NSSL Home > Scientific Publications > Technical Memorandum NSSL-107
There are two major components that enable the real-time delivery of the base data. The first is the Radar Interface and Data Distribution System (RIDDS) [Jain 95], which was created by the NSSL to extract base data from the NEXRAD radar product generator in real time. RIDDS consists of a Sun SparcStation 5/110 workstation, running special software developed by the NSSL that takes base data from the wideband-3 port or the RPG and outputs them as UDP packets to an Ethernet hub. A circular buffer on the RIDDS is used to hold several volume scans of data before it is transmitted.
The second component is the Local Data Manager (LDM) software developed by the UCAR Unidata program. LDM is a system for event-driven data distribution that acquires data from, and transmits data to other networked computers.
As shown in Figure 2.2, CRAFT involves connecting a RIDDS workstation to the radar, along with a personal computer running the Linux operating system and LDM software. The inexpensive PC requires only nominal memory (64 Mbytes) and disk space (2 Gigabytes). The data are compressed by the PC, and then sent to an optional multi-port router ($1,500) and finally to the Internet via a direct Internet connection, a T1 line, or a 56kb line. The data are then passed to a large LDM server on the Abilene backbone for delivery to multiple users through Internet.
|
| Figure 2.2 Data compression and transmission in CRAFT [Droegemeier 00] |
In addition to the two major components described above, a key aspect of CRAFT is the used of a data compression algorithm known as BZIP2. This algorithm is an off-the-shelf, public domain, lossless data compression algorithm. It compresses the base data, in packets of 100 radials, down to an average of 1/10th their original size. This translates to a data volume of between 40 and 90 gigabytes/day for the 120 NWS radars, compared to 165 gigabytes without any data compression. The aggregate compressed base data rate for the entire national WSR-88D network is only 6-8 mbps.