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SBAS DFMC SIS ICDEPO Inputs to IWG#26

[from Pro-SBAS studies]India - 05th Feb. 2014


2

Reminder from IWG#25

Enhanced ICD Strategy Analysis• Analysis of needs for enhanced ICD• Definition of enhanced ICD• Bandwidth Analysis • Performance analysis

Conclusion / possible way forward for SBAS DFMC ICD definition

17 January 2014 MSIL2


3


SBAS DFMC ICD1o Derived from (existing) SBAS L1 legacy ICDo Capable of sustaining high DFRE dynamics o Limited to 51SV (DOP limitation).Dynamic mask improves

but rather complex,possible safety issue,still DOP limitationo Possibility to extend to 91SV (altern. ICD with 2MT6+

general alert message), but necessitates to relax update rate (LTC) when >2 Const. Time To First Start impacted (wrt. MOPS L1 legacy)

SBAS DFMC ICD2 o Optimised (for bandwidth usage) o Capable of up to 91SV with TTFF o Simple to implement (System+User)o Yet

Rigid (max. 7 DFRE’s (possibly expandable to 8) update simultaneously in MTC, to repeat twice upon change)

Imposes DFRE stability constraints on the System (with possible performance penalty)

17 January 2014 MSIL2


21 April 2023 4


21 April 2023 PROSBAS

IWG#25 recommended to investigate possible unification of ICD1 and ICD2 in a unique and common (pre selected) “Enhanced ICD”, with an objective to gather the advantages of each of both ICD’s that is

• ICD1 like flexibility (SBAS&GNSS performances)/reactivity upon asynchronous event

• ICD2 like simplicity/BW efficiency/determinism

IWG Technical SubGroup (Stanford, EPO/Pro-SBAS) met in ESTEC in Dec.2013 (as per IWG#25 recom.)

The material which has fed this TSG is summarized in next slides, together with preliminary derivation of TSG meeting outcomes into a draft SBAS DFMC “Enhanced ICD” definition (working document)


5

Enhanced ICD Strategy Analysis

MSIL217 January 2014

IWG 25Study of asynchronous events

and of DFREI changes

Best option for Enhanced ICD

(IWG 26)

Analysis of needs for enhanced ICD (WAAS, EGNOS, SDCM)MT C limited to 7 DRFEI updates => Need for new mechanism in case of more DFREI updates

Enhanced ICD Bandwidth Analysis

Enhanced ICD Performance Analysis

Definition of Enhanced ICDIntroduction of new «MT 6 – like» messages


7MSIL217 January 2014

Low variable DFREsEx: WAAS

No need (tbc) for dynamic MT_6_1/2 mechanism. DFRE ICD

Variable DFREsEx: EGNOS

Need for dynamic MT_6_1/2 mechanism. Enhanced ICD

High variable DFREsEx: SDCM

Need for MT_6_1/2 mechanism

(more frequent usage than EGNOS case)

Num changes in

12-sec windo

w

Percentage of events (%)

1 Cons

(24 SVs)

2 Cons

(48 SVs)

3 Cons

(72 SVs)

4 Cons

(91 SVs)

More than 7 0 0.034 0.116 0.197

Num changes in 12-

sec window


1 Cons (24 SVs)

2 Cons (48 SVs)

3 Cons (72 SVs)

4 Cons (91 SVs)

More than 7 4.845 15.239 34.164 50.232

T. Walter, J. Blanch and P. Enge (2013), “Implementation of the L5 SBAS MOPS”, IONGNSS

2013 Proceedings.

Num changes in

30-sec windo

w


1 Cons

(24 SVs)

2 Cons

(48 SVs)

3 Cons

(72 SVs)

4 Cons

(91 SVs)

More than 7 0 0.0 0.0 0.0

Analysis of needs for enhanced ICD : DFREI changes


21 April 2023 The European GNSS Programme 8

Extraction of the Histogram of the number of UDRE changes in a 12-

seconds window for 1 to 4 constellations (UDREs from

EGNOS real data).



Remarks

Previous statistics do not discriminate between DFRE increase and decrease cases (e.g. if decrease, DFRE change may not be repeated through consecutive MTC)

Capability to accommodate up to 8 DFREi’s (instead 7) is now proposed in new ICD draft (pending 6*4 bits preamble is baselined)

Maintaining MTC for 18s (NPA Time-Out) upon DFREi change would increase the probability of update of > 7/8 DFREí

Analysis of needs for enhanced ICD: DFREI changes


10

Proposed definition of enhanced ICD (MT6.1/6.2)

Principle : introduce ICD1 MT6 like messages (MT6.1, MT6.2) into ICD2, to improve DFRE update flexibility• Up to 53 SV in mask (e.g. 24 GNSS1, 24 GNSS2, 5 Geos)

o MT6.1 is used in place of MTC (no MTC), thus eliminating DFRE update constraints inherent to MTC

o Handling of MT6.1 is similar to MT6 for SBAS L1 standard

• More than 53 SV in masko MTC is mandatory. It is used for alertso MTC format could be tailored to number of SV mask (allowing

more than 7/8 DFREi in spare place)o MT6.1/MT6.2 can be used in complement to MTC in situations of

multiple (>7/8) DFRE update, to minimize user impactso Handling of MT6.1/MT6.2 : see next slides

