A Novel Device for Autonomous Real-Time Precise Positioning with Global Coverage

D. Calle P. Navarro, A. Mozo, R. Piriz, D. Rodriguez, G. Tobias

Abstract:	Precise Point Positioning (PPP) is a relatively new positioning technique providing centimeter-level error. PPP processes dual-frequency pseudorange and carrier-phase measurements from a single user receiver, using detailed physical models and corrections, and precise GNSS orbit and clock products calculated beforehand (for example products from IGS, the International GNSS Service). PPP is different from other precise-positioning approaches like RTK in that no reference stations are needed in the vicinity of the user receiver. The only observation data that must be processed are measurements from the user receiver. Another advantage of PPP is that since the GNSS orbit and clock products are by nature global, the PPP solutions are also global, i.e., the PPP approach works for a receiver located anywhere on or above the Earth surface, and the resulting position is referred to a well-known terrestrial reference frame (normally ITRF). PPP can be applied at post-processing level and also in real-time applications, provided that real-time input orbits and clocks are available. One disadvantage of standard PPP however is its relatively slow convergence time, which is of the order of an hour for decimetric accuracy, as compared to nearly instantaneous convergence with centimetric accuracy in short-baseline RTK.In 2008 GMV introduced magicGNSS, a web application for high-accuracy GNSS data processing. magicGNSSis available online at http://magicgnss.gmv.com. A free account can be requested online for a one-month trial period. A PPP software module is available in magicGNSS, allowing the user the post-processing of dual-frequency static and kinematic RINEX measurement files. The PPP module supports GPS and GLONASS data. Satellite orbits and clock products are calculated internally in a transparent way for the user. GMV has developed an infrastructure for the generation of precise GPS and GLONASS orbits and clocks with very low latency in a first step, and in real time in a second step. The products generated this way are contributed to the IGS Real Time Pilot Project, and are also used to feed the PPP service, part of the web application magicGNSS.The product generation is based on an Orbit Determination and Time Synchronisation (ODTS) process, which runs typically every 15 minutes. This process receives as input dual-frequency code and phase measurements collected in real time from a world-wide network of IGS stations, using the NTRIP protocol. Then,they are pre-processed also in real time by a Pre-Processing and Validation module (PPV) and made available to the different algorithms. In parallel to the ODTS, another process estimates the clocks in real time taking as input the observations and the outputs from the last ODTS execution. There is a small latency in the delivery of the clock estimation, which is associated to the time that the algorithm waits for the arrival of the measurements from the station through the Internet, typically one or two seconds.The GPS and GLONASS satellites are processed together, in order to ensure a consistent solution. It is necessary to estimate an inter-channel bias when processing GLONASS data. This must be done in order to compensate for the different internal delays in the pseudorange measurements through the GLONASS receiver, associated to the different frequencies used by the different satellites. The real-time orbits and clocks are available as a data stream to real-time processing algorithms (such as real-time PPP), and stored in standard formats (SP3, clock RINEX) for offline use. In the near future the products will contain also additional clock biases to allow the resolution of integer carrier-phase ambiguities in PPP, for improved positioning accuracy andshorter convergence time.In parallel to the PPP product generation, GMV is developing a novel user terminal prototype for PPP, that can be connected to any GNSS receiver providing real-time raw measurements and navigation messages through a serial or USB port (in RTCM format), and can receive PPP orbit+clock corrections via GSM/GPRS (internet through mobile phone network) and also via Iridium (satellite based, with global coverage). This device is a self-contained, DC-powered case, containing mainly a Single Board Computer (SBC board) running Linux for the PPP client software, an additional board hosting the new Iridium 9602 modem, plus a LCD touch display for terminal control and visualization of results. The major advantage of this design is that the PPP user terminal can be connected to virtually any professional GNSS receiver in the market, since practically all of them are able to generate raw output in RTCM format via a serial or USB port.For remote locations without GSM/GPRS coverage, satellite communications via Iridium shall be used. The Iridium SBD service has a low, uniform global latency reported to be of the order of half a minute. PPP service via Iridium is based on clock corrections sent at a rate of one minute; this is believed to be a reasonable compromise between positioning performances and communications costs.This paper describes the implementation of GMV´s infrastructure for real-time PPP products and their usage in the new user terminal.
Published in:	Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011) September 20 - 23, 2011 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	699 - 706
Cite this article:	Navarro, D. Calle P., Mozo, A., Piriz, R., Rodriguez, D., Tobias, G., "A Novel Device for Autonomous Real-Time Precise Positioning with Global Coverage," Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011), Portland, OR, September 2011, pp. 699-706.
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