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© 2013 The MITRE Corporation. All rights reserved. Tim Cashin, Dmitri Baraban, Roland Lejeune SBAS IWG #24 Meeting CNES, Toulouse, France 23-24 January 2013 Dual-Frequency, Multi-Constellation (DFMC) Receiver Fallback Modes Progress Report
| 2 || 2 | Problem Statement Identify set of DFMC receiver fallback modes to be required –Objective: ensure adequate navigation when constellations and/or SBAS services do not support the normal mode –Normal mode: dual-frequency, multi-constellation Notes –“adequate” implies a trade off between navigation performance and difficulty of implementation –The level of navigation achievable will depend on the fallback mode Objective: LPV-200 when integrity is provided by SBAS © 2013 The MITRE Corporation. All rights reserved.
| 3 || 3 | Objective Provide basis for development of Minimum Operational Performance Standards (MOPS) for Dual-Frequency, Multi- Constellation (DFMC) user equipment –Current RTCA SC-159 plan is to develop MOPS for SBAS user equipment 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 –On-going IWG task to develop DFMC SBAS message structure capable of supporting up to 4 constellations –However, the scope of this study is limited to dual-frequency GPS/Galileo/SBAS user equipment This study does not preclude including other constellations in DFMC MOPS at a later time: “GPS + Galileo” could be replaced with “constellations in view” © 2013 The MITRE Corporation. All rights reserved.
| 4 || 4 | Basic Assumptions DFMC receivers 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 Only modes corresponding to “high probability – high benefit” scenarios are considered min. requirements –Avoids imposing “complex” design on all manufacturers –MOPS may include additional requirements for optional scenarios, if greater design flexibility is desired © 2013 The MITRE Corporation. All rights reserved. * In this briefing, L1 (C/A) is simply referred to as L1 since aviation has no plan to use L1C. **In this briefing, E5a is simply referred to as E5 since aviation has no plan to use E5b.
| 5 || 5 | Analysis Approach Receiver does not have to deliver the “best” solution in all scenarios, but should provide service with high probability whenever possible –With many satellites in view, some satellites can be ignored or used sub-optimally Therefore, solutions relying on complex satellite mixes are not considered minimum requirements Integrity provided by conventional RAIM when SBAS integrity is not available, or does not provide service –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 –RAIM mode will provide navigation for en route through non- precision approach (LNAV) operations only © 2013 The MITRE Corporation. All rights reserved.
| 6 || 6 | 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) –DF WAAS will not provide an L5-only service EU plans to modify EGNOS for dual-frequency service based on 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 plan DF SBAS implementations © 2013 The MITRE Corporation. All rights reserved.
| 7 || 7 | Possible Fallback Mode Types Multi-constellation (MC) to single-constellation (SC) –No additional UE requirements are needed For ex., if user equipment (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 SBAS-based to unaugmented navigation –If operating in en route (ER) through LNAV navigation and SBAS does not provide service, use RAIM Dual-frequency (DF) to single-frequency (SF) –Because of interference or transition to SF SBAS area Mixed modes –Combine SBAS-augmented and unaugmented satellites –Combine DF and SF satellites © 2013 The MITRE Corporation. All rights reserved.
| 8 || 8 | SBAS-based Integrity Modes © 2013 The MITRE Corporation. All rights reserved. Core Constellation Signals Integrity Source Yes/No*Comment GPS L1+L5 & Galileo E1+E5 SBAS L5Yes Nominal case inside service area of a DF SBAS that augments both constellations (or one of them). GPS L1 & Galileo E1SBAS L1Yes 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 E5SBAS L5TBDInterference 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. Is this mode a minimum requirement? *Yes/No column answers question: should a corresponding receiver mode be standardized as a minimum requirement? Note 1: DF SBAS is unlikely to exist before a full constellation of GPS L1/L5. So, no modes are included for scenarios mixing GPS L1-only SVs and GPS L1/L5 SVs. All Galileo SVs will be dual-frequency. In addition mixing SF and DF satellites would require correcting for, and accounting for the uncertainties in, inter-frequency biases, which would increase complexity. SF navigation should be available under most circumstances. Note 2: The receiver 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 3: The operational benefits of an SBAS L5-only service may be limited (larger ionospheric delay errors will limit the availability of LPV service); RAIM will support en route through LNAV navigation.
| 9 || 9 | RAIM-based Integrity Modes (No Satellite with SBAS Augmentation) © 2013 The MITRE Corporation. All rights reserved. Core Constellation Signals Integrity Source Yes/No* Comment GPS L1+L5 & Galileo E1+E5 RAIMYes Nominal case outside SBAS service area. GPS L1 & Galileo E1RAIMYes Nominal case outside SBAS service area when interference on L5/E5. GPS L5 & Galileo E5RAIMYesNominal 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: See Note 1 on page 8. Note 2: See Note 2 on page 8. Note 3: A 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, the receiver should 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 multi-constellation navigation using RAIM should generally always be possible.
| 10 | RAIM-based Integrity Modes (Some Satellites with SBAS Augmentation) © 2013 The MITRE Corporation. All rights reserved. Core Constellation Signals Integrity Source Yes/NoComment Some GPS L1+L5 with SBAS corrections + Some GPS L1+L5 without SBAS corrections & some Galileo E1+E5 with SBAS corrections + Some Galileo E1+E5 without SBAS corrections. RAIMNo 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, users 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 en route through non-precision approach. Degraded above case in which only L1/E1 is available RAIMNo DO-229D has optional mixed case for GPS L1-only. Scenario captures loss of L5/E5 to interference. Degraded above case in which only L5/E5 is available RAIMNoDO-229D has optional mixed case for GPS L1-only. Scenario captures loss of L1/E1 to interference. 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 estimating time differences between SBAS and each constellation as well as inter-frequency biases, which would increase complexity.
| 11 | Expected Levels of Navigation © 2013 The MITRE Corporation. All rights reserved. Core Constellation Signals Integrity Source Levels of Navigation GPS L1+L5 & Galileo E1+E5 SBAS L5ER/LPV-200 (perhaps autoland or LPV-100?). GPS L1 & Galileo E1SBAS L1 ER/LPV-200 (LPV-200 may not be available during some severe ionospheric storms). GPS L5 & Galileo E5SBAS L5ER/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 RAIM ER/LNAV/RNP 0.1. GPS L1 & Galileo E1RAIM ER/LNAV/RNP 0.1. GPS L5 & Galileo E5RAIMER/LNAV/RNP 0.1.
| 12 | Initial Thoughts on Mode Selection What mode to use for navigation? –In LPV mode, SBAS-based integrity is required Receiver 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, receiver 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 rule, and receiver may have to periodically check service from alternative modes © 2013 The MITRE Corporation. All rights reserved.
| 13 | Conclusions Based on this study, the “minimum requirement” modes include: –2 modes in which integrity is provided by SBAS A 3 rd mode (SBAS 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 –Estimated likelihood of postulated operational scenario –Intuitive judgment about the need for specific mode given a particular scenario in order to obtain service Anticipated benefits (in terms of service availability) of given scenarios have not been evaluated quantitatively © 2013 The MITRE Corporation. All rights reserved.
| 14 | © 2013 The MITRE Corporation. All rights reserved. Backup Charts
| 15 | 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) © 2013 The MITRE Corporation. All rights reserved.
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