GNSS receivers
GNSS receivers
RTK, PPP and autonomous
Let's talk about the types of positioning solutions. RTK – Real Time Kinematic (rtk technology). This is a relative method that measures not coordinates, but distances relative to a base, i.e., a receiver located at a point with known coordinates and transmitting measurements ("corrections"). Therefore, this method is strictly non-autonomous, with corrections transmitted through the internet or radio channels. The accuracy ranges from 4 to 25 mm CEP50 + 0.5 to 1 mm per km of distance to the base. There are two main variants: Single-frequency RTK is limited to an area with the same ionosphere and troposphere, meaning up to 10-20 km from the base. For example, NEO-M8P. Dual-frequency RTK is limited to an area with the same troposphere, meaning up to 70-100 km from the base. For example, ZED-F9P. Autonomous. This means a solution based solely on GNSS satellite data. It is called autonomous because the receiver makes this decision autonomously, without external corrections. The simplest solution is the single-frequency autonomous solution. Its accuracy is approximately 2.5-3 meters CEP50. Next is the dual-frequency autonomous solution. Remember that the dual-frequency receiver eliminates ionospheric distortions by itself? Typical accuracy is 1.2 - 1.5 meters CEP50. Differential. Take a receiver, place it at a point with known coordinates, and find the difference between what should theoretically be received and what is actually received. We call this difference a correction and transmit it. SBAS – Satellite-based augmentation systems. Corrections transmitted from geostationary satellites on the L1 frequency and received by any GNSS receiver are used. The accuracy is about 1.5 meters CEP50. SBAS satellites hovering over Europe are called EGNOS (over the USA - WAAS, over Japan - MSAS, over India - GAGAN, over China - BDSBAS, over Australia - SoufPan, over Russia - SDKM). The disadvantage is that most SBAS systems transmit corrections only for GPS, and only the operational Russian SDKM transmits corrections for both GPS and GLONASS. SLAS – Sub-meter Level Augmentation Service. This is a Japanese system with a small coverage area (only Japan), so it's very accurate, with a CEP50 ranging from 30 to 70 cm. SLAS can support all systems, but currently, it only transmits corrections for GPS and QZSS. Corrections are transmitted from QZSS satellites on the L1 frequency, so they are received by many receivers with QZSS support, such as ZED-F9P. GBAS and GRAS. These are exclusively for aviation applications, with corrections transmitted via VHF. GBAS horizontal accuracy is no worse than 16 meters CEP95, which means 7.7 meters CEP50. GRAS requirements are even lower, with a 2-mile range at altitude. This is a heavy legacy from 30 years ago and the era of selective authority when civilian receivers could not obtain solutions better than a few hundred meters. GPS and GLONASS are used. DGNSS. A maritime system with transmission via medium radio waves. It uses a separate receiver and a separate or combined antenna. Surprisingly, almost any receiver can work with such corrections. Accuracy is up to 1 meter CEP50. It is available in coastal areas and uses GPS and GLONASS. Although the system is designed for maritime use, it has other applications. For example, in Australia, the fastest ambulance is an airplane called the RFDS (Royal Flying Doctor Service). There are many remote farms with a good stretch of road and a GNSS receiver with a DGNSS transmitter. If a doctor is needed, the receiver is turned on, sends corrections to the airplane, and a light aircraft lands directly on the road. Interestingly, many GNSS innovations are first implemented in Australia. StarFire – a commercial correction system from John Deere, transmitted from satellites in the L-band. Accuracy is around 5 cm CEP50 for dual-frequency receivers, with GPS and GLONASS systems. This system's accuracy is based on the fact that a dual-frequency receiver tracks ionospheric errors, while transmitted corrections account for other distortions (orbit errors, clocks, code bias). StarFix, OmniStar, Atlas – other systems of commercial corrections, broadcasted from satellites. PPP – Precision Point Positioning. A set of methods providing accurate positioning through global corrections, transmitted via the internet or from a satellite. Almost all PPP variants require a dual-frequency receiver. Accuracy ranges from 5 to 50 cm CEP50, with convergence times between 10 and 30 minutes. For those familiar with the field, there is a crucial difference between PPP and other methods: PPP corrections are not just simple pseudorange corrections but have physical meaning, such as corrections to the satellite's position in