The SRA radar consists of a 20-inch Rexalite lens, a feed horn on the lens axis which looks up into a mechanical scanning mirror that mirror-images the feed horn to the focal point of the lens, a pulse modulator and RF exciter, receiver, 1.7KW Extended Interaction Amplifier (EIK) and the RT-Linux data system. The data system is the topic we will discuss here. Figure 3 is a photograph of the SRA scanner installed on the NOAA hurricane hunter. The fairing is removed in this photo.
Figure 4 is a block diagram of the SRA power system. The SRA requires an uninterruptible power source for Linux and the three differential GPS receivers and computers. Instead of an off-the-shelf UPS, we went with a 12-volt 25 AH “RG” (recombinant-gas) sealed aircraft battery as the prime power source for the system. This was chosen for two reasons:
We needed an uninterruptible power source, because aircraft are notorious for power dropouts during engine start and shutdown.
We needed to power our 12-volt GPS receivers for up to an hour before and after each mission without aircraft power applied.
We purchased a 12-volt input 150W PC power supply to power the data system. The battery can power the data system and the three GPS receivers for about two hours, or the GPS receivers alone for five hours. We located the battery in the rear of the custom data system housing.
Figure 4 depicts the wiring of our power system.
Figure 5 is a block diagram depicting the internals of the SRA data system. The computer is a single-board 200 MHz Pentium which plugs into a passive backplane with ISA and PCI slots. The CPU card contains PCI-VGA video, PCI-IDE controller, PCI fast-wide SCSI controller, 64MB of RAM, 512MB of cache, two serial ports, a parallel port and the CPU. A PCI 3c595 network card provides networking and a special-purpose ISA card loaded with PIC microcontrollers provides an interface to the radar systems. A 6.4GB EIDE disk drive is used as /dev/hda to hold Linux and for data storage. A backup SparQ 1.0GB removable drive is installed as /dev/hdb. The system has no floppy or CD-ROM drive. If a CD or floppy is needed, they are simply remotely mounted with NFS from one of the Linux laptops which have both. No keyboard or monitor is used for normal operations, though they can be plugged in if the need arises.
Initially, we used a 4.2GB SCSI drive, but that used too much electrical power. Early development was done using a 250W 117vac PC power supply. When we switched to the 12-volt input 150W power supply, we discovered we were over our power budget by 25 watts or so. During the boot process, the power consumed by the combination of the SCSI drive and the waveform digitizer would cause the power supply to “spike” the 5-volt source and cause a reboot. It took us several hours to find this problem. It would generally happen just as Linux began loading, due to the digitizer being powered up and the drive being accessed. During the DOS boot, the digitizer was not powered until after DOS booted and after the digitizer configuration program loaded and ran. Consequently, the loading of Linux was the straw that broke the camel's back. We finally settled on a 6.4GB EIDE disk drive for Linux and for data storage. The power consumption of the EIDE drive is substantially less than the SCSI and no perceptible difference is seen in performance of the data system.
Figure 6 is a photo of the SRA data system during development. It was in this “state” until just a few days before our first test flight on the NASA C-130. Figure 8 is a photo of the data system after packaging. Figure 7 shows the internal organization of the data system as viewed from the top rear. The enclosure is reversed from most rack mounts. We wanted to have ready access to the computer card connections without having to remove the rear rack cover. The only connections on the rear are for the GPS receivers and the battery charger. The black power supply under the data system in Figure 8 is our prime power supply/battery charger.
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