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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. Introduction. The ionospheric effect is a major error source for SBAS:

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T sakai k matsunaga k hoshinoo k ito enri t walter stanford university

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


Introduction
Introduction

  • 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.


Msas status
MSAS Status

  • 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)


Position accuracy
Position Accuracy

@Takayama (940058)

05/11/14 to 16 PRN129

@Takayama (940058)

05/11/14 to 16 PRN129

GPS

GPS

MSAS

MSAS

Horizontal

RMS 0.50m MAX 4.87m

Vertical

RMS 0.73m MAX 3.70m


Concerns for msas
Concerns for MSAS

  • 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.


Apv i availability of ioc msas
APV-I Availability of IOC MSAS

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.


Components of vpl
Components of VPL

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.


Problem less density of ipp
Problem: Less Density of IPP

  • 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.


Using external network
Using External Network

  • 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.


Available network geonet
Available Network: GEONET

  • 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.

GEONET station

MSAS station


Sample ipp distribution
Sample IPP Distribution

  • 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 Japan-shape IPP clusters; each cluster is corresponding to the associated satellite.


New algorithms
New Algorithms

  • (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.


Residual bounding 1
Residual Bounding (1)

  • 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


Residual bounding 2

Vertical

Delay

IPP measurements

Interpolated plane

for users

Confidence bound

Overbounding

largest residual

Largest residual

IGP i

IGP i+1

Location

Residual Bounding (2)

  • Determine GIVE based on the maximum of residuals at IPPs located within 5 degrees from the IGP.


Residual optimization
Residual Optimization

  • 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


Number of available ipps
Number of Available IPPs

  • 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.


Give by residual bounding 1
GIVE by Residual Bounding (1)

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.


Give by residual bounding 2
GIVE by Residual Bounding (2)

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.


Reduction of givei
Reduction of GIVEI

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.


Comparison with foc waas
Comparison with FOC WAAS

Planar Fit

(FOC WAAS)

Residual Bounding

(All GEONET sites)

  • FOC WAAS: Dynamic Rirreg, RCM, multi-state storm detector, and CNMP;

  • GIVE values derived by residual bounding are still smaller than FOC WAAS algorithms.


Residual optimization1
Residual Optimization

  • 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.


User position accuracy
User Position Accuracy

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.


Evaluation by prototype sbas
Evaluation by Prototype SBAS

  • 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).


User protection
User Protection

  • 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.


System availability
System Availability

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


Conclusion
Conclusion

  • 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.


Announcement
Announcement

  • 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