Transcript Document

Dual-Frequency, Multi-Constellation
(DFMC) Receiver Fallback Modes
Tim Cashin, Dmitri Baraban, Roland Lejeune,
James (JP) Fernow
RTCA SC-159 WG-2
12-13 March 2013
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Problem Statement
 Identify set of DFMC user equipment (UE) fallback modes
to be required
– Objective: ensure adequate navigation when or where core
constellations and SBAS do not support normal mode
 SBAS-augmented DFMC
 Notes
– “Adequate” implies a balance between navigation performance
and difficulty/cost of implementation
– The level of navigation service achievable will depend on the
fallback mode
 Objective: LPV-200 when integrity is provided by SBAS
– A manufacturer could choose to implement optional additional
fallback modes
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Objective
 Provide input to development of Minimum Operational
Performance Standards (MOPS) for DFMC user equipment
– Current RTCA SC-159 plan is to develop MOPS for SBAS UE
using GPS L1/L5 and Galileo E1/E5a
 SC-159 will develop GPS/SBAS L1/L5 MOPS in first step, then merge
these MOPS with Galileo E1/E5 MOPS from EUROCAE
– Relates to on-going SBAS IWG task to develop DFMC SBAS
message structure capable of supporting up to 4 constellations
– The scope of this briefing is limited to dual-frequency (DF)
GPS/Galileo/SBAS user equipment
 Handling additional core constellations in DFMC MOPS needs to be
done at a later time: “GPS + Galileo” could be replaced with
“constellations in view”
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Basic Assumptions
 DFMC UE will be able to track all of the following signals
from a large set of satellites in view
– GPS L1 (C/A)*, GPS L5, Galileo E1, Galileo E5a**, SBAS L1 and
SBAS L5
– Required fallback modes are assumed independent of the number
of receiver channels
– SBAS L1 message will augment L1 only (and possibly E1?)
– SBAS L5 message will augment DF satellites (L1/E1-L5/E5) and perhaps
L5/E5-only, in the event that an SBAS provider wants to offer SBASaugmented service (TBD)
 Only modes corresponding to “high probability – high
benefit” scenarios are considered minimum requirements
– Minimize required complexity of UE design
*In this briefing, L1-C/A is referred to as L1 since aviation has no plan to use L1C.
**In this briefing, E5a is referred to as E5 since aviation has no plan to use E5b.
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Analysis Approach
 UE does not have to use the “best” solution in all scenarios, but
should provide service with high probability when feasible
(balance of benefit and cost)
– With many satellites in view, some satellites can be ignored or used
sub-optimally
– Supporting complex mixed satellite solutions is not expected to
provide a benefit that justifies implementation cost
 Integrity provided by conventional RAIM when SBAS integrity is
not available, or does not provide service
– RAIM mode will provide navigation for en route (ER) through nonprecision approach (LNAV) operations only
– Advanced RAIM (ARAIM) capability may be included in the analysis at
a later time, but only if a complete and stable definition of ARAIM is
available
 Would be an option only
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SBAS Modernization Plans
 FAA plans to modify WAAS for dual-frequency (DF)
service based on GPS L1/L5
– Start of service still uncertain (due to requirement for 24 DF GPS
satellites), but unlikely before 2020
