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Satellite-based Augmentation System - A base station in the sky

December 21, 2020 14 min read

Satellite-based Augmentation System, GPS base station in the sky

 

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    Chris Marshall is a positioning engineer at FrontierSI, the co-creators of a Satellite-Based Augmentation System (or SBAS).

     

    Chris did mechatronics engineering (mechanical-electrical) at uni. He had no interest in the spatial field until he came across a subject that seemed interesting enough, “Applications of GIS.” The topic opened the doors to an unknown world and he’s been on a whirlwind journey ever since.

     

    WHAT’S SBAS?

     

    It’s a Satellite-Based Augmentation System to standard GPS or GNSS signals. It’s a service that improves the quality of positioning from GPS — from multiple meters down to sub-meter level.

     

    HOW DOES SBAS WORK!

     

    SBAS uses similar technologies to other high-precision correction services people might be familiar with. It leverages an entire network of continuously operating reference stations around the ground area.

     

    In Australia and New Zealand, it uses the data  from the network of Continuously Operating Reference Stations (CORS) and integrates all the different measurements to compute corrections for the region. Once the data is sent to the processing center, it’s uploaded to a satellite, followed by a downlink from that satellite to the end-users.

     

    The only thing they see is the satellite signal alongside the usual GPS or GNSS observations.

     

    IS THIS A BASE STATION… IN THE SKY?

     

    It’s a DGPS station in the sky, yes.  Why there?

     

    The satellite is geostationary. It sits above Australia and New Zealand at a certain elevation angle all the time. All you need is a sky view and you can see the satellite.

     

    Yes, the same technology can be applied from ground base stations.

     

    But there are a few notable advantages to being in the sky.

     

    WHAT NOTABLE ADVANTAGES?

     

    You could deliver the same service via radio signals from a ground-based system. Absolutely.

     

    The question is: what is your accuracy requirement?

     

    There’s a host of correction services available, depending on your accuracy requirements and the technology you have.

     

    There are also different ways of getting high precision signals.

     

    If you’re within an area with mobile coverage, you can even go a step further than SBAS and use real-time kinematic corrections directly from a core station. The limitation of that is that it will only work while you still have a data connection available to you — which, for most places in Australia, is limited to the mobile networks.

     

    If you run out of 3G or 4G data, you can’t get that uplink or downlink in two directions.

     

    SBAS is excellent because it’s a single direction. As long as you can see the satellite, you don’t need a data connection. You don’t need anything else — just an antenna and a GPS to pick up the signals.

     

    For Australia and New Zealand, if you’re anywhere in those countries or in their maritime zones, and you can see that satellite — you can get higher precision in your GPS.

     

    DO I NEED A SPATIAL RECEIVER? SOFTWARE? EXTRA HARDWARE?

     

    The short answer is no.

     

    You don’t need special hardware. The most basic implementation of the SBAS uses the same frequency as the standard GPS satellites do. It picks up the SBAS satellite just as another satellite to include in its computations.

     

    Some more advanced uses of SBAS and their implementation, such as the ones in Australia and New Zealand, are more forward-thinking. The newest signals and the more recent combinations of signals require different hardware.

     

    But at the basic level, SBAS can be picked up by simple hardware that doesn’t have to cost a lot. It probably already exists for many users. You may need a software or firmware upgrade to get it working, but many receivers currently in use are SBAS compatible.

     

    WHAT’S THE BENEFIT OF USING SBAS?

     

    For the man on the street with a mobile phone, probably not very much apart from their positioning accuracy being higher and bouncing less on the map.

     

    Reliability and availability are the two big players here.

     

    Signals are more reliable, and you can see the signals more of the time. The result you get from these signals will be more correct than if you were using augmented GPS.

     

    There are trade-offs, of course.

     

    What do you need it for? What’s your operational use case? What are you trying to do?

     

    Do you need the highest accuracy? The highest possible integrity? Do you need to know that there’s always positioning coming through your signal?

     

    It all depends on your industry and how you’re using the positioning data.

     

    SBAS provides a whole host of those benefits, with the most straightforward one being accuracy. You get a more precise location — if you’re looking at the map, it won’t put you on the opposite side of the road or through a building as often.

     

    THIS IS NOT A STRONGER SIGNAL. IT’S AN EXTRA SIGNAL FOR A MORE ACCURATE POSITION.

