LPI radar or data link performance is acomplex function of many variables. three main elements of LPIS performance affect a stealthy system's success.
- the actual design features incorporated into the emitting system specifically for LPI.
- The ESM stategy and corresponding implimentation utilized by threat forces. Hostile forces ESM strategy and implimentation are very important factors affecting any LPI design. Design stretegy begins with an analysis of individual threat receiver characteristics against which the LPIS may be deployed. The EOB deployment and location strategy for intercept receivers musT also be considered so as to create the most succesfull design features. additionally, the issue of whether or not individual interceptors are netted has a strong influence on the strategies an LPIS might use
- the geometry between the area of regard ( AOR ) and threat receivers.
There are seven main interceptability constraints :
- LPI system mainlobe power at the intercept receiver
- LPI system sidelobe power level at the intercept receiver
- Area of mainlobe and sidelobe on the ground or at a certain threat altitude.
- Time of the AOR illumination for mapping, tracking, or targeting
- Intercept receiver density and search time
- Intercept receiver detection response
- Power management strategy
Each constraints is describe in some detail, later as will be seen, engagement scenarios must be assumed so as to properly perform LPI evaluation, and more details are described later too. Insya’Allah.
The first 3 interceptability constraints are despicted in a simple figure below :
Ground power contours computed to determine interceptibility
LPIS (or constant altitude ) power contours can be computed to determine interceptability using a figure that assumes that interceptability receivers are distributed in on some regular grid on a surface plane perpendicular to the down vector beneath the emitter or at a constant altitude such as on orbiting surveillance aircraft. The power intercepted at each of these grid coordinates can be calculated based on the emitter geometry. This consists of repetitive calculation of the beacon Equation. The emitter if airborne has some flight path, altitude, and corresponding ground range from the ground track. The emitter, if surface based, has some altitude difference with the interceptor and some surface range to a point under the interceptor. Based on those known parameters, the emitter line of sight ( LOS ) is calculated for each grid coordinate in the geometry. The emitter power captured by intercept receivers located at each of these grid intersections then can be calculated. If one assumes that each of the interceptors has a specific sensitivity and that its antenna coincidentally is pointed at the emitter, one can calculate the area containing all those intercept receivers in which the emitter received signal is larger that the minimum threshold. Corresponding power contours can be interpolated from the grid points as shown in the picture above. The details of the calculation process are given later. Insya’Allah.
In addition, there are far sidelobes in the immediate vicinity of the emitter, and their area will be larger because of the emitter’s proximity to the ground :
platform altitude affects antenna "footprint"
At some range, the emitter horizon occurs and although there is a small amout of refraction that causes the emitter to see a little over the geometrical horizon, there is a horizon cutoff that occurs rather abruptly; beyond that range, the emitter antenna mainlobe no longer intercepts the Earth’s surface. In general, the low-altitude, shallow-with grazing-angle case creates the largest area in which the emitter can be detected and thus is the most dangerous from an LPI point of view. Of course, actual footprints created by emitters on a surface vary with surface shape, grazing, angle, antenna shape, and antenna sidelobe weighting. Three example footprints on a Earth are shown below :
footprint varies with antenna shape
In this particular case, three antennas inscribed inside a 37-in circle ( typical large fighter antenna diameter ) are compared. FOR this example, the surface range to the center of the mainbeam is 80 km, and the depression angle is 8 degree. The first antenna is a circular disk with Sonine weighting. It has substancially more gain; thus one intuition would be that it would have the best intercept footprint for a given detection performance. The second example is a square inscribed in the 37-in circle with 33-in sides and 40-dB Taylor weighting that is separable in two dimensions. The 3rd example is a diamond-shape antenna, again inscibed in the 37-in circle, with 40-dB separable Taylor weighting. The actual footprint area for each of these cases has been calculated and is shown below above below each footprint. What is surprising is that the circular amplitude weighting give rise to near boresight sidelobes that are above an interceptor threshold; hence the actual footprint area on the ground is 3234 km². In the cases of the inscribed square, even though it has lower gain and, hence, a broader mainbeam, the enhanced sidelobe performance gives rise to less than half the footprint area- a little over 1.6k km² . lastly, the 33-in diamond with a Taylor separable weighting gives rise to slightly less than 1.6k km² in the area. By the way, I will show it later that separable illumination functions are easier to manufacture, and so actual sidelobes are usually better implemented this way. Thus, it can be seen that the amplitude weighting, especially separable amplitude weighting functions, and actual antenna shape have a significant impact on the intercept footprint. Antenna footprints and their calculation, I will describe them in more detail later. The problem is primarily one of geometry and approximations to simplify calculations.