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Gen-system—a new experimental philosophy for EISCAT radars
Jourml Printed
of Afmospherrc and Terrestrial in Great Britain.
Physics, Vol. 48, Nos 9-10, pp. 777-785, 1986.
GEN-SYSTEM-a
OfX-9169/8653.00+ Pergamon Journals
.I0 Ltd.
new experimental philosophy for EISCAT radars TAUNO TUPUNEN
EISCAT Scientific Association S-981 27 Kiruna, Sweden (Receivedfor publication 29 April 1986)
Abstract-GEN-SYSTEM
is a code name for a new experimental design philosophy, a set of related correlator algorithms and an experiment library based on this philosophy. It is designed to obtain an easy way to develop powerful experiments having several different modulations in the same pattern or to have very powerful modulations in single channel experiments, needed sometimes in special applications. The experimental philosophy is discussed and a few examples given on practical experiment design.
1. INTRODUCLLON The code name GEN-SYSTEM arises from the word ‘general’, because it is a system developed especially for general purpose aurora1 zone incoherent scatter experiments. The first incoherent scatter experiments following the GEN philosophy were done in January 1985 with the EISCAT UHF system. The EISCAT facility is described, for example, by BARON(1984), to which the reader is referred for details. The most important design criterion in GEN-SYSTEM development was to obtain methods of running an incoherent scatter radar having parameters such as EISCAT in that mode which maximises information flow from the target as greatly as possible when using standard modulation methods. The output data format, compatibility with existing display and analysis software, etc., were not considered to be important, because if that compatibility is fully maintained the primary design goal cannot be obtained. It was, however, demanded that the system must be realized with the existing hardware. This was a limitation, because certain restrictions in the correlator design prevent some of the most effective modulation methods being used. The details of the GEN-SYSTEM cannot be handled here. The practical solutions are described in a report by TIJFWNEN(1985). Some discussion is given below about the general principles of the system and related experiment design. Although the description is limited here to those applications which are interesting for the EISCAT radars, the principles themselves are general and could be applied in other radar installations as well if the modulation, receiving and data processing systems allow. 2. THE MEASURINGPROBLEM In a general purpose aurora1 experiment a reasonable spatial and time domain response and measuring
accuracy must be obtained from about 70 km to about 500 km. The upper part of the target (above 200 km) can be measured by using long pulses (150-400 ps) in the most effective way and, as long as the obtained spatial resolution is acceptable, no other modulations exist which could compete with a simple long pulse in the statistical accuracy of the target autocorrelation function estimate. Thus the long pulse is naturally one of the needed modulations. If long pulses take quite a considerable part of the used RF duty cycle, then this modulation can be used to form the remote station part of the target illumination too, and no RF duty cycle is needed for remote station purposes only. By repeating the long pulse 2-4 times in a cycle one gives in practice 40-70% of the RF duty cycle to the long pulses, which gives good enough accuracy at the remote sites and allows monostatic measurements at very long ranges. The target cross-section as a function of range is measured by having a ‘power profile’ part in the modulation. The spatial resolution demand in the Dand &layers is, however, so high that the same power profile pulses cannot be easily used in the F-layer, because of signal-to-noise ratio limitations. Thus a good general purpose experiment needs at least two different power profiles with different resolutions. The power profile should be repeated several times, although in practice one has to limit the number of power profile pulses to 2-8 in a modulation cycle. A well designed general purpose experiment usually contains two different power profiles, both of which are repeated 24 times. E-layer autocorrelation function estimates cannot be obtained by using long pulses, because of the spatial resolution demand. The classical possibility is to use pulse codes, but then the signal-to-noise ratio becomes relatively poor. As a consequence, as large a portion of the RF duty cycle as possible has to be given to the pulse coded part in a general purpose
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T. TURUNEN
experiment. In practice this means 4 channels with pulse coded modulations, because this is the highest number of practical codes which can be programmed into the same transmission pattern, When multiplexing 4 codes together, at least 2 different codes must be used and in the simplest case the second code is a mirror image of the first (TURUNENand SILBN, 1984). If the elementary pulses used in the pulse code are further phase coded, then one can multiplex only 2 codes together and they cannot be identical if one uses practical parameters in the experiment design. Thus two sets of pulse codes are usually needed. A general purpose experiment has five different modulations, one set of long pulses, two sets of power profiles and two sets of pulse codes. If one does not need autocorrelation function estimates for the lower part of the ionosphere and spatial resolution is not of vital importance (or beyond practical possibilities when considering the available integration time), one can omit the pulse codes and high resolution power profiles. If one is interested only in the low altitude part, then in some cases one can leave out the long pulse part and low resolution power profiles, although it causes difficulties in obtaining the tristatic Doppler estimate. Eight channels are available in the EISCAT receiving system. Thus with the present hardware one can work using 8 frequencies which can be modulated independently. In some of the frequencies one can transmit more than one modulation in a measuring cycle if the modulations are such that they do not disturb each other. A commonly occurring total cycle is of the order of 12 ms in a heavily modulated general purpose experiment. Remembering that one must be able to calibrate every channel, the following practical rules can be established. (1) Long pulse modulation (150-400 ps) has to be sampled at long ranges too, and the background power estimate demands that the illuminated target is not nearer than about 1000-1500 km. This needs the whole measuring cycle and thus no other modulations can use the same channel. (2) High resolution (10-30 ps) and low resolution (40-80 ps) power profiles can share the same channel if the transmission gap after the low resolution power profile is long and the gap after the high resolution power profile is not too short. For example, if the high resolution power profile is followed by a 4 ms receiving period and the low resolution power profile followed by an 8 ms receiving period, no problems usually arise. The channel can be calibrated at the end of the low resolution power profile receiving period. (3) High resolution power profile and pulse code can be handled by the same channel if the power
profile is transmitted first and the receiving period following the transmission of the pulse coded waveform is long. In practical experiments the spatial response of the power profile and pulse code are programmed to be the same. The power profile does not disturb the pulse code at all, except by a tiny amount at zero lag, which contains self-clutter in any case. However, when transmitting in this mode one may lose the possibility of improving the power estimate by the deconvolution method (LEHTINENand HUUSKONEN, 1986). In the monostatic part of an experiment a single long pulse in a cycle produces a result much more rapidly than any other standard modulation, even if it is repeated in all the remaining 7 channels. However, this is not the case at the remote sites. Especially at Sodankyla, under some conditions the needed integration time for a given accuracy is of the same order of magnitude as the integration time needed in the monostatic pulse coded part. In order to get an accurate tristatic velocity, more than one long pulse should be transmitted if the long pulse parameters have been fitted to the monostatic measuring demands. Usually a good compromi~ is obtained by transmitting two long pulses in a cycle. If phase coding is not used, the modulations in a general purpose experiment are as follows : (1) Two long pulses ; (2) four pulse codes ; (3) two or four high resolution power profiles ; (4) two low resolution power profiles. The choice at point 3 depends on whether one shares the channels with the pulse codes or with low resolution power profiles. The former case leads to ‘IO-block’ modulation and the latter case to ’ IZblock modulation. In every channel one must sample one data vector for the background estimate, one for the noise injection estimate and one or two data vectors for the scatter data. In the monostatic part of the experiment the IO-block modulation produces 26 different data blocks and the 1Zblock modulation produces 28 different data blocks in a cycle. The total length of the input data vector is in practice 2000-6000 complex words. This data has to be handled in real time. In the present EISCAT system the necessary real time computations must be done by having only 16 control registers for the input side, 16 registers for the output side and about 50 free program steps in the processors of the correlator. The algorithms must be fast enough (2.54.8 MHzcomplex multiplication rate is obtained in practice) and they must be flexible enough in order to allow user-oriented experiment
119
GEN-SYSTEM
design. One must have a way to compress the data as much as possible, because the result must fit to the available result memory (which is 2047 complex 32 bits words at present in the EISCAT correlators and 4095 words in the future). The GEN-SYSTEM is developed to give an easy solution to the real time processing problem arising from the experimental reasoning given above. The remote station real time processing problem is simple. 3.
