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