– FAA currently has no plan to modify WAAS to augment Galileo (or
any other constellation besides GPS)
– DF WAAS will not be designed to provide an L5-only service*
 EU plans to modify EGNOS for DF service and augment
both GPS L1/L5 and Galileo E1/E5
– DF EGNOS release planned for ≈2020
– DF EGNOS may provide L5-only augmentation
 The need for this service is under evaluation
 Other service providers may also implement DF SBAS
*UE might be able to support SBAS-augmented L5-only ER through LNAV even if
SBASs do not send information intended for L5. SBAS information intended for L1 or
for L1/L5 might be useable, due to large HALs associated with ER/LNAV
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Possible Fallback Mode Types to Consider
(1 of 2)
 Multi-constellation (MC) to single-constellation (SC)
– Automatically supported with no additional UE requirements
 E.g., if UE in SBAS mode can use both GPS and Galileo monitored
satellites, it can also provide navigation if SBAS augments only one of
the constellations
 Similarly, if UE can support GPS + Galileo, it can support Galileo
 Not discussed further in this briefing
 Dual-frequency (DF) to single-frequency (SF)
– Because of interference or transition to SF SBAS area
 SBAS-augmented (DF or SF) to RAIM (DF or SF)
– If operating in en route (ER) through LNAV navigation and SBAS
does not provide service, use RAIM using uncorrected satellites
 Mixed modes (see next chart)
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Possible Fallback Mode Types to Consider
(2 of 2)
 SBAS-augmented mixed modes
– One SBAS augmenting both DF and SF satellites
– Two or more SBASs augmenting all SF or all DF satellites
– Combination of above two cases
 RAIM-based mixed modes
– RAIM using different core constellations, all DF or all SF
– RAIM using both SF and DF satellite measurements
– Combination of above two cases
 RAIM using both unmonitored satellites and satellites corrected
by one or more SBASs
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Additional Notes on Fallback Modes
 For en route through LNAV, UE can use single-frequency
ionospheric model on SF satellites
– Instead of SBAS ionospheric model
– Appropriate ionospheric error model required
– Consistent with DO-229 requirement
 Not allowed: mixing corrections from different SBASs on a
single satellite
– E.g., clock corrections from EGNOS and ephemeris corrections
from WAAS
– Consistent with DO-229 requirement
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SBAS-based Integrity Modes (augmented by
the same single SBAS at any given time)
Core Constellation
Signals
GPS L1/L5 & Galileo E1/E5
Integrity
Source
SBAS L5
Yes/No*
Comment
Yes
Nominal case inside service area of a DF SBAS that
augments both constellations (or one of them).
GPS L1 & Galileo E1
SBAS L1
Yes
Interference on L5/E5 inside service area of a DF SBAS
that augments both constellations (or one of them), or
inside service area of SF SBAS.
GPS L5 & Galileo E5
SBAS L5
TBD
Interference on L1/E1 inside service area of a DF SBAS
that augments both constellations (or one of them).
WAAS will not provide this service. Whether EGNOS
will provide it is still TBD. If any SBAS supports this
service, is this mode a minimum UE requirement?
Answer may depend on regulator’s assessment of
continuity without such a requirement.
*Yes/No column answers question: should a corresponding receiver mode be standardized as a minimum
requirement?
