Introduction

Satellite-based positioning and navigation play a vital and growing role in areas as diverse as transportation, agriculture, emergency services, engineering, mapping and mining. Over the next 5 years, there will be a surge of new navigation satellites launched: the US modernized GPS constellation, Russia’s revitalized GLONASS, European Union’s Galileo and China’s BeiDou systems, collectively referred to as Global Navigation Satellite Systems (GNSS). Furthermore, the deployment of Regional Navigation Satellite Systems (RNSS) and satellite-based augmentation systems (SBAS) brings additional satellites and signals to augment GNSS position, navigation and timing (PNT) capabilities. However, standalone GNSS navigation solutions—even with the large number of signals from multiple satellite constellations—can only provide positioning resolution of the order of several meters. This is not adequate to satisfy the stringent requirements of many PNT applications, particularly in mission- and safety-critical applications such as aviation requiring high-integrity information, maritime, land transportation and emergency services.

GNSS augmentation is a method to improve PNT accuracy, reliability, availability and continuity through the integration of external information into the calculation process. Real-time kinematic (RTK) and precise point positioning (PPP) were developed for high-accuracy positioning, which the mining, civil construction and precision agriculture sectors require; the SBAS service was established mainly to provide integrity assurance for aviation operations and submeter-level Differential-GNSS (DGNSS) for location-based services (LBS), maritime users and many other applications. These augmentation services have significantly improved GNSS PNT capability over the past several decades. Other augmentation methods have also been developed such as assisted GNSS and integrated GNSS and inertial navigation systems. However, the term “GNSS augmentation” is used here to specifically describe SBAS that support wide-area augmentation through the use of additional satellite-broadcast correction messages.

A report prepared for the Australian Department of Industry, Innovation, Climate Change, Science, Research and Tertiary Education in 2013 by ACIL Allen Consulting Inc. (ACIL Allen Consulting 2013) stated that “Augmented GNSS, which provides greater performance in terms of accuracy, integrity, availability and reliability, are delivering significant economic benefits in several key sectors of the economy, as well as environmental, safety and other social benefits.” In 2012, Australia’s GDP was between $2.3 billion and $3.7 billion higher than it would have been without accumulated productivity improvements arising from augmented GNSS compared to the GDP of 2000. Furthermore, the study found if Australia focused on the extension of augmentation services to increase adoption, an associated GDP increase is projected to be between $7.8 billion and $13.7 billion by 2020 (compared to the GDP of 2012).

GNSS augmentation systems can be divided into ground-based and satellite-/space-based augmentation systems. Both systems require ground-monitoring stations to verify the validity of GNSS signals and generate augmentation information to enhance accuracy, integrity, availability and continuity. The key distinction between ground- and satellite-based augmentations is the communication link used to disseminate information. Ground-based systems use radio beacons to transmit information to users via a variety of radio frequencies, from low-frequency (LF) to ultra-high-frequency (UHF) bands. For example, the current standard for delivering high-accuracy RTK augmentation in Australia is the licensed UHF band in the 450–470 MHz range (unlicensed spread spectrum in the 915–928 MHz and 2400–2483.5 MHz is also available with limited range and compatibility). As of February 2016, the Australian Communications and Media Authority (ACMA) Register of Radiocommunication Licenses shows over 26300 UHF (450–470) frequency licenses in Australia. Managing the frequency assignment and interference is increasingly becoming an issue in densely populated areas. The transmission range of UHF is generally limited to 10 km in optimal conditions. However, in practice a reduced range of a few km can be expected due to rough terrain and vegetation cover. To combat this reduction in range, radio repeaters can be used to extend the range of the data link. When using LF band transmission, as in the case of DGNSS, the augmentation data link stretches to 300 km or more, but with lower bandwidth and data rate. Internet or mobile broadcasting is also a ground-based solution, commonly used for delivering network-RTK augmentation. Depending on telecommunication coverage, it must deal with issues of connectivity, coverage and roaming charges (Elneser 2016). Communication and GNSS satellites on the other hand can deliver augmentation information via a satellite communication link. This mode of communication is well suited for wide-area and/or regional augmentation of GNSS and can be broadcast to a very large number of users across a broad coverage area. It can also provide homogenous positioning quality within a consistent reference frame using a single GNSS receiver.

