CURRENT IGS NETWORK |
The
"core" IGS network comprises that subset of the total IGS
network
whose data is regularly analysed by at least three Analysis
Centres on more than one continent. By the end
of 1998 it numbered approximately
80 stations -- Figure 1.
Figure 1. The core IGS
tracking network by late 1998.
The configuration of the IGS network at the end of 1998 is indicated in Figure 2. A comparison of Figure 3 in section 12.2.2 and Figure 2 below indicates a rapid growth of the network over the past years, with the network nearly doubling in size each year! The specifications for the site, the receiver and the data communications procedures are carefully defined by the IGS. By the end of 1998, more than 200 permanently operating stations were contributing data to the IGS. Many of these sites have been established by a small number of agencies. However, quite a number of the sites (particularly the new ones) have been established by governments to serve multiple functions, such as commercial base stations, fundamental datum stations, Integrity Monitoring stations, or stations generating "differential" corrections for real-time DGPS navigation users. We may expect to see such a trend to continue.
Figure 2. The total IGS
tracking network by late 1998.
The future growth of the IGS global network is aimed at "filling in the gaps" in coverage. These additional sites will contribute nothing to improved orbit products (the present network is more than adequate for this purpose), but represent a densification of the ITRs. The IGS is developing the logistical ability and the techniques to include many hundreds of well distributed GPS stations for the purpose of determining station coordinates and velocities as part of the ITRS. This densification will ensure that most GPS users will be within about 1000km of a precise reference point on which to precisely link their local or regional survey (or datum).
What of the future? The IGS has also launched several other initiatives related to tracking networks. One is the coordination of a global tracking experiment for the GLONASS satellites, that operated for several months from late 1998 until early 1999. It is at present unclear whether an operational tracking network will be established to service the GLONASS system in the same manner as for GPS. The second initiative relates to the tracking of low earth orbiting satellites (LEO). This is considered to be an important new service, however the nature of the tracking network will have to change. The most important aspect is high data rate tracking. At present data is collected at 120 second intervals, stored on site for 24 hours, and then downloaded to a Regional Data Centre (and, in turn, to a Global Data Centre). (Some experiments have been carried out whereby data is logged and transmitted as one hour files.) For LEO tracking (particularly in support of new meteorological experiments), 1Hz (once per second) data tracking is required. A set of about 20 globally distributed tracking stations has been identified, and will commence tracking operations in the near future.
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© Chris Rizos, SNAP-UNSW, 1999