A common refrain in the world of GNSS is the desire for “interoperability,” the use of signals from multiple systems without a decline — and potentially even an improvement — in the quality of results.
Achieving this depends on large part in establishing comparable parameters — particularly the geodetic references and timing systems — among the GNSSs along with a dense network of ground reference stations that can provide continuous, precise monitoring of satellites’ orbital positions.
As recent objections to the installation of Russian GLONASS monitoring stations in the United States have shown, however, achieving this is not a trivial matter politically. And the more than 20-year effort of the International GNSS Service (IGS) — formerly International GPS Service — to establish this capability provides a measure of the practical challenges.
The IGS has established a global system of satellite tracking stations, data centers, and analysis centers that puts high-quality GNSS data and data products on line in near–real-time to support a wide range of scientific and engineering applications and studies.
To better understand the issues and possible benefits of multiple GNSS monitoring, we called on Tim Springer, a member of the IGS Governing Board’s Executive Committee. Springer is a contractor with the European Space Agency at its European Space Operations Center in Germany and a principal in PosiTim UG.
Before the establishment of the IGS, GNSS observation campaigns typically called for one or two weeks in the field followed by about six months analyzing the data back in the office. In a pilot project launched in 1992, the IGS set itself the task of processing a full day of data every day.
At that time the still-incomplete GPS system had only about 20 satellites in orbit and the IGS global tracking network consisted of a mere 30 stations. But even with this reduced set of satellites and stations, generating a solution took between 6 to 24 hours of computer processing time.
A combination of dramatically reduced processing times and increased data transfer capacities via Internet have reduced the time needed to produce IGS products from days, if not weeks, of delay to near–real-time. This development has taken place very gradually, evolving from the weekly “final” products delivered with a 14-day delay after the start of the week, to the daily “rapid” products that become available 17 hours after the end of the day.
The next step was “ultra-rapid” products that are generated four times per day at 0, 6, 12, and 18 hours UTC and contain 24 hours of estimated and 24 hours of predicted orbit and clock products. This has led to the latest development: the IGS real-time service, realized in 2013, which delivers GPS+GLONASS orbit and clock correction in near real-time.
The IGS statistics show that roughly 25,000 users per month directly access IGS products and information through its Central Bureau. In the timeframe from 2010 to 2012, download statistics from CDDIS, the main archive of IGS products, revealed that 3.6 million product files are downloaded each month, of which 64 percent took place in the United States accessed from more than 11,000 different IP addresses. The most downloaded products are the IGS ultra-rapid products.