A report on
The front-end of
the Pulsar Instrumentation
for GMRT
A. A. Deshpande
Raman Research Institute
Bangalore 560080
Dec. 89.
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Date : Tuesday, October 17, 2000
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A report on the front-end of the Pulsar Instrumentation for GMRT
a) Pulsar Search (Untargeted):
incoherent
addition
of
all
As
I
the
32
untargeted search for pulsars.
understand
dish
it,
outputs
we
in
will
the
use
proposed
Then the effective collecting area
would be ~5500 sq.m. and a field of ~0.3 sq.deg. would be searched at
any one time.
the
Also, we will be using a bandwidth of 32 MHz to improve
sensitivity
of
the
search.
In
order
to
search
down
to
‘millisecond’ periods, the data need to be sampled at submillisecond
intervals.
In the discussion to follow it is assumed that ~250sec
sampling interval would suffice.
It is clear that the 32 MHz band needs to be split into a large
number of narrow spectral channels before detecting the signals from
each dish, to use “post-detection dedispersion”.
narrow
channels
needs
to
be
decided
A number of such
considering
the
effects
of
interstellar scattering and the final sampling interval (e.g. 250sec).
It can be shown that at frequencies ≳ 610 MHz, if the 32 MHz band
is split into 256 channels then the dispersion smearing over the narrow
channel width (~125 KHz) will always be less than the effecting pulse
smearing caused by the interstellar scattering and the finite sampling
interval put together.
Thus, there is no gain in using more than ~256
channels.
It is assumed that from each of the dishes two bands of 16 MHz for
each of the two circular polarizations will be available.
Thus, 4
bands of 16 MHz are parallelly sampled and are Fourier transformed
(using VLBA chips) separately to produce 2 sets (corresponding to the
two polarization channels) of 256 complex numbers (corresponding to the
256
spectral
compensations
channels)
every
are
before
made
4
sec.
the
We
Fourier
will
assume
transform
for
that
each
output.
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delay
dish
For the present purpose, we will need to compute the power (C2+S2
if
C+jS
is
the
complex
spectral
contribution)
over
256
spectral
channels from all 64 sets (32 dishes x 2 polarization) and add them
together to produce 256 outputs every 4 sec.
These outputs then will
be block averaged (integrated) over intervals of 256 sec (say).
The
data rate at the output of this stage will be then 1M samples/sec.
The
processing involved in this incoherent addition of the dish outputs as
outlined here will be performed on-line and will need a sufficiently
complex processing system.
The preprocessor for general search
It is worth noting that the two bands of 16 MHz (amounting to a
total of 32 MHz bandwidth) will, in fact, be processed separately (as
upper and lower side bands) in the front-end electronics system of the
GMRT.
Therefore, we will define the parameter of preprocessor required
only for one such band that corresponds to a maximum of 128 spectral
channels (see fig. 1).
However, then we will need two such processors.
This may be advantageous considering a possibility that only 16 MHz
bandwidth may be available initially.
In the present case, 12 bit (in (4,4,4) format) complex Fourier
transform (FT) output at the rate of 32 M samples/sec will flow out
corresponding to each polarization channel of each dish output.
First
task is to “detect” these outputs to obtain samples proportional to the
power in each spectral channel separately.
We will need 64 “detector”
modules (32 dishes x 2 polarizations) working at 32 MHz rate (for one
side band).
The detector module:
The detector module is required to sum the squares of the real and
imaginary parts of the complex number at the input.
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Such an operation
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performed in a conventional way would need fast multipliers and adders.
The input format where a 4-bit exponent is included, makes it difficult
to
obtain
a
binary
representation
of
the
input
in
a
short
time.
Therefore, we would attempt to perform this operation in a different
way.
It is possible to use a 4096 location PROM, that can be addressed
by the 12-bits (4, 4, 4) at the input and to read out the corresponding
preprogrammed desired output represented with say 4 bits.