The Australian Government through Geoscience Australia has developed a National Positioning Infrastructure (NPI) Plan which examines investment in domestic infrastructure to deliver accurate and reliable PNT information to users across Australia (Hausler 2014). One of the visions of the Australian NPI is to provide multi-GNSS products and services anywhere (outdoor) and at any time across the Australian landscape and its maritime jurisdiction. In an Australian perspective, large coverage gaps between existing radio and mobile/Internet communications infrastructure prohibit delivery of a nationwide augmented GNSS service using ground-based communications alone. As Australia is fortuitously situated in the GNSS “hotspot” where GNSS satellite visibility is at a global high (Dempster and Hewitson 2007; Rizos 2008), Australia will be able to take advantage of the opportunities provided by GNSS, RNSS and SBAS satellites to deliver improved GNSS PNT performance across the region. Therefore, options for satellite-based delivery of GNSS augmentation must be explored as part of the NPI implementation in order to ensure GNSS augmentation information can be delivered anywhere and at any time in Australia.

We present an overview of SBAS options for augmented GNSS services in Australia, and potentially New Zealand, with the aim to tease out key similarities and differences in their augmentation capabilities. SBAS can technically be classified into two user categories: (1) SBAS for aviation and (2) “non-aviation” SBAS. It is worthwhile to note that the focus is on L-band frequencies associated with GNSS. The advantage of transmitting corrections using an L-band frequency is that GNSS receivers are already equipped with L-band antennas and radio frequency front ends, which could simplify the reception of data, and that no new frequency licensing is necessary.

Satellite-based augmentation system (SBAS) for aviation

The term SBAS, in its strictest sense, refers to a civil aviation safety-critical system providing wide-area GPS augmentation by broadcasting augmentation information using geostationary (GEO) satellites. As its primary aim is to improve integrity, availability and accuracy of basic GPS signals for aircraft navigation, SBAS transmits (a) integrity, (b) ranging information, and (c) correction messages, which include satellite and ionospheric corrections, so that:

  1. (a)

    Integrity is enhanced by sending alerts to users to not track the failed satellites identified as having large signal errors.

  2. (b)

    Signal availability is improved as the SBAS satellite transmits additional L1 ranging signal.

  3. (c)

    Accuracy is enhanced through the transmission of wide-area corrections for range errors, such as satellite orbits, clocks and improved ionospheric information.

In addition to corrections and integrity data, SBAS satellites also transmit ancillary information such as timing and degradation parameters through messages encoded in the signal. SBAS deliver corrections free of charge for users to obtain improved GPS positioning performance in region of SBAS coverage. In fact, moist commercial GNSS receivers are SBAS-capable.

All aspects of aviation SBAS operation are defined in the radio technical commission for aeronautics (RTCA), minimum operational performance standard (MOPS) and are International Civil Aviation Organization (ICAO) compliant (Radio Technical Commission for Aeronautics 2006). Its standard determines the format of the messages, the integrity levels required for certain operations, and the methods to calculate event probabilities, which in turn determine if the required integrity for a certain operation is met. However, it should be noted that SBAS is widely used in a range of non-aviation applications.

SBAS L1 signal

SBAS GEO satellites transmit an L1 signal (1574.42 MHz), modulated with a coarse/acquisition (C/A) pseudo-random noise (PRN) code. This signal uses the same carrier frequency as the GPS C/A code signal and many other systems’ civil signals in the upper L-band. The SBAS L1 radiofrequency characteristics are shown in Table 1.

Table 1 Characteristics of the SBAS L1 radio frequency
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The SBAS performances are defined with respect to civil aviation navigation safety operations and requirements. Table 2 shows the different performance requirements, on which ICAO certification is based. As the ICAO certification completely specifies the functions and details of a SBAS, any system that does not follow the specification cannot be certified, and thus not allowed to be used for aircraft safety operations. For example, even though is it not explicitly stated, the specification continuously refers to “geostationary orbit.” Thus, satellites in other orbits would not be allowed, at least in the first generation SBAS.

Table 2 ICAO certification of performance requirements for various aviation operations (International Civil Aviation Organization 2008)
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Existing SBAS

Several countries have implemented SBAS, or are in various stages of deploying their own SBAS. The USA has the Wide Area Augmentation System (WAAS); the EU has the European Geostationary Navigation Overlay Service (EGNOS); Russia has the System for Differential Correction and Monitoring (SDCM); Japan has the Multi-functional Satellite Augmentation System (MSAS); India has launched the Geo-Augmented Navigation system (GAGAN); and Korea has approved and is developing its Korean Augmentation Satellite System (KASS). China, South Africa and South America are currently in the conceptual phases of design for their own systems. Figure 1 shows existing SBAS coverage.

