SBAS Fundamentals

Fundamentals
Title	SBAS Fundamentals
Author(s)	GMV
Level	Basic
Year of Publication	2011

A Satellite-based Augmentation System (SBAS) is a civil aviation safety-critical system that supports wide-area or regional augmentation – even continental scale - through the use of geostationary (GEO) satellites which broadcast the augmentation information. A SBAS augments primary GNSS constellation(s) by providing GEO ranging, integrity and correction information. While the main goal of SBAS is to provide integrity assurance, it also increases the accuracy with position errors below 1 metre (1 sigma).

The ground infrastructure includes the accurately-surveyed sensor stations which receive the data from the primary GNSS satellites and a Central Processing Facility (CPF) which computes integrity, corrections and GEO ranging data forming the SBAS signal-in-space (SIS). The SBAS GEO satellites relay the SIS to the SBAS users which determine their position and time information. For this, they use measurements and satellite positions both from the primary GNSS constellation(s) and the SBAS GEO satellites and apply the SBAS correction data and its integrity.

The augmentation information provided by SBAS covers corrections and integrity for satellite position errors, satellite clock – time - errors and errors induced by the estimation of the delay of the signal while crossing the ionosphere. For the errors induced by the estimation of the delay caused by the troposphere and its integrity, the user applies a tropospheric delay model.

SBAS Performances

The SBAS performances are defined with respect to the level of service that the system is designed to. The main source for SBAS performances comes from civil aviation navigation safety requirements and they are different for each civil aviation operation (see Table ICAO GNSS performances requirements ).

Typical Operation	Horizontal Accuracy (95%)	Vertical Accuracy (95%)	Integrity	Time-To-Alert (TTA)	Continuity	Availability
En-route	3.7 km (2.0 NM)	N/A	1 – 1 × 10-7/h	5 min	1 – 1 × 10-4/h to 1 – 1 × 10-8/h	0.99 to 0.99999
En-route Terminal	0.74 km (0.4 NM)	N/A	1 – 1 × 10-7/h	15 s	1 – 1 × 10-4/h to 1 – 1 × 10-8/h	0.99 to 0.99999
Initial approach, Intermediate approach, Non-precision approach (NPA), Departure	220 m (720 ft)	N/A	1 –1x10-7/h	10 s	1 – 1x10-4/h to 1 – 1x10-8/h	0.99 to 0.99999
Approach operations with vertical guidance (APV-I)	16 m (52 ft)	20 m (66 ft)	1 – 2 × 10-7 per approach	10 s	1 – 8 × 10-6 in any 15 s	0.99 to 0.99999
Approach operations with vertical guidance (APV-II)	16 m (52 ft)	8 m (26 ft)	1 – 2 × 10-7 per approach	6 s	1 – 8 × 10-6 in any 15 s	0.99 to 0.99999
Category I precisión Approach	16 m (52 ft)	6.0 m to 4.0 m (20 ft to 13 ft)	1 – 2 × 10-7 per approach	6 s	1 – 8 × 10-6 in any 15 s	0.99 to 0.99999

As indicated in the table above, the performance requirements are expressed in terms of four quantitative concepts, many of them to be interpreted as probabilistic figures:

• Accuracy: is expressed in term of a Navigation System Error (NSE) as the difference between the real position of the aircraft and the position provided by the airbone equipment. A SBAS assures the compliance with respect the accuracy requirements by providing to the user corrections to the satellite orbit and clock errors as well as to the ionospheric residual propagation error.
• Integrity: is defined by ICAO as a measure of the trust that can be placed in the correctness of the information supplied by the system. This general statement is traduced at the SBAS system level as the maximum allowable probability that the navigation position error exceeds alarm limit and the navigation system does not alert the pilot in a time less than the time to alert. The SBAS assures the integrity requirements by:
  • Providing to the user satellite and/or ionospheric alarms in order to inform the user to reject the corresponding satellite/ionospheric corrections in its positioning computation.
  • Providing to the user Horizontal and Vertical Protection Level information (HPL, VPL) in order to assess the availability of the system, by comparing these PLs with the corresponding Alarm Limits for a given phase of flight (see next tabe). The SBAS computes and broadcasts integrity bounds to the satellite orbit and clock (UDRE) corrections as well as to the ionospheric corrections errors (GIVE) so that the user is able to compute a PL that over bounds the navigation system error experienced by the user with the integrity risk requirement.

Operation	Horizontal AL	Vertical AL
En-route (oceanic/continental)	7.4 Km (4 NM)	N/A
En-route (continental)	3.7 Km (2 NM)	N/A
En-route, Terminal	1.85 Km (1 NM)	N/A
NPA	556 m (0.3 NM)	N/A
APV-I	40 m (130 ft)	50 m (164 ft)
LPV200	40 m (130 ft)	35 m (200 ft)
APV-II	40 m (130 ft)	20 m (66 ft)
Category I	40 m (130 ft)	15 to 10 m (50 ft to 33 ft)

• Continuity: is the probability that the specified system performance will be maintained for the duration of a phase of operation, presuming that the system was available at the beginning of that phase of operation and was predicted to operate throughout the operation. Lack of continuity means that the operation must be aborted (with the associated risk).
• Availability: is the probability that the navigation service is available at the beginning of the planned operation. A SBAS is considered available when the accuracy, integrity and continuity requirements are met and it is measured in terms of probability of the system being available for any given user at any given time. In practice, the availability is computed by measuring the probability of XPL being below XAL. It should be noted that a lack of availability is not a safety concern but prevents the nominal operation of the system, and implies an associated impact on the service operation status.

