GNSS performance monitoring

Abstract

Nowadays,
many people and organizations depend on Global Navigation Satellite Systems
(GNSS. Monitoring of GNSS is important to ensure the quality of GNSS
measurements and products. The availability of the signals-in-space (SiS) is an
essential part of the monitoring of GNSS, but it is not clear how availability
is defined and standards for monitoring are lacking. The main research question
for this thesis is therefore: Which are the key
performance indicators related to availability that unambiguously describe
sensor station and system performance in time, how can these be computed in an operational
manner, and how can they be presented in a condensed form to the stakeholders? This
includes an objective of defining unambiguous performance parameters for sensor
station and system, and address the considerations related to the definition. A
prototype software tool is created to study the algorithms and compute the key
performance indicators.

Availability
is in the basis a binary operation: a signal is available or unavailable. When
this is applied on daily
measurements, daily statistics can be computed. A signal is considered
available if the code, carrier phase and C/N0 measurements are present, and
meet certain standards. A signal is said to be expected if the satellite is
expected to transmit that signal, the receiver is configured to receive that
signal, and the signal is not blocked by objects in the signal’s path to the
sensor station. For this it’s needed to define and compute an elevation mask
for each station.

The
sensor station and system performance parameters are computed from a network of
sensor stations, using files in the Receiver Independent Exchange format (RINEX).
Four key performance indicators are defined for the sensor station performance.
The Daily Station Availability describes the part of the day that the station
is operational, the Daily Station Total Availability gives how well the station
receives, and the Effective Mean Elevation quantifies the elevation mask and
thus the location of the sensor station. These three parameters are summarized
into the Overall Station Quality parameter, which gives and overall performance
class to the sensor station.

For the
system performance a satellite is considered available if all signals are
received by a sensor station
of the monitoring network and the health status is healthy. The satellite is
considered unavailable if the signals are received by none of the monitoring
stations, while expected by at least two stations, or the health status is
unhealthy. The Daily GNSS Availability parameter gives the percentage of the
day that the satellite was available and the Daily Available number of
Satellites tells how many satellites were available during the day. 

The
parameters are computed for a period of 100 days. Results are presented using
color codes and by showing only
detailed information in case of anomalies or specific investigations. The
proposed key performance indicators showed to be very useful at pointing out
good performances or anomalies. While SiS availability gives much insight in
the performance, a monitoring tool can be improved when combined with other
performance aspects.