Thus,
the
“detection” can be performed with just one read operation from a PROM.
If the 12-bit input already includes the sign bits for the real and
imaginary parts then, we would need only a 1024 location PROM as the
sign bits are not important in the present case.
The choice of the
number of bits at the output as 4 is somewhat arbitrary although very
safe.
At this stage, the random fluctuation in the output around its
average value will be comparable to the average value itself.
We would
desire that the ALC (Automatic Level Control) in the analog section can
be
adjusted
to
ensure
that
the
average
power
in
each
channel
is
maintained close to that corresponding to the half counts of the 4-bit
output.
Summation of the detected outputs
In the next stage 64 inputs (that are proportional to the power and 4
bits each) have to be added together.
The 32 inputs corresponding to
each one of the polarization channels may be added together first and
then the two results can be combined.
This may have some advantage if
the two polarization channels need to be treated separately in future.
The final output will have fluctuations which are 8 times smaller than
the average value; thus requiring atleast 4 bit representation.
again,
Here
we may seriously consider use of PROMs to perform addition
instead of conventional methods.
In any bit-size manipulation (e.g.
While reducing number of bits for any intermediate result)
off can be realized easily with the use of look-up tables.
rounding
Also, these
would help in subtraction of some part of the mean value at suitable
stages.
The
addition
of
64
samples
may
(32+16+8+4+2+1) PROMs working at 32 MHz rate.
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at
the
most
require
63
If larger capacity PROMs
Date : Tuesday, October 17, 2000
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are used, then this number would reduce.
It may also be possible to
use ASIC (Application Specific Integrated Circuits) in place of PROMs
for the addition operations.
(8 bits each)
This module will output 128 channel data
every 4 sec.
Pre-integration
Depending on the pre-integration required (corresponding to the
final sampling time interval) these data need to be averaged in time.
Also, if the number of spectral channels are to be reduced for low DM
pulsar searches (relevant for searches at high galactic latitudes),
contributions
averaged.
of
successive
spectral
channels
need
to
be
block
It is worth noting, that the averaging over detector outputs
of N spectral channels is equivalent to having reduced the spectral
resolution by a factor N before detection, and averaging of N detected
outputs samples in time.
It is clear that at this stage of processing
some amount of flexibility in the pre-integration time and the final
number of spectral channels is essential.
The pre-integration time will in most cases be 256 sec.
However,
in some special cases we may choose it to be 128 sec or 512 sec.
Similarly the final number of spectral channels in most cases will be
128 (per 16 MHz band).
This number should be reducible by a factor of
2 or 4 in some special cases.
We can employ a scheme as shown in fig. 2 using fast memories and
adders to perform the required integration operation.
The output of
one of the memories will be read while the other is busy integrating
the incoming data.
The 16-bit outputs of this integrator need to be converted to some
reduced number of bits (1, 2, or 4), after subtracting long-term mean
values for corresponding spectral channels, for reducing the output
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Date : Tuesday, October 17, 2000
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bit-rate for recording.
Thus, we need a long-term integrator (
τ
≳ 10
seconds) that will supply the offsets that should be subtracted from
different channel data.
These offsets need to be determined with much
less error compared the rms noise deviations in the short integration
outputs, implying large time constants for the long-term integration.
A
time
constant
of
~10
sec
is
more
than
sufficient
from
the
consideration of the relative errors in the offset determination.
If
these offsets do not change appreciably during any single observation,
then
we
could
determine
them
only
throughout that observing interval.
in practice.
once
and
However,
use
for
subtraction
this may not be the case
Then a running long-term average should be subtracted.
However, in such a case we should also record the changing values of
the long-term averages, so that this data could be later examined for
very slow periodicities.
If the output is to be converted to one-bit, the one-bit will be
‘1’ if the integrated output is greater than the long-term mean and ‘0’
otherwise.
For converting the output to samples with two or more bits,
we will need some knowledge of the rms deviations due to the noise in
the output.
