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[73c35b]: / exps / cp1k / CP1KT.DESC Maximize Restore History
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% S P - E I - A L T C P 1 - T : D E S C %
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% Gudmund Wannberg, EISCAT HQ, February 27, 1991 %
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1. General information
The SP-EI-ALTCP1 experiment is a prototype for a next generation CP-1/CP-2
code. It makes extensive use of the new signal processing hardware which has
been installed in the EISCAT system over the past two years. The E region
data rate is increased by about 20 % in the power profile part and by as
much as 100 - 200 % in the spectral data. This is accomplished at the cost
of a minor reduction in Tromsē F region statistics (about -15 %), which is
caused mainly by the transmission of only one long pulse/IPP rather than
two, as in the current CP-1 versions. So as not to compromise the tristatic
velocity statistics, the remote sites are programmed to receive both the
long pulse and also the alternating code transmission which produces the
E region spectral data in Tromsē. This causes a net increase of the number
of spectral estimates per unit time at the remote sites of about 70 % relative
to the current CP-1.
The integration time is set to 5 seconds, which may be useful in highly
dynamic situations (auroral substorms etc.). IT MUST NOT BE CHANGED NEITHER
FROM THE EROS CONSOLE NOR THROUGH A CHANGE TO THE 'START-RADAR' COMMAND IN THE
:ELAN FILE; ANY CHANGES WILL CAUSE THE ALTERNATING CODE PART OF THE EXPERIMENT
TO FAIL. IF A DIFFERENT INTEGRATION TIME IS DESIRED THE TARLAN CODE GENERATOR
PROGRAM "ATC-1" MUST BE USED TO CREATE A NEW :TLAN FILE ADAPTED TO THE NEW
INTEGRATION TIME !!
2. System configuration
This experiment uses all eight channels of the receiver. The PDF filters
should be installed so:
Used for
CH1 and CH2 Linear phase, 25 kHz BW, lower crate F region PP
CH3 and CH4 Butterworth, 25 kHz BW, upper crate F region LPACF
CH5 to CH8 Linear phase, 25 kHz BW, lower crate D and E reg PP
+ alt. codes
The three channel groups should be internally balanced for gain to within
one decibel. It is permissible to completely attenuate out faulty channels
within a group in an emergency. This causes a data loss of 50 % per channel
in the F region groups but only a 25 % per channel loss in the D,E region
group.
In every IPP pulses are transmitted on four different frequencies, i.e.
one frequency each for the FPP and FACF groups and two frequencies for
the D,E region group (one for the PP and the other for the alternating code).
In the next cycle, all frequencies are changed and the pattern is repeated,
now of course using different receiver channels. Towards the end of each
cycle, all channels which are due for use in the next cycle are calibrated.
A new member of the 16 baud strong condition alternating code set is
transmitted every two IPPs, so it takes 64 IPPs to complete the set. At the
end of such a set, the frequency/channel pair used for the D,E region PP is
interchanged with the pair used for the alternating code (this happens about
twice a second). The result of this operation is that the D,E PP and the
ACFs measured with the alternating code are automatically intercalibrated
in an average sense. Decoding of the alternating code data is fully automatic
and performed by special purpose hardware in the correlator, which is
partly controlled from the correlator microprogram and partly from the
radar controller (the :TLAN and :TCOD files contain special 'bit' commands
to this effect).
CAUTION: Because of the intricate commutations of channels and frequencies
used in the experiment, it is inadvisable to attempt to change a frequency
or channel by editing of the :TLAN file. A special "TARLAN generator" program
has been written for this purpose. It is still not quite debugged and should
be used only in cooperation with the author, UGW at EISCAT HQ, who will be
pleased to offer help and advice.
