Dual-Frequency Multi-constellation Definition Document v3.07
Dual-Frequency Multi-Constellation Definition Document
v3.07
Prepared by:
Dual-Frequency Multi-constellation Definition Document v3.07
Forward
This Dual-Frequency Multi-constellation (DFMC) Space-based Augmentation System (SBAS) Definition Document draft was prepared to serve as the basis for Interoperability Working Group (IWG) review. This represents the preliminary outline and content for the DFMC Definition Document. Various IWG members have work plans to further develop content for this Definition Document. On completion of work plan items, modified sections of the Definition Document will be presented at IWG meetings for review, concurrence and approval.
Once the final document is completed, the above paragraph will be removed.
The Dual-Frequency Multi-constellation (DFMC) Space-based Augmentation System (SBAS) Definition Document was prepared by the Interoperability Working Group and approved at the IWG meeting on (TBD).
The document captures the minimum set of requirements that the IWG considers necessary to develop interoperable, international DFMC SBAS systems. These recommendations should serve as the basis for introduction of DFMC SBAS into the International Civil Aviation Organization Standards and Recommended Practices and the development of Minimum Operations Performance Standards.Since IWG is not an official agency of any of the member countries, the requirements recommended in this document may not be regarded as statements of official government policy of any member countries.
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Dual-Frequency Multi-constellation Definition Document v3.07
Table of Contents
Table of Contents
List of Figures
List of Tables
1Purpose and Scope
1.1Introduction
1.2Document Overview
1.3System Overview
1.4Operational Goals and Applications
1.4.1Operational Goals
1.4.2Intended Operations
1.4.3Operational Environment
1.4.4Operational Concepts
1.5Services
1.5.1Legacy L1 SBAS
1.5.2Dual Frequency L1/L5 Service
1.5.3Multi-Constellation Service
1.5.4Advanced RAIM Support Service
1.6Benefits
1.7Assumptions
1.8Reference Documents
2SBAS System Functional Requirements
2.1Performance
2.1.1Minimum navigation performance
2.1.2Surveillance Performance
2.1.3Allocation Methodology
2.2General Requirements
2.2.1Navigation Information
2.2.2Radio Frequency Interference
2.2.3Signal Processing Requirements
2.2.4Safety Strategy
2.3Services/Functions
2.3.1SF SBAS Functions
2.3.2DF SBAS Functions
2.3.3DFMC SBAS Functions
2.3.4Service Indications
3SBAS Subsystem Functional Requirements
3.1Non-Aircraft Elements
3.1.1Contribution to Ranging Accuracy
3.1.2Integrity
3.1.3Continuity
3.1.4Interaction with other SBAS
3.1.5Functional Requirements (Operations)
3.2Ranging Signal (SBAS Space Sub-systems)
3.2.1Ranging Performance
3.2.2Code-Carrier Coherence for L1 and L5
3.2.3Fractional Coherence between L1 and L5 Carriers
3.2.4Correlation Loss
3.2.5Short-Term End-to-End Carrier Frequency Stability
3.2.6SIS Phase Noise
3.2.7SIS L1/L5 Crosstalk
3.2.8Signal Deformation
3.3Aircraft Elements
3.3.1DFMC SBAS Capable Receiver
3.3.2Contribution to Ranging Accuracy
3.3.3Airborne Functional Requirements
3.3.4Integrity / Conditions for Use of Data
3.3.5Continuity
3.3.6Minimum Outputs
4SBAS Signal-in-Space Specification
4.1RF Specifications
4.1.1L5 Signal
4.1.2L1 Signal
4.2Messages
4.2.1Structure
4.2.2L5 Data Fields
4.2.3L1 Data Fields
4.2.4Commonality between L1 and L5 data
4.2.5Broadcast Requirements
4.2.6Message Broadcast / Processing Protocols
4.3Definition of Protocols for Data Application
4.4FAS Data
5Non Navigation System Functional Requirements
5.1SBAS Service Provider
5.2Air Navigation Service Provider (ANSP)
5.2.1Approved Operation Volume
5.2.2FAS Data Generation
5.2.3GNSS Monitoring / Acceptance
5.3Air Traffic Control
5.4AF/Maintenance
5.5GNSS Service Providers
6Contributors
7Acronym List
List of Figures
Figure 1 shows the expected performance for navigation and surveillance functions per flight operation.
List of Tables
Table 1 shows the End-to End SIS Phase Noise.