An estimate of these rms deviations due to noise can be
obtained knowing the long-term average value and the number of samples
effectively averaged after detection.
Using such an estimate of the
rms noise, we can obtain 2 bit representations of the output using an
arrangement where +ve deviations from the long term mean are sampled
with more steps compared to those used to sample –ve deviations (see
Clifton, Ph.D, Thesis, 1985 for more details).
The final outputs (1, 2, or 4 bits) of each spectral channel
obtained every pre-integration period will be stored in a buffer from
which this data will be read by some suitable acquisition system for
recording (e.g. VLBI Mark II recorder).
preprocessors
will
be
concatenated
to
Such buffers from the two
obtain
such
samples
from
maximum of 256 spectral channels every pre-integration period.
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a
b) Pulsar Search (Targeted)
In targeted searches for pulsars, our target field size may be
much smaller than that required in an untargeted search.
Coherent
addition of outputs from the dishes in the central square of GMRT may
have advantages for this purpose.
Assuming that ~16 dishes will be
placed in the central square (1 km x 1 km), an effective collecting
area of about 16000 sq. meters would be available to search a field of
only
~2
sq.
arcminutes
at
~600
MHz.
This
will
be
an
ideal
configuration for pulsar search in the cores of globular clusters.
Unfortunately, such a field would be too small to search for pulsars in
supernova remnants (SNR), in single search runs.
Therefore, we would
use in general, the incoherent addition of all 32 antenna outputs (as
described before) for pulsar searches in SNRs. In some specific cases,
target field sizes (due to some a priori information) may be as small
as ~2 sq. arcminutes and in those cases more sensitive search would be
possible with coherent addition.
For such coherent addition of outputs from the elements in the
central square, we would prefer to tap the sampler outputs after the
delay compensation (we assume that appropriate phase compensation would
be made in the RF front-end).
These 3 bit samples from each of the 16
dishes can be combined (again possibly using look-up tables) separately
to produce 4 outputs corresponding to 2 polarization channels for each
of the 2 bands of 16 MHz.
These 4 outputs will then be Fourier
transformed (using a separate set of VLBA or other suitable chips) to
obtain 128 channel data for each of the 4 outputs.
These outputs will
be ‘detected’ using suitable look-up tables as described in the earlier
section and the data of the two polarization channels will be combined
for each one of the 256 channels (see fig. 3).
explore the possibility of using ‘look-up’ tables.
Here also, we would
Beyond this point,
the data will be pre-integrated and final outputs (1,2 or 4 bits) will
be obtained using the scheme (and also the hardware) described in the
earlier section.
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c) Studies of known pulsars
For detailed studies of known pulsars, we would use the maximum
possible collecting area.
This means that we would necessarily go for
coherent addition of outputs from all the dishes of GMRT (i.e. the full
phased array mode).
baselines
at
fluctuations.
low
It may not be, however, possible to include long
radio-frequencies
due
to
not be true to begin with.
In many cases, this may
In such cases, we would first take-up an
exercise of determining the positions accurately.
step
phase
In the case of known pulsars, we can assume that their
positions are known to an arcsecond accuracy.
first
ionospheric
in
estimating
proper
motions
of
In any case, as a
pulsar,
we
would
be
determining the star positions more accurately than are required for
the
present
purpose.
To
minimize
the
complexity
of
the
interfacing/preprocessing hardware in the phased array mode, we will
tap the sampler
outputs after the
delay compensation.
Such 3-bit
samples from each antenna will be added together to finally produce 4
outputs corresponding to the 2 polarization channels for each of the 2
bands of 16 MHz.
The above requirements are similar to those in the case of the
phased central-square for targeted searches.
Therefore, we need to
just duplicate the hardware that would combine the outputs from 16
central square dishes, for combining the outputs from the rest of the
dishes (also ~16 in number).
The outputs at this stage can then be
correspondingly added together with the combined output of the centralsquare dishes.
