Lauri Wirola

Mass-market requirements for indoor positioning and indoor navigation

The next step in location services will be repeating the success of location services in outdoor use for navigation and local search in the indoor environment. In order to make indoor positioning and navigation reality in large scale not only compelling business opportunities must be present, but also the technology must be low-cost, low-power, have low maintenance expenses and require minimal amount of new infrastructure. This paper reviews the challenges and solution models for indoor positioning starting from the service level aspects, reflecting them to the architectural considerations and finally discusses selected technical implementation issues.

Bringing RTK to Cellular Terminals Using a LowCost Single-Frequency AGPS Receiver and Inertial Sensors

Today an ever-increasing number of handsets come equipped with a GPS receiver and some even with inertial sensors. Moreover, an even higher number of terminals are already capable of connecting to an add-on device with such capabilities. However, the full potential of these devices is not yet exploited. This paper introduces the mobile RTK (mRTK) solution, which can be included in the wireless standards to enable high-precision double-difference carrier phase positioning in handsets at no extra hardware cost. mRTK differs from the current OTF/RTK solutions in that it is a software-only solution using the hardware and wireless connections already existing in handsets. Moreover, the mRTK solution can utilize information from on-board inertial sensors. These are the key differentiating factors compared to the previous solutions. The paper shows that the sensors supplying information on baseline changes during the ambiguity initialization significantly assist the ambiguity resolution. A new communication protocol and messaging was defined in order to be able to exchange information between mRTK-capable handsets. The protocol includes reservations for additional GPS frequencies as well as for other Global Navigation Satellite Systems (GNSSs), such as Galileo. This protocol can be directly included in the wireless standards. Challenges in the current implementation include using only the L1 frequency for ambiguity resolution. Utilizing an L1-only receiver necessarily leads to penalties in the baseline accuracy due to inherent problems in the ambiguity resolution and validation. However, this paper shows that the baseline obtained is still better than the plain difference of positions. This paper shows that the mRTK solution significantly improves A-GPS performance. The mRTK solution also brings near-professional-quality positioning performance to the mass market. It would, therefore, be beneficial to include mRTK in wireless standards in order to expand A-GPS use cases in the short term and A-GNSS use cases in the long term.

Bandwidth and storage reduction of radio maps for offline WLAN positioning

Most of the existing mobile device positioning methods require data connectivity, i.e. they work in the mobile-assisted, or online mode. However, this consumes energy, induces transmission costs and results in unnecessarily long time-to-first-fix. These issues can be alleviated using mobile-based, or offline, mode. In this mode the device carries a subset of the global radio map in memory for fast positioning without data connection. The challenge of this approach is the large size of the offline radio map that needs to be downloaded, stored and updated periodically in the mobile device. This paper presents a method to find the significant APs in the global radio map and proposes using only those in offline positioning in order to compress the size of the required offline radio map. We also propose a method to further compress the size of the offline radio map by hashing the globally unique AP BSSIDs into locally unique shortened BSSIDs. We test the proposed methods with real-world data.

The effect of the antenna phase response on the ambiguity resolution

In order to get the best performance from carrier phase -based GNSS positioning methods in terms of accuracy and reliability the factors affecting the signal propagation must be characterized accurately. These carrier phase -based methods include precise point positioning (PPP) as well as real-time kinematic (RTK). While much focus has been put on atmospheric effects, the antenna effects are either ignored (low-end solutions) or handled by utilizing phase center offset and phase center variation (high-end solutions). The latter approach is typical in modern RTK equipment. Survey-grade antennas are designed to have such fine az-imuthal symmetry in the phase response that only elevation- dependent correction must be applied to the observations. This is referred to as the phase center variation. Moreover, the final baseline solution is corrected with the phase center offset in order to map the solution to a physical point in the antenna structure. The approach typically assumes that antennas of the same type have similar spatial response characteristics so that the same correction data can be applied to all the antennas of the same make. However, carrier phase -based techniques have been proposed for consumer-grade devices, in which the antennas are typically cheap, small and unoptimally positioned in the devices. In such cases the phase response may have high asymmetry both in azimuth and elevation and, hence, the current practices may no longer be sufficient. The unmodelled biases, amongst other, have impact on the probability of successful integer ambiguity fixing in RTK. This paper characterizes three antennas designed for GPS LI reception in terms of their magnitude and phase responses as a function of azimuth and elevation of the signal source. Two of the measured antennas were patches mounted in BluetoothTMGPS -receivers and one antenna was Trimble BulletTMIII that was measured for reference purposes. The phase responses are analyzed in the context of phase center offset and variation. The phase responses are then utilized in estimating the statistics of ambiguity fixing success rates. The measured antennas show varying performance in terms of phase response symmetry. The patches mounted in Bluetooth devices show approximately 70- and 49-degree variation in the phase response depending upon the direction of the signal. The lack of azimuthal symmetry prohibits the use of only elevation- dependent phase center variation tables and suggests the need for a full 3D table. The two antennas also show such differing responses that the use of a single PCV table for the antennas is not feasible. The bullet, however, shows only 4-degree variation and, hence, fine symmetry. Finally, even though the absolute variations in the phase responses are quite significant in antennas mounted in a Bluetooth GPS, the simulations show that these variations do not have a significant effect on the success rates for ambiguity resolution. This is because the probability of having a significant double difference bias turns out to be practically negligible.

Estimation of base station position using timing advance measurements

Timing Advance is used in TDMA (Time Division Multiple Access) systems, such as GSM and LTE, to synchronize the mobile phone to the cellular BS (Base Station). Mobile phone positioning can use TA measurements if BS positions are known, but in many cases BS positions are not in the public domain. In this work we study how to use a set of TA measurements taken by mobile phones at known positions to estimate the position of a BS. This paper describes two methods -- GMF (Gaussian Mixture Filter) and PMF (Point Mass Filter) for estimation of the BS position. Positioning performance is evaluated using simulated and real measurements. In suburban field tests, TA measurements suffice to determine BS position with an error comparable to the TA granularity (550m). GMF computes BS position much faster than PMF and is only slightly less accurate.

On the feasibility of adding carrier phase –assistance to cellular GNSS assistance standards

The 3GPP (Third Generation Partnership Project) Release 7 of GSM and UMTS cellular standards as well as SUPL2.0, used in IP networks, include major modifications as to how AGNSS (Assisted GNSS) assistance data is transferred from the network (cellular or IP) to the cellular terminal. Simultaneously position accuracy improvements may be introduced. One potential option is to use carrier phase -based positioning methods. This can be achieved integrally in the cellular network or by the use of Virtual Reference Stations and an IP network. The bulk of AGNSS devices will be singlefrequency due to additional cost associated with two RF front-ends. Hence, this study addresses the feasibility of single-frequency carrier phase-based positioning, making comparison with the dual-frequency case. The study shows that single-frequency carrier phase –based positioning is feasible with short baselines (<5 km) given that: 1) real-time ionospheric predictions are available and 2) there are enough satellites available. Namely, this requires hybrid-use of GPS and Galileo.