CORRELATORPROGRAMS
special modulations and to a certain extent it is usable in pulse-to-pulse correlation applications. The formula used to compute the UNIPROG matrix elements is E(n, m) = Z(n) * Z*(n + I- LZ),
(3.1)
where E(n, m) is the matrix element on row n and column m = n+/*LZ, Z(i) is an input data vector, i = 1, 2, . , LZ is lag increment in sample space and integer 1 is the lag parameter. The matrix has the only non-zero elements in every LIth diagonal in the upper triangle. The elements E(n, m) contain all the information obtainable in radar by using pulse codes with suitable selection of 1and LZ. If one adds G neighboring gates together, then a new smaller square matrix must be formed and its elements are
The GEN-SYSTEM correlator program package is built on the principle that one has to write an independent main program for every experiment. Different subroutine combinations (called ‘master programs’ in the GEN-SYSTEM) are available and the basic package contains several different master programs and some special programs. All the programs C&m) = 2 Z((k-l)*G+j)*Z*((k-1) ,= I can be written using a symbolic CORLAN language developed for the EISCAT correlators (T~~RUSTAD, *G+.j+(G.Z).LZ/G). (3.2) 1982). The computing subroutines are based on the fol- This matrix has the only non-zero elements in every LZ/G diagonal in the upper triangle and thus LZ/G lowing three basic algorithms. must be an integer. (1) UNIPROG. Although the UNIPROG approach is for most (2) LONGPULSE. applications the best one for pulse coded modulations, (3) REMOTE. some more advanced routines are included in the The UNIPROG is a further development of the GEN-SYSTEM correlator program package to ‘Universal Program’ and it is used for handling the handle pulse codes. These routines produce ready autocorrelation functions for the target volumes in the pulse coded parts of the modulation pattern by computing the ‘UNIPROG matrix’ (for details see, for case of 12 block modulation if the pulse coded part is based on certain 4 pulse codes or 5 pulse codes. They example, TURUNEN and SILBN, 1984). It also takes have not been written in a similar ‘user oriented way care of all the power profile computations. The only real difference compared with the original version is as the standard GEN-SYSTEM master programs, but that the GEN-SYSTEM version can be programmed can handle often only one modulation pattern. Two essentially different algorithms are used, but in printo add identically used channels and neighboring gates together in the result memory. Channel addition is ciple both of them do the UNIPROG matrix decoding in real time using essentially the formulation given simple, because one needs only to add the matrixes in detail by TURUNEN (1985). The benefits of this together in the accumulation process and this can always be done without any limitations. Addition of approach are simple and familiar output format the neighboring gates is done in the GEN-SYSTEM and a very compressed data structure, but, on the UNIPROG by adding together a given number of other hand, part of the information included in the UNIPROG matrix is lost, because one cannot the neighboring elements in the diagonals of the make use of the main diagonal (see LEHTINENand UNIPROG matrix. HUUSKONEN,1986). A necessary condition is that in every computed The GEN-SYSTEM LONGPULSE routine comdiagonal of the original matrix the number of elements putes the monostatic long pulse data with an algodivided by the number of neighboring elements added rithm which essentially differs from the ones used together is an integer. The resulting compressed earlier. It computes such that the arising absolute matrix has all the properties of the UNIPROG matrix, boundaries of the target volume are the same at every i.e. its elements can be used to form an autocorrelation lag. This program can also add different channels function estimate of a target volume when the target together if wanted. The program also has another new is illuminated by a pulse code. The GEN-SYSTEM feature. The spatial resolution can be programmed UNIPROG can also be used in applications utilizing
T. TURUNEN
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within the limits dictated by the modulation. The needed algorithm was mentioned by TURIJNENand SILBN (1984) as an application of the UNIPROG matrix, but at that time it was only possible to do the volume formation in the post processing by having the UNIPROG matrix as a primary datum. In the GEN-SYSTEM this is already done at the correlator level. The formula describing the algorithm is Y+* A(g,i) = 1 Z(M-t(g-1). V-i+k) k= 1 *Z*(M+(g-
1). V+k),
(3.3)
whereA@,i)istheithlag,i=0,1,2 ,..., M,ofthe target ACF estimate for the gth gate, g = 1, 2, . . , 9 max>using data samples ZG), j = 1, 2, . . . , gmoxV+ 244. V is called ‘volume index’ in the GEN-SYSTEM and gives the number of power points added together to form the zero lag estimate. M is the maximum lag to be computed. Gate indexing starts from 1. The peak-to-peak volume size at every gate is approximately S = (T+(V-
l)*t+STEP)*c/2,
(3.4)
where T = pulse length, t = sampling interval, STEP is the step response time of the receiver system and c is the velocity of light. The arising weighting factor WN(i) is for the white noise (background and noise injection) VW(i) = v+i.
(3.5)
The weighting factor WT(z) for the target contribution is in most cases, to a good accuracy, given by WT(i) = (V+Q.(T-i-t).