These 4 outputs will need to processed in a very
flexible way.
We will first try to list some definite aims for which this data
will be useful and then note some special requirements implied in terms
of the hardware for data processing.
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Date : Tuesday, October 17, 2000
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i)
To obtain high time resolution, high dynamic range average
pulse profiles with polarization.
ii)
To measure pulse arrival times.
iii) For single pulse studies, and studies of related topics e.g.
mode
changing,
(over
short
fluctuation
timescales)
intervening
medium,
spectra,
due
to
drifting
intensity
the
variation
effects
subpulses,
of
the
nulling,
microstructure etc..
iv)
Accurate
measurements
of
positions
of
pulsars
(VLBI)
for
estimation of proper motion.
v)
Measurements of HI emmission-obsorption profiles in pulsar
directions.
vi)
Measurements of radio spectra of pulsars over the entire
range of GMRT frequencies.
vii) Measurements of super-dispersion delays.
viii) To
obtain
improved
estimates
of
Dispersion
measures,
Rotation measures and to estimate the amount of scattering
in the interstellar medium.
ix)
Measurements of slow variation in the intensity, position,
etc.
of
pulsars
caused
by
refractive
effects
in
the
interstellar medium.
For many of the above mentioned observations, we can assumed that
the dispersion measure and the rotation measure values for a pulsar are
known.
Then it is convenient to “derotate” and dedisperse the pulsar
signal before recording the final output.
To do this, we would first
Fourier transform the combined output from all the dishes using VLBA
chips or some other suitable chips.
The number of spectral channels
could now be upto 512 per side band of width 1 to 16 MHz depending on
the center frequency of observation, the dispersion measure and the
time resolution required etc..
In this particular scheme, as we are
not
dedispersion,
attempting
pre-detection
the
time
resolution
achievable is limited to (N/BW); where N is the number of spectral
channels
over
a
bandwidth
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BW.
In
an
optimum
Date : Tuesday, October 17, 2000
configuration,
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the
dispersion smearing per spectral channel; i.e.
equal to N/BW.
KDM  BW 

 ; should be
f o3  N 
Considering only these two effects, the time resolution
Δt is given by
t 
2
2
 KDM   1  2
BW
 
f     where f 
3
N
 fo
  f 
Minimum value of Δt corresponds to the condition
2
 KDM 
2
  3
2f 
3

f
 fo 
i.e.
1
 KDM  2

f  
3

f
 o 
Thus
1
t min
1
 2KDM  2

2
  9x10 4  DM   sec .
 
3

f 3 
 fo 
 o 
where fo is the centre frequency in MHz and DM is the dispersion measure
in
Cm-3pc.
Fig. 4(a,b,c,d,e) shows the plots of Δtmin as a function of DM for
different values of fo.
We
also
indicate
the
smearing
scattering at different frequencies.
(τs)
due
to
the
interstellar
In individual cases this smearing
can differ from the indicated values by an order of magnitude in both
directions.
Assuming that BW can be equal to 1,2,4,8 or 16 MHz per side band
and the number of spectral channels per side band could be selected
from 64 to 512 (i.e. 64, 128, 256 or 512), the spectral resolution (Δf )
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Date : Tuesday, October 17, 2000
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could range from 2 KHz to 250 KHz.
achieve
the
time
resolution
This resolution range is enough to
indicated
Δtmin
by
at
frequencies except probably at 38 MHz (see fig. 4).
be noted that over the relevant
all
the
GMRT
However, it should
range of dispersion measures, the
effects of the interstellar scattering would dominate at 38 MHz, thus
relaxing the requirement of time resolution indicated by Δtmin.
The Δt Vs. DM plots (i.e. fig. 4) for different values of fo show
that suitable bandwidths and number of spectral channels can be chosen
to observe pulsar signals (at a given DM) with time resolution very
close to the theoretical Δtmin.