SBAS architecture

SBAS architecture

A SBAS is a safety critical system designed to augment one or several satellite navigation systems. In this sense, it is not a standalone system. The main layers of a general SBAS architecture are:

• Space segment comprising the geostationary satellites (GEO) with navigation payloads in charge of transmitting a GPS-like carrier signal with the SBAS information.
• Ground segment comprising all the ground elements in charge of the provision of the SBAS navigation message. The main elements are:
  • GEO satellite control center
  • Monitoring station network
  • Processing Facilities
  • Communication layer.
• Support segment comprising all the elements supporting the SBAS mission non-service-critical functionalities: configuration control, performance evaluation, maintenance and development, help desk, etc.
• User segment comprising all the user equipment needed to receive and use the SBAS information.

Space segment

The SBAS space segment is composed by several geostationary satellites in charge of broadcasting, over the service area, the SBAS navigation message. Typically, the SBAS satellites are multipurpose (commercial communication) satellites that carry out an additional navigation payload capable to generate a GPS-like signal that retransmit to the users the navigation message generated on-ground.

The SBAS GEO navigation payload is a transponder that relays the on-ground generated signal (upload to the GEO in the C-band frequency) to a transmitted signal in the L-band frequency band. For most of the navigation payloads, a second C-band downlink channel is available in order to improve the adjustment of the signal delay due to atmospheric propagation. However, first generation of navigation payloads only included a single-channel uplink, frequency translating repeater.

The new generation of SBAS navigation payload includes more advanced functionalities. Among them, it is worth noting:

• Increase of the emission bandwidth and broadcast power.
• Evolution to dual-frequency L1/L5 transponders.
• Evolution of payload from signal transponders to regenerative payloads in which the signal is entirely build up onboard (the GEO only receives from ground segment the navigation message to be modulated in the GEO signal).

SBAS Signal

The signal broadcasted by a SBAS GEO is subjected to the ICAO standards (see SBAS Standards). The SBAS satellite shall transmit a GPS-like L1 (1574.42 MHz) signal, modulated with a Coarse/Acquision Pseudo-Random Noise (PRN) code.

The SBAS L1 radiofrequency characteristics are:

Parameter	Description
Modulation	Bi-phase shift key (BPSK) modulated by a bit traincomprising the PRN code, and the SBAS data (modulo-2 sum).
Bandwidth	L1 ±30.69 MHz. At least 95% of the broadcast power will be contained within the L1 ±12 MHz band.
Ranging Codes	A PRN Code (Gold code) of 1 millisecond in length at a chipping rate of 1023 Kbps.
SBAS Data	500 symbols per second, module-2 modulated Effectively 250 bits per second
Power	Minimum power –131 dBm at 5 degrees elevation Maximum power –119,5 dBm

A convolutional encoding of the bits is performed with the following parameters:

Code Parameter	Value
Code Parameter	Value
Coding Rate	1/2
Coding Scheme	Convolutional
Constraint Length	7
Generator Polynomials	G1 = 171(oct); G2 = 133(oct)
Encoding Sequence	G1 then G2
Flush	No

SBAS Data Format

The specification of the SBAS message data format is contained in the ICAO SARPS Appendix B for the aspect related with the signal in space, as well as in the RTCA MOPS DO-229D for the minimum performance requirements applicable to the airborne SBAS receiver equipment.

Every SBAS provides ranging signals transmitted by GEO satellites, differential corrections on the wide area and additional parameters aimed to guarantee the integrity of the GNSS user:

• GEO Ranging: transmission of GPS-like L1 signals from GEO satellites to augment the number of navigation satellites available to the users.
• Wide Area Differential (WAD): differential corrections to the existing GPS/GLONASS/GEO navigation services computed in a wide area to improve navigation services performance. This includes corrections to the satellite orbits and clocks, as well as information to estimate the delay suffered from the signal when it passes through the ionosphere.
• GNSS/Ground Integrity Channel (GIC): integrity information to inform about the availability of GPS/GLONASS/GEO safe navigation service.

The SBAS delivers to the user the corrections and integrity data as well as some ancillary information (timing, degradation parameters, etc.) through messages encoded in the signal. The format of the messages is thoroughly explained in the article The EGNOS SBAS Message Format Explained.