Fig. 1
figure 1

Existing SBAS coverage provided by US WAAS, European EGNOS, Indian GAGAN, Russian SDCM and Japanese MSAS. Figure was adapted from GENQ Inc, 2015

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It is worthwhile to note that the coverage of SBAS is dictated by the availability of the SBAS signal and ground reference stations network for monitoring of ranging information and computing of satellite and ionospheric corrections. Currently operating SBAS can be referred to as first generation SBAS, as they currently only augment the GPS constellation with the exception of SDCM which augments both GPS and GLONASS. Although the current ICAO specification does cover augmentation of GLONASS, it does not make any reference to other GNSS or RNSS.

The U.S. WAAS covers the USA, Canada and Mexico, and was the first to be employed with its first satellite launched in 2003 (US Federal Aviation Administration 2008). As it is the oldest system and satellites reach their end of life, they are currently developing the next-generation SBAS, which will be discussed in “SBAS for Australia” section.

The Japanese MSAS and European EGNOS followed with their first satellites launched in 2005. MSAS was put in operation in 2007, but can only be used for non-precision approach (Fujiwara 2011). EGNOS started operation in 2009 for its open service, while its safety-of-life service started operation in 2011 (European Global Navigation Satellite Systems Agency 2015). Similar to WAAS, EGNOS is capable of providing full CAT-I (LPV-200) performance.

In 2011, the first Indian GAGAN satellite and the first Russian SDCM satellite were launched. GAGAN became operational after it received ICAO certification in 2014 (Aguilera et al. 2014; International Civil Aviation Organization 2015), even though its planned third satellite did not launch at that stage. Information about the operational status of SDCM is difficult to find, but all satellites have been launched and a 2014 meeting report indicates they aim for APV-II certification, but had not yet received this (International Civil Aviation Organization 2014).

The only SBAS not yet operational, but certain to be employed, is the Korean Augmentation Satellite System (KASS). Its development was initiated in 2002 (Kee 2014), is expected to provide open services in 2018, full operational capabilities in 2019–2020, and safety-of-life (APV-I) services in 2022 (Aguilera et al. 2014).

Tables 3 and 4 provide an overview of current SBAS. The “year” column shows the date when the first SBAS satellite became operational and the “total” column gives the number of satellites in the complete constellation. Within the “Australian visibility” column, “Y” means yes, “N” means no and “P” means partial. Partial visibility can exist in the case that only part of the constellation is visible to all of Australia or part of Australia.

Table 3 Overview of current SBAS constellations
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Table 4 Overview of SBAS satellites, their PRN, name, location, L-band signals that they transmit, satellite launch year and visibility in Australia
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It is also worthwhile to mention GMV’s magic SBAS test bed which can be used to support the design and implementation of an SBAS in a given region. It can be used in post-processed and real-time mode to demonstrate the feasibility and benefits of SBAS technology to potential user community. Another useful technology is the SISNeT (Signal-in-Space through Internet) developed by the European Space Agency (ESA) for relaying EGNOS messages. SISNeT is a technology that allows streaming of SBAS information in real-time over the Internet instead of a geostationary satellite.

SBAS for Australia

Australia is one of the few large Organizations for Economic Co-operation and Development countries without SBAS services, including both wide-area DGNSS, ranging and integrity services via satellites (Austroads 2013). According to a white paper produced by the Australian Space Industry Innovation Council in 2011 (Australia Department of Infrastructure and Transport 2011), Australia could consider a SBAS capability that is:

  1. 1.

    Solely owned and operated by Australia.

  2. 2.

    An extension of other existing SBAS, such as MSAS or GAGAN.

  3. 3.

    A global SBAS model, either implemented as part of a government or commercial arrangement.

Possible utilization of existing SBAS in Australia

In 2010 the possibility of developing an indigenous SBAS capability for Australia with a modest investment of USD$30 M arose, but became unlikely in 2011 (Collier et al. 2011). The option proposed was to add a GNSS augmentation payload to the Australian National Broadband Network (NBN) communication satellites, and to supplement ground infrastructure with ground reference stations and Master Control Stations. However, it was noted that, on the basis of information available in 2011, it was difficult to justify the significant investment involved in establishing a SBAS in Australia based on supporting aviation operations at smaller aerodromes alone. It was recommended that consideration of any future investment in SBAS would require a whole-of-Government approach. The significant cost of developing a SBAS could then be considered against potential benefits across a range of industries beyond aviation.