While selecting Δf in this manner, it is important to make sure
that the rotation of polarization vector across the band due to Faraday
rotation (in the intervening medium) is very small.
As the choice of
Δf depends on the observing frequency and DM, we plot in Fig. 5 the
maximum differential rotation across a spectral channel Vs. DM for
different observing frequencies, noting that the average line-of-sight
magnetic field = 1.232 (RM/DM) G (where RM is the rotation measure in
radians per meter square) and assuming that the magnitude of this value
in the worst case is about 0.5 G (See Fig. 6).
serious except at 38 MHz.
This effect is not
In any case at 38 MHz the interstellar
scattering dominating at high DMs (which are assumed to correspond to
high RMs) limit the effective time resolution rendering polarization
measurements across the pulse very difficult.
For each sideband, we will have two Fourier transformed outputs
corresponding
to
the
two
circular
polarizations.
The
different
spectral channel data will flow out sequentially at the rate of twice
the
bandwidth
(single
side
band)
used.
We
would
process
the
two
circular polarization channels to produce 4 Stoke’s parameters using a
fast
polarimeter.
The
Stoke’s
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parameters
need
Date : Tuesday, October 17, 2000
to
be
corrected
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for
Faraday
rotation
caused
by
the
intervening
medium.
The
corrected
parameters for all spectral channels will then be combined together
using a suitable dedispersion procedure, to output finally each of the
4 Stoke’s parameters at the rate of 2Δf.
Polarimeter/derotator
This module needs to work at 32 MHz rate when 16 MHz bandwidth
(per sideband) is used.
The two complex numbers input to this module
at any time correspond to the two circular polarization channels of a
’
’
’
Let these numbers be denoted by E R = C R+jS R
given spectral channel.
’
’
’
and E L = C L+jS L
(R = Right circular and L = Left circular).
Let the
phase correction (derotation) required due to the Faraday rotation to
one of the inputs (R) with respect to the other (L) be
rotate one of the vectors (i.e. R) by
Φ,
Φ.
We can first
such that
’ +j
’
ER = E Re Φ and EL = E L
Then the Stoke’s parameters (I,Q,U and V) are given by
I α ER
2
 EL

U α 2 ImE
and
2
 E' R
Q α 2 Re E R E L
*
EL
*
R
2
V α E L  E' R
2
2
 E' L
2
2
 E' R
2


 E' L
Where Re(a) and Im(a)indicate the real and imaginary parts of a.
’
’
The two inputs E R and E L
earlier.
The multiplication by
are in 4,4,4 format as encountered
ej Φ
can be performed using only data
over the first 8 bits (4,4), ignoring the exponent for the time being.
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Date : Tuesday, October 17, 2000
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’
That is, if E R
 (A’R,
’
’
’
’
’
B R, C R) where A R, B R, C R are 4 bits each, we will
need to phase rotate the complex no. AR+jBR by
AR+jBR=(A’R+jB’R)
If we use a look-up table to which
to read out
2(8+N)
(A’R,
(A’R,
Φ to
obtain
ejΦ.
B’R) and Φ will form the address
B’R), we will need a PROM with width 8 bits and depth
where N is the number of bits needed to represent
Φ.
N=7 is
sufficient considering the effective inaccuracies in phase due to 4-bit
representation of A and B.
This will mean that
discretely in steps of about 3o.
can be represented
Thus the PROM size will be 8 x 32k
’
’
The two exponents C R, and C L
bits.
Φ
can be added by using another look-
up table to obtain C= C R+C L+1. Then the parameters Q and U are given by
’
’
Q α 2c ( ARAL + BRBL )
U α 2c ( ALBR - ARBL )
The required four terms in the brackets can be obtained again
The outputs ARAL, BRBL with C can be used to
through look-up tables.
look-up a value of Q and ARBL, ALBR with C for a value of U.