Note 1: The UE may have different bias errors when tracking signals from different constellations due to
different signal structures and will have to account for such biases in computing protection levels.
Note 2: The operational benefits of an SBAS-augmented L5/E5-only service may be limited (larger ionospheric
delay errors will limit the availability of LPV service). As a backup, RAIM will support en route through LNAV
navigation including missed approach guidance with very high availability for multi-constellation UE
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RAIM-based Integrity Modes
(No Satellite with SBAS Augmentation)
Core Constellation Signals
GPS L1+L5 & Galileo
E1+E5
GPS L1 & Galileo E1
Integrity Yes/No*
Comment
Source
RAIM
Yes
Nominal case outside SBAS service area.
GPS L5 & Galileo E5
RAIM
Yes
Nominal case outside SBAS service area when interference
on L5/E5.
RAIM
Yes
Nominal case outside SBAS service area when interference
on L1/E1.
*Yes/No column answers question: should a corresponding receiver mode be standardized as a minimum
requirement?
Note 1: SF navigation is expected to provide ER through LNAV service under most circumstances, particularly for
MC UE. If single-constellation GPS-only UE exists, it should usually provide ER through LNAV service, although
availability may be less in case of a depleted GPS constellation. Availability on L5/E5 will be less than on L1 due
to larger ionospheric errors on L5/E5.
Note 2: See Note 2 on page 9.
Note 3: RAIM solution with multiple constellations will have to account for the difference in time reference
between the constellations. The Least Squares algorithm will need include one additional state for this purpose.
Note 4: For each multi-constellation scenario, UE will be able to automatically revert to single constellation
navigation when one of the constellations is missing. Both constellations will be operational in the DMFC
timeframe, and so ER through LNAV using RAIM will be supported with very high availability by MC equipment.
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SBAS-based Integrity Modes (augmented by
the same single SBAS at any given time)
Core Constellation Signals
Integrity
Source
Yes/No*
Comment
A mixture of SF and DF
satellites (whether GPS or
Galileo or both) augmented
by a single SBAS
SBAS L5
No
Limited benefit: Reverting to SF navigation will usually
provide service, although SBAS L5/E5-only LPV
availability will be lower than L1/E1-only. If approach
capability is lost, SF service will provide missed approach
guidance with very high probability. See following chart.
Complex: In general, both satellite and receiver biases
exists between SF and DF measurements. Unless the
net bias is assured to be negligible compared to
protection levels (PLs), UE must account for the bias.
One option of accounting for the bias would be to account
for the additional uncertainty in PLs.
Other options would be to estimate the bias and correct
either SF or DF satellite measurements for the net bias;
residual error would need to be reflected in PLs.
See backup charts for additional comments.
*Yes/No column answers question: should a corresponding receiver mode be standardized as a minimum
requirement?
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Limited Benefit of SBAS Augmentation of a
Mixture of SF and DF Measurements
 DF SBAS is unlikely to exist before 24 GPS L1/L5 satellites exist
 All Galileo satellites are DF
 Thus the only application of SBAS augmenting a mixed SF/DF