An alternative option for an Australian SBAS is to work in collaboration with neighboring countries to extend the service area of existing or newly developed SBAS, such as Japan’s MSAS, India’s GAGAN or Korea’s KASS systems. Currently visible in Australia are the full MSAS constellation, i.e., MTSAT-1R and MTSAT-2 satellites, two of the three SDCM satellites, two of the three GAGAN satellites (of which one visible in the west of Australia only), and the future KASS constellation. Even though these SBAS GEO satellites are visible in Australia, they cannot be used under current arrangements as the satellites do not transmit ionospheric information valid for the Australian region, i.e., the ionospheric data these SBAS GEO satellites transmit are for their own coverage region.

For an existing SBAS to work in Australia, it needs to have ground stations in Australia monitoring the satellite constellation for Australian-specific integrity and measuring ionospheric delay. In 2012, researchers from Electronic Navigation Research Institute (ENRI) in Japan conducted a study hypothesizing the expansion of the MSAS service area toward Australia through the installation of 15–20 ground stations in Australia and the submission of correction messages to Australian users through MSAS satellites. They confirmed the technical possibility of expanding the MSAS service area to Australia. In addition, they also noted two possible situations when considering the expansion of MSAS to Australia: (a) Australia has its own Master Control Station and the MSAS GEO satellite works as a transponder, independently of Japan, meaning that Australia has a specific PRN code different from MSAS; or (b) the Master Control Station currently located in Japan will service both countries simultaneously by using a single PRN signal.

Augmented GNSS using “non-aviation” SBAS

There are several “non-aviation” augmentation services delivered via satellite-based communication channels to support many GNSS PNT applications using positioning techniques such as wide-area DGNSS and PPP. In most instances, these services are also known as SBAS. However, to highlight their differences, these services will be referred herein as “non-aviation” SBAS.

The augmented GNSS services are not aviation SBAS compliant due to:

  1. 1.

    Differences in data message structures.

    The message structure used in “aviation-style” SBAS are defined in the RTCA format, while the message structure for DGNSS, RTK and PPP methods are defined by RTCM (Radio Technical Commission for Maritime Services), or a proprietary format in the case of commercial service providers.

  2. 2.

    Different frequencies used for transmission of corrections.

    The existing aviation SBAS signals are broadcast from SBAS GEO satellites using the L1 frequency, which share similar design to the GPS L1C/A signals. As all aspects of SBAS operation are defined in the RTCA MOPS (RTCA/DO-229), and are ICAO compliant, aircraft flying from the U.S. to Europe and Japan will be fully compatible with the European and Japanese SBAS. “Non-aviation” SBAS on the other hand uses other L-band frequencies, e.g., Japan’s Quasi-Zenith Satellite System (QZSS) L6 signal or commercial communication satellites providing L-band communication like Inmarsat. There is no standardization with respect to “non-aviation” SBAS.

  3. 3.

    Absence of the extra ranging signals from the GEO satellites.

    The L1C/A code transmitted by the aviation SBAS GEO satellites can be used as an additional ranging signal thus further improving the availability of the service. “Non-aviation” SBAS uses L-band channels as communication links only without additional ranging signals.

  4. 4.

    Missing integrity data and monitoring.

    Given that SBAS is implemented primarily for the civil aviation sector, one of the most important functions for aircraft landings and/or any safety- and mission-critical applications is “integrity”—the ability to provide timely warnings when the system is providing erroneous information and should not be used. Although GNSS satellites broadcast integrity messages as part of their navigation message, the latency of the message is not adequate for aviation use. Thus, SBAS transmits, in addition to ranging signal and correction information, integrity data to support safety-critical application in the aviation sector. “Non-aviation” SBAS services on the other hand do not typically transmit integrity data that meets the specification for safety-of-life applications. “Non-aviation” SBAS services mainly focus on delivering accurate positioning solutions, and guaranteed levels of availability and integrity (reliability) remain secondary considerations when accuracy is the key driver (Collier et al. 2011).

