GPS World, December 2009

DIRECTIONS 2010 However all this capability is forsaken or lost by continued use of interfaces chosen poorly or from outdated standards undue consolidation within isolated equipment packaging overextended proprietary rights and limited demonstrably flawed validation methods Drop Demands for Full Fix An immediate explosion of benefits can follow from acceptance of partial information Countless examples could be cited but two obvious ones suffice Within GPS or GNSS not all space vehicles SVs would be simultaneously affected by scintillation ionospheric disturbance effects vary with both location and time A similar case holds for multipath Data from some SVs could be rejected by decisions made external to a receiver without forcing rejection of all Central processing not within any one equipment box has always offered potential for other sources distance measuring equipment or DME and so on to make up for incomplete sets of SV data My broad goal here is to take advantage of information not currently used and to prescribe corrective strategies That GPS availability has thus far been more than satisfactory to a multitude of users but that could change objective has not been widely pursued due to perceived lack of urgency GPS availability has thus far been more than satisfactory to a multitude of users but that could change Availability Enhancements For about two decades the industry was effectively guided by a strong preference for the trait whereby every data refresh event was self sufficient A major reason for this was protection against gradual veering a snapshot sequence is less sensitive than a continuously evolving path estimate The cost of course is forfeit of benefits conferred by the sequences history More recently a middle ground was sought to mitigate the resulting loss subfilters used as much new data as possible while making some use of knowledge from an estimators covariance matrix I promptly endorsed that approach and sought to carry it to the limit A single measurement receiver autnomous integrity monitoring RAIM resulted offering an independent integrity test for each separate observation Despite its rigorous derivation the technique is quite simple in practice Further it bridges a gap that formerly separated integrity test from optimal estimation while also having significant advantages over conventional RAIM separation translates to independence from other satellites and therefore from geometry effective DOP of unity ability to use different error variances for different observations for example with nonuniformity in signal strength and or elevation With this discussion we have clearly left the realm of well known subjects with self evident prescriptions Much of what follows likewise falls into the category of relatively obscure methods Beyond Position Oriented A time history of GNSS observations with or without an inertial measurement unit IMU inherently carries dynamic information A file with observational history from multiple sources of course enables the aforementioned explosion of benefits The obvious immediate offerings include closing of data lapses via information sharing intrinsic backup with automatic activation vast reduction of latency effects for example from 200 meters to less than 1 meter at 400 knots after 1 second with easily obtainable velocity accuracy below 1 meter second formation of 1 sigma projected future error within reason Beyond these lie once again some lesser known techniques including a few that are virtually nonexistent in operation at the time of this writing With GNSS the full potential of dynamics calls for a revisit of carrier phase Carrier Phase Developments Rather than pursuit of unnecessary subwavelength fixes for aircraft for example with 20 meter wing span moving at 400 knots the true value of carrier phase in flight lies in enhanced dependability Sequential changes in carrier phase over 1 second provide excellent dynamics information with or without an IMU Recognition of this opportunity led to the concept of segmentation whereby position is determined separately from dynamics Carrier phase sequential changes with ambiguities unresolved can provide precise 1 centimeter second RMS with IMU decimeter second without streaming velocity independent of position Dead reckoning then provides a priori position correctible by pseudoranges One advantage of this scheme is subtle with 1 second phase change propagation effects generally at 1 centimeter or less no mask is needed The geometry benefit is obvious and flight experience has verified it This raises another segmentation characteristic the single measurement integrity testing is applicable to each carrier phase sequential change and to each pseudorange separately and independently These capabilities are untapped in essentially all operational systems air land and sea and all stand to gain Yet another opportunity can be added ability to sustain operation even if every SV has repetitive data gaps This advantage is best JAMES L FARRELL worked for 31 years at Westinghouse in design simulation and validation of navigation and tracking programs He continues teaching and consulting for private industry the Department of Defense and university research through Vigil Inc GPS World December 2009 www gpsworld com 10