Development of SBAS

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The ENRI has been developing SBAS (satellite-based augmentation system) which augments the performance of satellite navigation systems such as GPS. Recently, the ENRI has joined the standardization activities for the next generation SBAS and is transmitting the live signal of the next generation SBAS as an experiment of QZSS (quasi-zenith satellite system), Japanese national satellite navigation project.


Augmentation System

The satellite navigation systems such as GPS and GLONASS, called core systems, might have insufficient performance in terms of safety. For example, there is the specification for position accuracy derived by GPS, however, the accuracy is not guaranteed at any place anytime and sometimes a large error occurs.

An augmentation system prevents such a condition. For this purpose, augmentation system generally provides the following information to user receivers:

The ICAO (International Civil Aviation Organization) is responsible of the international standards of augmentation systems used for civil aviation applications. The SBAS (satellite-based augmentation system) is one of ICAO augmentation systems providing navigation means with sufficient integrity to aircraft users.

Nowadays some satellite navigation systems are available for use as shown in Table 1. We need an augmentation system to use them for civil aviation apllications.

Table 1 Satellite Navigation Systems (As of 2018)
NameOperated bySince# of SatellitesStatus
GPS USA 1993 31 Launching modernized satellites
GLONASS Russia 1996 24 Launching modernized satellites
Galileo EU 2017 18 30 satellites by 2019
BeiDou China 2015 24 35 satellites by 2020
QZSS Japan Not Yet 4 Will begin operation in 2018


L1 SBAS (Current SBAS)

The current SBAS was introduced for civil aviation in 2002 as an international standard augmentation system. It transmits augmentation signal to the continental-wide service area via geostationary satellites like INMARSAT. The current standards define only SBAS on L1 frequency (L1 SBAS), while the next generation SBAS using L5 frequency will be called L5 SBAS. The L1 SBAS is sometimes called SCSF (single-constellation single-frequency) SBAS because it can augment L1 C/A signal of GPS or L1SP signal of GLONASS only.

Japan has been operating its own SBAS called MSAS (MTSAT-based Augmentation System) with Himawari-7 (or officially MTSAT-2) geostationary satellite. We can use SBAS worldwide with operational systems shown in Table 2.

Table 2 The operational SBAS systems (As of 2018)
NameOperated bySince# of satellitesStatus
WAAS USA 2003 3 Upgrading to dual-frequency
MSAS Japan 2007 1 Switch GEO to QZS-3 in 2020
EGNOS EU 2011 3 Develping DFMC SBAS
GAGAN India 2014 3
SDCM Russia Not Yet 3 Will operate by 2020
BDSBAS China Not Yet 3 (plan) Will operate by 2020
KASS Korea Not Yet 1 to 2 (plan) Will operate by 2022

The current operational system, MSAS V1, will be reconstructed as MSAS V2, because the MTSAT-2 geostationary satellite will reach its end-of-life. The MSAS V2 will transmit an augmentation signal via QZS-3 geostationary satellite.


L5 SBAS (Next Generation SBAS)

The satellite navigation systems are easily affected by the ionosphere especially in the low latitude regions including Japan. In order to overcome this limitation and make satellite navigation available anywhere in the world, a measure is usage of multi-frequency signals. It is possible to eliminate ionospheric effects uging multi-frequency signals because the effects of ionosphere is dependent upon signal frequency.

The next generation SBAS uses multi-frequency signals to mitigate ionospheric effects. It also augments multiple core constellations to increase the number of satellites for positioning. The new version of SBAS is called MCMF (multi-constellation multi-frequency) SBAS or DFMC (fual-frequency multi-constellation) SBAS. GPS is transmitting L1. L2. and L5 frequency signals, however, we can use L1 and L5 only because L2 frequency is not protected for aviation use.

The ICAO is now conducting the standardization of the new version of SBAS, called L5 SBAS. The L5 SBAS transmits augmentation signals on 1176.45 MHz as same with GPS L5. It augments core constellations, i.e., GPS, GLONASS, Galileo, and/or BeiDou, using combination of signals shown in Table 3. Note that the receivers have to receive dual frequency signals from core constellations, while L5 SBAS augmentation signal is transmitted on L5 frequency.

Table 3 Combination of core constellation signals
Core ConstellationL1-Band SignalL5-Band SignalRemarks
GPS L1 C/A L5-Q
GLONASS L1OC L3OC CDMA signals
Galileo E1-C E5a-Q
BeiDou B1C B2a

L5 SBAS transmits augmentation messages listed in Table 4. For example, MT32 contains clock and orbit corrections and covariance matrix for a satellite to be augmented. Note that Fast Correction and Ionospheric Correction transmitted in case of L1 SBAS are removed.

The receivers have to receive dual frequency signals with combination shown in Table 3 and compute the ionosphere-free linear combination pseudorange by:

Ionosphere-Free Linear Combination
for position computation (PR: Ionosphere-free linear combination, fL1, fL5: Frequency of L1 and L5, respectively, PRL1, PRL5: Pseudorange measured on L1 and L5).

Table 4 L5 SBAS Augmentation Messages
Message TypeNameTransmission Interval (s)
31 Satellite Mask 120
32 Corrections and Covariance Matrix 120 for each satellite
34`36 Integrity Information 6
37 Degradation Parameters 120
39 and 40 SBAS Ephemeris 120 for each satellite
42 Timing Offset 240
47 SBAS Almanac 120 for each satellite

Table 5 summarizes the characteristics of L5 SBAS in comparison with L1 SBAS.

Table 5 Comparison of L5 SBAS to L1 SBAS
ItemL5 SBASL1 SBASRemarks
RF Specifications Frequency 1176.45 MHz 1575.42 MHz Same with GPS
Bandwidth 20 to 24 MHz ≥ 2.2 MHz
Modulation BPSK Potentially QPSK
Spreading Code PRN 120 to 158 Expanded from 120 to 138 for L1 SBAS
Symbol Rate 1 Ksps 500 sps
Encoding
1/2 FEC + Manchester Encoding
(No NH Coding)
1/2 FEC Constraint K=7 for FEC
Data Rate 250 bps
Message Length 250 bits
Preamble 4-bit 6-pattern 8-bit 3-pattern Synchronizes to GPS subframe
CRC Length 24 bits
Augmentation Function Core Systems
GPS, GLONASS, Galileo,
BeiDou, and SBAS
GPS, GLONASS, and SBAS
Total # of Satellites 210 214
Augments simultaneously up to 92 51
Pseudorange Augmented
L1+L5
Ionosphere Free Linear Combination
L1 No L5-only mode
Corrections Clock and Orbit
Fast Correction, Clock,
Orbit, and Ionosphere
No SA
SBAS Satellite Any Orbit Geostationary Orbit

Note that L5 SBAS service is independent from L1 SBAS service. Each SBAS provider may transmit L1 SBAS and L5 SBAS, transmit L1 SBAS without L5 SBAS service, or transmit L5 SBAS without L1 SBAS service. Any information shown above is subject to change because the standardization activity for L5 SBAS is currently ongoing.


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