source for SBAS:
– The ionospheric term is dominant factor of protection levels;
– Necessary to reduce GIVE values not only in the storm condition but
also in the nominal condition to improve availability of vertical guidance.
• The problem is caused by less density of IPP samples:
– The current planar fit algorithm needs inflation factor (Rirreg) and
undersampled threat model to ensure overbounding residual error;
– Solution: integrating the external network such as GEONET and CORS;
– Developed a GIVE algorithm suitable to such a situation.
• Evaluated a new GIVE algorithm with GEONET:
– 100% availability of APV-II (VAL=20m) at most of Japanese Airports;
– Still protects users; No HMI condition found.
ION GNSS 25-28 Sept. 2007 - ENRI
MSAS Status
SLIDE 2
• All facilities installed:
– 2 GEOs: MTSAT-1R (PRN 129) and
MTSAT-2 (PRN 137) on orbit;
– 4 GMSs and 2 RMSs connected with
2 MCSs;
– IOC WAAS software with localization.
• Successfully certified for aviation use:
– Broadcast test signal since summer
2005 with Message Type 0;
– Certification activities: Fall 2006 to
Spring 2007.
• Began IOC service on Sept. 27 JST
(15:00 Sept. 26 UTC).
Launch of MTSAT-1R (Photo: RSC)
ION GNSS 25-28 Sept. 2007 - ENRI
SLIDE 3
Position Accuracy
GPS
@Takayama (940058)
05/11/14 to 16 PRN129
MSAS
Horizontal
RMS 0.50m MAX 4.87m
GPS
@Takayama (940058)
05/11/14 to 16 PRN129
MSAS
Vertical
RMS 0.73m MAX 3.70m
ION GNSS 25-28 Sept. 2007 - ENRI
Concerns for MSAS
SLIDE 4
• The current MSAS is built on the IOC WAAS:
– As the first satellite navigation system developed by Japan, the design
tends to be conservative;
– The primary purpose is providing horizontal navigation means to aviation
users; Ionopsheric corrections may not be used;
– Achieves 100% availability of Enroute to NPA flight modes.
• The major concern for vertical
guidance is ionosphere:
– The ionospheric term is dominant factor
of protection levels;
– Necessary to reduce GIVE to provide
vertical guidance with reasonable
availability.
ION GNSS 25-28 Sept. 2007 - ENRI
APV-I Availability of IOC MSAS
SLIDE 5
MSAS Broadcast
06/10/17 00:00-24:00
PRN129 (MTSAT-1R)
Test Signal
Contour plot for:
APV-I Availability
HAL = 40m
VAL = 50m
Note: 100% availability
of Enroute through NPA
flight modes.
ION GNSS 25-28 Sept. 2007 - ENRI
Components of VPL
SLIDE 6
VPL
Ionosphere
(5.33 sUIRE)
Clock & Orbit
(5.33 sflt)
MSAS Broadcast
06/10/17 00:00-12:00
3011 Tokyo
PRN129 (MTSAT-1R)
Test Signal
• The ionospheric term is dominant component of Vertical Protection Level.
ION GNSS 25-28 Sept. 2007 - ENRI
Problem: Less Density of IPP
SLIDE 7
• Ionospheric component: GIVE:
– Uncertainty of estimated vertical ionospheric delay;
– Broadcast as 4-bit GIVEI index.
• Current algorithm: ‘Planar Fit’:
– Vertical delay is estimated as parameters of planar ionosphere model;
– GIVE is computed based on the formal variance of the estimation.
• The formal variance is inflated by:
– Rirreg: Inflation factor based on chi-square statistics handling the worst
case that the distribution of true residual errors is not well-sampled; a
function of the number of IPPs; Rirreg = 2.38 for 30 IPPs;
– Undersampled threat model: Margin for threat that the significant
structure of ionosphere is not captured by IPP samples; a function of
spatial distribution (weighted centroid) of available IPPs.
ION GNSS 25-28 Sept. 2007 - ENRI
Using External Network
SLIDE 8
• Integrating the external network to the SBAS:
– Increase the number of monitor stations and IPP observations
dramatically at very low cost;
– Just for ionospheric correction; Clock and orbit corrections are still
generated by internal monitor stations because the current configuration
is enough for these corrections;
– Input raw observations OR computed ionospheric delay and GIVE from
the external network; loosely-coupled systems.
