SBAS DFMC SIS ICD
EPO Inputs to IWG#26
[from Pro-SBAS studies]
India - 05th Feb. 2014
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
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
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Reminder from IWG#25
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
SBAS DFMC ICD1
o Derived from (existing) SBAS L1 legacy ICD
o Capable of sustaining high DFRE dynamics
o Limited to 51SV (DOP limitation).Dynamic mask improves
but rather complex,possible safety issue,still DOP limitation
o 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
o
o
o
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Optimised (for bandwidth usage)
Capable of up to 91SV with TTFF
Simple to implement (System+User)
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)
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Reminder from IWG#25
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 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)
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Enhanced ICD Strategy Analysis
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IWG 25
Study of asynchronous events
Best option for Enhanced ICD
and of DFREI changes
(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
Definition of Enhanced ICD
Introduction of new «MT 6 – like» messages
Enhanced ICD Bandwidth Analysis
Enhanced ICD Performance Analysis
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Analysis of needs for enhanced ICD : DFREI changes
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Low variable DFREs
Variable DFREs
High variable DFREs
Ex: WAAS
Ex: EGNOS
Ex: SDCM
No need (tbc) for dynamic MT_6_1/2
mechanism. DFRE ICD
Need for dynamic MT_6_1/2
mechanism. Enhanced ICD
Need for MT_6_1/2 mechanism
Num
chan
ges
in 30sec
wind
ow
More
than
7
Percentage of events (%)
1 Cons
2 Cons
3 Cons
4 Cons
(24
SVs)
(48
SVs)
(72
SVs)
(91
SVs)
0
0.0
0.0
0.0
Num
chan
ges
in 12sec
wind
ow
More
than
7
(more frequent usage than EGNOS case)
Percentage of events (%)
1 Cons
2 Cons
3 Cons
4 Cons
(24
SVs)
(48
SVs)
(72
SVs)
(91
SVs)
0
0.034
0.116
0.197
Num
chang
es in
12sec
windo
w
More
than 7
Percentage of events (%)
1 Cons
(24 SVs)
2 Cons
(48 SVs)
3 Cons
(72 SVs)
4 Cons
(91 SVs)
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.
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Extraction of the Histogram of the
number of UDRE changes in a 12seconds window for 1 to 4
constellations (UDREs from
EGNOS real data).
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Analysis of needs for enhanced ICD: DFREI changes
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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
9 April, 2015
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Proposed definition of enhanced ICD (MT6.1/6.2)
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
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 mask
o MTC is mandatory. It is used for alerts
o 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 impacts
o 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
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Proposed definition of enhanced ICD - Messages
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Proposed definition of enhanced ICD (MT6.1/6.2)
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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
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Proposed definition of enhanced ICD (MT6.1/6.2)
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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
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Proposed definition of enhanced ICD (MT6.1/6.2)
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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.
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Proposed definition of enhanced ICD
Robustness upon message loss
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 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
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Bandwidth Analysis - Scenarios
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
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
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Bandwidth Analysis - Scenarios
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1st 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)
2nd Set of simulations
•
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)
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Bandwidth Analysis
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First Set of simulations (Enhanced ICD)
Objective: compute bandwidth usage for complying with time-tofirst-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
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Bandwidth Analysis
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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.
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Bandwidth analysis
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Second Set of simulations
Objective: to compute message update rates for 85% bandwidth usager (relaxing MTD UR)
No Iono
Iono ECAC
Iono ECAC+AFI
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Bandwidth analysis
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Summary of the results
(for 85 % bandwidth)
Number Const.
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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
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Performance analysis
Extrapolation of the linear fit
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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.
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Performance analysis
Extrapolation of the linear fit
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 Analysis on extrapolation of the linear fit has been performed for
Enhanced ICD using as input MT_D Update Intervals computed in
bandwidth analysis
No Iono
Model
Regime
Optimistic
DeltaT_
FC (sec)
Sigma_
Delta _F
C (m)
40
0,0906
79
0,1077
119
0,1245
151
Enhanced
ICD
Nominal
Pessimistic
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Model
Regime
Optimistic
0,1331
40 0,1258
79
0,1511
119
0,1756
151
0,1906
40
0,1534
79
0,1991
119
0,2441
151
Iono ECAC+AFI
Iono ECAC
0,2685
Enhanced
ICD
Nominal
Pessimistic
DeltaT_
FC (sec)
Sigma_
Delta _F
C (m)
47
0,0943
94
0,1140
143
0,1310
181
47
94
143
181
47
94
143
0,1416
0,1315
0,1603
0,1869
0,2034
0,1631
0,2160
0,2625
181
0,2858
Model
Regime
Optimistic
Enhanced
ICD
Nominal
Pessimistic
DeltaT_
FC (sec)
Sigma_
Delta _F
C (m)
53
0,0968
107
0,1195
163
0,1363
207
53
107
163
207
53
107
163
0,1492
0,1352
0,1682
0,1962
0,2140
0,1698
0,2307
0,2767
207
0,2988
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Performance analysis
Estimation of VPL Performances
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
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
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Performance analysis
Estimation of VPL Performances
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 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)
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Performance analysis
Estimation of VPL Performances / mission targets
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PRELIMINARY ASSESSMENT OF ENHANCED ICD BASED ON MAXIMUM VPL VALUES
Constellations
GPS
GPS+GAL
GPS+GAL+GLO
GPS+GAL+
GLO+COMP
LPV-200 (35 m)
CAT I auto. (15m)
CAT-I auto (10 m)
VPL performance with Enhanced ICD (from preliminary simulations)
LPV-200 VAL=35m always achievable
Cat 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)
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Conclusion / possible way forward for ICD definition
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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 (
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Conclusion / possible way forward for ICD definition
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
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
9 April, 2015
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