3. Data dump structure, SP-EI-ALTCP1-T
The data dump contains five different blocks of data, namely
i) An ungated power profile for the D and E regions,
also serving as zero lags for the ACFs in block (iii),
ii) A "variance profile" computed from the undecoded
samples taken from the alternating code modulation,
iii) Incomplete ACFs derived from the alternating code,
iv) A gated, coarse resolution power profile for the
F region,
v) Conventional long pulse ACF data.
The dump is 2036 complex double integers long ; the SCANCOUNT value is
stored in dump address 2035.
3.1 D and E region power profile block
This block contains ungated power profile data which also doubles as
the zero lags for the ACFs stored in block (iii). Specifically, there are
90 (S+B) gates at addresses 0000-0089
60 B gates at addresses 0090-0149
10 (B+C) gates at addresses 0150-0159
Gate no. 9 at address 0008 coincides in range with A/C ACF gate no. 1
in block (iii). Data from two frequencies/channels are added; this is the
only block in the dump that employs channel adding!
3.2 Variance profile data block
This block contains 76 power estimates at addresses 0160-0235. Its use
is further described below.
3.3. Alternating code ACF block
This block is very straightforward. It contains
61 S gates (lags 1-15) at addresses 0240-1154, IT weighting, Ns = 16
2 B gates (lags 1-15) at addresses 1155-1184, IT weighting, Ns = 16
IT = inverse triangular, w(l) = Ns - l where l is the lag index
No channel adding is applied (one channel only per IPP).
NOTE: these gates contain NO zero lag - this cannot be unambiguously
estimated from the alternating code. Note also that the mean of the back-
ground is zero if the algorithm works properly, so background subtraction
is neither necessary nor recommended in this block (it would just increase
the variance !).
Recently, the RTGRAPH has been upgraded so that if <NO. OF SKIP WORDS>:=-1
a zero lag is extrapolated from the data and the ACF displayed with this
included. This allows you to display also power spectra with a proper
baseline level. The present :GDEF file for -ALTCP1 uses this feature.
3.4. F region power profile block
This block contains (S+B), B and (B+C) gates that exactly match those
of the current CP-1-I-T low resolution power profile in range coverage,
gate separation and range extent - only P-P resolution and the order of
the subblocks are different:
80 (S+B) gates at addresses 1181-1260
40 B gates at addresses 1261-1300
6 (B+C) gates at addresses 1301-1306
Gate adding is used; three neighbouring estimates sampled 10 us apart
are added to form each gate in the output. No channel adding is applied.
3.5. Long pulse ACF block
The gating parameters used in this block are identical to those of the
current CP-1-I-T LPACF, which should facilitate the adaption of existing
analysis software to the new program. Specifically, the block contains
21 S+B gates (lags 0-25) at addresses 1307 - 1852, GS weighting, Ns = 15,
6 B gates (lags 0-25) at addresses 1853 - 2008, GS weighting, Ns = 15,
1 B+C gate (lag 0 ) at addresses 2009 - 2034, GS weighting, Ns = 15.
GS = Gen-System , w(l) = Ns + l where l is the lag index
NO adding of channels is used in this block.
NOTE: the only real difference between this data block and the output of
a CP-1-I-T experiment is the ORDER of the sub-blocks, plus the fact that
only one channel is received per IPP.
4. Data properties
4.1. D/E region power profile
This is a very simple power profile data vector. It is derived from two
21 us pulses transmitted immediately after each other on two different
frequencies in the first half of each IPP. The frequency used for the first
pulse is later used for the alternating code which is transmitted in the
second half of each IPP. The frequencies are commutated according to a
relatively complicated scheme from one IPP to the next, but this ensures
that all frequencies used for the PP pulses are eventually also used for
the alternating code; hence the power profiles and the ACFs computed from
the code are automatically gain balanced and the PP can be used as a zeero
lag profile for the ACF series (which does not contain lag 0).
The PP channels are sampled at a 1/21 us rate and, as mentioned above,
two channels are added in the dump. No gate adding is performed. The
filters are 25 kHz linear phase ones. The range coverage is 64.65 - 345.00
km and the range increment/gate is 3.15 km.