     

    Imagine an urban area, or urban canyon, where there are buildings on both sides of you. Your sky view is limited to a narrow corridor through these buildings. In these circumstances, GPS struggles. The accuracy you get out of the system depends on the geometry of the satellites you’re observing all around you.

     

    If those satellites’ geometry is limited to a narrow strip of the sky, you can’t resolve your position anywhere near as well; you end up with significant errors. People using GPS in severely urban areas often find it jumping hundreds of meters all over the place, or sometimes putting them inside buildings or 10 stories high in the air. This is mostly because of areas like multipaths.

     

    With SBAS, these are still factors, and multipaths are still going to be an issue. But if you correct for common error sources, you get a more reliable service, even with fewer GPS satellites visible to your receiver.

     

    It’s not a silver bullet. It doesn’t fix all the issues with reception, but if you’re under a sky view that can see the SBAS satellite, you get a better result than otherwise.

     

    WHY IS A GEOSTATIONARY SATELLITE A GOOD CHOICE FOR THIS?

     

    GPS, as we know, is a constellation of satellites that are continually moving around. SBAS is not something we can just add to existing constellations.

     

    Why?

     

    The corrections for the SBAS are regional.  The SBAS I’m talking about here is being developed for Australia and New Zealand as a footprint. We’re using data from within Australia and New Zealand core sites to determine the corrections.

     

    There are similar SBAS systems around the world. The first one was the US WAAS (Wide Area Augmentation System) that’s been up for over a decade. That was the first SBAS, purely for the North American region.

     

    Since then, others are being developed, such as the European, Russian, Indian, and South Korean ones.

     

    Each of them will rely on a distinct set of input core stations to get the corrections for that region. Potentially, these technologies could go global.

     

    The current form of SBAS is tied to a particular region.

     

    WHAT’S THE JOURNEY OF THE DATA LIKE?

     

    It all starts with the satellites that are always putting out signals. The ground stations are continuously operating reference stations throughout the entire region (Australia and New Zealand). They receive all the satellites — they themselves are a very good quality GPS receiver. They pick up all the signals and locate themselves based on what they can see.

     

    The cors sites determine their own positions. They also know, for a fact, their last surveyed position to a high degree of accuracy. Based on the difference from the computer position at each core site versus the known location from survey across the entire network, you can work out a factor of how much error is being brought into the system — from transmission from the satellite down to the ground segment.

     

    Mostly, that’s correcting for ionospheric errors. Once the signals are picked up at the core sites, they use that difference in positions to work out the ionospheric error. They send that through to a ground processing station. That station takes all the data and puts out corrections for the entire SBAS region based on where in that region you might be.

     

    Thus, the more core sites across the region, the better. There’s a recommended minimum number, but in Australia and New Zealand, more than enough exist. In some world regions, it’s a struggle to get enough cors sites up to get the data through and build an SBAS.

     

    We’re lucky that there are plenty of core sites across our region because they’re used for various other surveying and precise positioning tasks. We’re already mature in that space, and this has made it simple to transition data streams forward and use them for SBAS purposes.

     

    The ground processing station computes the corrections. It’s sent to an uplink center where the uplink center’s only job is to talk to the geostationary satellite. The communications uplink happens; it sends the correction messages, which are then immediately downlinked from the geostationary satellite and broadcast on the same frequencies as the standard GPS.

     

    DOES THIS REPLACE RTK?

     

    No.

     

    There are a lot of RTK (Real-Time Kinematic) use cases where you do need that three-to-five-centimeter accuracy and precision.

     

    SBAS can’t get there yet.

     

    However, it can get down to a decimeter level with one of the new signals that we’ve been trialing out — that’s almost good enough. Depending on your use case, there will be circumstances when the simplicity of a system not requiring two-way communications and still having that decimeter level accuracy will be beneficial.

     

    ISN’T RTK SUPPORTED BY THE SAME NETWORK THAT’S SUPPLYING DATA TO THE SBAS?

     

    There are multiple ways you can do RTK.

     

    Networked RTK uses a network of core stations.

     

    Simple RTK uses single-base station RTK — you connect to the local cores that you’re near to and take a direct feed for correction data.

     

    Doing so, you’ll get to that three-to-five-centimeter level, but you also gain an additional factor based on the distance from you to the base station.

     

    If you’re 10 kilometers from the base station, you’ll get a decent three to five centimeters (sometimes, even higher accuracy). But as you go out to 100-150 kilometers from the core site, the accuracy you get tapers off and slowly degrades.