= (l+i/I’).(l--i-t/T).
s-1 A(j) = 1 Z(nk=l
1 +k)*Z*(n-
1 +k+j),
(3.8)
where A(j) is the autocorrelation function estimate at lagj,j=O, 1,2, . . . . and Z(n) is the first input data point. S is the number of samples. The upper limit for j is programmable. A simplified formula describing the calibration autocorrelation function computation in the algorithm REMOTE is
(3.6)
If this is normalized to unity at zero lag one obtains NWT(i)
The remote station data is handled by a correlator program GEN-REMOTE. This contains an essentially similar long pulse routine with triangular weighting, as used earlier in long pulse computations for the computation of the autocorrelation function estimate when the target is illuminated. For calibration it contains a routine which computes the background and noise injection estimates using identical weight at every lag (boxcar weighting). The main benefit is that one can improve the accuracy of the background estimate, because the receiver system can sample the calibration data almost all the time when the volume is not illuminated and the whole computational power can be used. In spite of this, the output data vector can be compressed into a very compact form and in practice only 100-200 output data points are needed to describe the whole information, independently of the number of channels used in the experiment. There are several different possibilities to do the necessary timing check when using the REMOTE routine. This routine is also suitable for system calibration purposes. The formula used to compute the long pulse autocorrelation function in the beamwidth-limited case (remote station application) is, in a simplified form,
C(j) = $ Z(n-l+k).Z*(n-l-tkfj), li= 1
(3.9)
(3.7)
The obtained raw target ACF after background subtraction must be divided by the given weighting function in order to get a boxcar weighted target ACF estimate. The properties of the GEN-SYSTEM LONGPULSE routine are as follows : (1) The absolute boundaries of the volume are the same at every lag and the major contribution, say 75%, comes from a volume which is not a strong function of lag in practical cases. (2) All the obtainable information in the sampled data is used up to the given maximum lag. (3) The spatial resolution can be programmed (because V is a programmable parameter).
where C(j) is the calibration ACF estimate at lagj and P is a programmable parameter giving the number of products summed together, which is the same at every lag. In the GEN-SYSTEM correlator programs one has thus two different long pulse routines, one for the modulation dependent volume formation and the other for the beamwidth-limited case (these two cases are in the earlier programs handled by the same algorithm which is essentially identical with the beamwidth-limited case). The remote station routine also has a special calibration algorithm producing boxcar weighted autocorrelation functions. The pulse coded part can be computed by using special algorithms producing boxcar weighted auto-
GEN-SYSTEM
correlation functions or by UNIPROG, producing a matrix format. In some cases special experiment-oriented algorithms are needed. One such application arises if one uses pulse-to-pulse correlation and the modulation methods are very complicated. Such an example is given in the next section. 4. GEN-PROGRAMS LIBRARY
The GEN-PROGRAMS library contains ready experiments based on GEN-SYSTEM philosophy. The full library cannot be described here. Three examples are given to show how the GEN-SYSTEM philosophy is used in real experiments. All the experiments described below have also been used in real experiments in 1985. The first example shows the structure used in ‘12block mode’. This kind of experiment has 12 modulations in a cycle divided into three transmitting periods, as shown in Figs. 1 and 2. The first phase consists of a group of four power profile pulses (EPPG) of relatively high resolution. The sampling
covers the D-, E- and lower part of the F-layers. Immediately when the sampling is ready one transmits a group of four pulse codes (MPG) using the same frequencies and same channels. These are sampled to cover the &layer and lower F-layer. The transmitter parameters and sampling must be defined so that the power profile gates and pulse code gates have the same spatial response and part of the power profile gates coincide in range with the pulse code gates. In this situation one can directly use the power profile data as zero lag in the autocorrelation function estimate without any further checking and balancing. The part of the power profiles which are beyond the range covered by the pulse coded modulations are simply used as power profiles. In the third phase one transmits the long pulse group (LPG) for the Player part of the experiment and low resolution power profiles intended mainly for F-layer purposes (FPPG) but which sample the whole altitude range from the D-layer to the upper parts of the F-layer. The LPG also forms the remote station part of the experiment. The sampling and calibration windows can be seen
zi*3 a) EPPG. All pulses 19 ps
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b) MPG, All pulses 19 ps
c) LPG + FPPG. Pulse lengths 350 and 50 ps
Fig. 1. The three modulation phases needed in ‘IZblock’ modulation mode. (a) EPPG is the E-layer power protile group ; (b) MPG is the multipulse group ; (c) LPG is the long pulse group and FPPG is the F-layer (low resolution) power profile group. The LPG also forms also the remote station modulation. In the example the pulses in the MPG are 19 ns and the shortest gap is 21 ps. The lag increment is 40 ns. The frequency (FtkF7) and channel (CHlCH8) selections can be used in a practical experiment.
T. TURUNEN
782
q TRANSMIT [7
RECEIVE
q CALIBRATE
CH1 CH2 CH3 CH4 CH5 CH6 CH7 CHE I
I
I
I
1
2
3
L
III
5 TIME
I
I
I
I
I
II
6
7
8
9
10
It
I
I
I
12
(msl
Fig. 2. Transmitting and receiving in the 1Zblock mode using the elements shown in Fig. 1. The parameters
are essentially taken from a library program GEN-SB.
from Fig. 2, where the timing and parameters are essentially the ones used in the library program GEN8B. Several library programs are written in this way. The multipulse group (MPG) is in the example formed by combining four 4-pulse codes (code names GEN8 and GEN-9), but similar programs exist for a combination of 3-p&e and $-pulse codes (GEN4 and GEN-7) and for a combination of four 5-pulse codes (GEN-2, several versions). Program versions exist which handle the data in the ‘standard GEN-SYSTEM mode’ or in a readily decoded mode if only 4-pulse codes or .5-pulse codes are used. If the ‘standard’ mode is used, then one has to produce two UNIPROG matrixes for the E-layer part. If decoding versions are used one needs to produce only one set of autocorrelation functions, because all the E-layer data can be added together at the correlator level. Inde~ndent of the output format, the data in the library versions is rather heavily oversampled and two or more neighboring gates added together in the correlator. In the simplest versions the output data of the ‘1Zblock mode’ experiment contains low altitude power profiles, E-layer autocorrelation functions, Flayer autoco~elation functions and low resolution power profiies. All these four sets of data have a corresponding calibration part in exactly the same format. If one does not want to transmit in IZblock mode, one can omit the EPG and transmit two power profiles multiplexed into the MPG sharing the channels with the FPPG. This ‘lo-block mode’ always demands a relatively complicated channel balancing procedure,
but it can be programs for a higher duty cycle and produces absolutely clutter-free main diagonals of the UNIPROG matrixes, opening up the possibility for deconvolution methods. On the other hand, it can, under some conditions, become disturbed by enhanced plasma line activity (for further details see TURUNEN,1985). The example shown in Fig. 3 is an arrangement to obtain very long range coverage. One has two transmitter phases which both contain the same low resolution power profile part formed by using four channels and frequencies and two long pulses, which have different channels in the first and second phases. When the first two channels see an illuminated target the other two channels are sampling calibration data. Figure 3 describes the library program GEN-5, which is also the basis of one version of EISCAT common mode program 3 (CP-3-D). The library also contains technically rather similar experiments where the low altitude part uses phase coded puise (GEN-6 and GEN-12) and also special experiments where all the transmitter power is used for the multipulses and high resolution power profiles (e.g. GEN-3). The two examples given above describe experiments which resemble the original EISCAT common mode programs CP-1 and CP-3 from the target sampling and spatial resolution point of view, but the way the radar is used to solve the measuring problem is much more effective. The third example does not have any corresponding earlier program of similar solution. It is the pulse-to-pulse correlation program GEN-11 from the GEN-PROGRAMS library. Because this
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GEN-SYSTEM a
TRANSMIT
q
RECEIVE
fl
CALIBRATE
0
2
CHl CH2 CH3 CHL CH5 CHS CH7 CHB
I4
6
6 TIME
10
12
11
16
16
(ms)
Fig. 3. Transmitting and receiving arrangements in a long range experiment. Long pulses are 350 ps and power profiles 70 /AS.The parameters arc taken from the common mode experiment CP-3-D.
