An equipment for measuring partial reflections using the FM-CW-radar principle at 3.18 MHz, installed at the Ionospheric Observatory Juliusruh of the CISTP (HHI), is described. The linear FM-chirp of 325 kHz bandwidth is Gaussian-weighted in amplitude and gives a height resolution of 1.5 km (chirp length is 0.6 sec). Preliminary results are presented for the first observation period in winter 1982/83

Priese, J.

Singer, W.

English

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° 4,
I 35 -2045 7 39
HEASUREHENTSor PARTIAL REFLECTIONS A." 3.18 }filZ
USING TIlE CW-RADAR TECHIIIQUE
J. Prie'seand W. Singer
' Academy of Sciences of the CDR
Central Institute of Solar-Terrestrial Physics
(Heinrich Hertz Institute)
DDR-1199 Berlin, GDR
An equipment for measuring partial reflections using the FH-C_-radar
principle at 3.18 _lz, recently installed at the Ionospheric Observatory
Juliusruh of the CISTP (HHI), is described. The linear FH-chirp of 325 kHz
bandwidth is Gaussinn-weighted in amplitude and gives a height resolution of 1.5
km (chirp length is 0.6 see). Preliminary results are presented for the first
observation period in winter 1982183.
INTRODUCTION
A partial reflection experiment ras put into operation in December 1982 at
Juliusruh (geographic coordinate 54.63"H, 13.38"£, L = 2.62) and is firstly
_ applied to study different D-region parameters such as electron density, height
of reflecting layers, and fading periods. This method gives an important
enlarge=eat of the well-known groundbased methods such _s Al/A3-absorptlon
measuronents, indirect phase-height measurements and meteor-wind observations,
which are being used in our Institute for diagnostics and _onitoring of the
mesosphere mad lower thermosphere.
Ii,STRUI_NTATION
The f_ctioual scheme of the transceiver equip_,entis given in Figurr I.
The chirp signal is generated by an ultra-linear VCO, the frequency ripples of
which being smaller than 300 Hz over the whole sweep width (further parameters
are listed in Figure 2). High llnearity is obtained by matched forming of the
controlling voltage following a power series of third order in ti=e (KALASS,
1977; KALASS et at., 1981). -
The envelope of the signal amplitude is frequency weighted by Gausslan
baud-pass-filters in both directions of transmitting and of reveiving; the 3
dB-band widths being 66.4 kHz. The Gaussian filters designed by TIH_L (1979,
::: 1980) approximate the ideal Gauss-envelope with ripples smaller 0.2 dB down to
-70 dB (the ends of he truncated chirp). The filters are realized by band-
passes of 14th order.
The amplified signal is transmitted by an array consisting of 4 x 4
horizontal orthogonal pairs of _alf-wave dipoles installed approximately 18
above the dissipative ground (n_ - 30 + j 170); the dipoles of each pair being
crossed with a distance of I m. A gain of 16.5 dB (for one polarization)
referred to the isotropic radiator has beta obtained by usins the co=puled
values of the attached Sonnerfeld-problem (PRIESE and SCH)_EIDE_IEINZE, 1983)
together with the measured current distribution as well as the resulting
radiation resistance of each dipole (PRIESE, 1980, 1981). The beam-widths (27"
in the E-plane, 28.5° in l{-plane)have been measvred by _eans of a helicopter
(PRIESE, 1981). The orthogonal arrays are e.:citedin phase quadrature and
opposite polarization are obtained be reversing one of the feed systems.
The receiving array is separated about 4 wavelengths from the transmitting
antenna and consists of 2 x 2 ortho_otml pairs of resonant loops. The sum
signal will be amplified by 40 dB in an amplifier of low intermodulation
https://ntrs.nasa.gov/search.jsp?R=19850012147 2020-03-22T18:38:40+00:00Z
,:o ORK)_.'LLP#,,C.:,.j,
OF POOR QUALIT'_
++++
++++ (D (I)
Jr+++
Jr+i+ (D (I)
' LJ,__,,,.
_.Jup _tt II
• : ,e,,.z,
Figure I. Functional scheme of the transceiver equiv_ent.
Figure 2. Signal diagr_n.
f
I
i
41
distort_ (for the first observation period a half-wave dipole could be used
only), n_ter mixing of the undelayed with the reflected and therefore delayed
sigrml, the audio frequency components of the mixer spectrum contains the
information of height and amplitude of the parti&l reflection. Following the