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Dual-Frequency Multi-constellation Definition Document v3.07
1Purpose and Scope
1.1Introduction
The objective of this document is to identify the characteristics of both the airborne and non-airborne elements critical to the interoperability of augmentation systems procured and operated by different countries with equipment manufactured to common standards. As such, this document delineates recommendations for a minimum set of requirements for a Dual Frequency Multi-Constellation (DFMC) Space Based Augmentation System, a Global Navigation Satellite System (GNSS) differential correction system available over large areas. This document provides the basis for modification of the International Civil Aviation Organization (ICAO) Standards and Practices (SARPs) and development of advisory group (industry) Minimum Operational Performance Standards (MOPS) for airborne equipment.
This document identifies the message interface and protocols to support the DFMC SBAS. It presumes that the DFMC SBAS service will exist on the L5 frequency. While the DFMC SBAS service might use modifications of the legacy L1 SBAS message structure, this document and effort is not intended to modify the L1 message interface. If there is a desire to modify the L1 interface or include DFMC SBAS capability on the L1 interface, then it will be necessary to develop an additional update to the SARPs to address those changes.
Instantiations of Single Frequency L1 SBAS (Legacy) augmentation systems developed in accordance with the SARPS were not fully consistent with airborne equipment developed in accordance with the MOPS. Recent efforts within RTCA identified discrepancies between the SARPs and MOPS and proposed changes so that the SARPs and MOPS are consistent. This document attempts to address the key components of a DFMC SBAS that are required for inclusion in the SARPs and MOPS.
1.2Document Overview
Section 1 of this document provides an overview of DFMC SBAS and provides information needed to understand the rationale for DFMC SBAS characteristics and requirements that are stated in the following sections.
Section 2 addresses functional requirements that apply to the system as a whole.
Section 3 addresses element functional requirements. These standards specify the required performance under standard environmental conditions.
Section 4 identifies the interface between the non-airborne and airborne elements to include the Radio Frequency (RF) layer, the message structure, and protocols for use of the message data.
Section 5 identifies roles or assumptions on other participants in the DFMC SBAS concept.
1.3System Overview
DFMC SBAS is similar to the Single Frequency L1 Only SBASsystem in operation today. DFMC SBAS is a wide-area differential system that will provide corrections for one or more core Global Navigation Satellite System (GNSS) constellations. These differential corrections apply to a dual-frequency user who mitigates ionospheric delay through direct line of sight delay measurements from each ranging source. The non-airborne element uses a network of ground-based reference receivers to monitor GNSS ranging signals, calculates differential corrections at a central computing facility, and relays the corrections in a standard message format to the airborne user through a geostationary satellite. The airborne element receives GNSS ranging signals and the corrections information andthen applies the corrections in the process of calculating a position solution. The airborne element also calculates an associated protection level. Integration of the airborne element into an airframe permits use of the position and protection level information for navigation and guidance information, enabling en-route through precision approach operations.
DFMC SBAS has several notable differences from the SBASs currently in operation. First, DFMC SBAS uses two frequencies from the ranging sources. This permits use of an iono-free pseudorange measurement, thereby removing the largest potential error source in the single-frequency SBAS. Use of dual-frequency essentially shifts the responsibility of ionosphere delay mitigation from ground-based corrections to direct mitigation by the airborne user. It effectively combines the ground’s basic and precision correction functions into a single iono-free correction function. Second, DFMC SBAS considers the use of multiple GNSS constellations as an inherent part of the SBAS design and develops protocols for the use of multiple constellations. The design considers the processing by the ground and airborne elements and includes changes to the message interface to enable multi-constellation capability. Third, DFMC SBAS identifies the applicable reversionary modes that the SBAS design supports. With multiple frequencies and constellations, many options are possible. The identification of reversionary modes for DFMC will reduce the potential behavior incorporated in airborne equipment, leading to more predictable user behavior.
1.4Operational Goals and Applications
1.4.1Operational Goals
The operational goals of DFMC SBAS are to achieve significant improvements in service flexibility, safety and user operating cost over legacy L1 SBAS by providing:
a) Vertical guidance in geographic areas and during conditions where legacy L1 SBAS is not operational, i.e., in equatorial areas and in other areas where theionosphere introducesvariations in delay that cannot be adequately modeled or bound.[1]
b) Improved availability by usingranging sources with two frequencies to provide direct ionosphere delay measurement and by usingranging sources from multiple constellations to ensure adequate ranging source geometry at all times.
c) Provide better level of service through reduction of calculated Vertical Protection Levels (VPLs). DFMC SBAS should be able to provide VPLs in the 10-12 m range. Additional effort if required to determine whether this better level of service can be the basis for additional capability, such as Autoland.