solution differs greatly from earlier approaches to the problem it is handled here in slightly more detail. In D-layer experiments one wants to measure both the target cross-section and its autocorrelation function. The coherence time is long and thus the correlation has to be measured by illuminating the target by pulses transmitted in a few millisecond periods. The data is naturally received immediately after every pulse. In GEN-11 the target is illuminated at 2.222 ms intervals. The spatial resolution demand is very high and therefore GEN-11 uses a 13 bit Barker code with 7 ~LSbaud length which gives 1050 m spatial resolution. The problems in D-layer experiments are as foliows :
(1) The experiment has to give analysable data, even when the signal-to-noise ratio is much below 1% . (2) One has to eliminate the distant mode clutter, which is often much stronger than the D-layer signal. (3) One has to eliminate the receiver recovery effects arising from the operation of protection switches, transmitter pulse leakage to the receiver front end and strong atmospheric clutter from low altitudes. (4) The noise temperature of the receiving system may differ from the usual level after the transmitter pulse. (5) The gain may vary a little due to the shock given by the transmitter pulse. (6) When measuring using a very narrow final bandwidth, the possibility of contamination from mains harmonics is high. All the effects listed above in fact exist and many of
them often give a contribution comparable to the target return. In GEN-11 the clutter is eliminated by changing the modulation after every transmitted pulse and the gain stability problems are largely eliminated by approximating the target power by the real part of the lag at 112 ps delay, where the expectation value for white noise is zero in the experimental arrangements used. The mains harmonics and offset voltage disturbances are, in the main part of the experiment, eliminated by designing the modulation such that half of the target contribution has positive and half of it has negative sign for a given volume and given lag. The modulation pattern needed is shown in Fig. 4. One Barker coded double pulse is transmitted having a variable gap between the pulses and the latter pulse is reversed in phase. For the D-layer target the last transmitted delay is used to measure a point in the autocorrelation function near zero lag. The mean value of the transmitted delay is 112 ps. The previous four transmitted double pulses do not contain the delay of the pulse illuminating the D-layer and this gives the clutter cancellation. The possible tiny compression side lobe leakage in the experiment is calibrated ‘by computing the delay, which is one baud length longer than the one used in the pulse illuminating the D-layer. This is a slightly approximate method, but works well enough for the clutter and also gives s;rstem offset voltage and hum contamination estimates for the 112 ,USdelay. The white noise contribution has zero expectation va:tie and thus possible gain variations operate only
T. TLRUNEN
784 +PULSE
-PULSE
a
TRANSMIT
0
RECEIVE
@ CALIBRATE
0
P2 c
2.222 ms---,
fgl
P3
PL TOTAL CYCLE
P5 Lb2.222
Pl
P2
ms
Fig. 4. A special pulse-to-pulse correlation modulation and transmitting and receiving arrangements. For details see text.
on the signal, not on the sum of signal and noise. Note that the noise level is usually two orders of magnitude
higher than the signal level. In the experiment a double pulse illuminates the target twice. Thus one can have, at a11longer delays, four possible delayed products, which all have, approximately, a delay of 2.222 n ms, n = 1, . . . , 43. Because of the inverted phase in the later pulse, two of these products are positive and two are negative. They have to be integrated separately. The process is free from spatial ambiguities, except in every fifth lag in positive products (the same modulation repeats in every fifth pulse). Summing all the four products together after reversing the negative signs is equivalent to 26 bit compression and in the process possible low frequency disturbances in the receiver system are cancelled. The background power needed in the variance estimates and the noise injection power needed to get the gain must be measured by computing a true zero lag. Because no instants without target signals exist, the noise injection is added into every other receiving period and the power with and without the noise injection are integrated separately. The measuring algorithm GEN-11 has already been successfully used in experiments, and a reasonable accuracy is obtained even from the undistur~d solarproduced D-layer with the EISCAT UHF system in
a few minutes integration time, but the algorithm has been mainly developed for forth~~ng VHF radar applications. The algorithm also has other versions than the one described. The special correlator program written for GEN11 takes care of all the computations needed in the real time processing. Because of the changing modulation pattern, the program is not trivial and it is one of the examples when the standard ‘general purpose’ approach cannot give a solution to the measuring problem.
SUMM~Y The GEN-SYSTEM is developed for a radar having extensive modulation possibilities and several parallel receiving channels which can be independently used at different frequencies. The main design criterion has been to make use of the transmitter duty cycle as well as possible and to allow multimodulation experiments allowing practical spatial and time domain resolution everywhere in the possible ranges of the measurement and in all parts of the target having different measuring demands. Together with the methods, a ready experiment library has been developed, which in many applications offers a solution to the measuring problem.
GEN-SYSTEM author is very grateful to the Director of EISCAT, Dr MURRAYBARON,for his great interest in this work. He followed the work carefully and made valuable comments. I also thank Dr WALTER SCHMIDTfor fruitful discussions and Dr PETERCQLLIS for reading and commenting on the manuscript. EISCAT is supported by: Centre Acknowledgements-The
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National de la Recherche Scientifique (CNRS), France ; Suomen Akatemia (SA), Finland ; Max-Planck-Gesellschaft (MPG), Federal Republic of Germany; Norges Almenvitenskapeliga Forskningsrid (NAVF), Norway; Naturvetenskapliga ForskningsrHdet (NFR), Sweden ; Science and Engineering Research Council (SERC), United Kingdom.
REFERENCES
BARONM. TURUNENT. and SIL~NJ. LEHTINEN M. and HUUSKONEN A. Reference is also made to the following
TURUNENT. T~RUSTADB. W.
1984 1984 1986
J. atmos. terr. Phys. 46,469. J. atmos. terr. Phys. 46, 593. J. atmos. terr. Phys. 48,781.
1985 1982
EISCAT Technical Note 85/44. EISCAT Technical Note 82/36.
unpublished material:
of Afmospherrc and Terrestrial in Great Britain.