The success of SBAS is attributable to the fact that it is an open and free service that removes the need for separate hardware to receive SBAS messages, as well as the use of a common L1 frequency that leads to ease of signal reception. Vast majority of commercial GNSS receivers including those in smartphones for LBS applications are SBAS-capable, which means that it can provide higher positioning accuracy in region of SBAS coverage. As aviation SBAS must comply with RTCA MOPS standards and be ICAO compliant, all systems are compatible and interoperable. A user with a standard GPS receiver can benefit from the same level of service and performance whether located in the WAAS or EGNOS coverage area. “Non-aviation” SBAS services, on the other hand, were mostly developed on an ad hoc basis and differ predominantly with respect to their delivered positioning accuracies and their targeted applications, e.g., offshore positioning versus precision agriculture. Figure 2 shows accuracies comparison of different GNSS positioning techniques, e.g., SBAS, DGNSS, PPP and RTK.

Fig. 2
figure 2

Comparison of positioning accuracies provided by SBAS, DGNSS, PPP and RTK methods. Figure was adapted from NovAtel Inc, 2015. It should be noted that the positioning accuracies provided by some positioning techniques such as DGNSS and RTK are a function of baseline length

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Commercial satellite-based augmentation services

While SBAS for aviation requires ICAO certification, a range of commercial ventures exist providing commercial GNSS augmentation services via satellite communication to support various PNT applications. Table 5 lists the companies providing increased accuracy positioning services using GEO satellites transmitting L-band frequencies. Many of these offered services use the SBAS designation to identify their augmentation services. However, they should be considered separate from the government-operated aviation SBAS as discussed.

Table 5 Commercial GNSS augmentation services that deliver correction information through satellite communication channels
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Unlike aviation SBAS, these commercial SBAS services are not standardized and cannot be ICAO certified. This means not all GNSS receivers are able to receive the augmentation signals, and dedicated hardware and subscriptions are required. OmniSTAR is the largest satellite augmentation service provider, and there are 33 receivers from different manufacturers that can decode OmniSTAR messages. Trimble RTX services work on 17 Trimble receivers; the NavCom StarFire service on two NavCom receivers; C-Nav services on two C-Nav receivers; and Fugro Starfix services only work on the one Fugro StarPack GNSS receiver. The augmentation services of Veripos, TerraStar and Novatel can be used on a select number of receivers from multiple manufacturers. Neither Veripos, TerraStar, nor Novatel provides a list, and thus, users should check for service availability from the receiver manufacturers. In particular, the receiver must be able to receive and demodulate the augmentation information.

While SBAS uses the L1 to transmit augmentation information, commercial providers lease frequency transponders on GEO communication satellites. The services are delivered using different L-band frequencies, depending on the service and region. For example, the Trimble CenterPoint RTX service is provided through a range of different frequencies and data transmission speeds as shown in Table 6.

Table 6 Trimble RTX service broadcasts augmentation information on different L-band frequencies and baud rates
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Table 7 lists the communication satellites utilized by commercial service providers to deliver augmentation services to their customers. Further investigation confirms that all companies use the same satellites, which is the Inmarsat satellite series.

Table 7 List of the communication satellites utilized by commercial GNSS service providers to deliver augmentation correction information
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Table 8 provides a summary of the currently available commercial communication satellites between 100°E and 180°W, which provide L-band transmission communications. These satellites could transmit augmentation corrections to GNSS PNT users in Australia and New Zealand. Excluded in the table are existing aviation SBAS satellites visible to Australia: SDCM Luch-5A, MSAS MTSAT-1R and MSAS MTSAT-2 satellites already covered in Table 4. It is apparent there are only a few L-band communication satellites in this region, which most likely means that they are already allocated for specific services. The only satellites having available L-band channels visible to Australia are Inmarsat-4F1 and Inmarsat-3F3.

Table 8 List of geostationary communication satellites between 100°E and 180°W, which transmit L-band signals
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Next-generation SBAS

As stated previously, the current SBAS coverage is limited by the availability of localized ionospheric corrections derived from ground reference receivers, and the present SBAS services only support L1 frequency and mostly single-constellation positioning, i.e., GPS, which further limits the availability and continuity of the SBAS service. Key SBAS providers such as WAAS and EGNOS are already planning improvements to expand their coverage areas and improve their performance. In addition, next-generation GNSS and RNSS satellites will have augmentation capabilities to improve the performance of GNSS. This section provides an update of next-generation SBAS including GNSS and RNSS satellites with their augmentation capabilities.

