© 2013 the mitre corporation. all rights reserved. tim cashin, dmitri baraban, roland lejeune,...

© 2013 The MITRE Corporation. All rights reserved.

Tim Cashin, Dmitri Baraban, Roland Lejeune, James (JP) Fernow

RTCA SC-159 WG-2

12-13 March 2013

Dual-Frequency, Multi-Constellation (DFMC) Receiver Fallback Modes

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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 SBAS-augmented 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 non-

precision 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

© 2013 The MITRE Corporation. All rights reserved. For internal MITRE use

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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

Integrity Source

Yes/No* Comment

GPS L1/L5 & Galileo E1/E5 SBAS L5 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 Integrity Source

Yes/No* Comment

GPS L1+L5 & Galileo E1+E5

RAIM Yes Nominal case outside SBAS service area.

GPS L1 & Galileo E1 RAIM Yes Nominal case outside SBAS service area when interference on L5/E5.

GPS L5 & Galileo E5 RAIM Yes Nominal case outside SBAS service area when interference on L1/E1.


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)



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.


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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)



Yes/No* Comment

Augmentation of satellites by multiple SBAS

Whether all satellites are DF, all satellites are SF, or mixed SF/DF

SBAS L1 No Optional in DO-229 using SBAS L1. UE must account for the different in SBAS Network Times between the different SBASs.

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.


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RAIM-based Integrity Modes (Mixture of DF and SF Satellites and/or SBAS-Augmented and Unmonitored)



Yes/No Comment

Mixture of SF and DF satellites

RAIM No Little incremental benefit over RAIM using L1/E1-only or RAIM using L5/E5-only.

Some satellite with SBAS corrections + Some satellites without SBAS corrections

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.

Combination of above 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

Integrity Source

Levels of Navigation


SBAS L5 ER/LPV-200 (perhaps autoland or LPV-100).

GPS L1 & Galileo E1 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).


Levels of Navigation


RAIM ER/LNAV/RNP 0.1.

GPS L1 & Galileo E1 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-to-satellite 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

– 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


iononoisemultipathLnoisemultipathLff

f

,,5,,12

521

25

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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.


1

1

0

0

0

0

1

1

3,2,1,

3,12,11,1

321

131211

mnmnmn

nnn

nnnG

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

© 2013 the mitre corporation. all rights reserved. tim cashin, dmitri baraban, roland lejeune,...

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