LTE Positioning and RTK: Precision down to the centimeter
Precise positioning based on global navigation satellite systems (GNSS) has become increasingly relevant for commercial use cases across different areas. Why? GNSS receivers are now more affordable and available than ever – existing most commonly within mobile handsets. GNSS positioning is based on information about signals and positions of multiple satellites, and is often assisted by information from mobile devices, provided by cellular network operators. Real Time Kinematic (RTK) is a technology that greatly improves precision of GNSS positioning, narrowing it from a few meters to mere centimeters. However, the challenge of RTK is in supporting all different devices that will be used in mass-market use cases, for example autonomous vehicles. LTE networks can now meet this challenge and provide users with precise positioning support.
Let us walk you through the recent 3GPP LTE additions that make this possible.
Automotive application, in particular autonomous vehicles, is a key new area for precise positioning. The positioning component can be one of several sensors in a vehicle. Relative positioning information sensors such as, radar, lidar, and vision will be key for collision avoidance, for example. For the sensors in the car, the positioning estimation would only be relative for that particular vehicle, whereas precise position is something that can be important in collaborative use cases when positioned observations, as well as vehicle positioning and planned trajectories, are shared. Other application areas which are likely to benefit from precise positioning are land surveying, construction, farming, outdoor mining, maritime and near ground drone traffic control. With such an expanding set of use cases, there is a need for efficient and scalable provisioning of assistance data.
The 3GPP LTE positioning scope prior to 2018 focused on support for E911 and IoT positioning, while it is also possible to provide some GNSS assistance data to connected devices on request. The positioning scope changed considerably with the recently completed 3GPP LTE Release 15, which features efficient and scalable provisioning of positioning assistance data to support precise GNSS positioning at centimeter accuracy levels.
View the video below to see the precision of RTK-based positioning in action at a client implementation. The demo took place in summer 2018.
Precise positioning based on GNSS
The main reason why GNSS-RTK enables precise positioning is the use of carrier phase measurements instead of only code phase measurements as is used by more traditional GNSS receivers.
Code phase in this context means time of arrival measured by correlating each received satellite signal with a replica which is based on the same pseudo-random code as the satellite signal. This enables a time of arrival estimation at a fraction of the length of a code symbol, corresponding to even less than a meter. Information about the available signals and satellite orbits can be retrieved from the navigation message from the satellites. Alternatively, the information can be provided as assistance data via some ground-based service provider, for example, the cellular operator.
The carrier phase, in contrast, is obtained when tracking the carrier frequency component of the satellite signal. It can be estimated at an accuracy of a few millimeters. The problem is that all periods of the carrier signal are identical, so the number of integer wavelengths between the device and the satellite is unknown. The integer number of wavelengths per satellite is determined via advanced signal processing and supported by assistance data. The GNSS-RTK assistance data is defined by the Radio Technical Commission for Maritime Services (RTCM) special committee 104 – the same one that defined differential GPS and GLONASS. Today, the assistance can be provided to a subscribing device from a server via the end-2-end IP-based signaling protocol NTRIP (Networked Transport of RTCM via Internet Protocol).
GNSS RTK is based on accurate satellite signal measurements from a reference station or a network of reference stations. The carrier phase measurement per satellite depends on the true carrier phase and the unknown integer number of wavelengths. It is subject to noise in the form of clock biases at the receiver and satellite, as well as delays in the ionosphere and troposphere. The GNSS-RTK assistance data can be provided either in observation state representation (OSR) or state space observation (SSR). In the case of OSR, the server provides carrier phase measurements from one or more reference stations at accurately known positions. The reference station may be physical, or it may represent a position (non-physical reference station) at which the server has interpolated carrier phase measurements based on data from surrounding physical reference stations. Alternatively, in case of SSR, the server estimates the regional error contributions such as clock biases, orbit errors and atmospheric delays, and provides the estimated error parameters to the device. In any case, the server needs regular information about the device position to understand a suitable scope of the assistance data.
Efficient provisioning of GNSS-RTK assistance data via LTE Positioning Protocol (LPP)
In Release 15, 3GPP has defined GNSS-RTK assistance data coding that extends LPP. The protocol can be transported between a device and a location server via either a secure user plane location (SUPL) protocol or via control plane signaling in the network. The latter means that it is possible for the radio network to be aware of- and prioritize the assistance data over other general traffic. In both user plane and control plane transport, the network is aware of the position of the serving base station. This means that the network has a crude position estimate of the device, enough for determining the suitable configuration of the GNSS-RTK assistance data. In comparison, assistance data provisioned via NTRIP needs the device to regularly feedback its position.
The figure below illustrates the GNSS-RTK provisioning via LPP unicast to three devices. A Network RTK (NRTK) server obtains measurements from physical reference stations (black), interpolate the measurements at locations corresponding to non-physical reference stations (grey), and provide the information to the location server. The location server will identify the relevant assistance data scope for each of the devices depending on their crude positions.
Positioning assistance data broadcasting and encryption
In scenarios in which there are many UEs within a region that require the same set of positioning assistance data, then the unicast approach explained previously may not be efficient. In order to allow assistance data provisioning scalable to a large number of subscribing users, 3GPP has also specified positioning assistance data broadcast. In this mode, the location server encodes, and optionally encrypts, positioning assistance data relevant for the coverage area of each base station. The compiled positioning system information blocks (Pos SIBs) are regularly sent to the corresponding base station, which transmits the data via a system information broadcast. Since the base station position is known, it can be used to compile the set of relevant positioning assistance data. The location server also sends the corresponding decryption keys to the Mobility Management Entity (MME).
Capable devices will retrieve decryption keys from the MME based on their subscription scope in the home subscriber server (HSS). Thereby, it is possible to differentiate between devices and enable positioning assistance data as a service to subscribing users. The device will retrieve a broadcasted list of Pos SIBs from the base station to understand what positioning assistance data is being broadcasted, to determine which information it can decrypt and which it can use. The used encryption keys are valid for one or more tracking areas, and contain expiration triggers. When the device moves into a tracking area where it misses a valid key, or stored keys have expired, the device needs to retrieve new keys from the MME.
The below figure illustrates provisioning of encrypted broadcasted positioning assistance data, where some UEs have obtained a valid key from MME to allow broadcast data decryption, while others cannot access that information.
The GNSS-RTK support discussed in this blogpost and standardized in LTE Release 15 would definitely benefit not only LTE but also 5G networks. GNSS-RTK assistance data signaling is also supported by New Radio (NR) devices as per Rel.15 agreement, and dedicated NR refinements and adaptions can be part of the recently started Rel.16 NR Positioning study item together with NR specific positioning methods.
Want to know more about standardization of positioning solutions? Check out our earlier blog posts Indoor positioning enhancements in LTE standardization and IoT positioning in LTE standardization.
Visit our Setting the standards page to get an overview of standardization.