Physics, Vol. 48, Nos 9-10, pp. 777-785, 1986.
GEN-SYSTEM-a
OfX-9169/8653.00+ Pergamon Journals
.I0 Ltd.
new experimental philosophy for EISCAT radars TAUNO TUPUNEN
EISCAT Scientific Association S-981 27 Kiruna, Sweden (Receivedfor publication 29 April 1986)
Abstract-GEN-SYSTEM
is a code name for a new experimental design philosophy, a set of related correlator algorithms and an experiment library based on this philosophy. It is designed to obtain an easy way to develop powerful experiments having several different modulations in the same pattern or to have very powerful modulations in single channel experiments, needed sometimes in special applications. The experimental philosophy is discussed and a few examples given on practical experiment design.
1. INTRODUCLLON The code name GEN-SYSTEM arises from the word ‘general’, because it is a system developed especially for general purpose aurora1 zone incoherent scatter experiments. The first incoherent scatter experiments following the GEN philosophy were done in January 1985 with the EISCAT UHF system. The EISCAT facility is described, for example, by BARON(1984), to which the reader is referred for details. The most important design criterion in GEN-SYSTEM development was to obtain methods of running an incoherent scatter radar having parameters such as EISCAT in that mode which maximises information flow from the target as greatly as possible when using standard modulation methods. The output data format, compatibility with existing display and analysis software, etc., were not considered to be important, because if that compatibility is fully maintained the primary design goal cannot be obtained. It was, however, demanded that the system must be realized with the existing hardware. This was a limitation, because certain restrictions in the correlator design prevent some of the most effective modulation methods being used. The details of the GEN-SYSTEM cannot be handled here. The practical solutions are described in a report by TIJFWNEN(1985). Some discussion is given below about the general principles of the system and related experiment design. Although the description is limited here to those applications which are interesting for the EISCAT radars, the principles themselves are general and could be applied in other radar installations as well if the modulation, receiving and data processing systems allow. 2. THE MEASURINGPROBLEM In a general purpose aurora1 experiment a reasonable spatial and time domain response and measuring
accuracy must be obtained from about 70 km to about 500 km. The upper part of the target (above 200 km) can be measured by using long pulses (150-400 ps) in the most effective way and, as long as the obtained spatial resolution is acceptable, no other modulations exist which could compete with a simple long pulse in the statistical accuracy of the target autocorrelation function estimate. Thus the long pulse is naturally one of the needed modulations. If long pulses take quite a considerable part of the used RF duty cycle, then this modulation can be used to form the remote station part of the target illumination too, and no RF duty cycle is needed for remote station purposes only. By repeating the long pulse 2-4 times in a cycle one gives in practice 40-70% of the RF duty cycle to the long pulses, which gives good enough accuracy at the remote sites and allows monostatic measurements at very long ranges. The target cross-section as a function of range is measured by having a ‘power profile’ part in the modulation. The spatial resolution demand in the Dand &layers is, however, so high that the same power profile pulses cannot be easily used in the F-layer, because of signal-to-noise ratio limitations. Thus a good general purpose experiment needs at least two different power profiles with different resolutions. The power profile should be repeated several times, although in practice one has to limit the number of power profile pulses to 2-8 in a modulation cycle. A well designed general purpose experiment usually contains two different power profiles, both of which are repeated 24 times. E-layer autocorrelation function estimates cannot be obtained by using long pulses, because of the spatial resolution demand. The classical possibility is to use pulse codes, but then the signal-to-noise ratio becomes relatively poor. As a consequence, as large a portion of the RF duty cycle as possible has to be given to the pulse coded part in a general purpose
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experiment. In practice this means 4 channels with pulse coded modulations, because this is the highest number of practical codes which can be programmed into the same transmission pattern, When multiplexing 4 codes together, at least 2 different codes must be used and in the simplest case the second code is a mirror image of the first (TURUNENand SILBN, 1984). If the elementary pulses used in the pulse code are further phase coded, then one can multiplex only 2 codes together and they cannot be identical if one uses practical parameters in the experiment design. Thus two sets of pulse codes are usually needed. A general purpose experiment has five different modulations, one set of long pulses, two sets of power profiles and two sets of pulse codes. If one does not need autocorrelation function estimates for the lower part of the ionosphere and spatial resolution is not of vital importance (or beyond practical possibilities when considering the available integration time), one can omit the pulse codes and high resolution power profiles. If one is interested only in the low altitude part, then in some cases one can leave out the long pulse part and low resolution power profiles, although it causes difficulties in obtaining the tristatic Doppler estimate. Eight channels are available in the EISCAT receiving system. Thus with the present hardware one can work using 8 frequencies which can be modulated independently. In some of the frequencies one can transmit more than one modulation in a measuring cycle if the modulations are such that they do not disturb each other. A commonly occurring total cycle is of the order of 12 ms in a heavily modulated general purpose experiment. Remembering that one must be able to calibrate every channel, the following practical rules can be established. (1) Long pulse modulation (150-400 ps) has to be sampled at long ranges too, and the background power estimate demands that the illuminated target is not nearer than about 1000-1500 km. This needs the whole measuring cycle and thus no other modulations can use the same channel. (2) High resolution (10-30 ps) and low resolution (40-80 ps) power profiles can share the same channel if the transmission gap after the low resolution power profile is long and the gap after the high resolution power profile is not too short. For example, if the high resolution power profile is followed by a 4 ms receiving period and the low resolution power profile followed by an 8 ms receiving period, no problems usually arise. The channel can be calibrated at the end of the low resolution power profile receiving period. (3) High resolution power profile and pulse code can be handled by the same channel if the power
profile is transmitted first and the receiving period following the transmission of the pulse coded waveform is long. In practical experiments the spatial response of the power profile and pulse code are programmed to be the same. The power profile does not disturb the pulse code at all, except by a tiny amount at zero lag, which contains self-clutter in any case. However, when transmitting in this mode one may lose the possibility of improving the power estimate by the deconvolution method (LEHTINENand HUUSKONEN, 1986). In the monostatic part of an experiment a single long pulse in a cycle produces a result much more rapidly than any other standard modulation, even if it is repeated in all the remaining 7 channels. However, this is not the case at the remote sites. Especially at Sodankyla, under some conditions the needed integration time for a given accuracy is of the same order of magnitude as the integration time needed in the monostatic pulse coded part. In order to get an accurate tristatic velocity, more than one long pulse should be transmitted if the long pulse parameters have been fitted to the monostatic measuring demands. Usually a good compromi~ is obtained by transmitting two long pulses in a cycle. If phase coding is not used, the modulations in a general purpose experiment are as follows : (1) Two long pulses ; (2) four pulse codes ; (3) two or four high resolution power profiles ; (4) two low resolution power profiles. The choice at point 3 depends on whether one shares the channels with the pulse codes or with low resolution power profiles. The former case leads to ‘IO-block’ modulation and the latter case to ’ IZblock modulation. In every channel one must sample one data vector for the background estimate, one for the noise injection estimate and one or two data vectors for the scatter data. In the monostatic part of the experiment the IO-block modulation produces 26 different data blocks and the 1Zblock modulation produces 28 different data blocks in a cycle. The total length of the input data vector is in practice 2000-6000 complex words. This data has to be handled in real time. In the present EISCAT system the necessary real time computations must be done by having only 16 control registers for the input side, 16 registers for the output side and about 50 free program steps in the processors of the correlator. The algorithms must be fast enough (2.54.8 MHzcomplex multiplication rate is obtained in practice) and they must be flexible enough in order to allow user-oriented experiment
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GEN-SYSTEM
design. One must have a way to compress the data as much as possible, because the result must fit to the available result memory (which is 2047 complex 32 bits words at present in the EISCAT correlators and 4095 words in the future). The GEN-SYSTEM is developed to give an easy solution to the real time processing problem arising from the experimental reasoning given above. The remote station real time processing problem is simple. 3.