orbit, satellite clocks, and relative signal delays from different antennas. Since these corrections have physical meaning, they are the same for any point on Earth. This is called SSR - State Space Representation. PPP solutions can be obtained from both PPP-AR and PPP-RTK corrections. SouthPAN PVS – As usual, Australia is ahead of the curve. SouthPAN is an Australian SBAS, and PVS (PPP via SouthPAN) is a service for transmitting corrections on the L5 frequency from SouthPAN satellites. The accuracy is less than 40 cm CEP50 horizontally and up to 55 cm CEP50 vertically. GPS and GALILEO systems are used. The service is very new, launched on September 26, 2022 (currently in experimental operation). The solution improvement time is estimated at up to 80 minutes, meaning the quality of the solution will improve for 80 minutes after starting. It is highly likely that this system will be used directly by receivers, and Europe and the USA will launch their counterparts. Moreover, PVS has a chance to become an ICAO standard. BEIDOU PPP-B2b. This is the transmission of PPP corrections on the B2b frequency from three geostationary Beidou satellites. GPS, GALILEO, GLONASS, and BEIDOU systems are supported. The accuracy is 10 cm CEP50, with a convergence time of 20 minutes. The coverage area is the Asia-Pacific region. PPP-AR – PPP with phase ambiguity resolution. In addition to the standard PPP correction set, phase corrections and VTEC (Vertical Total Electron Content) for single-frequency measurements are added. This improves accuracy to 2-15 cm CEP50 and reduces convergence time to 3-15 minutes. RTCM3 SSR and IGS SSR – two similar yet slightly different formats for transmitting PPP-AR corrections via the internet. Typically, PPP-AR calculation is not implemented by the receiver itself but by a separate processor. All satellite systems are supported. In the future, the formats are planned to be expanded to transmit PPP-RTK corrections. As these are open standards, the characteristics depend on specific correction sources. MADOCA-PPP – Multi-GNSS Advanced Orbit and Clock Augmentation - Precise Point Positioning. It is clear that the two purely Japanese systems (SLAS and CLAS) were not enough for the Japanese, and from September 30, 2022, they began testing another PPP-AR system called MADOCA, with the signal also transmitted on L6 from QZSS satellites. Currently, corrections are provided for GPS, QZSS, GLONASS, and GALILEO, with BEIDOU support expected later. Unlike SLAS and CLAS, this is a global system for the entire Asia-Pacific region, available wherever QZSS satellites can be received. Currently, the accuracy is better than 15 cm CEP50, with a convergence time of 30 minutes, and plans to improve to 10 minutes. GALILEO HAS — High Accuracy Service. This is a global European-made PPP-AR system, with plans to expand to PPP-RTK for the European region in 2024. The transmission comes from GALILEO satellites on the E6 frequency, and corrections are provided for GPS and GALILEO satellites. The accuracy is 10 cm CEP50, with a convergence time of 5 minutes globally and 100 seconds in the future for the PPP-RTK mode in Europe. SVO EVI – System for High-Precision Determination of Ephemeris-Time Information. This is a future Russian PPP-AR system with transmission on L3 from geostationary Luch satellites. It was planned for 2025, but the prospects are very uncertain. The achieved accuracy is 30 cm CEP50, with a planned improvement to 10 cm. Fugro Starfix G4 – A global commercial PPP-AR service supported by Fugro receivers. The transmission comes from geostationary satellites, and corrections are provided for all systems. The accuracy is 5 cm CEP50. PPP-RTK — this is PPP with accuracy close to RTK. To achieve this, troposphere data (ZTD – Zenith Tropospheric Delay) and a more accurate ionospheric model STEC (Slant Total Electron Content) are added to PPP-AR. As a result, accuracy improves to 1-6 cm CEP50, and convergence time drops to 1 minute. PointPerfect — paid (i.e., encoded) PPP-RTK corrections from geostationary satellites and over the internet in the SPARTN format. The accuracy is 2.5-5 cm CEP50. They provide service in Europe, the USA, South Korea and Australia. It can work as autonomous solution, not requiring the internet, and it is implemented in ready-to-use ZED-F9P and ZED-F9R receivers (plus a NEO-D9S correction receiver). For the receiver two chips and two antennas are needed (the second one for corrections), the satellite angle above the horizon is relatively low for Northern Europe, and while GPS, GALILEO, and GLONASS are supported, BEIDOU is not. CLAS – Centimeter Level Augmentation Service. Another Japanese system, also transmitting signals from QZSS satellites, but unlike SLAS, it offers centimeter-level