JED - June 2012 - (Page 53)

EW 101 Spectrum Warfare – Part 14 Digital Communication By Dave Adamy continued Figure 11. In this technique, a single sensor (IR, UV, or visible light) (or set of sensors) is directed through the angular area of interest. The spacing of the lines in the raster is close enough to provide the required resolution of the picture in the vertical dimension. The horizontal resolution is determined by the angular movement between the samples of the data from the sensor. In analog video, this sampled data has a frame synchronization pulse at the beginning of each picture captured; and a line synchronization pulse at the beginning of each line in the raster pattern. For commercial television (in the US), there are 575 lines in the raster and 575 samples taken per line. Every second line (alternating) is sent 60 times per second. This captures 30 full pictures per second. In Europe, there are 625 raster lines and 625 samples per line. Every second line is sent 50 times per second, yielding 25 full pictures per second. In either case, this allows “full motion video” because the human eye can only see a new picture 24 times per second. This analog video signal requires a bandwidth of just under 4 MHz in full color. By digitizing the output of the scanned sensor, a digital video signal is produced Figure 12 shows the other approach to capturing imagery data. In this case, there are a number of imagery sensors in an array. Each sensor captures one pixel of the picture. The outputs of these sensors are sequentially sampled and digitized to form a serial digital signal suitable for transmission. The bit rate of the digital signal is determined by the formula: Bit rate = frames per sec x pixels per frame x bits per pixel A standard, full resolution digitized video signal has 720 by 486 pixels per picture with 16 bits for each pixel. This makes 720 x 486 x 16 bits per picture. The Journal of Electronic Defense | June 2012 L ike the last two months in this series, we will continue the figure and table numbering. LINK MARGIN SPECIFICS Link margin is the difference between the minimum signal level in the receiver for proper link connectivity and the actual signal level received as the link is configured. Table 2 shows the items that need to be considered in calculating the link margin. This table was adapted from a similar table in the textbook, Introduction to RF Propagation by Dr. John Seybold (Wiley, 1958) The “subtotal” items in this table are related by the following two formulas: RSP = ERP – TPL + TRG Where: RSP = received signal power ERP = effective radiated power TPL = total path loss TRG = total receiver gain NLM = RSP – RSS Where NLM = net link margin RSP = received signal power RSS = receiver system sensitivity 53 DIGITIZING IMAGERY An important issue in net-centric warfare is the transportation of imagery from the point of origin to the point at which an operator or other decision maker needs to access the information carried in the imagery. The imagery can be from a large part of the electromagnetic spectrum: visible light, infrared or ultraviolet. There are two basic approaches to the capture of imagery. One way is to scan an area using a raster scan as shown in SENSOR MOVED THROUGH RASTER PATTERN SERIAL BIT STREAM PIXELS SEQUENTIALLY SAMPLE PIXELS SERIAL BIT STREAM Figure 11: If imagery is sensed using a raster scan, the intensity of each color in each pixel is digitized into a serial bit stream. Figure 12: If the imagery sensor has a sensor array, the intensity of each color is digitized for each pixel and output as a serial bit stream.

Table of Contents for the Digital Edition of JED - June 2012

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
Future EW: Next Gen Jammer
Technology Survey: Spectrum Analyzers
EW 101
AOC 2012 Election Guide
AOC News
Index of Advertisers
JED Quick Look

JED - June 2012