1.4.2Intended Operations
1.4.2.1Horizontal Navigation
DFMC SBAS will support horizontal position determination and horizontal protection using differential corrections for flight operations that control altitude with barometric reference. These flight operations include oceanic, enroute, terminal and non-precision approach operations.
1.4.2.2Horizontal and Vertical Navigation
DFMC SBAS will support horizontal and vertical position determination and protection using differential corrections for flight operations that require horizontal and vertical position control using GNSS. These flight operations include non-precision approach with vertical guidance and precision approach operations (Localizer Performance with Vertical Guidance (LPV)).
1.4.2.3Surveillance
DFMC SBAS will support surveillance requirements in support of Automated Dependent Surveillance Broadcast (ADS-B) or equivalent systems.
1.4.2.4Autoland
As an optional operation, DFMC SBAS design should consider support for Autoland. The autoland requirements have not been determined and validated at this time, but ongoing work intends to identify the requirements for SBAS-enabled autoland.
1.4.2.5Surface Movement
As an optional operation, DFMC SBAS design should consider the ability to provide protection levels applicable to surface movement. The surface movement requirements have not been determined and validated at this time.
1.4.3Operational Environment
The operational environment is similar to the operational environment for legacy L1 SBAS. The constituent entities in the DFMC SBAS are included below.
1.4.3.1Global Navigation Satellite System (GNSS) constellations
A GNSS constellation is a set of satellites that provide ranging signals on at least two frequencies with published interfaces and standards adhered to by the GNSS provider. The GNSS includes a ground monitoring network to monitor and control the satellites. The Interoperability Working Group (IWG) identified four GNSS constellations for consideration in DFMC SBAS.
a) Global Positioning System (GPS): Operational system using code division multiple access (CDMA) with a L1 C/A signal (1575 MHz), undergoing modernization to add L2C and L5 services. The United States operates GPS.
b) Globalnaya Navigatsionnaya Sputnikovaya Sistema(GLONASS): Operational frequency division multiple access (FDMA) system operating on L1 (1602 MHz) and L2 (1246 MHz), undergoing modernization to add CDMA signals on a frequencies to be determined. The Russian Federation operates GLONASS.
c) Galileo: A future system using CDMA on the E1 and E5a frequencies, with the first four satellites of a 27 satellite constellation in orbit. System initial operational capability is planned for 2014, with full capability expected by 2020. The European Union operates Galileo.
d)BeiDou Satellite System (BDS): A future system using CDMA on B1 and B5 frequencies. The system provides initial Asian capability, with full capability expected by 2020. The People’s Republic of China operates BDS.
1.4.3.2SBAS Service Providers
Several different service providers will operate DFMC SBASs. The Service Providers decide which GNSS constellations their respective SBAS will augment. The service providers procure and deploy the ground monitoring network and the geostationary satellites in accordance with ICAO and industry standards. The SBAS service providers coordinate with other Air Navigation Service Providers (ANSPs) to coordinate use of the SBAS by other ANSPs.
1.4.3.3Air Navigation Service Providers (ANSPs)
The ANSP are responsible for the approval and performance of navigation within their respective airspace. The ANSP coordinates with the SBAS service provider to facilitate approval of the SBAS within their airspace. The ANSP approves procedures using SBAS in their airspace.
1.4.3.4Ground Monitoring Network
The ground monitoring network receives GNSS signals at multiple ground stations. A central site processes the signals received by the ground monitor stations to determine the applicable orbit and clock corrections for the monitored GNSS satellites. Some ground systems might determine ionospheric corrections to support single-frequency users.
1.4.3.5Geostationary (Ranging) Satellites
Geostationary satellites and their respective GEOUplink Subsystems(GUSs) providethe communications link between the ground monitoring network and the avionics user. For some SBAS, the communications link can provide an additional ranging signal.
1.4.3.6Aircraft
DFMC SBAS aircraft will have avionics that receive and process the dual-frequency signals from GNSS satellites and the L1 and/or L5 signals from SBAS satellites.
1.4.3.7Pilots
Pilots have a limited role with respect to operation of the DFMC SBAS equipment. Pilots select the use of DFMC SBAS equipment or alternate navigation equipment. Pilots monitor the proper operation of the equipment and respond to alerts or warnings provided by the equipment. Pilots might input navigational plans (waypoints) for some equipment. Pilots will select and verify the approach for approach operations or select a null-approach when desiring navigation without approach operations.Pilots are not involved with equipment mode operation for legacy L1 SBAS equipment. Pilots are not expected to be involved with individual satellite selection. The role of pilots in selection of the operational SBAS system is under discussion. Pilots are not involved with SBAS system selection in legacy L1 SBAS equipment.