satellite set would in case of RFI affecting a subset of satellites
(infrequent, it is hoped)
Other reversionary modes provide high availability of ER/LNAV
– SF-only SBAS-L1/E1-augmented service
 In addition SBAS L1/E1 supports high LPV availability in middle latitudes
– RAIM using unmonitored SF or DF satellites
 Though RAIM availability for single constellation L5-only or E5-only is not
extremely high, due to larger iono error on L5/E5
 In any case, first SF/DF option (see previous page) is not expected

to provide a large increase in availability (relative to other
reversionary modes) due to the large uncertainty in the SF/DF bias
The same is true of the second SF/DF option (bias estimation)
– Uncertainty is large relative to LPV VPL/HPL because the estimate of
bias depends on SBAS SF iono error
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SBAS-based Integrity Modes (augmented by
the multiple SBASs at the same time)
Core Constellation Signals
Integrity
Source
Yes/No*
Comment
Augmentation of satellites
by multiple SBAS
SBAS L1
No
Optional in DO-229 using SBAS L1. UE must account for
the different in SBAS Network Times between the
different SBASs.
Whether all satellites are DF,
all satellites are SF, or mixed
SF/DF
Limited benefit. This mode would support only ER
through LNAV. ER through LNAV could also be
supported with high availability using augmentation based
on one of the SBASs or based on RAIM ignoring SBAS
augmentation
Complex: requires accounting for time difference either
by reflecting additional uncertainty in PL or via estimation
and correction (and reflecting residual uncertainty in PL).
See backup charts for additional comments.
*Yes/No column answers question: should a corresponding receiver mode be standardized as a minimum
requirement?
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RAIM-based Integrity Modes (Mixture of DF and SF
Satellites and/or SBAS-Augmented and Unmonitored)
Core Constellation Signals
Mixture of SF and DF
satellites
Some satellite with SBAS
corrections + Some
satellites without SBAS
corrections
Combination of above
Integrity Yes/No
Comment
Source
RAIM
No
Little incremental benefit over RAIM using L1/E1-only or
RAIM using L5/E5-only.
RAIM
No
DO-229D has an optional mixed case for GPS L1-only.
Conditions will exist outside SBAS service areas where
some SVs are augmented and others are not. However, in
the multi-constellation environment, MC UE will typically
see 15 or more satellites. So, modes using a mix of
corrected and uncorrected satellites may not be necessary
in order to ensure adequate service for ER through LNAV.
Even GPS-only UE would see little increase in availability
compared to RAIM using all unmonitored satellites.
RAIM
No
Little incremental benefit over RAIM using L1/E1-only or
RAIM using L5/E5-only.
Note 1: A mix of GPS L1-only and GPS L1/L5 satellites will exist after Galileo IOC constellation is operational
(2015?). However, DFMC user equipment is unlikely to exist before most GPS SVs are broadcasting L1+L5
(2020?). Galileo SVs will be dual-frequency. So, a scenario mixing SF and DF satellites is not considered
necessary in DFMC timeframe as ER/LNAV navigation will highly likely be available from an SF solution using all
SVs or from a DF solution using the subset of DF SVs.
Note 2: Using mixed scenarios as described above would require accounting for and/or estimating time
differences between SBASs, GPS, and/or Galileo time and/or inter-frequency biases. Any of these would
increase complexity. See back charts.
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Expected Levels of Navigation
Core Constellation Signals
GPS L1+L5 & Galileo
E1+E5
GPS L1 & Galileo E1
Integrity
Source
SBAS L5
Levels of Navigation
ER/LPV-200 (perhaps autoland or LPV-100).
SBAS L1
ER/LPV-200 (LPV-200 may not be available during
some severe ionospheric storms).
GPS L5 & Galileo E5
SBAS L5
ER/LPV-200 (However, level of LPV-200
achievable is unknown and probably limited even
during quiet ionospheric conditions).
Core Constellation Signals
Integrity
Source
Levels of Navigation
GPS L1+L5 & Galileo
E1+E5
GPS L1 & Galileo E1
RAIM
ER/LNAV/RNP 0.1.
RAIM
ER/LNAV/RNP 0.1.
GPS L5 & Galileo E5
RAIM
ER/LNAV/RNP 0.1.
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Thoughts on Mode Selection
(1 of 2)
 What mode to use for navigation?
– In LPV mode, SBAS-based integrity is required
 UE should take advantage of all DF or all SF monitored satellites
 In DF SBAS area (and no interference), DF navigation should generally
be preferred; however, availability of SF navigation may be better in
some circumstances
– In ER/LNAV mode, UE will have to chose between SBAS and
RAIM
 Largest satellite subset of same category is most likely to provide
service when tracking a mix of DF, SF, monitored and non-monitored
satellites
 However, this is not an absolute
 DO-229 states that UE “should compute HPLFD and HPLSBAS based on
all useable satellites for each HPL to determine which set of satellites
and HPL provides the best performance (i.e., smallest HPL)”
– There may be circumstances when this is not the best choice, e.g., if
one of the satellites used in the smallest HPL is about to set
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Thoughts on Mode Selection
(2 of 2)
 DO-229, Section 2.1.1.6, requires UE to switch to a new set of
satellites within the time-to-alert when a change to the selected
set of satellites is necessary

