ION GNSS 2012
Session D5: Multi-Constellation User Receivers

Title: Integration of A Consumer Single-chip Receiver for Multi-Constellation GNSS
Author(s): C-T. Weng, C-W. Chen , S-Y. Huang, K-I. Li, Mediatek Inc., Taiwan
Date/Time: Friday, September 21, 2012, 9:43 a.m.
Room: 108/109 (NCC)

While the number of GNSS satellites is steadily increasing as the system providers execute their launch programs, it is common already for GPS chip set vendors to support both GPS and the others satellite navigation systems. The combination of multi-constellation will provide greatly improved performance for the end user. This paper introduces the design concept and practical trial testing results of a one-chip multi-constellation GNSS receiver based on the viewpoint from manufacture and consumer level. The integrating multi-constellation GNSS performances with availability, improved accuracy, faster time to first fix and an outstanding decoding performance with -151dBm that is the best performance in commercial product, will be then presented. Especially, an additional output in this paper will be a pilot study of the signal processing of the BEIDOU -2 which China has released the test version ICD for its Compass (BEIDOU -2) GNSS system last year.
This article briefly outlines the difficulties of integrating these non-compatible systems, including GPS, GALILEO, GLONASS, QZSS, and BEIDOU, and offers an economic solution in the mass market where cost is the most important key, but performance demands in terms of sensitivity, time-to-first-fix, power consumption, and availability are extreme. One of the aims is to provide improved indoor and urban canyon availability for mass-market by using all available GNSS satellites. Extreme integrity is required to overcome the hardware incompatibility issues of GLONASS, that is, its frequency division multiple access (FDMA) signal rather than the code division multiple access format used by GPS, different centre frequency, and different chipping rate, all without adding significantly to the silicon cost of the receiver chipset. Due to the non-constant group delay characteristic of the GLONASS RF front-end, the inter-channel hardware biases vary across the different GLONASS channels. Traditionally, it is well known to utilize the received GPS signals and/or the received GLONASS signals to offset group delay errors in the received GLONASS signals. However, this method cannot perform well and takes longer computations under single GLONASS operation. In addition, system independent processing delay, called inter-channel biases, from different GNSS system will also induce uncertainties in navigation results. For easy to use the consumer products, aspects of a novel method and system for calibrating group delay errors and inter-channel bias in a combined GPS/GLONASS or single GLONASS will be presented. The preliminary results indicate a significant improvement from 22 meters to 8 meters in 3D position RMS error.

The second problem encountered in system independent receivers is in the different time systems employed by each of the satellite navigation systems. It has become a standard practice to solve for the time differences within the receiver´s navigation solution via a combination of receiver clock corrections and/or time offsets. In this paper, the subsequent use of these time offsets will provide a more accurate navigation solution than without them. However, the problem with using the time offsets is that they pose an additional integrity risk because they are also potential sources of faults. Thus, a proper RAIM method has been adopted to deal with the use of the time offsets for multiple constellation solution. Mathematical models to account more reliable solution for the time differences with and without the time offsets by using additional measurements will be presented in this paper. For the time being, the ICD fails to describe the B1 signal´s modulated data, particularly the satellite almanac and ephemeris elements used to compute Compass satellite orbits and clock offsets, that are essentially for incorporating these new signals into the algorithms for determining position and time of receivers. However, it is easily to overcome by updating the software to convince the positioning service.

Most electronic devices would require efficient power consumption for a longer operating time. Satellite position computation is a time-consuming task in the user position computation process. Different constellations transmit their own format of the orbital information. GPS, Galileo and QZSS transmit a Keplerian model for satellite orbit which is useful over 2 to 4 hours. GLONASS transmits satellite state vector (position, velocity and acceleration) at a particular time and is valid over +/-15 minutes interval. Hence, GLONASS satellite orbit computation requires a numerical integration of an orbital force model with satellite state vector. In this paper, different approaches were evaluated to compare their accuracy, processing complexity and effect of integration step size. In our observations, force model accuracy over the integration period and integration duration significantly causes the error in satellite state vector to accumulate over time resulting in poor user accuracy. A lesser integration duration with smaller step size is ideal for limiting error growth in the satellite state vector. This requires the receiver to integrate every second for all the satellites to be used in position which puts a considerable burden on the processor. Numerical integrators were evaluated to measure the integration error growth over 1-hour duration at different strides. The impact of MIPS complexity on the integration duration for different step sizes will be shown then.

With latest semiconductor processing, manufactures are making the overall GNSS receiver both smaller and less power-consumption. The ability to build a complete multi-constellation GNSS receiver with a costly bill of materials and covering a smaller PCB will mean that smaller and sleeker receivers can be built into an ever-wider range of end applications, with the reduced power consumption further saving space by reducing the space needed for battery power sources. Thus, to overcome the issues of increased sensitivity and rapid TTFF by massively increasing the number of correlators applied to each receiver channel will become impossible. In this paper, a crucial for the scheduling correlators utilization among different GNSS systems have been adopted for achieving fast TTFF. Eventually, the works include a more complete analysis of the integration in terms of different satellite navigation systems and different operating environments. Besides integration, testing results can also be carried out for TTFF, accuracy, continuity and availability.

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