1.4.4Operational Concepts
1.4.4.1Overview
In DFMC SBAS, the user receives ranging signals and navigation data on two frequencies from a sufficient number of GNSS satellites and associated integrity and augmentation information from one or more geostationary satellites to develop and iono-free position solution that supports horizontal navigation, horizontal and vertical navigation, and surveillance requirements during flight operations. A DFMC SBAS ground system develops and monitors the integrity and augmentation signals for the geostationary satellite broadcast.
1.4.4.2General Concepts
1.4.4.2.1Augmented Constellations
The DFMC SBAS concept provides the capacity to correct any offour constellations and associated augmentation systems, provided the total number of satellites across all systems does not exceed 210 satellites. For consideration in the DFMC SBAS framework, constellations need to provide global service using at least two frequencies. The anticipated constellations are GPS, Galileo, GLONASS, and BeiDou, provided that the constellations progress as currently planned. There is no minimum number of constellations that a service provider must augment.Since there is no minimum augmentation requirement, neighboring SBAS might augment different constellations. Airborne equipment should be capable of switching ranging sources when transition from one SBAS service to another. Airborne equipment should address position biases between different SBAS systems during the transition.
1.4.4.2.2Number of concurrently augmented satellites
While the DFMC SBAS concept provides for the capability to identify 210 ranging sources, it is neither practical nor necessary to augment this number of ranging sources concurrently. Most SBAS only observe a portion of the augmented constellations at a time, based on the extent of the ground monitoring network. Current development efforts suggest the ability to broadcast 91 active augmentations concurrently. This corresponds with the maximum expectation number of augmentations observable to a ground monitoring network augmenting 4 constellations. There is a trade-off in the augmentation rate of any one ranging source versus the total number of augmented ranging sources.
1.4.4.2.3SBAS Signal in Space Data Content
DFMC SBAS will have two available frequencies for distribution of SBAS data, namely the L1 and the L5 frequencies. Since the L1 frequency supports legacy L1 SBAS users, its message formation will be virtually identical to the format and function currently identified in the ICAO SARPS and the RTCA MOPS. The L5 frequency provides more latitude, since the interface on L5 is subject to definition in the DFMC development process. The L5 frequency can either be tailored to provide a unique Iono-free Differential Correction function, or it can be a closer approximate of the L1 format applicable to both the dual frequency and L5 user. The current expectation is that equipment operating in DFMC mode will decode the L5 SBAS messages while equipment operating in the legacy L1 SBAS mode will decode the L1 SBAS messages. .
1.4.4.2.4Integrity Information
The ground monitoring network installed by an SBAS service provider will monitor GNSS satellites in view of the network and form corrections and integrity messages. The ground monitoring network will broadcast the corrections to users via GEO satellites. The DFMC SBAS system should provide an indication of “Not Monitored” for satellites not sufficiently observed and should either provide corrections and integrity data or indicate “Do Not Use” for satellites that it can observe. . The ground monitoring network will monitor the actual broadcast as part of satisfying the integrity requirement for the user. It is anticipated that the message capability for the corrections messages will be sufficient that each service provider can broadcast integrity and corrections information for all monitored GNSS satellites through each of its operational GEO satellites. Thus all the information available from one SBAS service provider can be obtained from a single GEO, and additional GEOs from that provider only provide back-up information to the original GEO.
1.4.4.2.5Receiver Autonomous Integrity Monitoring (RAIM)
It is anticipated that airborne equipment will have RAIM functionality. RAIM functionality might provide reversionary capability or it might provide an alternate capability when it provides better service than available SBAS systems. The current RAIM concept has embedded assumptions about the constellation performance. Prior to use with constellations other than GPS, it will be necessary to revalidate or update these assumptions. It might be desirable or necessary to provide different assumptions for each constellation used in RAIM. It might be desirable to provide different assumptions whether RAIM is being used with an iono-free position solution or single-frequency solutions using either of the frequencies. Scientists are assessing an Advanced RAIM (ARAIM) algorithm for multi-constellation use that requires validation of assumptions on a constellation basis. Scientists are also assessing ARAIM capability to provide the vertical navigation function. ARAIM might require additional messaging from the SBAS GEO.