• Necessary modifications:
– A new algorithm to compute vertical ionospheric delay and/or GIVE is
necessary because of a great number of observations;
– Safety switch to the current planar fit with internal monitor stations when
the external network is not available.
ION GNSS 25-28 Sept. 2007 - ENRI
Available Network: GEONET
GEONET station
MSAS station
SLIDE 9
• GEONET (GPS Earth Observation
Network):
– Operated by Geographical Survey
Institute of Japan;
– Near 1200 stations all over Japan;
– 20-30 km separation on average.
• Open to public:
– 30-second sampled archive is
available as RINEX files.
• Realtime connection:
– All stations have realtime datalink
to GSI;
– Realtime raw data stream is
available via some data providers.
ION GNSS 25-28 Sept. 2007 - ENRI
Sample IPP Distribution
SLIDE 10
• A snap shot of all IPPs
observed at all
GEONET stations at an
epoch;
• GEONET offers a great
density of IPP
observations;
• There are some Japanshape IPP clusters;
each cluster is
corresponding to the
associated satellite.
ION GNSS 25-28 Sept. 2007 - ENRI
New Algorithms
SLIDE 11
(1) Residual Bounding:
– An algorithm to compute GIVE for given vertical delays at IGPs;
– Vertical delays are given; For example, generated by planar fit;
– Determine GIVE based on observed residuals at IPPs located within 5
degrees from the IGP; Not on the formal variance of estimation;
– Improves availability of the system.
(2) Residual Optimization:
– An algorithm to optimize vertical delays at IGPs;
– Here ‘Optimum’ means the condition that sum square of residuals is
minimized;
– GIVE values are generated by residual bounding;
– Improves accuracy of the system.
ION GNSS 25-28 Sept. 2007 - ENRI
Residual Bounding (1)
SLIDE 12
• An algorithm to compute GIVE for given vertical delays at IGPs:
– The MCS knows ionospheric correction function (bilinear interpolation)
used in user receivers, Iv,broadcast(l,f), for given vertical delays at IGPs
broadcast by the MCS itself;
– Residual error between the function and each observed delay at IPP,
Iv,IPPi, can be computed;
– Determine GIVE based on the maximum of residuals at IPPs located
within 5 degrees from the IGP.
Vertical delay for user
Observed delay at IPP
ION GNSS 25-28 Sept. 2007 - ENRI
SLIDE 13
Residual Bounding (2)
Vertical
Delay
IPP measurements
Interpolated plane
for users
Confidence bound
Overbounding
largest residual
Largest residual
IGP i
IGP i+1
Location
• Determine GIVE based on the maximum of residuals at IPPs located within 5
degrees from the IGP.
ION GNSS 25-28 Sept. 2007 - ENRI
Residual Optimization
SLIDE 14
• An algorithm to optimize vertical delays at IGPs:
– Vertical delays at IGPs can also be computed based on IPP
observations as well as GIVE values;
– Again, define residual error between the user interpolation function and
each observed delay at IPP, Iv,IPPi;
– The optimum set of vertical delays minimizes the sum square of
residuals; GIVE values are minimized simultaneously;
– The optimization can be achieved by minimizing the energy function
(often called as cost function) following over IGP delays (See paper):
Function of IGP delays
ION GNSS 25-28 Sept. 2007 - ENRI
Number of Available IPPs
SLIDE 15
• The histogram of the
number of IPPs
available at each IGP
(located within 5 deg
from the IGP);
• For 68% cases, 100 or
more IPPs are
available;
• Exceeds 1000 for 27%
cases.
ION GNSS 25-28 Sept. 2007 - ENRI
GIVE by Residual Bounding (1)
SLIDE 16
Planar Fit
Residual Bounding
(All GEONET sites)
• Histogram of computed
GIVE values in typical
ionospheric condition
for two algorithms;
• Residual bounding with
GEONET offers
significantly reduced
GIVE values;
• Blue lines indicate
quantization steps for
GIVEI.