     

    This is not a factor with SBAS — you can pick up the same accuracy level anywhere in the country or the maritime zones.

     

    All you need is to see the sky clearly. How good is your receiver? How good is the rest of your hardware? If you have all the pieces in place, you’ll get corrected data through and you’ll get down to the correct level of accuracy.

     

    SBAS decouples from the core infrastructure directly; you leverage it without managing the core site itself.

     

    WHAT ARE SOME USE CASES FOR SBAS?

     

    Use cases for SBAS have been a focus of ours over the last three or four years.

     

    There’s been a considerable body of work done around the space, known as the SBAS Test-bed. It specifically looked at different use cases to put them into organizations, sectors of industry, and then attempted to quantify what those benefits would be in the future.

     

    Unsurprisingly, the largest industry to benefit from SBAS would be agriculture.$820 million were projected in feed and fertilizer savings for farmers because of the enhanced pasture utilization that you can get from SBAS enabled virtual fencing. The four largest sectors that would benefit from SBAS have over $1 billion in valuation, projected over the next 30 years.

     

    VIRTUAL FENCING IS A FASCINATING CONCEPT

     

    The idea of having a virtual line, or a virtual boundary within which things are held, can be applied to many sectors — construction, resources, and agriculture, of course.

     

    One thing we trialled in the SBAS Test-bed was SBAS enabled collars on livestock. Cows were given collars with GPS receivers.  We draw a line across a paddock. If the cows approached that line, they would get a sound alarm that would beep at them.  A gentle beep and never any harm done. The cows learned over time that if they don’t go there, it won’t beep at them. A behavior formed. If you have enough of the herd wearing these collars, they stay away from that region.

     

    The end goal of this is to get all your cows lined up along the field and slowly move your line forward. All your cows grazing exactly where you want them to be over time — a large lawnmower effect.

     

    SBAS virtual fencing can get you there.

     

    But that’s just one application.

     

    I CAN SEE THIS FOR AUTOMATED OR AUTONOMOUS VEHICLES DOING FARM WORK

     

    Spraying, seeding, spreading… Usually, these are done with precision GPS or RTK.

     

    SBAS can be good enough for some tasks where you don’t need the highest level of accuracy, in which case you’ll use an RTK solution.

     

    When absolute accuracy isn’t so much of an issue, SBAS is good enough to use in many industries.

     

    A farmer will not worry about marking a point in their paddock, coming back to it in 5 years, and it’s not precisely there.

     

    But they will care about driving their tractor down the road, and when they come back on the same route in 20 minutes, it will take them somewhere else.

     

    WHO IS THIS NOT FOR?

     

    SBAS has its advantages over standalone GNSS and some advantages over RTK corrections.

     

    There is no substitution for the level of accuracy you can get with a networked RTK solution or being close to a cors station using a single RTK solution. This, of course, usually needs equipment — RTK equipment is more difficult to get than something that can run SBAS.

     

    When you need three-to-five-centimeter accuracy, nothing stops you from going out and doing that at a fairly low cost already.

     

    If you live in Australia or New Zealand, both their geospatial agencies provide free data streams through those services. Geoscience Australia, through the AUS core service, provides a direct feed to your local core site.

     

    If you have a compatible receiver, you can get free corrections at that three-to-five-centimeter level.

     

    WHAT’S THE FUTURE SBAS? IS THIS THE END-GAME?

     

    There are a few different next steps here.

     

    But first, let’s see how we got here.

     

    SBAS is “underway” in its current implementation. Continuous testing is already in place, with the SBAS Test-bed completed between 2017-2019. It was run by FrontierSI and several industry partners, Geoscience Australia, Land Information New Zealand, and the technology partners who helped run the service.

     

    An economic benefits analysis showed a potential $7.6 billion benefit from the service over the next 30 years. It’s significant enough to prompt the Australian and New Zealand governments to invest in the service.

     

    The service is under development and on its way right now. (Do sign up for the Geoscience Australia newsletters for the official word on the when and the how.)

     

    We don’t yet know what the final implementation will look like. But we can make educated guesses based on the technologies and tests we did in the last few years.

     

    The standard L1 SBAS service is the simplest to pick up and the most compatible with the broadest range of equipment.

     

    More future-focused solutions, such as a dual-frequency multi-constellation SBAS, use two major satellite constellations and correct them (GPS and Galileo) to provide a slightly higher accuracy. But most importantly, a higher integrity than a standard SBAS solution.