CORRELATORPROGRAMS
special modulations and to a certain extent it is usable in pulse-to-pulse correlation applications. The formula used to compute the UNIPROG matrix elements is E(n, m) = Z(n) * Z*(n + I- LZ),
(3.1)
where E(n, m) is the matrix element on row n and column m = n+/*LZ, Z(i) is an input data vector, i = 1, 2, . , LZ is lag increment in sample space and integer 1 is the lag parameter. The matrix has the only non-zero elements in every LIth diagonal in the upper triangle. The elements E(n, m) contain all the information obtainable in radar by using pulse codes with suitable selection of 1and LZ. If one adds G neighboring gates together, then a new smaller square matrix must be formed and its elements are
The GEN-SYSTEM correlator program package is built on the principle that one has to write an independent main program for every experiment. Different subroutine combinations (called ‘master programs’ in the GEN-SYSTEM) are available and the basic package contains several different master programs and some special programs. All the programs C&m) = 2 Z((k-l)*G+j)*Z*((k-1) ,= I can be written using a symbolic CORLAN language developed for the EISCAT correlators (T~~RUSTAD, *G+.j+(G.Z).LZ/G). (3.2) 1982). The computing subroutines are based on the fol- This matrix has the only non-zero elements in every LZ/G diagonal in the upper triangle and thus LZ/G lowing three basic algorithms. must be an integer. (1) UNIPROG. Although the UNIPROG approach is for most (2) LONGPULSE. applications the best one for pulse coded modulations, (3) REMOTE. some more advanced routines are included in the The UNIPROG is a further development of the GEN-SYSTEM correlator program package to ‘Universal Program’ and it is used for handling the handle pulse codes. These routines produce ready autocorrelation functions for the target volumes in the pulse coded parts of the modulation pattern by computing the ‘UNIPROG matrix’ (for details see, for case of 12 block modulation if the pulse coded part is based on certain 4 pulse codes or 5 pulse codes. They example, TURUNEN and SILBN, 1984). It also takes have not been written in a similar ‘user oriented way care of all the power profile computations. The only real difference compared with the original version is as the standard GEN-SYSTEM master programs, but that the GEN-SYSTEM version can be programmed can handle often only one modulation pattern. Two essentially different algorithms are used, but in printo add identically used channels and neighboring gates together in the result memory. Channel addition is ciple both of them do the UNIPROG matrix decoding in real time using essentially the formulation given simple, because one needs only to add the matrixes in detail by TURUNEN (1985). The benefits of this together in the accumulation process and this can always be done without any limitations. Addition of approach are simple and familiar output format the neighboring gates is done in the GEN-SYSTEM and a very compressed data structure, but, on the UNIPROG by adding together a given number of other hand, part of the information included in the UNIPROG matrix is lost, because one cannot the neighboring elements in the diagonals of the make use of the main diagonal (see LEHTINENand UNIPROG matrix. HUUSKONEN,1986). A necessary condition is that in every computed The GEN-SYSTEM LONGPULSE routine comdiagonal of the original matrix the number of elements putes the monostatic long pulse data with an algodivided by the number of neighboring elements added rithm which essentially differs from the ones used together is an integer. The resulting compressed earlier. It computes such that the arising absolute matrix has all the properties of the UNIPROG matrix, boundaries of the target volume are the same at every i.e. its elements can be used to form an autocorrelation lag. This program can also add different channels function estimate of a target volume when the target together if wanted. The program also has another new is illuminated by a pulse code. The GEN-SYSTEM feature. The spatial resolution can be programmed UNIPROG can also be used in applications utilizing
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within the limits dictated by the modulation. The needed algorithm was mentioned by TURIJNENand SILBN (1984) as an application of the UNIPROG matrix, but at that time it was only possible to do the volume formation in the post processing by having the UNIPROG matrix as a primary datum. In the GEN-SYSTEM this is already done at the correlator level. The formula describing the algorithm is Y+* A(g,i) = 1 Z(M-t(g-1). V-i+k) k= 1 *Z*(M+(g-
1). V+k),
(3.3)
whereA@,i)istheithlag,i=0,1,2 ,..., M,ofthe target ACF estimate for the gth gate, g = 1, 2, . . , 9 max>using data samples ZG), j = 1, 2, . . . , gmoxV+ 244. V is called ‘volume index’ in the GEN-SYSTEM and gives the number of power points added together to form the zero lag estimate. M is the maximum lag to be computed. Gate indexing starts from 1. The peak-to-peak volume size at every gate is approximately S = (T+(V-
l)*t+STEP)*c/2,
(3.4)
where T = pulse length, t = sampling interval, STEP is the step response time of the receiver system and c is the velocity of light. The arising weighting factor WN(i) is for the white noise (background and noise injection) VW(i) = v+i.
(3.5)
The weighting factor WT(z) for the target contribution is in most cases, to a good accuracy, given by WT(i) = (V+Q.(T-i-t).