introduction to chirp theory by KLAUDER et al. (1960) it is well known that an
unweighted chirp-process results in an audio frequency spectrum with range-
sidelobes of the sin x/x-type. A sidelobe-compressiot_ is absolutely necessary
in order to avoid confusion of the spectra of the partial reflection levels
' among one another as well as to suppress the sidelobe spectra of the strong E-
layer reflection and the mutual coupling between the antennas respectively.
Following a first idea of KLAUDER et al. (1960), Keiser introduced
Gaussian weighting as a rigorous method to reduce the sidelobes, and prepared
the basic parameters of chirp, filters and transceiver (see reports by BREMER et
al., 1973; KALASS etal., 1981). Furthermore, the chirp process results in the
important compression gain of the signal compared with a constant-frequenCy
pulse of the same envelope. Power gain N and delay resolution _ are given for
an infinite chirp by
N = _/m • AT, A_ : _ • m/K 2.
Based on the desired values of _ = I0 _s (corresponding to a height resolution
of 1.5 km) and a compression gain of N = 43 dB Lhe signal processing parameters
shown in Figure 2 have been derived. Additional efforts have been made to study
the effects of sweep truncation as well as af Ehase and amplitude distortions of
the chirp (KALASS et al., 1981).
Last not least, the _udio frequency spectrum of a detached partial
reflection level is obtained (after some omissions) in the form
T 2} _ [f____ 10:Vs]2}Aout(f) _ exp{- _ [NA_f - _] = exp{- 5.4 Hz
The 3.4 dB width of this spectrum is about 5.4 Hz, so the Fourier-analyzer must
have the same frequency resolution and is hardware realized by active low-pass
filters of 2.7 Hz bandwidth (KALASS, 1977) following the outputs of the
quadrature demod,Llators. The spectrum of each sweep is analyzed for 30 height
channels from 47.5 l_n to 91 km with a stepsize of 1.5 km. The A/D converter has
a resolution of II bits. For each sweep the mnplitude height profile is
recorded on magnetic tape. The data reduction will later be done on an all-
" " " purpose KRS 4200 computer. ""
Yhe equipment has two different modes of operation. In the continuous mode
sweeps are transmitted with a repetition frequency of 1.25 Hz, in the inter-
mitt_nt modt I0 consecutive sweeps are emitted at the beginning of each minute.
7n both modes it is possible to observe either with a fixed polarization
(ordinary or extraordinary) or with polarization switching.
PRELIMINARY RESULTS
Data have been evaluated for days with high radio wave absorption (winter
anomaly) and low absorption in Dec_ber 1982 and January 1983 to investigate the
variability of electron density in winter. For winter anomaly conditions the
data of December 24, 1982 and January 12, 1983 are presented. On these days,
A3-abso_ption values on 3 measuring paths exceeded the monthly median value by
about 4 to 8 dB and Al-absorption values in the HF-range (2 ?Oiz)by about 6 dB.
As example for conditions of low absorption the data of December 27, 1982 and
January 21, 1983 are used, when LF-absorption was about 5 dB and MF-absorption
about 3 dB below the monthly median (all data for solar zenith angle X = 78.5°).
42
I.
In Figure 3, time averaged height profiles (median values) of the
a_plitudes of the ordinary (o)- and extraordinary (x)-_ode are shetn, for the
days mentioned above. The height profiles are smoothed by running averages over
3 successive values. For low absorption the A×-height profiles exceed the
noise level above 74.5 k_ and attain a maximum at about 80 km (on Jar,. 21 the
noise level was enhanced by two times). For winter anomaly conditio.nn, signals
, exceeded the noise level already at lower heights (on Dec. 24 at 70 kin; on Jan.
12 at 67 _). In the height profiles ,naxi_a occur on Dec. 24 at 79 kin and on
Jan. 12 at 76 k_. By analysis of the autocorrelation functions for continuous
measurements with fixed polarizations, the half-zmplitude width:- of the auto-
correlation functions were found of the order of seconds, increasing with
decreasing heights.
2t Jan 198J \x
2: L_c
70 _ U _.A. ',_a-,.,,ur
90
O0
12Jan198.;
•1._,4 i "
20 50 Ioo BOO 500
REL4TIYE ..fE.ILE
Figure 3. Averaged amplitude profiles of the
extraordinary and ordinary component (h'A:
day with winter anomaly in absorption).
The basic theory for the differential-absorption experiment was given by
CARDNERand PAgSEY (1953), modified by BELROSE and BURKE (1964), assuming that
observations of weak echoes of high frequency radio waves scattered from the
lover ionosphere are caused by Fresnel reflection from discontinuities in the
refractive index. Following :here papers, the height distribution of electron
density has been derived from the observed ratio A /Ao of the bnckacattered
_=aplltudesof the extraordinary and ordinary magne[o-_onic components using the
43
improved quasi-longltudinal approximation to the exact Sen-l_yllerexpression of
the refractive index after FLOOD (1980), The hgight profile of r_onoenergetic
collision frequency is asst_ed as vm - 7.5 x I0 J x p, the pressure d_ta being
taken from CIRA 72 includin_ seasonal variations.
In Figure 4 the derived electron density profiles (solar zenith an!,.les
. about 78") are shown for the data of Figure 3. Noise correction has _et_
performed for the _plitude data by using the noise in the height rant.e55 to 60
km_ where no ionospheric si}',nalsare expected. At the ,Jppcrend of the derived
profiles, the electron density values are uncertain by a factor of about 1.5 due
to the low signal-to-noise ratio o_ the extraordinary component.
• _._ • X,,"$?*
' " --CgJJ-,'OICUT 3_J::_o,_
It J8 - It 1I UT 1553_'P_'_
ZtJe.nt9_3 .
.... CJ44 - /OZSLrr l_YJ.tx"O/tsi z I , *
So _.,_. _*'_,\
;_ _ _. _8.7" ..1...- /
70 ---- r,._l ,_ - lt ZS UT J,"*J$_',_ov_.r
_0 "\
........ • .... * ......
80
/ --- _.I_I- IO01ur 6_.r_-_'es
• " "" _'0 .... 113_-_213tJ_ 803_.,r_'vez
i l _ ,I i i ' *
£L[CTRON DENSITY, _N "J
Figure 4. Electron-density profiles for winter
conditions using the a_plitude data of Figure
3 (NA: day w_th winter anomaly).
A comparison between the electron density profiles evaluated tot hi8h
absorption conditions vith the profiles der£_ed [or Io_ absorptio_ sl_s a
marked increase in electron density between ?6 and 85 km in the presence of
winter anomaly.
CONCLUDING RE_IA_KS
A few examples of preliminary results have been presented, obtained by a
new partial reflection equir_cnt using the _-radar principle durins winter
4.
44
1982/83. For the four days evaluated here it was possible to derive electron
density profiles in the height range 72 to 90 k_. Under rioter anomaly
conditions the electron density profiles show a _rked increase between 76 and
85 k_ by a factor of about 2 to 6 compared to days without winter anomaly.
To check the reliability of the derived profiles it will be of interest to
compare the observed sbsorptien values in the LF- and }iF-rangewith the
theoretical calculated absorption values [or these electron density profiles.
Before the forthcomin_ s_er observatLon period the prov_siottal receiving
antenna v_ll be replaced by the designed receiving array. In future it ia
planned to i_ple_ent a partial reflection drift equipment.
REFERENCES
Belrose, J. g., H. J. Burke (1964), J. Geephys. Reg., 6q, 2799.
Bremer, J., d. Keiser, J. Priese, W. ginger, d. Taubenhcim and E. Weiss (1973),
Unpublished report, Central Institute of Solar-Terrestrial Physics, Berlin-
Adlershof. GDR.
Flood, W. A. (1980), Radio gci., 15, 797.
Gardner, F. P, and J. F. Pavaey (1953), J. Atmos. Terr. Phys., 3, 321.
Kalass, P. (1977)j Thesis_ liochschulefur Verkehrswesen "Friedrich Lilt',
Dresden (unpublished).
Kalass, D., J. Keiser, A. Himalski, J. Prieae, N. Singer, J.-G. Stsngenberg,
II.Tinpl and N. Vietzke (1981), Unpublished report, Central Institute of
Solar-Terrestrial Physics, Berlin-Adler_hof, GDR.
Klaudet, J. R., A. C. Price, S. Darlington and W. J. Albersheim (1960), Bell
gyst_._Technical Journal, 39, 745.
Priese. J. (19S0). GDR-Patcnt NP HOI Q/198373.
_f-'_ Priese, J. (1981), Nachrichtentechnik/Elektronik, 31, 372. •
Prlese, J. and G. Schneiderheinze (1983), IIIII-STPReport (in press), Central
Institute of golar-Terrestrinl Physics, Berlin-Adlershof, GDR.
Ti_npl. I1. (1979), Nachrichtentechnik/Elektronlk, 29, 188, 283, 458.
Timpl, II.(1980), GDR-Patent NP I103I1/206263.
) ®


Measurements of Partial Reflections at 3.18 Mhz Using the CW Radar Technique

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