In terms of implementation• Introduction of MT6.1/MT6.2 should not deeply affect the

foundations/basic mechanisms, nor the simplicity of ICD2

• Moreover MT6.1/MT6.2 could be SBAS optional mechanism 17 January 2014 MSIL2



Proposed definition of enhanced ICD - Messages


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MT6.1/6.2 dynamic mechanism (when more than 53 SV)

• System computes DFREIs each epoch and checks all SVs in mask, if DFREI has changed with respect to any previously broadcast DFREi that is still valid (12 s or 18 s (TBC))

• A change in DFREI for each individual satellite with MTC is maintained during 12 s (i.e. timeout of the integrity in PA)

NB: The case of NPA users (with an integrity timeout of 18 seconds) should be discussed

• If the System detects that more than 7(or 8) DFREIs have to be changed, then MT6.1/6.2 mechanism can be activated in place of MTC, thus mitigating user performance impact


14



More than 53 SVs in mask: MT6.1 & MT6.2

No Alarm, first case• System detects change of more than 7 DFREIs at epoch when MT_C is not to be

broadcast - > MTC substituted by MT6.1 & 6.2

• Robust upon message loss: Unchanged DFRECIs and Changed DFREI sent two times within 12 s (or 18 s - TBC). If both information are lost, receiver sets the SV to Not Monitored


15



More than 53 SVs in maks: MT6.1 & MT6.2

No Alarm, second case• System detects more than 7 DFREI changes at epoch when MT_C is to be

broadcast

• MTC is substituted by MT6.1 and next epoch, MT6.2 is broadcast. • (To be discussed) Even if MT6.2 is sent 7 epochs after the previous MTC, it is

ensured to send the integrity of all SVs twice in 12 s since another MT6.1 is sent the epoch before next MTC and this MTC is substituted by MT6.2. This mechanism avoids loss of service that could be induced by MTC broadcast instead of MT6.1/6.2.

• (Alternatively) MTC could be sent in priority after 6s.


16

Proposed definition of enhanced ICDRobustness upon message loss


Robustness upon integrity message loss:• MT_C:

o Non-alarm MT_C lost without DFREI change: robustness ensured due to 12 sec DFREI timeout (PA)

o Non-alarm MT_C lost with DFREI changes: robustness ensured since DFREI change maintained during 12 sec. It is ensured that numerical changing DFREIs are sent twice in 12 sec. If both messages are lost, the receiver will have the DFREI timed-out and the corresponding SV would be set internally to Not Monitored

• MT_6_1/2: It is ensured that the new DFREI values are sent twice in 12 sec

Robustness upon alarm + message loss: As in an alarm situation, MT_C is sent 4 consecutive times, robustness upon message loss is ensured

Additionally, the System will be in charge of computing any possibility for a receiver to have misleading information, due to any possible valid message loss combination. In case this happens, the System shall send the corresponding 4 consecutive MT_C alarm messages

Robustness upon loss of other messages : OBAD(MTD), IOD mechanisms


17

Bandwidth Analysis - Scenarios


Different scenarios have been prepared for Enhanced ICD Bandwidth analysis, extrapolation of the linear fit analysis and VPL performances analyis.

Main difference between scenarios

• variable MTD update rate to increase BW margins (e.g. to be capable to adapt to SBAS with variable DFRE dynamics, to tune BW margins (e.g. for additionnal parameters (e.g. iono for L5 back-up), alarms)).

• TTFS impact to be assessed


18

Bandwidth Analysis - Scenarios


1st Set of simulations

2nd Set of simulations

• Time-to-first-start limit (SBAS L1 legacy like) => Messages update interval imposed at 120 s (incl. for MT D), except integrity (MT C) and iono. (MT 18, 26, 30) messages

• Up to 100% bandwidth could be used with a deterministic behavior

• Scheduler algorithm should not be constrained by standardization (SBAS System dependent). Simplicity of Enhanced ICD allows non-deterministic scheduler with deterministic behavior

• Simulations limited to SBAS with Low Variable DFREs (WAAS like) and Variable DFRE (EGNOS like)

• No a priori time-to-first-start (i.e. relaxed compared to L1 Legacy)

• Compute MT_D Update Intervals for leaving 15% free bandwidth

• Simulations for any type of SBAS (incl. With high dynamic DFRE like SDCM)


20

Bandwidth Analysis


First Set of simulations (Enhanced ICD)

Objective: compute bandwidth usage for complying with time-to-first-start (TTFS).NB: BW budget could be refined to account for MT_E at higher rate (e.g.60s) and relaxed MT_F, MT_G or MT_H

NoIono

Result: Bandwidth usage not larger than 100% in NoIono case even for 4 constellations


21

Bandwidth Analysis


First Set of simulations (Enhanced ICD)Iono ECAC

Result: Bandwidth usage larger than 100% in Iono ECAC in 4 const. case

Iono ECAC+AFI

Result: BW usage > 100% in Iono ECAC+AFI for 3 and 4 const. L5-only back-up cannot be implemented unless TTFS constraint is relaxed.


