GPS World, April 2014

EXPERT ADVICE FIGURE 1 Typical flow of data within the GPS correction system operational processing systems for correcting satellite clock and ephemeris information Each of these systems tends to utilize particular data sources and often output their results in different message structures and encodings One such system for correcting the timing and positioning of GPS satellites is Estimation and Prediction of Orbits and Clocks to High Accuracy EPOCHA Currently navigation and timing improvements are only uploaded to the satellites and GPS devices once a day To improve the EPOCHA system the National Geospatial Intelligence Agency NGA is researching the logistics and benefits of updating the navigation and timing information at much shorter time frames for example every 2 15 minutes The corrected satellite clock and state data can then be sent to the satellites to the processing centers to improve geolocation of real time or archived positions or remotely sensed observations and to devices in the field to improve real time position measurements A processing system in widespread use for applying these corrections to positional measurements is the open source GPS Toolkit GPSTk This software was used in OWS 9 to demonstrate the processing of SWE Common encoded GPS data within a Web enabled environment As shown in Figure 1 the data flowing between archiving and processing components exist in a wide variety of formats Currently these message streams consist of message structures defined through various documents some of which have restricted access Additionally these streams and the messages they contain are being encoded in various formats including for example a binary exchange format BINEX a systemspecific XML schema an HDF5 file format several text based formats and others The message components within each of these formats are inconsistent even though two messages may describe similar information Often a processing system is required to read data and output results in multiple formats and to understand the inconsistencies between them By forcing different software and processing systems to support multiple message structures and data formats the current system inhibits the effective use of these data by requiring several format specific readers and writers to be developed in the appropriate software language C C Java Python as required by each application system providing inconsistent message structures between the data used or produced by different processing systems requiring meticulous and thus error prone human interpretation of the data components based on the limited documentation provided for each creating lack of interoperability with regard to using data designed for or produced by a different particular processing system and discouraging development of new and innovative software and processing solutions The Engineering Report addresses the feasibility of using the OGC SWE Common Data v20 standard to support all message and data streams within future generations of the GPS operational network In particular the effort focuses on message streams that provide input to and output from the processing systems responsible for providing improved position and time accuracy within the GPS network Here are the benefits of the SWE Common Data standard The data can be fully described in a machine and humanreadable XML document providing data type units constraints semantics quality labels and so on and an unambiguous definition of both the data structure and encoding of messages records The data values themselves can be encoded in highly efficient binary or ASCII text blocks or streams A single software application is able to read any data described in SWE Common data Any process can be described in SensorML using SWE Common as inputs outputs and parameters Any SensorML defined process can participate in easilydefined executable workflows The Engineering Report describes the formats and how they were encoded and the Web services created to move data between various GPS processing systems FIGURE 2 FIGURE 2 Collection of SWE services providing on demand access to all GPS related data in the project GPS World April 2014 www gpsworld com 12