Magnetic Recording For Near Real Time Reconnaissance Applications

Magnetic Recording For Near Real Time Reconnaissance ApplicationsW. D. KesslerData Systems Division, Fairchild Weston Systems Inc. P.O. Box 3041, Sarasota, Florida 33578AbstractThe definition of recording for "near real time reconnaissance" applications is reviewed along with the requirements for a recorder to be used in this application. Next the storage medium (magnetic tape) and its importance is discussed. Following this, the recording technologies available to achieve the required results are pre­ sented. Both longitudinal fixed head and helical scan rotary head technologies are considered along with some of their merits and demerits. Finally, a brief look into the future of this storage technology is presented.IntroductionMany airborne reconnaissance systems under consideration today will require data to be returned to the base station in near real time; that is data will be obtained and stored while over the target then very soon thereafter will be telemetered to the desired locations. In some cases, the data will be edited while airborne, either while still over the target or shortly thereafter so that only selected portions need be transmitted. This will require additional data manipulating or time tagging which further complicates the data storage system. In either case a record of the full mission will be required for complete analysis at the base, and for archiving. These new requirements levy more stringent requirements on the airborne data storage systems.This paper considers the role of the magnetic tape recorder/reproducer as the pri­ mary onboard data storage system.RequirementsIt is convenient to diagram the input data requirements on the chart of Fig. 1 . This is a plot of recording time vs digital data rate in MBPS. The box encloses the generally accepted E.G. Sensor output rates. The lower left shows the requirement of 80 MBPS for about 200 seconds, and the same data rate for 1 hour describes the lower right point. The upper two points are defined by the requirement for 400 MBPS for each of 200 seconds and 1 hr. Most requirements fall within this rectangle. The dashed diagonal lines define constant capacity (data rate x time). For instance, 400 MBPS for 30 minutes requires almost 10^ bits, while five hundred MBPS for 200 sec. only requires about lO^-*- bits. Also from this it is clear that most requirements are covered by about 5 X 1011 bits. The function of other lines on the chart will be described later.It is noted that while this is a representation of the data rate generated by the sensors it is not necessarily the recording requirement because data compression before recording may be used. A compression of 4 to 1 is not unusual, and we might create another block displaced downward by a factor of 4 on our chart. However, since this is not always true and to keep from over simplifying, the original limits (without data compression) will be used.A significant and demanding requirement for reconnaissance operation is that reproduction of data is often required to be at a tape speed considerably different than that at which it was recorded. These reproduce speeds are not so easily portrayed on a chart because of the large number of record/reproduce ratio possibili­ ties. For some cases, such as those using low bandwidth, low frequency data links, reproduce at the slowest practical speed is required; other times when a wide band high frequency data link is used the fastest practical reproduce speed may be desirable. For the general case we must assume a wide range of record/reproduce ratios must be accommodated, although not necessarily in one application. Furthermore it is no longer sufficient to consider the recorder as a simple data storage system within the aircraft. It may be necessary to use two recorders, reproducing on one while recording on the other to edit the data before transmitting it to the base sta­ tions .