accuracy and is transmitted on the L6 frequency instead of L1. The accuracy ranges from 1 to 4 cm CEP50, supports all systems, and the coverage area is Japan. It is also implemented in ZED-F9P receivers but requires an additional NEO-D9C receiver. KPS-CLS — Korean Positioning System – Centimeter Level Service. A South Korean system similar to CLAS but designed for South Korea. The launch is planned for 2029. Hexagon TerraStar C Pro – A global commercial service supported by Novatel receivers. The transmission comes from geostationary satellites, and corrections are provided for all systems. The accuracy is 1.5 cm CEP50, with a convergence time of 3 minutes. Trimble CenterPoint RTX – A global commercial PPP-AR service with the ability to work in PPP-RTK mode for the USA, Canada, and Europe. Supported by Trimble receivers, the transmission comes from geostationary satellites, and corrections are provided for all systems. The accuracy is 2 cm CEP50, with a convergence time of 3-5 minutes globally and 1 minute in regions supporting PPP-RTK. Swift Navigation Skylark – A commercial service with transmission exclusively via the internet. Supported by Swift receivers, the service covers the USA, Europe, South Korea, Japan, and Australia. It supports GPS, GALILEO, and BEIDOU systems. The accuracy is 4 cm CEP50, with a convergence time of 5 seconds to 25 cm. © Eltehs SIA 2023
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Stability and Accuracy
It's time to talk about the accuracy of positioning solutions and how it's measured. The best method is to plot all the measurement results on a map and mark a circle where a portion of these measurements fall. Depending on the portion chosen, it will be called different terms. For example, if we choose a circle where 50% of the points fall, it's called CEP (Circular Error Probable) or CEP50. Unlike measurement times, we can distinguish between short-term solution stability (how closely points cluster within 3-5-10-30 minutes) and long-term stability. Satellites orbit the Earth in approximately 11 hours and 56 minutes, and after this period, satellite conditions repeat. Long-term stability is usually measured over 24 hours, or two orbits. If you measure very stable solutions for a few hours, you may encounter solid Earth tides that can affect height by up to 30 centimeters and horizontal measurements by up to 5 centimeters. When measuring over a year or more (and very accurately), you can even detect the drift of continents (or the movement of tectonic plates) and the slow sliding of mountains towards the sea. These are millimeters and centimeters but can be measured. So, when you're given very accurate coordinates (accurate to millimeters), ask when and during which moon phase they were measured. It's essential to distinguish between solution stability and accuracy. Stability refers to the size of the circle that most solutions fit into. Accuracy, on the other hand, is the distance from the center of this circle to the true coordinates. In urban canyons, there are situations where accuracy can be worse than stability. Imagine a tall, massive concrete building to the north, like the Great Wall of China, with an angle of about 80 degrees from the horizon to the roof. To the south, there's a smaller building with a 40-degree angle to the roof. You won't be able to receive signals from satellites to the north due to the building blocking them. You also won't directly receive signals from the southern satellites, but you can receive their reflected signals off the northern building. As a result, the southern satellite signals will travel a longer path, causing the positioning solution to shift to the north. Stability and accuracy largely depend on the parameter "elevation mask angle," which limits the reception of low-elevation satellites. On the one hand, low satellites provide more coordinate information (while high satellites provide more altitude and time information). On the other hand, low satellite signals are noisy due to the troposphere, and often not only direct but also reflected signals are received. The mask varies from 5 to 15 or more degrees for different conditions, and for timing solutions, it's recommended to be even higher, up to 20-30 degrees. What we've discussed refers to post-measurement (aposteriori) evaluations. The information the receiver provides as solution characteristics is prior data (apriori). These are calculated from residual deviations in the matrix. In other words, the solution is computed using the least squares method, and the discrepancies in measurements from individual satellites compared to the average solution are transformed into an accuracy parameter. © Eltehs SIA 2023
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