22

Bandwidth analysis


No Iono

Iono ECAC

Iono ECAC+AFI

Second Set of simulations Objective: to compute message update rates for 85% bandwidth usager (relaxing MTD UR)


23

Bandwidth analysis


Summary of the results(for 85 % bandwidth)

Number Const.MT_D Update Interval (in seconds)

NoIono ECAC ECAC+AFI

1 Const. 40 47 53

2 Const. 79 94 107

3 Const. 119 143 163

4 Const. 151 181 207


24

Performance analysisExtrapolation of the linear fit


Enhanced ICD: linear clock model

-At IWG #25, it was shown that linear clock model is a better method for clock estimation than RRC method used in UDRE Alternative ICD.

-In present analysis of extrapolation of the linear fit in Enhanced ICD, it is assumed GPS-like clock performances.

-Glonass SV clocks have been also been studied (IWG 25) showing larger Delta_FC values with respect to GPS SVs with a factor ~2.


25

Performance analysisExtrapolation of the linear fit


Model Regime DeltaT_FC (sec)

Sigma_Delta _FC (m)

Enhanced ICD

Optimistic

40 0,090679 0,1077

119 0,1245

151 0,1331

Nominal

40

0,125879 0,1511

119 0,1756151 0,1906

Pessimistic

40 0,153479 0,1991

119 0,2441

151 0,2685


Sigma_Delta _FC (m)

Enhanced ICD

Optimistic

47 0,094394 0,1140

143 0,1310

181 0,1416

Nominal

47 0,131594 0,1603

143 0,1869181 0,2034

Pessimistic

47 0,163194 0,2160

143 0,2625

181 0,2858


Sigma_Delta _FC (m)

Enhanced ICD

Optimistic

53 0,0968107 0,1195163 0,1363

207 0,1492

Nominal

53 0,1352107 0,1682163 0,1962207 0,2140

Pessimistic

53 0,1698107 0,2307163 0,2767

207 0,2988

No Iono Iono ECAC Iono ECAC+AFI

Analysis on extrapolation of the linear fit has been performed for Enhanced ICD using as input MT_D Update Intervals computed in bandwidth analysis


26

Performance analysisEstimation of VPL Performances


Same model as presented at IWG#25 has been used for VPL performances

VPL mean, minimum and maximum values have been computed from DeltaT_FC and DeltaT_IP for all the Scenarios


33

Performance analysisEstimation of VPL Performances


Performance of «enhanced ICD» are equivalent to those of ICD2 (i.e. improves up to 4 const., despite MTD update rate relaxation)

VAL values around 10-15m can be targeted with enhanced ICD for ≥ 2 const. (tbc with more refined model)


34

Performance analysisEstimation of VPL Performances / mission targets


VPL performance with Enhanced ICD (from preliminary simulations)

LPV-200 VAL=35m always achievableCat I VAL=15m necessitates at least 2 constellations in the SBAS System. Cat I VAL=10m limit seems possible target (tbc) with ≥2 const. (yet more refined performance model should be built); better margins with 3 or 4 const.More demanding missions (e.g. Cat II), if targetable, would necessitate fine tuning (e.g. DFRE scaling) and possible complementary features (e.g. receiver DRAIM for TTA)


21 April 2023 35

Conclusion / possible way forward for ICD definition

21 April 2023 PROSBAS

ICD1 and ICD2 can actually be unified through “Enhanced ICD”, which provide the advantage of both (w/o their inherent constraints and/or limitations) and discarding some drawbacks

• Performance could be optimised towards demanding VAL’s (in the range of 10m, and possibly less (TBC))

• SBAS MOPS L1 legacy TTFS can be met (providing L5 iono BU with ARAIM or SBAS NPA, not with SBAS PA) without imposing system design constraint (e.g. MT_D broadcast only when SV visible)

• Possibility to handle dynamic DFRE situations (>7/8 simultaneous update) thus mitigating user penalty and enabling fast recovery (e.g. upon general constellation alert, SNT instability etc.)

• Robustness (e.g. upon message loss)

• Simplicity very close to ICD2 for system + user (yet to be consolidated with user manufacturers); much simpler than ICD1 (



Draft “Enhanced ICD” has been derived for EGEP internal usage (HISTB/NISTB) capitalising on JASMIN/PRO-SBAS material and gathering comments (collected along Dec.2013/Jan 2014) from Stanford, MITRE, GMV Pro-SBAS, EGNOS V3 Industry and CNES

Pending further consolidation loop with IWG partners (after IWG#26), this draft “Enhanced ICD” could serve as first step towards SBAS DFMC ICD definition

Conclusion / possible way forward for ICD definition

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