A plasma sheath enveloping a reentry vehicle would affect performances of on-board antenna greatly, especially the navigation antennas. This paper studies the effects of reentry plasma sheath on a GPS right-hand circularly polarized (RHCP) patch antenna polarization property during a typical reentry process. Utilizing the algorithm of finite integration technique, the polarization characteristic of a GPS antenna coated by a plasma sheath is obtained. Results show that the GPS RHCP patch antenna radiation pattern distortions as well as polarization deteriorations exist during the entire reentry process, and the worst polarization mismatch loss between a GPS antenna and RHCP GPS signal is nearly 3 dB. This paper also indicates that measures should be taken to alleviate the plasma sheath for maintaining the GPS communication during the reentry process.
A reentry vehicle reentering the Earth’s atmosphere at a high Mach number speed is enveloped by a plasma sheath due to the shock wave heating of surrounding air and the ablation of heat shield materials, causing the ionizing of air molecules and heat shield materials [
The characteristic of on-board antennas covered by a reentry plasma sheath has been studied by several authors for several decades. Swift et al. [
It is believed that the plasma sheath affects not only antenna admittance characteristics and radiation power pattern but also the polarization property. Although, the radiation property and impedance characteristic of plasma-covered antenna have been studied well. However, the deteriorations of antenna polarization property have not been payed much attention to and few works of literature have concentrated on the changes of GPS RHCP patch antenna polarization property in the reentry plasma environment. Consequently, analysing the effects of reentry plasma sheath on GPS patch antenna polarization property is also significant to the reentry blackout research and on-board electromagnetic systems design.
In this paper, a practical inhomogeneous plasma sheath profiles data from NASA’s report and a dynamic stratified medium modeling method according to electron density profiles are adopted. The RHCP gain, polarization property, and polarization mismatch loss of a GPS RHCP microstrip patch antenna covered by a plasma sheath at different altitudes during a typical reentry process are analyzed utilizing the algorithm of finite integration technique.
Generally, a plasma sheath consists of equal numbers of positive ions and free electrons together with a large number of neutral particles. It is well known that the plasma can be characterised by two parameters which are electron density and collision frequency. The natural oscillation frequency of free electrons is the plasma frequency, and the frequency of free electrons colliding with ions and neutral particles is called the collision frequency. The relation between electron density
In practice, the reentry plasma sheath is inhomogeneous and varies with different altitudes. As indicated in the technical note [
Electron density profiles at different altitudes for RAM-C vehicles (reprinted courtesy of NASA).
The collision frequency distribution of reentry plasma sheath perpendicular to vehicle surface is believed to be approximately uniform [
Collision frequencies in different typical altitudes (
Altitude/kft | 70 | 82 | 100 | 156 |
Collision frequency/GHz | 23.00 | 13.18 | 5.71 | 0.42 |
Altitude/kft | 175 | 202 | 233 | 250 |
Collision frequency/MHz | 175.0 | 49.92 | 11.82 | 5.37 |
The reentry plasma sheath is reflective, refractive, lossy, and frequency-dispersive medium. Thus, accurate analysis of the interaction of reentry plasma sheath with patch antenna is quite complicated. However, by some simplification, the inhomogeneous reentry plasma sheath can be modeled approximately by several adjacent homogeneous thin plasma slabs according to the electron density distribution profile. The degree of accuracy of this modeling method depends upon the number of slabs chosen to approximate the actual electron density and collision frequency distributions. To achieve excellent simulation accuracy and prevent the simulation mesh from being superabundance, the electron density discrepancy between the adjacent thin plasma slabs is limited to be less than 10%. As a result, in the region where the gradient of electron density distribution is large, the plasma slab width is thinner than the plasma slab width in the region where the gradient of electron density distribution is small.
Accordingly, the electromagnetic characteristic of reentry inhomogeneous plasma sheath can be established by the plasma frequency
In which
A schematic diagram of the stratified model which divided an electron density distribution profile of 250 kft into 40 layers is shown in Figure
Inhomogeneous plasma sheath stratified models.
An RHCP square patch antenna [
Plasma-covered antenna simulation models.
The finite integration technique (FIT) was proposed in 1977 by Thomas Weiland and was elaborated in the literature [
Here the finite integration time domain algorithm is used to calculate electromagnetic characteristic of an RHCP patch antenna covered by reentry plasma sheath, and the results will be discussed in the following section.
The RHCP gain, polarization property, and polarization mismatch loss of a RHCP GPS microstrip patch antenna covered by plasma sheath at different altitudes during a typical reentry process are given below.
As the coordinate system for the antenna simulation is shown in Figure
RHCP gain (plane of phi = 0 deg) at different altitudes.
RHCP gain (plane of phi = 90 deg) at different altitudes.
Figure
Figure
Figures
Since reentry plasma sheath affects GPS patch antenna-realized gain pattern greatly, the antenna polarization property deteriorations need to be studied as well. When GPS antenna elevation is lower than 60 degrees, the signal transmitted by GPS satellites could not be received efficiently; thus, only the polarization property of antenna elevation less than 60 degrees is studied in this paper. Figures
Axial ratio (plane of phi = 0 deg) at different altitudes.
Axial ratio (plane of phi = 90 deg) at different altitudes.
It can be seen that GPS patch antenna polarization property deteriorates a little only at the altitude of 70 kft; however, at the other altitudes especially 175 kft, and 156 kft the antenna polarization property deteriorates immensely. Consequently, the reentry plasma sheath causes the depolarization effect on the GPS antenna receiving performance, and then polarization mismatch loss may occur between the GPS signal and GPS antenna. Additionally to the degradations of realized gain, polarization mismatch losses reduce the antenna receiving ability further more. Therefore, polarization mismatch losses caused by the reentry plasma sheath should be studied in the following section.
Assuming that an RHCP GPS signal is transmitted to this plasma-covered GPS patch antenna, it may cause polarization mismatch loss between the receiving antenna and the RHCP GPS signal due to GPS patch antenna polarization property deteriorations. Figures
Polarization mismatch loss (plane of phi = 0 deg) at different altitudes.
Polarization mismatch loss (plane of phi = 90 deg) at different altitudes.
Noticing that the polarization mismatch loss is below 0.5 dB at the altitudes of 70 kft, for the plasma sheath is not severe enough, measures should be taken to alleviate the plasma sheath for maintaining the GPS communication during the reentry process.
This paper utilizes a dynamic stratified modeling method to establish electromagnetic simulation models of reentry inhomogeneous plasma sheath and then studies the effects of the reentry plasma sheath on a GPS RHCP patch antenna polarization property. The electron density distribution and collision frequency data got from the NASA report are used to assess the performance of the GPS patch antenna; as a result, the process of RAM-C GPS reentry blackout can be reproduced basically from the perspective of GPS patch antenna performance degradation. Moreover, in addition to the degradations of antenna-realized gain pattern, antenna polarization property deteriorations and polarization mismatch losses are also important in the reentry vehicle GPS communication system designing. Consequently, it should attract much more attention to antenna polarization property in the reentry vehicle electromagnetic system design. Finally, the results indicate that it should take measures to alleviate the plasma sheath for maintaining the GPS communication during the reentry process; meanwhile, in order to calculate the on-board antenna radiation property affecting the entire reentry vehicle and overall plasma sheath accurately, an electrically large simulation technique should be proposed in the future.
This work has been supported by the Fundamental Research Funds for the Central Universities, 72125087.