Mobile RTK: Using Low-Cost GPS and Internet-Enabled Wireless Phones
An ever-increasing number of mobile handsets come equipped with GPS and some with inertial sensors. However, these single-frequency units do not exploit the higher accuracy possible with real-time kinematic (RTK) techniques. Now a group of Nokia researchers are developing a software-only RTK solution using the hardware and wireless connections already existing in mobile phones.
Government regulation such as E911 and the promise of location-based services (LBS) are the biggest drivers for integrating positioning capability into mobile phones. The increasing sophistication of applications and refinement of map databases are continually tightening the accuracy requirements for GNSS positioning. In particular, location-based games and features such as “friend finder” sometimes require better accuracy than what is achievable with state-of-the-art network-assisted GPS (A-GPS) platforms.
Cellular standards for GPS assistance data exist for both control plane and user plane protocols. These protocols carry information that help the integrated GPS receiver to improve its sensitivity, speed up signal acquisition, and especially reduce the time to first fix. However, these approved standards do not contain sufficient information for the receiver to do carrier phase positioning.
Until now, no compelling reason existed for adding carrier phase positioning related features into cellular standards so that they could employ real-time kinematic (RTK) techniques. Generally, RTK-enabled devices on the market are expensive and intended primarily for geodetic and survey applications. Also, there has been no real need in the cellular world for the accuracy RTK provides. With evolving LBS applications, however, this situation is changing.
This article describes a solution called mobile RTK (mRTK), a system specifically designed and implemented for the cellular terminal use. Its design incorporates low-cost single-frequency A-GPS receivers, Bluetooth (BT) communications, and inertial sensors.
Basically, the technique involves exchanging measurements in real-time between two units — one designated as the reference and the other as the user terminal — and producing the best possible estimate of the baseline between the terminals using RTK techniques. We are developing the solution so that in the future it will be possible to add any other Global Navigation Satellite System (GNSS) measurements in addition to GPS measurements — or even instead of GPS measurements.
Using a simulator, we shall provide data that show it is possible to enable high-precision, carrier phase-based positioning in handsets with minimal additional hardware costs. Further, we shall describe some of the protocol aspects and especially the aspects of adding support for mRTK messaging to already existing cellular standards — GSM and UMTS. We believe that the mRTK solution will bring high performance to the mass market.
Moreover, additional GPS signals, such as L2C and L5, and other GNSSes such as Galileo will become operational in the near future. Consequently, it would be very beneficial to begin incorporating mRTK into the pertinent wireless standards now so that the infrastructure and the service providers will be ready when business opportunities present themselves.
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mRTK Solution Overview
We are designing the mRTK solution to work with low-cost, off-the-shelf GPS receivers with certain requirements (for example, the ability to report carrier phase measurements and data polarity). Therefore, performance degradations are expected in terms of time to ambiguity resolution, accuracy, and achievable baseline length.
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Testing the System
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Cellular Protocol Aspects
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Future work with the existing testing protocol includes more testing, especially field testing, and testing with different signal conditions and satellite constellations. The testing protocol itself should be modified with new features such as the VRS service. Using VRS, the baseline can always be kept very short, and accurate absolute positioning is available everywhere using mRTK.
One of the ideas that also need to be further developed is peer-to-peer protocols. In those protocols the mRTK measurements would be transmitted directly from one terminal to another without the use of a server in between.
As an example, this kind of protocol could be embedded into voice-over-IP (VoIP), in which the data channel for the voice encoding is already open and could easily accommodate other data transmissions that do not have strict real-time requirements, such as mRTK. Other peer-to-peer protocol means would exist, for instance, in WLAN, where the terminals are connected to the same subnet and would be able to open direct connections to each other.
The solution we have presented holds a lot of potential. Especially with the forthcoming satellite systems (e.g., Galileo and modernized GPS), the solution will significantly improve the accuracy of positioning in the mobile terminal. Nonetheless, the standardization of the mRTK features will require a lot of joint effort among terminal and network manufacturers and cellular operators.
For the complete story, including figures, graphs, and images, please download the PDF of the article, above.
ManufacturersThe mRTK prototype platform uses the iTrax03/16 GPS OEM receiver manufactured by Fastrax Ltd., Vantaa, Finland. The accelerometer is an LIS3L02DQ from STMicroelectronics, Geneva, Switzerland. A GSS7700 GPS/SBAS simulator from Spirent Communications, Paignton, Devon, United Kingdom.
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