This invention relates to improvement in bistatic passive radar systems and methods and is directed particularly to using a bistatic passive radar system to determine the altitude and slant range of a selected target, such as a moving airborne target, that is illuminated by a host transmitter, such as a two dimensional scanning radar that is located remotely from the bistatic passive radar system.
In cross-referenced patent application No. 304,458, there is described a bistatic passive radar system in which the distance D (as therein identified) between a target illuminating scanning radar located on the ground (therein called a host transmitter) and a bistatic passive radar receiver (also located on the ground) is determined. This is accomplished by providing means for accurately measuring the azimuth angle B between a selected ground target being illuminated by the host transmitter and the direct line between the host transmitter and the bistatic receiver and, using this angle B along with other data, to accurately determine the range distance D to the host transmitter. To accomplish this purpose a repositionable single antenna or a second directional antenna as used with the bistatic radar system to measure the angle B.
To find the correct value of D, an iterative technique is used by which the angle of the latter antenna, directed towards the target, is displayed as a B cursor on a plan position indicator (PPI) of the passive radar system; and using a PPI display bistatic range correction circuit, the distance D is manually varied on the PPI display until the B cursor bisects the video image of the selected target on the display. When the video image of the selected target is bisected by the B cursor, the distance D on the PPI display is correct.
This iterative method of passively determining the distance D is based upon the assumption that the selected target (reflector of opportunity) is either a piece of ground clutter or an object at such a low elevational angle that it would be considered in a horizontal plane, since selected targets at substantial elevational, angles such as aircraft, would cause the measured distance D to be erroneous. Also, assuming that the distance D can be determined and correctly indicated on the PPI display, the slant range and altitude of a selected airborne target are not apparent from the display of the video image of the target on the display.
To illustrate this latter point, attention is directed to FIG. 1, herein, which illustrates the geometrical situation in three dimensions in relation to where the video image of a selected airborne target is displayed on the bistatically corrected two-dimensional PPI display described in cross-referenced patent No. 304,458. A moving airborne target at position T is directly above point P on the ground. A video image of the target would not be displayed on the PPI display as being at point P, however. The path of the radar signal from the position of the host transmitter Tx to the position of the bistatic passive radar system Rx is Rs+S, which is longer than the path Rc+C in the horizontal plane of the host p transmitter and the radar system. As a result, a video image of the airborne target is falsely displayed at position F on the two-dimensional PPI display. Even though the azimuth angle xcfx86 to position F corresponds to the correct azimuth angle xcfx86 to point P beneath the target, the slant range S and the altitude H of the selected target in relation to the position Rx of the radar system are not apparent from the display of the video image of the selected target at the false position F on the two-dimensional PPI display.
The present invention provides an improved system and method of using a bistatic passive radar system in conjunction with a host transmitter that is a determinable distance D from the radar system, for determining the distance D, and for displaying video images of a selected target, the position of the radar system and the position of the transmitter on a display. The invention is characterized by using the display to determine the slant range S and the altitude H of the selected target relative to the position of the radar system.
Various features of the invention are described in the description of the preferred embodiments.