GPS World - May 2008 - (Page 48)

INNOVATION | Algorithms & Methods Making a Difference with GPS Time Differences for Kinematic Positioning with Low-Cost Receivers Johannes Traugott, Dennis Odijk, Oliver Montenbruck, Gottfried Sachs, and Christian Tiberius LET' S REVIEW. Most radio signals consist of a carrier wave that is modulated in some way. This includes the GPS satellite signals. The pseudorandom-noise ranging codes and the navigation message are modulated onto the L-band carriers using binary biphase modulation. A GPS receiver uses the ranging codes to determine its distance from multiple satellites and then, through the process of multilateration, its position. But what about the carrier phase? Is it just a means to convey the ranging codes and navigation message? Definitely not. A GPS receiver determines its velocity as well as its position and it does this not by differencing sequential code-based positions, which would not be very accurate, but rather by measuring the Doppler shift of the received carrier. But the carrier can be used in other ways too. In fact, it can be used for determining positions, just like the code, but with much higher precision. Over 20 years ago, surveyors and geodesists devised ways to make use of recorded measurements of INNOVATION INSIGHTS with Richard Langley the phase of the received carriers to determine accurate relative positions between If you time-difference a roving receiver and a base or reference receiver at a known location. The technique phase measurements, was enhanced over the years, evolving into RTK the ambiguity disappears. an approach known asAs itsor real-time kinematic positioning. name suggests, RTK is usually employed in real time using auxiliary radio communications (often cell-phone-based) between the base and rover receivers. However, RTK-style positioning can also be used to postprocess collected data, achieving the same high-accuracy standards. But one of the difficulties with the RTK approach is resolving the so-called carrier-phase ambiguities. One cycle of the carrier looks just like the next, so how can you determine the exact number of cycles in the carrier between the satellite' s antenna and the receiver' s antenna? Well, it can be done, but even with increasingly sophisticated techniques, there is a limit to how far away a rover can be from the base station. Isn' t there a way to get rid of the integer ambiguity problem? There is. If you time-difference sequential carrier-phase measurements, the ambiguity actually disappears! As we' ll see in this month' s column, you can determine accurate relative positions using time-differenced carrierphase measurements. But there are some caveats. Read on. ª Innovationº is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments and topic ideas. To contact him, see the ª Contributing Editorsº section on page 8. 48 GPS World | May 2008 G PS is rapidly maturing into a system that provides positioning information to wide range of civil users. This phenomenon is driving both the rapid growth of location-based applications and the development of receiver technology targeting the needs of a true mass market. These receivers, designed to operate reliably under adverse conditions and in different environments, are typically based on miniaturized, lowcost modules. They are commonly restricted to modest accuracy (meter range) and low navigation update rates (1 hertz, or a few Hz at most). Using primarily L1 code ranges for onboard navigation, some receivers also provide raw measurement data sampled at up to 10 Hz and additionally containing L1 carrier-phase ranges. The quality of the latter is usually sufficient to successfully apply relative phase-based processing techniques such as real-time kinematic (RTK) positioning. With such an approach, centimeter-level accuracy, as required for certain scientific applications, becomes possible with mass-market receivers. However, RTK requires the use of a base or reference receiver, typically within a range of 10 to 20 kilometers. Moreover, some kind of static or dynamic initialization is required in order to solve for phase ambiguities. Instantaneous ambiguity resolution can only be accomplished under exceptionally good conditions. Besides complicating the measurement set-up and the data processing, these constraints can prohibit the application of phase-based differential techniques in certain situations. In response to these challenges, we have investigated an approach for achieving high-accuracy positions based on measurement time differences from a single low-cost receiver. The idea of time differences is to exwww.gpsworld.com http://www.gpsworld.com

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GPS World - May 2008 Contents Out in Front Letters to the Editor Expert Advice GPS III Contract Award Now a Reality? Near-Space Location Boost RTK Crops Up in Precision Ag Safety Afoot 50+ Leaders to Watch Making a Difference with GPS Product Showcase Advertisers Index & Company Directory Marketplace Classifieds Seen & Heard

GPS World - May 2008

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