Techniques and methods for adaptive single antenna radar system polarization optimization for anti jam and anti clutter applications

Antenna polarization refers to the electric field orientation of a radio wave with respect to the earth's surface. It is advantageous for a radar system to be able to alter its antenna polarization due to the varying nature of target geometries, clutter, and unwanted jamming. This novel approach offers a system in which antenna polarization is adapted to match an optimal state determined from the changing electromagnetic environment.


INTRODUCTION
Radars are used in commercial and militar remotely sense objects using electromagnetic transmit energy and sense its return after the en by objects within the radar antenna's field of v the object's location, speed, size, and other d The radar senses the desired return (reflected energy) from the objects the radar is intending as undesired returns from clutter (returns buildings, and other unwanted objects) and ja which intentionally or unintentionally radi degrades the radar's performance). There are techniques and coding schemes which exist to and clutter. However, to the knowledge of the no current techniques which can adaptively co jammers by optimizing antenna polarization in Polarization refers to the electric field orien wave with respect to the earth's surface. P include vertical, horizontal, slant 45, right h hand circular, and elliptical. For example, a ve radio wave has an electric field that oscillates line (relative to the surface of the earth) ove polarization is characterized by the polarizatio emits. Slant 45 polarizations refer to polariz oriented on a 45 degree slant relative to the between vertical and horizontal. Circular polarizations have an electric field vector wh time (the electric field vector traces out an ellip time).
Antennas most efficiently receive the same they transmit. A vertically oriented antenna t polarization and most efficiently receives vert If a vertically polarized antenna is used to rec This unique system which reduce jamming is shown as a block dia system consists of the typical p transceiver: the antenna, transmitter/ generic radar system which provide of the radar system are not discusse system could take many forms wi processing capabilities. The generic blocks. The first block of the new system i block. This block consists of any an determines that jamming or clutter i jam/clutter detection could take on m Single mization for ions ce approximately 20dB or m because the antenna is not the wave you are trying to ttle of the wave) [1]. This is ovel system, which controls tion to find the minimum arized jamming. Essentially are used to detect the ming, and then the radar's ltered in response. With the ligned to the polarization of ts of clutter or jamming d.
N SYSTEM DESCRIPTION es the impact of clutter or gram in Fig. 1. The radar pieces of a generic radar /receiver, and the rest of the es signal processing. Details ed in this paper as this radar ith various front ends and radar system has three new n Optimization for Anti-Jam and Radar s the Jam/Clutter Detection nalog or digital method that is present in the return. The many approaches. Jamming could be detected by using an analog or digital approach to detect a higher than expected receiver noise floor and/or any unusual targets which possess unrealistic dynamics, unexpected or changing radar cross section (RCS), unexpected or changing locations, and other abnormalities. Clutter could be detected by determining the location of returns and correlating them to where natural obstructions should be located.
When jamming or clutter is detected, the power level of the unwanted return is stored in memory. The polarization of the radar antenna(s) is then altered either electrically or mechanically to yield a new radar polarization by the Polarization Control block. The new receive levels from jamming or clutter are determined by the Jam/Clutter Detection block and fed to the Memory block. If the new jamming or clutter received power levels are smaller (have been diminished by the new radar polarization orientation), then this polarization orientation is stored as being an improvement over the old orientation and will be used as the optimal polarization orientation unless a repeat of this process yields a new optimal orientation.
It is worth mentioning that all real desired radar targets will also have a radar cross section (RCS) that varies with polarization and, therefore, varying returns levels for various radar polarizations [1]. If the system detects a greatly reduced return from desired targets for various polarization orientations this should be incorporated into the decision for the "optimal" polarization orientation of this radar system at that instance in time. Essentially the "optimal" polarization orientation should be a joint decision between having larger return levels from desired realistic targets and having diminished return levels from jamming and clutter.

