Using SBAS to Enhance GBAS User
Availability: Results and Extensions
Sam Pullen, Ming Luo, Todd Walter, and Per Enge
Stanford University

spullen@stanford.edu
EIWAC 2010

Tokyo, JapanPaper EN-030
12 November 2010

Motivation• GBAS availability is limited by the “geometryscreening” implemented to mitigate hypotheticalworst-case ionospheric anomalies.
– Ground screening has severe impact on CAT I LAAS.
– Airborne screening used in GAST-D (for CAT III) remains
driven by ionospheric threats.

• External information is the key to removing thisconstraint.

• Three approaches have been envisioned:
1) Use of certified SBAS where it now or will exist
2) Use of uncertified “COTS” monitoring networks

3) Use of space weather forecasts and “nowcasts”
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Severe Ionospheric Anomaly in CONUS
on 20 November 2003
20:15 UT

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21:00 UT

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Time Variation of Ionospheric Delay on
20 November 2003

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Resulting Ionospheric Anomaly Threat
Model for CONUS

Slant iono. gradient bound (mm/km)

425
375

Flat 375mm/kmFlat 425mm/km

Linear bound:ybnd (mm/km) = 375 +50(el-15)/50

300200(note: plot notprecisely toscale)

Also bounds on:
Front speed wrt. ground: ≤ 750 m/s
Front width: 25 – 200 km
Total differential delay ≤ 50 m

100

515

30

45

6590

SV elevation angle (deg)

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Worst-Case Impact on CAT I GBAS
Maximum Ionospheric Error in Vertical (MIEV) at Memphis(24-Satellite SPS-Standard GPS Constellation)

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Use of External Information in GBAS

6Parameter Inflation Required to Remove
Unsafe Subset Geometries
Use spr_gnd / P-value inflation algorithm described in{Ramakrishnan, et al, ION NTM 2008}

10-meter
VAL

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SBAS to Augment GBAS (1):Today’s SBAS Coverage
Source: T. Walter, et al, ION ITM 2010

MSAS
WAAS

SBAS LPV
Availability

EGNOS

SBAS to Augment GBAS (2):Future SBAS Network Expansion
Source: T. Walter, et al, ION ITM 2010

SBAS to Augment GBAS (3):
SBAS by 2025 (GPS L1-L5 w/Expansion)
Source: T. Walter, et al, ION ITM 2010

SDCM
WAAS

SBAS LPVAvailability

EGNOS

GAGANMSASGBAS Use of WAAS GIVE Values

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GIVE Method Case 1:
All IGP’s are “Good”

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GIVE Method Case 2:
One IGP is “Not Observed”

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GIVE Method Cases 3 and 4:
One IGP is Either “Neutral” or “Bad”“Neutral” Case

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“Bad” Case

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Validation via UIVE at Local Area
Monitor (LAM) Site
Source: J. Rife, et al, IEEE PLANS 2006

• UIVE > 13 m threshold violated (briefly)  0.6 % of the time

• Proposed rules for GBAS are somewhat stricter
 Need to retain geometry screening as a backup mode
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SBAS UDRE for Clock/Ephemeris
Monitoring
UDRE
Value

UDRE
Integer

GBAS
Class.

Ephemeris
MDE≤ 50.0 m

0 – 12Good500 m

150.0 m

13

OK1500 m

Not
Monitored14

Neutral

GBAS value( 2700 m)

Do Not Use

15Do NotUseExcludefrom Use

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Ensuresthatephemeris
threat neverlimits CAT Iavailability

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SBAS SDM: Support of GBAS Approvalof New Satellites
Recent Work of Dr. R. Eric Phelts at Stanford
600
Metric 1
Metric 2
Metric 3
Metric 4
Max % Difference

500% Difference

400

Maximum of 4
SDM metrics used
to demonstratelack of signal
deformation

300

200

10000510152025303520 61 33 34 35 36 48 48 38 39 46 58 43 41 55 56 53 54 59 51 45 47 60 24 25 26 27 44 57 30 52 23 G1 G2
M R A A A A M M A A R M R R M R M R R R R R R A A A A R M A M A

PRN – SVN – Block Type
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Alternatives to SBAS
• Despite expected growth of SBAS, some GBAS sites
will lie outside good SBAS coverage– Also, no assurance that all SBAS systems will satisfy GIVE
and UDRE performance assumptions.

• Two alternatives are worth considering:– Running SBAS-like algorithms on outputs of existing,“uncertified” receiver networks– Using space weather products now being developed:» “nowcasts” of the real-time situation» 3 – 6 hour (?) look-ahead forecasts• Both of these alternatives require replacements forhigh-integrity SBAS processors and datalinks
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Use of Regional Networks:
Australian Example

 Enhancements of existing ground receiver networks cansubstitute for SBAS in specific regions.
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Concerns with Use of ExternalInformation (1)• The original “benefits case” for GBAS assumes thateach site operates independently, as do ILS and MLS.
– GBAS precision approaches are “ILS-lookalike”.

• Therefore, GBAS cannot rely upon externalinformation.
– GBAS-only methods run in the background at all times.
– For example, when SBAS cannot guarantee that no threat
exists, existing GBAS algorithms still provide required
integrity assurance, but with reduced availability.

• Even if GBAS can operate without external
information, the FAA is hesitant to rely on it to meetadvertised performance benchmarks.
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Concerns with Use of External
Information (2)• Non-aviation substitutes for SBAS are technicallyfeasible but require guarantees that information
provided is “safe” to civil aviation standards.
• Existing receiver networks must be “certifiable”
without requiring “SBAS” levels of coding andredundancy (otherwise, not cost-effective).
• Beyond network outputs, automated data-transfer
mechanism to each GBAS site must also be certified.
− NextGen mission statement highlights the future importanceand data-sharing networks, but most ATM information hasless direct safety impact (or does it?)− Understanding and providing this capability should be part of
NextGen/SESAR/etc. and should not be limited to GNSS.
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Summary• The use of external information is the most costeffective near-term way to enhance GBAS availability.
– Guarantee absence of ionospheric anomalies» Enhance precision approach availability» Enable other uses of GBAS (“DCPS”)– Monitor GNSS satellites to much tighter tolerances• SBAS is the most convenient way to obtain thisinformation– Algorithms and datalinks are already certified– Information delivered in timely manner on L1 frequency• Where SBAS is not suitable, alternatives exist, butnew safety certification is needed.
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Questions?
• Thank you for your attention。
ご清聴は、ありがとうございました。

• Questions are welcome!
質問だったら、遠慮しないで、英語にも日本語にも伺ってください。

Backup Slides follow…

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