Data Acquisition

Let us first estimate the maximum data rate produced by the GMRT correlator. For each antenna there are four associated data streams (two sidebands USB and LSB, in each of two polarizations RR and LL) each of 16 MHz bandwidth. Therefore, one requires four sampling points per antenna with sampling rate of 32 MHz. The present correlator has only 60 sampling points, and handles only one side band. For each stream there is a corresponding FFT unit in the correlator. This FFT unit carries out a 512 data point transform in real time, i.e. one 512 point transform every $16 \mu$sec. The 512 point transform corresponds to 256 complex numbers, i.e. 256 frequency channels. The time to acquire 512 data samples, i.e. $16 \mu$sec is called a FFT cycle. The number of distinct pairs of antennas (including an antenna with itself, i.e. self correlations) that can be made from 30 antennas is $30 \times (30+1)/2 = 465$. Therefore, 465 Multiplier and Accumulator (MAC) units are required to correlate all data from 30 antennas. Each MAC unit accepts 4 data streams i.e. two polarizations from two antennas (it makes no sense to correlate USB with LSB) and multiplies them to produce 128 complex numbers each for two polarizations, or 256 values for one polarization. In either case, it is 256 complex numbers, which the MAC units sum for a duration of a STA (Short Term Accumulation) cycle, which is 4096 FFT cycles. One STA cycle is equivalent to $4096 \times 16\mu$sec $= 66$ms. The MAC data format is such that 4 bytes encode one complex number. The actual number of MACs in the correlator is $176\times 3$ (there are 3 Racks with 176 MAC units each) or $528$, i.e. there are $528-465$ redundant MACs. Therefore the total amount of data produced per second per side band is $528\times (1{\rm sec}/66 {\rm ms}) \times 256 \times 4~~ {\rm bytes} = 8$MB. The total data including both side bands would be $= 16$MB/sec.

$16$MB/s is a huge data rate to be sustained on any general purpose machine, or to be stored on any media. This means that would would need another piece of hardware in the correlator to carry out Long Term Accumulation (LTA). Such a hardware element was planned, but has not been implemented so far. Instead, in the present correlator system, the STA cycle has been configured for 8192 FFT cycles, i.e. the STA cycle duration is 132 ms. This brings down the sustained data rate per side band to 4 MB per second. Even this requires a special interface cards to input the data into the general purpose machine for processing. The host computer currently used for the DAS is a pentium based machine running the Linux operating system.