III. ADAPTIVE POLARIZATION SYSTEM EXAMPLES
To show how this system is useful, a possible radar scenario with unwanted clutter is shown in Fig. 2. The radar transceiver transmits a vertically polarized wave. This wave hits the intended target, which in this example is an airplane being tracked for commercial or military purposes. The wave also hits an unintended target such as a vertically oriented building, group of trees, mountain, radio tower, etc. The radar return is shown in the graph at the top right and shows that there are sufficiently strong returns from the intended target, but even stronger returns from the unwanted clutter. This example shows the object causing clutter is vertically large and relatively smaller in the horizontal direction. This indicates that it will return a larger signal for a vertically polarized radar and a smaller signal for a horizontally polarized radar.  Fig. 3 shows what would happen if the radar transceiver rotated its polarization to horizontal. The return from the clutter is decreased as its size in the horizontal direction is smaller than that of the vertical direction. Also, the return from the intended target increases as the airplane is larger in the horizontal direction than it is in the vertical direction. The unwanted return is decreased by altering the antenna's polarization. This same example scenario is also applicable for a scenario where the unwanted energy is from a jamming transmission. This scenario shows how altering the radar transceiver's polarization can reduce the impact of clutter or jamming. The advantages of having a radar system that can dynamically change its polarization become even more apparent with a jammer example. For a monostatic radar system, the received power from a given target return can be found using (1) [3,4].
is the radar's transmit amplifier output power, is the radar's antenna gain, σ is the RCS of the reflecting object, λ is the wavelength of the radiated wave, and R is the distance between the radar and the reflecting object. Th not account for atmospheric losses, but is s example.  (1) indicates the power receive reflecting the radar transmission. However, th receive unwanted energy from a jamming transmitting energy which interferes with the receive. The amount of power that the radar unwanted jammer is found in (2) [3,4]. jammer's transmit amplifier output power, the radar's antenna, is the gain of the jam is the wavelength of the radiated wave, between the jammer and the radar.  (1) and (2) do not take into account mismatch loss which this proposed radar a advantage of. Equation (1) does not need mismatch term since the radar is using the s transmit and receive (i.e. the antenna can differently than itself). Equation (2) requ mismatch to be incorporated as a loss into the accounts for any disorientation between the ra the jammer antenna. This polarization mismat a loss in the power received from the jam jammer antenna and radar antenna possibly polarization orientations (e.g. horizontal and v (3) is the resulting jammer power equation antenna polarization mismatch loss term shows how antenna polarization mismatch loss linearly polarized antennas where θ is the between the radar and jammer antennas [5].
mismatch loss. In y polarized and eceive antenna is signal with no antenna is again polarized wave. now oriented as ctive antenna size hough the receive antenna in Fig. 4B is the same siz Fig. 4A, its receive gain is reduc effective height in the vertical plan cos(θ).  This new system takes advantage of the term in (5). The radar system can alter its polarization to purposefully find the polarization mismatch angle between the radar and jammer antennas (or clutter) that minimizes the power received from the clutter or jammer (J) by maximizing the polarization mismatch loss ( ). Fig. 6 shows values of J and S for various polarization mismatch angles (θ). The solid line shows the received power from the desired object which for this example remains relatively constant. The dashed line shows the received power from the jammer which is greatly reduced for certain angles (θ). The plot in Fig. 6 results from an example scenario where the values in (1) and (3)   Real antennas have a limited polarization loss when orthogonal to the wave polarization. The orthogonal polarization mismatch loss is known as co-pol to cross-pol ratio (co/x-pol) and is usually at most 15-20dB for linearly polarized antennas. Fig. 6 represents a system with an ideal (infinite) co/x-pol ratio. Fig. 7 shows the resulting J/S ratios for the jamming and target return signal levels from Fig. 6 as well as two more realistic polarization mismatch loss models which cut-off at 15dB and 20dB co/x-pol ratios respectively.

IV. CONCLUSION AND FUTURE WORK
This work presents a novel radar antenna system which dynamically changes polarization in the presence of clutter and jamming. The methods described use a sense, observe, decide, and react decision process to optimize the antenna polarization state. The optimal antenna polarization state is one that minimizes the received power levels from polarized clutter and jamming signals. Future work could be performed to develop a functional unit which demonstrates the decision process and control of an antenna's polarization in the presence of polarized clutter or jamming.