4.2. Variance profile
This data block is included in the hope that it may become useful in the
analysis of the alternating code ACF data. It contains a series of raw
power estimates derived from all the samples used by the decoding process
(there are 76 of them in this case of a 16 bit code and 61 gates) and so
it can be understood as an "ambiguous lag-0 profile". An advanced analysis
routine might attempt to estimate the true variance of each lagged product
in the decoded ACFs by starting from this ambiguous power profile.
The first point in the variance profile corresponds to a sample taken
as the leading edge of the coded pulse passes 90 km range. As the modulation
is 336 us long, the power in this point is an average over 50.4 km, and
the lower boundary of this first point is just below 40 km range. Some
ground clutter may be expected to occur in the first few points. This,
if present, will also affect the first few ACF gates. It remains to be
seen to what degree this corrupts the data.
4.3. Alternating code ACF
The alternating-code ACF data is derived from a 16 baud, strong condition
alternating code set using a baud length of 21 us. A total of four different
frequencies and four receiver channels are used for this modulation, and
each of the 32 sequences that make up the code set is repeated on
two different frequencies. It takes slightly less than one-half second
to complete the full set; this is consequently also the time during which
the target (ie the ionosphere) must be assumed to be stationary in an
average sense, for the technique to produce dependable results.
Sampling is at 21 us/sample, filters used are 25 kHz linear phase.
The lag extent is from 21 - 315 us and the lag increment 21 us.
Fitting for velocity should be straightforward - the zero lag is not
used anyway in the velocity estimator. Fitting for density requires an
estimate of the zero lag. This can be taken from the power profile in
block (i), which is measured through the same receiver channels and on the
same frequencies as the alternating code - it is consequently automatically
intercalibrated with the nonzero lags and can be patched into the zero lag
locations of the ACFs after proper scaling. The relative weight of the PP
is 2, but remember that it must be background subtracted also !
PP gate no. 9 corresponds to ACF gate no. 1, which is at 89.95 km range.
The range increment/gate is 3.15 km.
4.4. F region power profile
This power profile is mainly intended as a replacement for the 2*29 us
power profile in the current CP-1-I-T. The number of gates, range coverage
and gating all are identical to the CP-1-I version. The modulation is
slightly different - as there is more freedom with respect to how long
the transmit pulses can be made in -ALTCP1, we have decided to use just
one, 40 us pulse per IPP rather than the two, 29 us pulses of CP-1. The
p-p range resolution of this modulation is thus slightly coarser than
in the previous version, about 9 km, but the FWHM is only marginally in-
creased. Only one channel per IPP is used.
4.5. Long pulse ACF
The signal carrying part of this block is arranged to behave exactly
the same as the CP-1-I-T, i.e. the series of gates is computed with a
volume index of 15 and a max. lag of 25. The first gate is centered on
150 km range. Six background gates are computed instead of five; they
are all fully independent.
The noise calibration part of this data is quite different from that of
CP-1-I - it contains a computed zero lag with the proper weight relative
to the other gates, but all higher lags are forced to zero in the output.
There is a reason for this: If one computes only the zero lag, only 15
noise samples are required, but if one were to compute the full ACF from
samples taken during the noise injection period, 73 samples would be
required! Omitting everything but the zero lag computation thus enables
us to save a considerable amount of time, namely (73-15)*10 us = 580 us,
in the calibration phase of the experiment. This represents a significant
increase in the effective duty cycle of the experiment. At the same time,
the output format is consistent with that of the other ACFs in the block.
Note, however, that in RTGRAPH it is impossible to display the calibration
gate using background subtraction - all lags except lag 0 will come out
negative in this case......
The sampling increment is 10 us, hence the lag extent is from 0 - 250 us.
25 kHz Butterworth filters with a nominal risetime of 24 us are used, which
leads to oversampling and a substantial correlation between neighbouring lags.