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[PEM13-P07] Observation of Doppler frequency shift and time of flight of a temporally modulated HF wave propagating through ionosphere
Keywords:Doppler frequency shift, time of flight, temporally modulated HF wave, continuous Doppler sounding, cross-correlation function, maximum likelihood method
Ionosphere perturbed by vibrating atmosphere is being observed by continuous Doppler sounding (CDS) of reflective HF waves. Usually used for CDS is an HF wave of a single frequency, though it does not serve for time of flight (ToF) observation in principle. Actually, ionosonde estimating reflection height of a sounding wave is incorporated with CDS for three-dimensional analysis of gravity wave propagation in ionosphere [1]. A temporally modulated HF wave does serve for observation of both Doppler frequency shift (DFS) and ToF as shown below by an ionosonde pulse propagating through ionosphere.
A model of a pulse emitted by an ionosonde and a set of its replicas of shifted frequencies are used to generate a model cross-correlation function of shifted frequency and ToF. An intensity of a cross-correlation is shown in Fig. 1 for a pulse of a pulse-width 80μsec and center frequency of 10MHz. When a cross-correlation function of those replicas and received samples of an ionosonde pulse is well approximated by a model cross-correlation function, DFS and ToF of the received pulse is well estimated by the maximum likelihood method.
A set of samples of a pulse train emitted by the Kokubunji ionosonde operated by the National Institute of Information and Communications Technology are received in Amagasaki about 380km away from the emitter. Cross-correlations of some pulses in the train and their replicas of zero frequency shift are plotted as a function of the emission frequency and ToF of each pulse in Fig. 2. This graph is equivalent to an ionogram. Intensity of a cross-correlation of a pulse of center frequency 8.12MHz, which is slightly smaller than observed maximum usable frequency 8.14MHz, and its replicas is shown in Fig. 3 as a function of DFS and ToF in the limited range denoted by the black box in the Fig. 2. It is interpretated that the received signal mainly consists of high-angle (Fernstrahlung) propagation and low-angle (Nahstrahlung) propagation in the F-region. Fernstrahlung propagation is well shown to have larger DFS than Nahstrahlung propagation with assist of their separation in ToF.
The model cross-correlation of the model pulse and its replicas of shifted frequencies is subject to restricted resolution defined by wave form of the model as shown in Fig. 1. Maximum likelihood estimation of DFS and ToF is subject to another restriction due to signal to noise ratios of received signals. Actually, some structures such as observed in a sporadic-E layer (Fig. 4) are out of these restrictions. Within these restrictions a temporally modulated HF wave does serve for observation of both DFS and ToF. Our strong interest is attracted by call marks in Morse code emitted by CDS emitters in Taiwan [2]. Their cross-correlations are expected to serve for analysis of ionosphere that they propagate through.
References:
[1] J. Chum and K. Podolská, “3D Analysis of GW Propagation in the Ionosphere,” Geophys. Res. Lett., vol. 45, 2018.
[2] “Multipoint Continuous Doppler sounding system,” Basic description of the system developed at the Institute of Atmospheric Physics, Academy of Sciences, Czech Republic.
Figure captions:
Fig. 1: An intensity plot of a model cross-correlation of a model pulse and its replicas of shifted frequencies against frequency shift (horizontal axis) and time lag (vertical axis).
Fig. 2: An intensity plot of cross-correlations of each pulse of emission frequency (horizontal axis) and its replica of time lag after its emission (vertical axis).
Fig. 3: An intensity plot of cross-correlations of received signal and model replicas of frequency shift (horizontal axis) and ToF (vertical axis).
Fig. 4: An intensity plot of cross-correlations of received signal and model replicas of frequency shift (horizontal axis) and ToF (vertical axis).
A model of a pulse emitted by an ionosonde and a set of its replicas of shifted frequencies are used to generate a model cross-correlation function of shifted frequency and ToF. An intensity of a cross-correlation is shown in Fig. 1 for a pulse of a pulse-width 80μsec and center frequency of 10MHz. When a cross-correlation function of those replicas and received samples of an ionosonde pulse is well approximated by a model cross-correlation function, DFS and ToF of the received pulse is well estimated by the maximum likelihood method.
A set of samples of a pulse train emitted by the Kokubunji ionosonde operated by the National Institute of Information and Communications Technology are received in Amagasaki about 380km away from the emitter. Cross-correlations of some pulses in the train and their replicas of zero frequency shift are plotted as a function of the emission frequency and ToF of each pulse in Fig. 2. This graph is equivalent to an ionogram. Intensity of a cross-correlation of a pulse of center frequency 8.12MHz, which is slightly smaller than observed maximum usable frequency 8.14MHz, and its replicas is shown in Fig. 3 as a function of DFS and ToF in the limited range denoted by the black box in the Fig. 2. It is interpretated that the received signal mainly consists of high-angle (Fernstrahlung) propagation and low-angle (Nahstrahlung) propagation in the F-region. Fernstrahlung propagation is well shown to have larger DFS than Nahstrahlung propagation with assist of their separation in ToF.
The model cross-correlation of the model pulse and its replicas of shifted frequencies is subject to restricted resolution defined by wave form of the model as shown in Fig. 1. Maximum likelihood estimation of DFS and ToF is subject to another restriction due to signal to noise ratios of received signals. Actually, some structures such as observed in a sporadic-E layer (Fig. 4) are out of these restrictions. Within these restrictions a temporally modulated HF wave does serve for observation of both DFS and ToF. Our strong interest is attracted by call marks in Morse code emitted by CDS emitters in Taiwan [2]. Their cross-correlations are expected to serve for analysis of ionosphere that they propagate through.
References:
[1] J. Chum and K. Podolská, “3D Analysis of GW Propagation in the Ionosphere,” Geophys. Res. Lett., vol. 45, 2018.
[2] “Multipoint Continuous Doppler sounding system,” Basic description of the system developed at the Institute of Atmospheric Physics, Academy of Sciences, Czech Republic.
Figure captions:
Fig. 1: An intensity plot of a model cross-correlation of a model pulse and its replicas of shifted frequencies against frequency shift (horizontal axis) and time lag (vertical axis).
Fig. 2: An intensity plot of cross-correlations of each pulse of emission frequency (horizontal axis) and its replica of time lag after its emission (vertical axis).
Fig. 3: An intensity plot of cross-correlations of received signal and model replicas of frequency shift (horizontal axis) and ToF (vertical axis).
Fig. 4: An intensity plot of cross-correlations of received signal and model replicas of frequency shift (horizontal axis) and ToF (vertical axis).