Measurements and simulation of ionospheric scattering on VHF and UHF radar signals: Coherence times, coherence bandwidths, and S₄

Abstract

[1] Irregularities in the electron density of the ionosphere cause phase and amplitude scintillation on transionospheric VHF and UHF radar signals, particularly at lower radio frequencies. The design of radar and other transionospheric systems requires good estimates of the coherence bandwidth (CB) and coherence time (CT) imposed by a turbulent ionosphere. CB and CT measurements of the equatorial ionosphere, made using the Advanced Research Project Agency Long-range Tracking and Identification Radar 158 MHz and 422 MHz phase coherent radar located on Kwajalein (9.4°N, 167.5°E), are presented as a function of the two-way S 4 scintillation index at 422 MHz The log linear regression equations are CT = 1.46 exp(À1.40 S 4 ) s at 158 MHz and CT = 2.31 exp(À1.10 S 4 ) s at 422 MHz. CT also varies by a factor of 2-3 depending on the effective scan velocity through the ionosphere, v eff . The CT and CB, as a function of S 4 , have been compared to those from the Trans-Ionospheric Radio Propagation Simulator, a phase screen model. A close agreement is achieved using appropriate values of v eff and midrange values of phase spectral index and outer scale. Validation of CB is, however, limited by insufficient radar chirp bandwidth. Formulating the model in terms of the two-way S 4 index (an easily measurable parameter) rather than more fundamental phase screen parameters (which are difficult to obtain), improves its utility for the systems engineer. The frequency dependencies (spectral indices) of S 4 and of CT are also presented to allow interpolation and some extrapolation of these results to other frequencies.

Cubic Phase Distortion and Irregular Degradation on SAR Imaging Due to the Ionosphere

“…The coherence of signals can be seriously affected by scintillation [22], [23], which can reduce the resolution of SAR imaging. Previous papers about its effects on SAR imaging are studied in detail; an analytical study is proposed by Ishimaru et al [7] and also studied by others [24]- [28].…”

Dual-tone radio interferometric positioning systems for multi-target localization using a single mobile anchor

“…We assume three active targets (I=3 and i=0,1,2), and the basic frequency of each dual-tone signal is f 0 =1 MHz, f 1 =1.1 MHz, f 2 =1.2 MHz. The frequency difference g b =50 kHz, which is smaller than the typical channel coherence bandwidth at the VHF band [22]. The directly sampling frequency f s =3 MHz.…”

V/UHF space radars: Spatial phase decorrelation of transionospheric signals in the equatorial region

“…As soon as more measurements are collected in different configurations, the dependence of this relation with location, hour, season, and sunspot number can be checked. Cross‐evaluation against phase screen model simulations [ Rogers et al , 2009a, 2009b] or theoretical calculations [ Rino , 1979] will also likely provide further insights into this relationship. This is likely to be the subject of a later paper.…”

“…In a previous study it was found that in equatorial regions, decorrelation of a SAR due to ionospheric scintillation is likely to occur for ∼40% of the time in the evening hours between 1800 and 2400 LT at sunspot maximum [ van de Kamp et al , 2009]. In other recent studies, measurements using the ALTAIR radar on Kwajalein Atoll and associated modeling using the Trans‐Ionospheric Radio Propagation Simulator (TIRPS) have quantified the coherency bandwidth and coherency time of VHF and UHF signals propagating through the equatorial ionosphere [ Cannon et al , 2006; Rogers et al , 2009a, 2009b].…”