ION GNSS 25-28 Sept. 2007 - ENRI
GIVE by Residual Bounding (2)
SLIDE 17
Planar Fit
Residual Bounding
(All GEONET sites)
• Histogram of computed
GIVE values in severe
storm condition for two
algorithms;
• The result is not so
different from case of
typical condition.
ION GNSS 25-28 Sept. 2007 - ENRI
Reduction of GIVEI
SLIDE 18
Planar Fit
Residual Bounding
(All GEONET sites)
• Histogram of 4-bit
GIVEI index broadcast
to users;
• Lower limit of GIVEI is
10 for planar fit;
• Residual bounding can
reduce GIVEI as well as
GIVE values.
ION GNSS 25-28 Sept. 2007 - ENRI
Comparison with FOC WAAS
SLIDE 19
Planar Fit
(FOC WAAS)
Residual Bounding
(All GEONET sites)
• FOC WAAS: Dynamic
Rirreg, RCM, multistate storm detector,
and CNMP;
• GIVE values derived by
residual bounding are
still smaller than FOC
WAAS algorithms.
ION GNSS 25-28 Sept. 2007 - ENRI
Residual Optimization
SLIDE 20
• Histogram of difference
of IGP delays with and
without residual
optimization;
• Adjustment of IGP
delay stays 0.052m;
• In comparison with
quantization step of
0.125m, the effect is
little.
ION GNSS 25-28 Sept. 2007 - ENRI
User Position Accuracy
SLIDE 21
Planar Fit
(RMS = 1.47m)
Residual Bounding
(RMS = 1.10m)
Residual Optimization
(RMS = 1.10m)
• User vertical position
error at Tokyo in typical
ionospheric condition;
• Residual bounding
improves user position
accuracy, while residual
optimization is not
effective so much.
ION GNSS 25-28 Sept. 2007 - ENRI
Evaluation by Prototype SBAS
SLIDE 22
• Prototype SBAS software developed by ENRI (NTM 2006):
– Computer software running on PC or UNIX;
– Generates the complete 250-bit SBAS messages every seconds;
– Simulates MSAS performance with user receiver simulator;
– Available as an MSAS testbed; Measures benefit of additional monitor
stations and evaluates new candidate algorithms.
• Integration with the proposed algorithms:
– Scenario of vertical ionospheric delay and GIVE is generated based
on GEONET archive data with application of the proposed algorithms;
– The prototype generated augmentation messages with ionospheric
corrections induced as the scenario;
– Tested for typical ionospheric condition (July 2004) and severe storm
condition (October 2003).
ION GNSS 25-28 Sept. 2007 - ENRI
User Protection
SLIDE 23
PPWAD Simulation
03/10/29-31
3011 Tokyo
Condition:
Severe Storm
Algorithm:
Residual Bounding
(All GEONET sites)
• Users are still
protected by this
algorithm during
the severe storm.
ION GNSS 25-28 Sept. 2007 - ENRI
System Availability
SLIDE 24
PPWAD Simulation
04/7/22-24
Condition:
Typical Ionosphere
Algorithm:
Residual Bounding
(All GEONET sites)
Contour plot for:
APV-II Availability
HAL = 40m
VAL = 20m
ION GNSS 25-28 Sept. 2007 - ENRI
Conclusion
SLIDE 25
• Introduced new algorithms and usage of the external network to
mitigate ionospheric threats:
– Algorithms for bounding ionospheric corrections based on optimization of
residual error measured by dense monitoring network;
– Integration of GEONET as an external network.
• Evaluation by prototype SBAS software:
– Reduced GIVEI enables 100% availability of APV-II flight mode
(VAL=20m) at most of Japanese airports;
– No integrity failure (HMI condition).
• Further investigations:
– Consideration of threats against the proposed algorithms;
– Reduction of the number of stations required for residual bounding;
– Temporal variation and scintillation effects.
ION GNSS 25-28 Sept. 2007 - ENRI
Announcement
SLIDE 26
• Ionospheric delay database will be available shortly:
– The datasets used in this study; and
– Recent datasets generated daily from August 2007;
– Each dataset is a file which consists of slant delays observed at all
available GEONET stations with 300-second interval; Hardware biases
of satellites and receivers are removed;
Access to URL:
http://www.enri.go.jp/sat/pro_eng.htm