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.