– Prior to the setting of a satellite below the mask angle, it may be
desirable for UE should search for an alternate set of satellites to
switch to
A few years ago, Airbus flight tests of SBAS indicated that
switching from one SBAS to another SBAS or to GPS over the
Atlantic Ocean resulted in reported position jumps
– Caused concern among pilots
– Position jumps were likely due to the UE not switching satellites until
HPL exceeded HAL
 Thus UE tolerated poor geometry, resulting in significant position error
– For DFMC MOPS, a requirement should be considered to minimize
position jumps
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Conclusions
 Based on this study, the “minimum requirement” modes
include:
– 2 modes in which integrity is provided by SBAS
 A 3rd mode (SBAS-augmented L5-only service) remains TBD
– 3 modes in which integrity is provided by RAIM
 Decision to include (not include) a mode was based on
– Timeframe of DFMC receiver, DF SBAS, and DF core
constellations
– Estimated likelihood of postulated operational scenario
– Engineering judgment about the need for specific mode given a
particular scenario in order to obtain service, and secondarily,
about the cost (complexity) of implementing the mode
 Anticipated benefits (in terms of service availability) of given scenarios
have not been evaluated quantitatively
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Backup Charts
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Previous Studies
 EUROCAE WG62 Combined GPS/Galileo/SBAS Receiver MOPS
Skeleton briefed SC159 WG2 in June 2010
– File: “Laurent Azoulai RTCA SC159 WG2_STANDARDS WP2300
GPS_Galileo_MOPS_skeleton_intro.ppt”
 EUROCAE WG62 study briefed to SC159 WG2 in June 2009
– File: “EUROCAE WG62 STANDARDS combinations def and roadmap.ppt”
 EUROCAE WG62 ConOps briefed to SC159 WG2 in January 2008
– File: “Presentation 4 GPS_GALILEO_ConOps Issue 3 draft 0.71.doc”
 EUROCAE WG62 study briefed SC159 WG2 in May 2007
– File: “Future GNSS_Rx_MOPS_WG62_RTCA May2007.ppt” (old)
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Complexity of SBAS-based Integrity Modes
Using Mixed SF/DF Satellite Measurements
 Assume the case of a single SBAS augmenting a mixture of SF and
DF satellite measurements
– In general both a satellite and a receiver bias exists between SF and
DF measurements
 Receiver bias is (in modern receivers) common to all measurements of a
given type (SF or DF)
 In the case of augmentation by a single SBAS, the SBAS correction would
make the satellite bias common to all measurements of a given type
– In fact, current WAAS derives corrections from reference-receiver-tosatellite DF measurement combinations
– But in order to account for the biases, UE must either
 (Method 1) Inflate satellite range error uncertainty estimates enough to
ensure that the PL bounds the error from all sources including the biases
 (Method 2a) Estimate the net bias by differencing SF and DF
measurements for DF satellites, then correct the SF or DF measurements
with the bias, and account for remaining error when computing HPL/VPL, or
 (Method 2b) In position solution, solve separately for user clock SF offset
from SBAS Network Time (SNT) and for user clock DF offset from SNT
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Complexity of Integrity Modes Using
Measurements Different Time References
 A solution mixing SBAS-augmented and unmonitored satellite
measurements, or augmented by different SBASs, must account
for the difference in time reference among SBAS(s) and core
constellation(s)
– A simple solution in which modeled range error uncertainties are
increased by a fixed, conservative error bound on the time
difference would somewhat reduce the potential operational
benefit from that mode
 This solution would also impose a requirement on service providers to
verify that actual differences in time references (currently 50 ns
maximum between L1 SBASs and GPS time) are compatible with the
assumed modeled uncertainty
– Estimating the time difference(s) would add complexity (especially
if multiple core constellations are used), and the uncertainty in the
estimate(s) would have to be conservatively accounted for when
computing PLs
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Methods of Estimating Difference in Time
References: Example for L1 and L1/L5
 Method 2a: use either a snapshot or a lag-filtered weighted
difference between measurements
– It might not remain constant; a UE fault might corrupt it
 This fault must be detected or prevented from affecting estimate
– Difference in L1 and L1/L5 range measurements includes
 f 52 
 2
   L1,multipath ,noise   L 5,multipath ,noise    iono
2
 f1  f 5 
– Standard deviation of error in difference includes iono (significant)
as well as a factor times error in airborne multipath/noise and in
clock and the range component of ephemeris error
– Significant uncertainty in estimate of time difference results in
increased protection levels and reduced availability
 Benefit is especially reduced for LNAV/VNAV, LPV, and LP
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Methods of Estimating Difference in Time
References
 Method 2b: augment the matrix of direction cosines with an
extra column(s) to solve separately for different time references
– Uses one more range measurement to estimate the additional
state than if the additional UE clock offset from time reference
were not estimated. This reduces availability.
 12
 13
  11
 



 n2
 n3
  n1
G

 n1, 2  n1,3
 n1,1


 

 nm ,1  nm , 2  n m ,3
0
 

1 0
0 1

 
0 1
1










Direction cosines to
satellites with
measurements with
one time reference
Direction cosines to
satellites with
measurements with
another time
reference
If both SF and DF measurements to the same satellite are included, the correlation
must be accounted for in the weighting matrix
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