Next-generation aviation SBAS

Next-generation SBAS satellites will incorporate transmission of a second civilian signal in the protected aeronautical band (the L5 signal) and incorporate measurements from new GNSS constellations. EGNOS SES-5 and ASTRA 5B satellites, for example, are capable of transmitting dual-frequency signals compatible with GPS L1/L5 and Galileo E1/E5 signals.

The L5 frequency band is especially suitable for safety-of-life applications because users are not allowed to interfere with their signals. The dual-frequency (L1 and L5) system will also be fully robust against ionospheric gradients that currently limit vertical guidance at times of severe ionospheric disturbances. In fact, the largest benefit of the next-generation dual-frequency SBAS system is that the service coverage area can be extended farther away from the ground reference station network by taking advantage of both L1 and L5 frequencies. It is expected that next-generation SBAS will improve accuracy, availability and continuity of GNSS performance (EU-US Cooperation on Satellite Navigation Working Group C 2010).

GPS has already launched 12 Block IIF satellites transmitting the L5 signal and is scheduled to achieve L5 full operational capability (FOC) with 24 satellites broadcasting the signal by 2024 (US Government 2014). GLONASS has started to broadcast CDMA signals at both the L1 and L5 frequencies. The Galileo and BeiDou constellations are currently being deployed and also will broadcast in both the L1 and L5 (or near L5) bands. The Japanese Quasi-Zenith Satellite System (QZSS) has launched 1 satellite in 2012, with 3 additional satellites to be launched by 2018, and complete a 7-satellite constellation by 2023. All QZSS satellites will transmit the L5 signal. The use of multi-constellation satellites and receivers will provide additional redundancy, thereby improving the systems’ reliability and availability. This is especially important for liability critical applications such as those for intelligent transport system (ITS).

The official message specification and ICAO certification of dual-frequency multi-constellation SBAS are not yet in place. The SBAS Interoperability Working Group (IWG) recently agreed on the common GNSS SBAS message design. It is worth noting that the procedure for next-generation SBAS to become fully operational is in fact a lengthy one. The Federal Aviation Administration (FAA) of the USA originally planned to enter the dual-frequency next-generation SBAS phase in 2014, with completion planned in 2019. However, as the launch schedule of the newer GPS satellites has changed, the integration schedule has consequently been modified and is now divided into two phases. The first phase, planned to take 5–7 years, focuses on infrastructure improvements to enable the use of the L5 signal. The second phase, also planned to take 5–7 years, follows the declaration of final operational capability (FOC) of the GPS L5 signal and focuses on the implementation of the dual-frequency user capability, e.g., dual-frequency multi-constellation SBAS receivers operating on two frequencies (Lawrence 2015). As such, dual-frequency SBAS is expected, at the earliest, to be fully operational by 2024.

The use of dual-frequency SBAS, as well as expansion of the ground reference network, shows significant potential to create a global LPV-200 level SBAS coverage with a vertical alert limit of 35 m and horizontal alert limit of 40 m (Walter et al. 2010). Figures 3, 4 and 5 show the various possible situations of dual-frequency SBAS with the hypothetical expansion of the ground reference stations network.

Fig. 3
figure 3

Status of current aviation SBAS. (Top) Ground reference stations network of the five SBAS : WAAS, EGNOS, MSAS, GAGAN and SDCM. (Bottom) The LPV-200 availability of the five combined dual-frequency SBAS (Walter et al. 2010)

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Fig. 4
figure 4

Expansion of ground reference station network for WAAS, EGNOS and MSAS into the southern hemisphere to improve performance and coverage of dual-frequency GPS-only SBAS. (Top) Hypothetical expansion of the ground reference stations network of the current SBAS. (Bottom) The LPV-200 availability of the combined dual-frequency SBAS with hypothetical expansion of ground stations (Walter et al. 2010)

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Fig. 5
figure 5

Improvement to coverage provided by integrating additional Galileo constellation to a GPS-only system. (Top) Availability of the five combined dual-frequency SBAS (i.e., WAAS, EGNOS, MSAS, GAGAN and SDCM) with both GPS and Galileo using the current SBAS ground reference stations. (Bottom) The LPV-200 availability of the five combined dual-frequency SBAS with both GPS and Galileo together with hypothetical expansion of ground stations in the southern hemisphere (Walter et al. 2010)

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Adding additional GNSS constellations to improve the coverage and availability of existing SBAS LPV-200 service also provides benefits (Walter et al. 2010). Figure 5 (left) shows the improved coverage when adding Galileo satellites to a single GPS-only system using the existing reference stations network as shown in Fig. 3 (left). The additional satellites have potential to fill in the coverage gaps in the northern hemisphere and provide more reliable coverage well beyond the reference stations network. Figure 5 (right) shows the availability of the service coverage area when using the combined dual-frequency SBAS with both GPS and Galileo together with hypothetical expansion of the ground stations network in the southern hemisphere as shown in Fig. 4 (left). Global service coverage could potentially be obtained with a multi-constellation dual-frequency SBAS using the expanded network of ground reference stations.

