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T. Sakai, K. Matsunaga, K. Hoshinoo, K. Ito, ENRI T. Walter, Stanford UniversityPowerPoint Presentation

T. Sakai, K. Matsunaga, K. Hoshinoo, K. Ito, ENRI T. Walter, Stanford University

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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:
- 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

- 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

@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

- 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

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

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

- 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

- 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

- 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

- 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

- (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)

- 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

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

- 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

- 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)

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)

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

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

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 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

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

- 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

- 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

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

- 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

- 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

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