Report of GPS-99 Session 12
Issues of data quality management and hardware/software technological problems in GPS

The presentation in this session clearly indicated the continuing quest for GPS technologies that provide high data quality at high data rates that enable rapid solutions providing a number of data products. Several presentations (Namie & Yasuda; Martin & Jahn; Doeller) dealt with the development of real-time kinematic (RTK) systems for accurate (cm-level) real-time positioning within a region. System details vary between countries, but all employ GPS receivers that also receive real-time clock, atmosphere, and ionosphere corrections encoded on microwave signals. The RTK corrections are calculated using data from a network of reference GPS sites. Collecting the reference data, performing the solutions, communicating the RTK corrections to the transmission centers, and transmitting the RTK corrections, occurs in a few seconds.

Several presentations addressed reducing errors that limit accuracy in near real-time processing. Zumberge et al. discussed the limitations imposed by selective availability (SA). Extrapolating satellite clock corrections under SA is a major source of error in near real-time processing. The problem is pushing the rate of data acquisition towards the 1Hz regime. Fang et al. discussed another very important novel approach that make use of a 24-hr moving window that steps every hour, it is possible to achieve accuracy of 10-30 cm in orbital position, and 1-2mm in precipitable water vapor, over a period of 1-2 hrs following the solution. Stowers et al. discussed data handling issues entailed in such high data acquisition rates for a global network. Issues of accuracy for post real-time processing were addressed by several presentations. The theme in these presentations was increased reliability in data processing.

Simons et al. discussed the choices that one makes in GPS data analysis and the effects that they have on position estimates. Sadly, these choices, as well as analysis software used, still have (after more than a decade of research) non-trivial effects on position determinations. More work is required in this area. De Jong & Teunissen discussed a data quality assessment method based on hypothesis testing. This approach leads to an internal measure of reliability represented by a Minimal Detectable Bias (MDB). Under certain circumstances, the MDB can be reduced to ~20% of a cycle. A poster by Kristansen also addressed statistical methods for assessing data quality. Postfit residuals were used to determine station-dependent data weighting (phase and pseudorange). This scheme significantly reduced "outliers".

In general, the presentations in this session were quite forward-looking, both in terms of applications and techniques. Topics addressed within talks included: new or enhanced GNSS's (Martin & John; de Jong & Tuenissen); evolution towards an European standard for RTK (D?ller); advanced data processing scheme for future applications (Fang et al.; Stowers et al.); systems integration (Fujii & Tanaka); and new applications requiring near real-time analysis whose accuracy approaches that achievable in post-processing (Zumberge et al.; Fang et al.; Stowers et al.)

Akio Yasuda

Session 1
Session 2
Session 3
Session 4
Session 5
Session 6
Session 7
Session 8
Session 9
Session 10
Session 11
Session 12
Session 13
Session 14