= (l+i/I’).(l--i-t/T).
s-1 A(j) = 1 Z(nk=l
1 +k)*Z*(n-
1 +k+j),
(3.8)
where A(j) is the autocorrelation function estimate at lagj,j=O, 1,2, . . . . and Z(n) is the first input data point. S is the number of samples. The upper limit for j is programmable. A simplified formula describing the calibration autocorrelation function computation in the algorithm REMOTE is
(3.6)
If this is normalized to unity at zero lag one obtains NWT(i)
The remote station data is handled by a correlator program GEN-REMOTE. This contains an essentially similar long pulse routine with triangular weighting, as used earlier in long pulse computations for the computation of the autocorrelation function estimate when the target is illuminated. For calibration it contains a routine which computes the background and noise injection estimates using identical weight at every lag (boxcar weighting). The main benefit is that one can improve the accuracy of the background estimate, because the receiver system can sample the calibration data almost all the time when the volume is not illuminated and the whole computational power can be used. In spite of this, the output data vector can be compressed into a very compact form and in practice only 100-200 output data points are needed to describe the whole information, independently of the number of channels used in the experiment. There are several different possibilities to do the necessary timing check when using the REMOTE routine. This routine is also suitable for system calibration purposes. The formula used to compute the long pulse autocorrelation function in the beamwidth-limited case (remote station application) is, in a simplified form,
C(j) = $ Z(n-l+k).Z*(n-l-tkfj), li= 1
(3.9)
(3.7)
The obtained raw target ACF after background subtraction must be divided by the given weighting function in order to get a boxcar weighted target ACF estimate. The properties of the GEN-SYSTEM LONGPULSE routine are as follows : (1) The absolute boundaries of the volume are the same at every lag and the major contribution, say 75%, comes from a volume which is not a strong function of lag in practical cases. (2) All the obtainable information in the sampled data is used up to the given maximum lag. (3) The spatial resolution can be programmed (because V is a programmable parameter).
where C(j) is the calibration ACF estimate at lagj and P is a programmable parameter giving the number of products summed together, which is the same at every lag. In the GEN-SYSTEM correlator programs one has thus two different long pulse routines, one for the modulation dependent volume formation and the other for the beamwidth-limited case (these two cases are in the earlier programs handled by the same algorithm which is essentially identical with the beamwidth-limited case). The remote station routine also has a special calibration algorithm producing boxcar weighted autocorrelation functions. The pulse coded part can be computed by using special algorithms producing boxcar weighted auto-
GEN-SYSTEM
correlation functions or by UNIPROG, producing a matrix format. In some cases special experiment-oriented algorithms are needed. One such application arises if one uses pulse-to-pulse correlation and the modulation methods are very complicated. Such an example is given in the next section. 4. GEN-PROGRAMS LIBRARY
The GEN-PROGRAMS library contains ready experiments based on GEN-SYSTEM philosophy. The full library cannot be described here. Three examples are given to show how the GEN-SYSTEM philosophy is used in real experiments. All the experiments described below have also been used in real experiments in 1985. The first example shows the structure used in ‘12block mode’. This kind of experiment has 12 modulations in a cycle divided into three transmitting periods, as shown in Figs. 1 and 2. The first phase consists of a group of four power profile pulses (EPPG) of relatively high resolution. The sampling
covers the D-, E- and lower part of the F-layers. Immediately when the sampling is ready one transmits a group of four pulse codes (MPG) using the same frequencies and same channels. These are sampled to cover the &layer and lower F-layer. The transmitter parameters and sampling must be defined so that the power profile gates and pulse code gates have the same spatial response and part of the power profile gates coincide in range with the pulse code gates. In this situation one can directly use the power profile data as zero lag in the autocorrelation function estimate without any further checking and balancing. The part of the power profiles which are beyond the range covered by the pulse coded modulations are simply used as power profiles. In the third phase one transmits the long pulse group (LPG) for the Player part of the experiment and low resolution power profiles intended mainly for F-layer purposes (FPPG) but which sample the whole altitude range from the D-layer to the upper parts of the F-layer. The LPG also forms the remote station part of the experiment. The sampling and calibration windows can be seen
zi*3 a) EPPG. All pulses 19 ps
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b) MPG, All pulses 19 ps
c) LPG + FPPG. Pulse lengths 350 and 50 ps
Fig. 1. The three modulation phases needed in ‘IZblock’ modulation mode. (a) EPPG is the E-layer power protile group ; (b) MPG is the multipulse group ; (c) LPG is the long pulse group and FPPG is the F-layer (low resolution) power profile group. The LPG also forms also the remote station modulation. In the example the pulses in the MPG are 19 ns and the shortest gap is 21 ps. The lag increment is 40 ns. The frequency (FtkF7) and channel (CHlCH8) selections can be used in a practical experiment.
T. TURUNEN
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q TRANSMIT [7
RECEIVE
q CALIBRATE
CH1 CH2 CH3 CH4 CH5 CH6 CH7 CHE I
I
I
I
1
2
3
L
III
5 TIME
I
I
I
I
I
II
6
7
8
9
10
It
I
I
I
12
(msl
Fig. 2. Transmitting and receiving in the 1Zblock mode using the elements shown in Fig. 1. The parameters
are essentially taken from a library program GEN-SB.
from Fig. 2, where the timing and parameters are essentially the ones used in the library program GEN8B. Several library programs are written in this way. The multipulse group (MPG) is in the example formed by combining four 4-pulse codes (code names GEN8 and GEN-9), but similar programs exist for a combination of 3-p&e and $-pulse codes (GEN4 and GEN-7) and for a combination of four 5-pulse codes (GEN-2, several versions). Program versions exist which handle the data in the ‘standard GEN-SYSTEM mode’ or in a readily decoded mode if only 4-pulse codes or .5-pulse codes are used. If the ‘standard’ mode is used, then one has to produce two UNIPROG matrixes for the E-layer part. If decoding versions are used one needs to produce only one set of autocorrelation functions, because all the E-layer data can be added together at the correlator level. Inde~ndent of the output format, the data in the library versions is rather heavily oversampled and two or more neighboring gates added together in the correlator. In the simplest versions the output data of the ‘1Zblock mode’ experiment contains low altitude power profiles, E-layer autocorrelation functions, Flayer autoco~elation functions and low resolution power profiies. All these four sets of data have a corresponding calibration part in exactly the same format. If one does not want to transmit in IZblock mode, one can omit the EPG and transmit two power profiles multiplexed into the MPG sharing the channels with the FPPG. This ‘lo-block mode’ always demands a relatively complicated channel balancing procedure,
but it can be programs for a higher duty cycle and produces absolutely clutter-free main diagonals of the UNIPROG matrixes, opening up the possibility for deconvolution methods. On the other hand, it can, under some conditions, become disturbed by enhanced plasma line activity (for further details see TURUNEN,1985). The example shown in Fig. 3 is an arrangement to obtain very long range coverage. One has two transmitter phases which both contain the same low resolution power profile part formed by using four channels and frequencies and two long pulses, which have different channels in the first and second phases. When the first two channels see an illuminated target the other two channels are sampling calibration data. Figure 3 describes the library program GEN-5, which is also the basis of one version of EISCAT common mode program 3 (CP-3-D). The library also contains technically rather similar experiments where the low altitude part uses phase coded puise (GEN-6 and GEN-12) and also special experiments where all the transmitter power is used for the multipulses and high resolution power profiles (e.g. GEN-3). The two examples given above describe experiments which resemble the original EISCAT common mode programs CP-1 and CP-3 from the target sampling and spatial resolution point of view, but the way the radar is used to solve the measuring problem is much more effective. The third example does not have any corresponding earlier program of similar solution. It is the pulse-to-pulse correlation program GEN-11 from the GEN-PROGRAMS library. Because this
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GEN-SYSTEM a
TRANSMIT
q
RECEIVE
fl
CALIBRATE
0
2
CHl CH2 CH3 CHL CH5 CHS CH7 CHB
I4
6
6 TIME
10
12
11
16
16
(ms)
Fig. 3. Transmitting and receiving arrangements in a long range experiment. Long pulses are 350 ps and power profiles 70 /AS.The parameters arc taken from the common mode experiment CP-3-D.