Next-generation GNSS and RNSS satellites with augmentation capabilities

GNSS and RNSS satellites such as Galileo, GLONASS, BeiDou and QZSS will transmit augmentation signals with the aim to improve the performance of GNSS. These augmentation signals offer extra data transmission channels and spreading code-encrypted signals purely for civil purposes. One of the advantages of using GNSS satellites for transmission of augmentation corrections instead of GEO satellites is that the coverage at high latitudes (above 60°) is significantly improved with respect to that of GEO satellites.

European Union’s Galileo

The Galileo program plans for a complete 30-satellite system by 2020. In addition to the free open service provided by E1 and E5 navigation signals, Galileo satellites also deliver high-accuracy positioning capabilities (e.g., PPP) for paying commercial users using the data (E6b) and pilot (E6c) component transmitted in the E6 (or L6, 1278.75 MHz) frequency band (Hernandez et al. 2015). The Galileo commercial service is mainly based on the E6b and E6c signals, which permits the transmission of 448 bps per satellite and spreading code encryption for authentication (Rodriguez et al. 2014). Early test results indicate that positioning accuracies at the decimeter level using a standalone receiver with two-day-old orbit and clock predictions can be achieved (Hernandez et al. 2015). Also, research conducted by Thales Alenia Space and the French Center National d’Etudes Spatiales (CNES) in 2015 has demonstrated the feasibility of using the E5b signal (250 bps) transmitted by the EGNOS ASTRA 5B satellite payload to deliver a PPP solution (Charlot et al. 2014). The Astra 5B satellite will replace the transponder on Inmarsat 3F2 satellite once the Astra 5B satellite enters EGNOS service planned in late 2016.

Russia’s GLONASS

The first GLONASS-K satellite launched in 2011 transmits CDMA signals in addition to the system’s traditional FDMA signals. Of particular interest is the new GLONASS L3 CDMA signal centered at 1207.14 MHz, sharing the same frequency as Galileo E5b signal in the protected aeronautical frequency band. Apart from the SDCM SBAS technology development, PPP service is also planned for transmission on the L1/L3 GLONASS bands by 2018 enabling high-accuracy positioning and navigation services (Stupak 2013).

China’s BeiDou

The GNSS of the People’s Republic of China is known as the BeiDou Satellite Navigation System (BDS), or simply BeiDou. BeiDou has gone through two phases of regional navigation satellite system development (Ding 2011), i.e., BeiDou-1 (Compass Satellite Navigation Experimental System) and BeiDou-2. The BeiDou-2 system began offering regional navigation satellite service in the Asia–Pacific region in December 2012 with six GEO, five IGSO and four MEO satellites. The global navigation satellite system service will be delivered by the BeiDou-3 system. The BeiDou-3 constellation will consist of five GEO satellites, 27 MEO satellites and three IGSO satellites, totaling 35 satellites. Although the original plan aims for completion of the BeiDou-3 constellation by 2020, rapid development of both the ground and space components suggests BeiDou-3 might be finished by as early as 2018.

China has plans to provide its own SBAS service, called Satellite Navigation Augmentation System (SNAS) (Ding 2011). However, there is little information publicly available on the development of SNAS at the time of writing (2016).

Japan’s Quasi-Zenith Satellite System (QZSS)

The Japanese QZSS program has launched one IGSO satellite in 2012, and plans are in place to launch three additional satellites (2 IGSO + 1 GEO) by 2018 and complete a seven-satellite constellation by 2023. QZSS is intended as a regional augmentation system for GNSS, aimed at enhancing the availability and the performance of GNSS PNT in Japan and its coverage area. QZSS satellites will transmit navigation signals that are fully compatible and interoperable with GPS, i.e., L1C/A, L1C, L2C and L5. In addition to the navigation signals, QZSS will also transmit two augmentation signals known as L1S and L6 and one experimental augmentation signal in 2018 known as L5S.

The L1S signal will be compatible with the aviation SBAS system, providing submeter accuracy wide-area corrections as well as integrity from the GEO satellite to support safety-of-life services. In fact, the MSAS SBAS signal that is currently transmitted from MTSAT satellites operated by the Japanese Ministry of Land, Infrastructure, Transport and Tourism will be transferred to QZSS for transmission via the L1Sb signal using the QZSS geostationary satellite around 2020. The Civil Aviation Bureau of Japan will be responsible for the provision of the L1Sb SBAS signal.

The L6 signal is aimed at providing centimeter-level real-time augmentation service to support high-accuracy positioning and navigation applications such as surveying, precision agriculture and machine guidance. The QZSS L6 signal is transmitted on a 1278.75-MHz carrier frequency, the same frequency as the Galileo E6b signal. The L6 signal is unique in that it has a 2000-bps data capacity, eight times that of an L1 SBAS signal (250 bps). The QZSS L6 signal has sufficient data capacity to deliver real-time positioning accuracies of ±5 cm in the horizontal component and ±10 cm in the vertical component using PPP techniques as demonstrated in Australia and New Zealand (Choy et al. 2015; Harima et al. 2015).

Conclusion

We provided an overview of potential SBAS availability in Australia and New Zealand, with the intent to identify key similarities and differences in their augmentation capabilities. SBAS can be divided into two user categories, namely aviation SBAS and “non-aviation” SBAS. Aviation SBAS is an ICAO-certified civil aviation safety-critical system providing wide-area GNSS augmentation by broadcasting augmentation information using GEO satellites. As its primary aim is to improve integrity, availability and accuracy of basic GNSS signals for aircraft navigation, aviation SBAS transmits integrity, ranging information and correction messages that include satellite and ionospheric corrections. Examples of aviation SBAS are WAAS, EGNOS, SDCM, GAGAN and MSAS.

On the other hand, “non-aviation” SBAS support numerous GNSS PNT applications using positioning techniques such as wide-area DGNSS and PPP with lesser ground reference station density. These systems are not ICAO compliant because of the difference in data message structures, difference in signal frequencies used to transmit the corrections, absence of the extra ranging signals from the GEO satellites and absence of integrity data that meet certification requirements by ICAO. Most of these “non-aviation” SBAS are commercial in nature and operated by companies such as Trimble, Fugro and Veripos. It must be stressed that these services mainly focus on delivering accurate positioning solutions through wide-area DGNSS and/or PPP services, and guaranteed levels of availability and integrity therefore remain secondary considerations when accuracy is the key driver (Collier et al. 2011).

It is expected that next-generation SBAS and GNSS will improve accuracy, integrity, availability and continuity of GNSS performance. Next-generation GNSS and RNSS such as Galileo, BeiDou and QZSS transmit augmentation signals in the L1, L5 and L6 frequency bands. These augmentation signals have data capacity to deliver a range of augmentation services such as SBAS, wide-area DGNSS and PPP, to meet the demands of various industry sectors. For example, from 2018 the Japanese QZSS satellites will transmit on the L6 signal augmentation corrections to support centimeter-level RTK service in Japan. The current bandwidth on Galileo E6b signal also permits transmission of PPP corrections from Galileo satellites to enable centimeter-level positioning. In addition, there are well-developed plans to put in place next-generation dual-frequency multi-constellation SBAS. Multi-constellation GNSS increases robustness against potential degradation of core satellite constellations. The use of the second L5 signal offers improved resistance against interference as operations can still proceed when access to one of the signal is lost. This further improves availability and continuity of services. The use of dual-frequency signals also increases robustness against ionospheric gradients that currently limit vertical guidance in times of severe ionospheric disturbances. In fact, the greatest benefit of dual-frequency multi-constellation SBAS is that the service coverage area can be extended farther away from the ground reference station network as well as using a sparse density of ground stations.

Australia is fortuitously situated in the GNSS “hotspot” (Dempster and Hewitson 2007; Rizos 2008). It has the advantage over North America and Europe of being able to receive signals from all next-generation GNSS and SBAS satellites. Therefore, the opportunity to access these signals as a form of satellite-based augmentation delivery system in Australia offers substantial benefits to improve the performance of GNSS PNT, which could lead to significant increases in the productivity of many industries. The next stage of the work is to assess the level of improvement provided by next-generation SBAS and GNSS augmentation systems in the region to Australian GNSS users.