The
’
’
values of |E R|² and |E L|² can be looked-up separately as discussed in
earlier
sections
to
find
correspond to (I+V)/2,
the
(I-V)/2,
total
Q,
power.
U.
Thus
the
outputs
will
We would also compute I and V
explicitly, making the possible number of outputs 6.
For simultaneous
two-frequency observations, the two-frequency data will be available
through
the
polarization
two
channels
channels.
which
In
this
otherwise
correspond
case,
total
the
to
power
the
two
outputs
corresponding to (I+V)/2, and (I-V)/2 will be used as corresponding to two
observing
frequencies
and
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the
other
outputs
Date : Tuesday, October 17, 2000
will
be
ignored.
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A
simplified block diagram of this module is shown in Fig. 7.
shown
here
is
just
one
of
the
many
possible
The scheme
schemes.
If
fast
multipliers are available, then the corresponding lookup circuitry for
phase rotation and other complex multiplications could be replaced by
such devices.
A
maximum of 4 of the 6 possible
outputs will be
processed further for dedispersion at any time.
Dedispers:
We will need in general 4 dedispers per side band.
For single
frequency polarization observation inputs to all the 4 units (for 4
Stoke’s
parameters)
will
be
predetermined delay gradient,
dedispersed
according
to
a
common
while for dual frequency observations
only two units will be used and will have different delay gradients for
dedispersion.
Here, we will discuss a possible design for such a unit.
The input to
this unit will be from the polarimeter and different frequency channel
data arrive sequentially every 1/(2BW) in time.
The aim here is to combine different frequency channel data after
appropriate delay corrections to individual frequency channel outputs.
If Δf
is the bandwidth of each spectral channel then the effective
sampling time/channel is 1/(2Δf) and this will also be the minimum
sampling time at the output of such dedisperser.
If the ith spectral
channel is to be delayed by Di samples, then the output is
i max
A out  j   A in i,  j - D i 
i 1
Where
Ain(i, j) is
the ith channel sample at time
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Date : Tuesday, October 17, 2000
jΔtsamp .
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Dedispersing in a conventional way, i.e. by introducing suitable
delays in the paths of Ain and summing the delayed versions would
involve a large number of hardware components, as we have a large
number of spectral channels.
We propose the following design to reduce
the hardware requirement considerably.
If Dmax is the maximum delay in
units of sampling interval, then we can use 3 memory units each of
depth L
> Dmax.
accumulate
Two of the memory units [(1, 2) or (2, 3) or (3, 1)] will
Aout while the output from the 3rd [i.e. 3 or 1 or 2
respectively] is being read.
After (LΔtsamp) time the first of the two
memories used for accumulation will be read, and will output L samples
of Aout as shown in Fig. 8.
As a sample Ain(i, j) will be added to
Aout(J+Di), we will need to know values Di.
These Di values for i upto 512
can be predetermined using DM value, Δtsamp etc.
available on a separate memory.
any sample Ain(i, j)
and can be made
Then the address of the location where
should be added (where j is modulo 3L+1) is J+Di
which is also modulo 3L+1.
Each cycle, consisting of:
get address --> read data memory --> add new sample -->
write back
into the memory, needs to be completed within about 30nsec in the
extreme case, and this should be possible with the speeds achievable
with the components now available in the market.
The data output rate from each dedisperser could be as high as
2sec/sample.
some
cases
memory.
is
Further integration in time to reduce the data rate in
possible
by
manipulating
the
addresses
to
the
data
A factor of 2 reduction in the data output rate is always
posssible wihtout significant loss of resolution in time.
This will be
important when Δf ~ 250 KHz.
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If
two
sidebands
are
used,
the
corresponding
data
will
be
dedispersed separately and the final outputs can be combined together
with suitable delay using a variable shift register in one path.