Ground Mission segment

The ground mission segment is probably the most critical part of a SBAS. The main functionalities to be provided are:

• The Monitoring and Data Collection of the primary satellite system to be augmented. Its function is to collect the main input data for the SBAS mission. This is performed by a network of GNSS receivers densely distributed through the service area. The network of monitoring stations should be designed in order to fulfil:
  • Correct coverage of the SBAS entire service area.
  • Monitor correctly the ionosphere in the service area.
  • Densely enough to assure the accuracy of the provided correction and well as to provide an adequate level of redundancy of observables in order to compute the integrity products.

Additionally, it is highly desirable to complement the monitoring network with station located out of the service area in order to correctly monitor the SBAS GEO satellites as well to improve the observability of long-term varying effects as the satellite orbit determination, satellite manoeuvres, etc.

Each monitoring station shares a common set of characteristics/components: o Dual frequency (L1/L2) receivers with geodetic quality. o Atomic frequency standard (Caesium/Rubidium/H-Maser). o Able to track all in view GPS and GEO satellites o Allocated in site conditions with good local environment condition in terms of multipath and radio frequency interference o Geo-referenced to WGS-84 (or ITRF) within a 1-3 cm accuracy o Compliant with relative stringent processing capabilities:  1 Hz data acquisition  Embedded data quality checks to remove misleading data  Data processing and broadcast within a few milliseconds – Integrate robustness against known threats (e.g. “Evil” waveforms detection). • The Data Processing element of the SBAS ground segment is in charge of: o Processing all input data from the Ranging and Monitoring Data Stations o Estimate the satellite corrections, ionospheric model and error variance terms o Perform a dedicated integrity assessment on the SBAS and GPS signals o Format the outputs according to the SBAS standards

Support Mission Segment

The SBAS Support Segment collects all the elements need to support the development and operation of an SBAS system. These elements are not related with the provision of the SBAS service but they are needed as external support functionalities. They SBAS support needs can be listed as:

• The design, development and validation phases
• The deployment and operation
• The certification process
• The maintenance and troubleshooting
• The certification of applications

Between the different SBAS Support elements it is worth noting:

• End to End Simulators in charge of simulating data under controlled conditions (reference errors, emulated feared events) in order to emulates the real system behaviour (delays and communication problems, algorithm, broadcasting features, etc).
• Service Volume Simulators in charge of evaluating the overall theoretical SBAS expected performances over the service area under a set of controlled conditions
• Performance Analysis Tools in charge of measure all the performance concepts at:
  • system level by analyzing the Signal in Space information and checking the accuracy, integrity, continuity and availability features but using concepts based in the errors available at pseudorange domain.
  • user point of view level, by analyzing user date (including local conditions) and applying user-related algorithm (see SBAS standards).
• On-line monitoring tools for tracking the system behaviours in real time and collecting system failure and anomalies. This element also supports the system operation by predicting the expected behaviour of the system and provides warning as system outages or degradations. It also provides warning to external users.
• Archiving tools supporting the data archiving. The data comprises both all the internal info generated by the system as well as external data (satellite reference orbit and clock data, ionospheric information, outages, etc).
• Application test benchs consisting in tools to support application by generating test scenarios for application certification as well verification of correct performances. They are able to plug different application software or equipment for homologation purposes.

User segment

The SBAS user segment comprises all the user equipment that makes use of the SABAS Signal in Space (SIS). The SBAS User segment is in fact not under the control of the SBAS service provider as it is driven by the SBAS application market.

In general, the SBAS service operator provides different service levels aiming at different market sectors, namely an Open Service, a Safety of Life service (SoL) and a Commercial Service.

For the Safety-of-Life (SoL) service, the SBAS user equipment shall be compliant (certified) against several standards. For instance, civil aviation SBAS equipment shall demonstrate (see SBAS standards):

• Full compliance to RTCA SBAS MOPS DO-229 ()
• Full compliance to the RTCA SBAS MOPS 228 and 301 (antenna requirements)
• Compliance to RTCA TSO (C190, C145b, C146b) for SBAS equipment.
• Compatibility to other avionics equipment, in particular Flight Management Systems (FMS).

The SoL civil aviation certified equipment is in the highest rank with respect its cost. There exist a large number of certified receivers manufacturers worldwide both in the US (GARMIN, Honeywell, Rockwell Collins, General Avionics, etc) and in Europe (see complete list in EASA homepage).

The Open Service (OS) is targeted to low cost, general purpose GPS equipment that use the SBAS SIS to provide the user with an enhanced accuracy performance in comparison with ones provided by a standalone GPS positioning system. In comparison with the certification requirements of the user equipment above, user equipment shall not be fully compliant with the RTCA MOPS DO 229 processing rules but can only make use of the processing algorithm that make use of the accuracy corrections provided by the SBAS SIS.

Finally, some SBAS service providers (see EGNOS) includes the provision of the information computed by the SBAS ground element (input raw data, corrections, integrity information) by a dissemination mean different that the SBAS GEO link (generally, using terrestrial telecommunication networks). This constitutes the Commercial Service. This market sector comprises professional users (land and geodesy applications, maritime or terrestrial transport applications) that are not subject to the integrity and latency requirements needed in the SoL service.