     

    Lastly, there’s a PPP service. It can get down to the decimeter level, notably after a convergence period.

     

    There are trade-offs with each of these services. But the key with the entire program is that it’s free.

     

    The end goal of all this is a service that anyone in Australia and New Zealand can pick up and apply to whatever operational use they may have, with no real barriers to entry.

     

    All this harkens back to SBAS originally being designed for aviation use as an inherent aviation technology.

     

    How can you charge people for access to something like this when you’re trying to improve their safety by improving their positioning quality?

     

    A pilot landing on a runway can’t suddenly be asked for his login credentials. He’d be in real trouble. Thus, these services are free and open to anyone who needs them.

     

    SBAS services have their place in this precise positioning ecosystem. So do RTK corrections. By making them available and accessible to all sectors or anyone who wishes to use them within the country, we are here to uplift spatial capability throughout the nations. This will allow people to do more, to get the technology in their hands, and try new things out to see what works.

     

    Ultimately, that’s where innovation comes from. It’s ordinary people getting their hands on something shiny and new, getting excited about it, and trying it out to see what happens.

     

    IT’S INCREDIBLE HOW GPS IS SHAPING OUR LIVES

     

    There’s an entire industry built around location services.

     

    But there are multiple sides to it — locations and data collection. There’s always a privacy concern.

     

    Suppose you’re tracking me to a high accuracy. Does that mean that everyone knows where I am all the time, to a better accuracy?

     

    The underlying data sovereignty and privacy is still an issue and will continue to be an issue in the future. But it’s worth noting that the benefits you accrue from knowing where things are to high accuracy far outstrip the immediate concern people have for “am I on this side of the street versus a couple meters away.”

     

    Privacy is always going to be a concern.

     

    Security is inherent to the SBAS system. Specifically, if we’re talking about aviation use, which is where the goal is.

     

    SBAS is ultimately slated for integration into the Civil Aviation Standards. It is already defined in the Standards used internationally, and there’ve been SBAS systems in the US for over a decade.

     

    But before it can be used reliably for those purposes, we need secure ground stations. There are still things that need to be done over the next few years.

     

    SBAS signals for ground users are projected to come back online in Australia in around 2021, according to early indications. For aviation, we’ll have to wait until about 2025.

     

    That’s purely to allow security to be developed, the background infrastructure to be built, and to ensure the system is as reliable as possible.

     

    HOW IS THIS USEFUL FOR AVIATION?

     

    Ultimately, in aviation, users are not concerned about centimeters of accuracy. They’re more worried about safely landing a plane on a runway. The primary use case of this system is not even accuracy.

     

    Rather, it’s the integrity and the availability of the system.

     

    For landing a plane, the pilot is not worried about one or two meters left or right. They are afraid if they’re coming in for an approach that the system drops out or tells them unreliable information. Those are the things that need to be verified.

     

    There are strict standards for these things, and a very high level of certainty is required for the aviation use case.

     

    If you are a positioning user from Australia or New Zealand and are excited about these upcoming signals, Chris and his team need your input. You can sign up for a questionnaire online to help tailor these services to meet the needs of all sectors of industry.

     

    SBAS has an intimidating name, but its simplicity is the beauty of it. It increases the accuracy and precision of locating yourself.

     

    In 2019, I recorded an episode on smartphones and their role in precise geolocation and what that would look like in the future. During the episode, we talked about positioning as a service. The difference between that episode and what we’ve just been talking about now is how positioning as a service is obviously a pay-to-play thing.

     

    But SBAS feels much more like positioning as an infrastructure. When I think of infrastructure, I think of roads, water pipes, and the internet. The value proposition there is clear.

     

    For positioning as an infrastructure, the value proposition is simple: a common understanding of talking about location.

     

    That’s it.

     

    By improving that, it will be more precise and more accurate. And yet, the value created by that is estimated at $7.6 billion over the next 30 years.

     

    Sure, a lot can change in 30 years, and we don’t know what kind of services and products will be based upon this or be possible because of this.

     

    What does this mean for data collection?

     

    A dot in space — even a very accurate, precisely located dot has no real value. But if we use the ability to create accurate and precise dots to capture features and to capture data, or to update datasets like OpenStreetMap, we can then propagate them into other datasets and create other information.

     

    It’s difficult to understand how much value is going to be created from this. Who knows what the knock-on effect of better data over 30 years will be?

     

    Be sure to subscribe to our podcast for weekly episodes that connect the geospatial community. 

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