High-speed recording needs more than just storage capacity

Dec. 1, 2005
Applications in airborne intelligence, surveillance, and reconnaissance have always demanded that recording systems capture large amounts of data.

By Ben Ames

HOUSTON - Applications in airborne intelligence, surveillance, and reconnaissance have always demanded that recording systems capture large amounts of data. In 2000, designers at VMETRO Inc. in Houston helped place a sealed, ruggedized, four-disk JBOD on the U-2 aircraft.

Since then, engineers have sought ways to use ruggedized rotating media to replace the more expensive and inflexible solid-state flash memory data-recording systems, says Tom Bohman, vice president of business development for recording products at VMETRO. JBOD sands for “just a bunch of disks.”

Now VMETRO engineers have created a mobile, 1-gigahertz, ultrawideband digital data recorder for applications such as signals intelligence and electronic intelligence, electronic countermeasures and electronic support measures, surveillance, radar analysis, and software-defined radio.

“Just five years ago it was unthinkable that a mobile system could perform sustained recording at the 2-gigabytes-per-second rate needed for an ultrawideband digital data recorder, so virtually all such recorders used limited amounts of on-board memory to provide a few seconds of snapshot recording,” he says.

The U.S. Air Force U-2 surveillance plane uses ruggedized disk drives from VMETRO to store data from its powerful sensors.
Click here to enlarge image

Now, advances in commercial mass storage, digitizer, and recording engine technology have enabled the ultrawideband (UWB) recorder.

The density (and hence capacity) of hard drives has grown from 9 gigabytes in 2000 to 36 gigabytes, then 73 gigabytes, 146 gigabytes, and 300 gigabytes in late 2004. It is heading toward 500 gigabytes in the standard 3.5-inch drive form factor, Bohman says.

Massive capacity in an extremely small space is very important to the UWB Data Recorder, transferring data to an array of drives at a sustained 2 gigabytes per second. That is 120 gigabytes per minute or 7.2 terabytes per hour.

In 2000 one hour of recording would have required 200 36-gigabyte hard drives. That capacity is now possible using just 24 of the 300-gigabyte drives and fits into a pair of 2U, 12-disk JBOD units, Bohman says.

More important to recording rates of 2 gigabytes per second is the hard-drive performance or data-transfer rate. All disks include cache memory that allows a burst rates to the drive nearly equal to the speed of the drive interface, but ultimately the sustained throughput to each disk is the speed with which data is transferred to the platter.

In 2000 the fastest enterprise disk allowed a sustained rate of 20 megabytes per second, meaning that the recorder would need to stripe 100 drives to achieve the performance needed. Today the enterprise-class drive can sustain more than 60 megabytes per second, reducing to less than 36 the number of drives that can fit into just 6U of rack space, and has the capacity to record up to 1.5 hours of ultrawideband data.

Hard-drive interface-technology advances are also important to the practical implementation of the UWB recorder. Two important aspects of the drive interface are the cables and the throughput to the drive’s cache memory.

In 2000 parallel interfaces such as IDE/ATA and SCSI dominated, while the thinner and longer Fibre Channel serial interface has gained favor today, says Bohman.

The primary task of the CPU processor in the UWB Recorder is to control the data flow from the digitizer to the storage. The system also needs a high-end board-level digitizer, a nd a data interface to the recording engine.

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