Maximum of 4 outputs (assuming 16 bits each) that are related to
the Stoke’s parameters can be recorded on to a suitable medium (e.g.
mag. Tape 6250 bpi, 100 ips) quite comfortably except when high time
resolution is to be retained at very high frequencies (at 1400 MHz and
partly at 610 MHz) for very low dispersion measure pulsars.
cases we may either compromise by
In such
reducing the time resolution to
reduce the effective output data rate or use gating over main pulse and
(possible) interpulse region again reducing the effective data output
rate.
This
data
can
then
be
processed
off-line
using
the
central
computing facility.
The polarimeter + dedisperser described here will cater to the
requirements for:
i)
high time resolution, high dynamic range studies of pulse
profiles with polarization.
ii)
Some
single
pulse
studies
e.g.
studies
of
mode
changing
intrinsic fluctuation spectra, drifting subpulses, nulling.
iii) Estimation of average pulse energy density over the range of
GMRT frequencies and to study slow variations in intensity.
iv)
Estimation of the pulse smearing due to scattering in the
interstellar medium.
v)
Measurements of super-dispersion delays using two frequency
observations.
For accurate estimations of DM and also for measurements of HI
emission/absorption in directions of pulsars we would like to obtain
average pulse profiles separately in each spectral channel.
If we
assume
to
that
the
pulse
profile
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in
each
spectral
Date : Tuesday, October 17, 2000
channel
is
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be
obtained over about 128 bins, then we would need (512 x 128 = ) 64K
memory locations to store and integrate the data.
With two such large
memories and a fast adder such averaging can be realised.
This folded
data at a much reduced rate can be recorded on a magnetic tape.
studies
of
the
time
and
frequency
structure
of
the
For
interstellar
scintillation also, a synchronous averager as above will be useful.
Here the averaging is performed over times shorter than the correlation
time scales of the scintillation.
we
can
employ
polarimeter.
4
such
For estimation of Rotation measures,
synchronous
averagers
for
4
outputs
of
the
The phase rotation of one of the circular polarizations
in the polarimeter is then not performed.
The polarimeters, dedispers, and synchronous averagers described
above need to be controlled by a suitable microprocessor based system
or a PC.
This control system can be in turn controlled by the Master
control system for observations. The main jobs of the required control
system would be to determine the phase shifts in the polarimeter, delay
gradients
in
the
dedispersers,
the
addresses
during
synchronous
averaging and also to control the configurations required for different
applications.
For
microstructure
dedispersion
is
studies
essential.
and
Such
a
timing
studies,
dedisperser
initially for a smaller bandwidth (1 or 2 MHz).
will
coherent
be
designed
Details related to
this will be worked out later.
VLBI
on
pulsars
to
obtain
accurate
position
information
is
important for estimation of proper motions and for measurements of slow
position changes due to refractive effects in the interstellar medium.
Details
relating
to
this
aspect
need
to
be
discussed
S.Ananthakrishnan.
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with
Here, I have tried to list (roughly in the order of priority) some
of the pulsar observational programmes with GMRT.
Final priorities
will need to be discussed and decided.
i]
Targeted pulsar search using the dishes in the central square
available by 1991.
This would need the hardware to combine the dish outputs after
samplers, 4 FT boards, 4 detectors, 2 pre-integrators + bit selectors,
VLBI mark II type recording system and play back system with a suitable
interface to the computing machine to be used for data processing.
Initially, we may build the hardware required for only one side
band of 16 MHz and/or reduce the number of spectral channels by a
factor of 2.
Then, it will be possible to record the data on to high
density magnetic tapes, till the time when a VLBI mark II type recorder
becomes available for use.
Data processing may be initially carried out using the central
computing facility or any other better facility available by then.
ii]
Studies of known pulsars: VLBI, timing, microstructure.
This may initially be done using only 1 side band of 2 MHz.
The
hardware to combine the dish outputs after samplers and VLBI mark II
type recording system will be used.
For timing and microstructure
studies a coherent dedisperser will be used.
iii] Other
polarization,
studies
interpulse
of
known
emission,
pulsars:
energy
Average
spectra,
pulse
profiles,
single
pulse
studies, etc.
This
would
need
polarimeters,
dedispersers
(post-detection)
addition to the hardware that would be used for targeted searches.
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in
iv]
Untargeted search: (After sufficient number of dishes are
available for use).
This would need 128 detectors and two combining networks to add 64
detector outputs each.
The preintegrator and the hardware beyond that
stage would be same as that for targeted searches.
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**************************************************************
***** Insert the Diagrams ---------------------------- *******
**************************************************************
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Computational Requirements for Pulsar Search with GMRT
In
the
case
of
untargeted
search,
256
spectral
channel
data
(corresponding to 32 MHz bandwidth) will be sampled every 256 sec for
about 4 minutes, on each field.
This will correspond to 256 M samples
of data (per field) that will need to be processed in a time comparable
to the observing time.
If we adopt the approach where the data is dedispersed first and
then different dedispersed outputs are Fourier transformed (1-D), then
we would need to produce and process about ~700 dedispersed outputs.
These outputs would cover a dispersion measure (DM) range of 0 to
~3000 cm-3 pc and would have appropriate steps in DM such that pulsar
signals with dispersion measures in between the discrete values will
not suffer any significant additional smearing.
Each dedispersed output would typically have 106 samples.
number
of
samples
in
high
DM
outputs
averaging and coarse sampling.
can
be
reduced
by
The
suitable
The dedispersion operation for 256
spectral channels to produce ~700 dedispersed channels may need even in
the best case ~5 x 109 fixed-point operations (additions).
This data then is to be Fourier transformed separately.
As the
number of data points in higher DM channels would be smaller than 106,
we will assume for simplicity that the FT computational requirements in
~500 FFTs of 106
our case are equivalent to about
points each.
number of floating-point operations would then be ~40 x 109.
computation
of
amplitude
spectra
(single–side)
would
require
The
The
~109
floating-point operations.
Harmonic averaging upto 32 harmonics (1,2,4,8,16,32) and scanning
for significant periodicities would require ~35 x 109 floating-point
operations.
Total
number
of
floating-point
operations
could
estimated to be ~80 x 109.
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be
If this analysis needs to be performed in about 4 minutes (which
would
be
the
observing
time
per
computing power of ~300 MFLOPS.
field)
then
it
corresponds
to
a
If we increase the individual spectral
channel width, so that we would have only 128 channel data, then the
computing power requirement would come down to ~150 MFLOPS.
It may be
possible to relax the requirements in MFLOPS by allowing more time(than
observing time) for analysis.
However,
it may be pointed out that in
the estimates given above, we have not included the requirements for
search in doppler acceleration that may need an order of magnitude
increase in the computing power.
Therefore, it is better if we aim for
a computing power ≳ 300 MFLOPS which then with some compromises can be
used for search in doppler acceleration also.
I
have
been
thinking
about
many
possible
ways
to
meet
the
computational requirements and I will soon send you a separate note
describing them.
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MAJOR CHANGES SINCE THE FIRST REPORT (dt. December 1989)
i)
Outputs of FT boards will be used for producing both the
phased and the incoherent array outputs.
ii)
The
frequency
of
operation
of
the
hardware
for
pulsar
observation will now be 16 MHz (uniform rate) instead of 32
MHz (Burst/uniform).
iii) Both the phased and the incoherent array outputs will be
produced parallely and will be made available for other uses
also (e.g. VLBI etc.).
iv)
For
coherent
dedispersion,
where
much
longer
FTs
(32K
points) are needed, the phased array output will be suitably
inverse
Fourier
Transformed
to
obtain
time
sequence
over
separate bands of 2.5 MHz.
v)
We will assume that the Exabyte recorder (to be used to
record the correlator outputs) will be available initially
to record the data from the pulsar receiver also.
vi)
The final diagram is shown on the next page.
Desh
21st Aug. ‘90
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