solution differs greatly from earlier approaches to the problem it is handled here in slightly more detail. In D-layer experiments one wants to measure both the target cross-section and its autocorrelation function. The coherence time is long and thus the correlation has to be measured by illuminating the target by pulses transmitted in a few millisecond periods. The data is naturally received immediately after every pulse. In GEN-11 the target is illuminated at 2.222 ms intervals. The spatial resolution demand is very high and therefore GEN-11 uses a 13 bit Barker code with 7 ~LSbaud length which gives 1050 m spatial resolution. The problems in D-layer experiments are as foliows :
(1) The experiment has to give analysable data, even when the signal-to-noise ratio is much below 1% . (2) One has to eliminate the distant mode clutter, which is often much stronger than the D-layer signal. (3) One has to eliminate the receiver recovery effects arising from the operation of protection switches, transmitter pulse leakage to the receiver front end and strong atmospheric clutter from low altitudes. (4) The noise temperature of the receiving system may differ from the usual level after the transmitter pulse. (5) The gain may vary a little due to the shock given by the transmitter pulse. (6) When measuring using a very narrow final bandwidth, the possibility of contamination from mains harmonics is high. All the effects listed above in fact exist and many of
them often give a contribution comparable to the target return. In GEN-11 the clutter is eliminated by changing the modulation after every transmitted pulse and the gain stability problems are largely eliminated by approximating the target power by the real part of the lag at 112 ps delay, where the expectation value for white noise is zero in the experimental arrangements used. The mains harmonics and offset voltage disturbances are, in the main part of the experiment, eliminated by designing the modulation such that half of the target contribution has positive and half of it has negative sign for a given volume and given lag. The modulation pattern needed is shown in Fig. 4. One Barker coded double pulse is transmitted having a variable gap between the pulses and the latter pulse is reversed in phase. For the D-layer target the last transmitted delay is used to measure a point in the autocorrelation function near zero lag. The mean value of the transmitted delay is 112 ps. The previous four transmitted double pulses do not contain the delay of the pulse illuminating the D-layer and this gives the clutter cancellation. The possible tiny compression side lobe leakage in the experiment is calibrated ‘by computing the delay, which is one baud length longer than the one used in the pulse illuminating the D-layer. This is a slightly approximate method, but works well enough for the clutter and also gives s;rstem offset voltage and hum contamination estimates for the 112 ,USdelay. The white noise contribution has zero expectation va:tie and thus possible gain variations operate only
T. TLRUNEN
784 +PULSE
-PULSE
a
TRANSMIT
0
RECEIVE
@ CALIBRATE
0
P2 c
2.222 ms---,
fgl
P3
PL TOTAL CYCLE
P5 Lb2.222
Pl
P2
ms
Fig. 4. A special pulse-to-pulse correlation modulation and transmitting and receiving arrangements. For details see text.
on the signal, not on the sum of signal and noise. Note that the noise level is usually two orders of magnitude
higher than the signal level. In the experiment a double pulse illuminates the target twice. Thus one can have, at a11longer delays, four possible delayed products, which all have, approximately, a delay of 2.222 n ms, n = 1, . . . , 43. Because of the inverted phase in the later pulse, two of these products are positive and two are negative. They have to be integrated separately. The process is free from spatial ambiguities, except in every fifth lag in positive products (the same modulation repeats in every fifth pulse). Summing all the four products together after reversing the negative signs is equivalent to 26 bit compression and in the process possible low frequency disturbances in the receiver system are cancelled. The background power needed in the variance estimates and the noise injection power needed to get the gain must be measured by computing a true zero lag. Because no instants without target signals exist, the noise injection is added into every other receiving period and the power with and without the noise injection are integrated separately. The measuring algorithm GEN-11 has already been successfully used in experiments, and a reasonable accuracy is obtained even from the undistur~d solarproduced D-layer with the EISCAT UHF system in
a few minutes integration time, but the algorithm has been mainly developed for forth~~ng VHF radar applications. The algorithm also has other versions than the one described. The special correlator program written for GEN11 takes care of all the computations needed in the real time processing. Because of the changing modulation pattern, the program is not trivial and it is one of the examples when the standard ‘general purpose’ approach cannot give a solution to the measuring problem.
SUMM~Y The GEN-SYSTEM is developed for a radar having extensive modulation possibilities and several parallel receiving channels which can be independently used at different frequencies. The main design criterion has been to make use of the transmitter duty cycle as well as possible and to allow multimodulation experiments allowing practical spatial and time domain resolution everywhere in the possible ranges of the measurement and in all parts of the target having different measuring demands. Together with the methods, a ready experiment library has been developed, which in many applications offers a solution to the measuring problem.
GEN-SYSTEM author is very grateful to the Director of EISCAT, Dr MURRAYBARON,for his great interest in this work. He followed the work carefully and made valuable comments. I also thank Dr WALTER SCHMIDTfor fruitful discussions and Dr PETERCQLLIS for reading and commenting on the manuscript. EISCAT is supported by: Centre Acknowledgements-The
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National de la Recherche Scientifique (CNRS), France ; Suomen Akatemia (SA), Finland ; Max-Planck-Gesellschaft (MPG), Federal Republic of Germany; Norges Almenvitenskapeliga Forskningsrid (NAVF), Norway; Naturvetenskapliga ForskningsrHdet (NFR), Sweden ; Science and Engineering Research Council (SERC), United Kingdom.
REFERENCES
BARONM. TURUNENT. and SIL~NJ. LEHTINEN M. and HUUSKONEN A. Reference is also made to the following
TURUNENT. T~RUSTADB. W.
1984 1984 1986
J. atmos. terr. Phys. 46,469. J. atmos. terr. Phys. 46, 593. J. atmos. terr. Phys. 48,781.
1985 1982
EISCAT Technical Note 85/44. EISCAT Technical Note 82/36.
unpublished material: