Shared Risk Observing Program

The Shared Risk Observing (SRO) program allows observers access to capabilities that can be set up via the Observation Preparation Tool and run through the dynamic scheduler (without intervention), but are not well tested. In semester 2013A this program provides a number of new correlator capabilities:

1. Data rates as high as 60 MB/s and integration times as short as 50msec.
2. Use of the 3-bit samplers in any frequency band, with subbands which can be narrower than 128 MHz. All subbands within a baseband must be contiguous, and must have the same bandwidth.  Note that it makes little sense to use the 3-bit samplers for L and S bands, where the 8-bit system cover the entire available observing band with higher dynamic range and better sensitivity.
3. Mixing 3- and 8-bit basebands (3-bit AC and 8-bit BD, or vice versa) in the same correlator configuration.
4. A simple spectral sweep mode using recirculation to provide 512 channels in dual polarization (RR, LL) for each of 32 64-MHz subbands spanning two 1 GHz basebands.
• Recirculation is a technique whereby narrowband data are fed back into a cross-multiplier chip multiple times to obtain more lags, and hence more spectral channels.
• This particular recirculation configuration provides 2 x 32 x 512= 32768 correlations per baseline.
5. More flexible Baseline Board stacking, with the ability to assign any number of Baseline Board pairs (BlBPs) to each subband, rather than only powers of two. These configurations are significantly more complex, and the observer is urged to read the section on spectral line capabilities (above) with care. Some examples of possible shared risk configurations are given below.
6. Access to the 3-bit samplers for observations with (up to 3) subarrays.

Those proposing for Shared Risk Observating must use the Shared Risk version of the Resource Catalog Tool (SRCT) to define their correlator set-up(s) instead of the GOST. Note however that the SRCT does not fully enforce the SRO restrictions outlined here, particularly for 3-bit observations, so some configurations allowed by the SRCT may correspond to Resident Shared Risk observations.  Those in doubt are encouraged to get in touch with NRAO staff well before the proposal deadline.

Shared risk observations: spectral line configurations

Arbitrary Baseline Board stacking using the 8-bit samplers is allowed under the Shared Risk Observing (SRO) program for semester 2013A. The basic capabilities are given in the table above (Subband Bandwidth and Spectral Resolution Options), the difference being that the number of Baseline Board pairs assigned to a subband (n_BlBP) can be any integer up to 64, rather than only a power of two. Similarly the interaction between BlBP stacking the number of independent subbands is as described above, but this interaction is more subtle and potentially confusing in the shared risk case.

To give some idea of the possibilities, the following tables give two examples of possible shared risk configurations. The SRCT display shows how the Baseline Boards are used to process the individual subbands. The cyan boxes show the Baseline Boards used to process data from baseband A0/C0, while the yellow boxes show Baseline Boards used to process data from baseband B0/D0.

Shared Risk Configuration Example #1
Baseband	Subband	Pol'n products	Spectral channels	nBlBP
A0/C0	sb0	RR	10240	40
A0/C0	sb1	LL	768	3
A0/C0	sb2	RR,LL	2176	17
B0/D0	sb0	RR	256	1
B0/D0	sb1	RR,LL	384	3
SRCT display:

Shared Risk Configuration Example #2
Baseband	Subband	Pol'n products	Spectral channels	nBlBP
A0/C0	sb0	RR	4352	17
A0/C0	sb1	RR, LL	1152	9
B0/D0	sb0	RR,RL,LR,LL	192	3
B0/D0	sb1	RR, LL	4480	35
SRCT display:

Note that the individual subbands can have different bandwidths, and those bandwidths may be chosen completely independently of the number of spectral channels in each subband. So for instance a subband with bandwidth 2 MHz and 1152 spectral channels would have a channel separation of 2 MHz/1152= 1.736 kHz; but the observer could equally well choose a bandwidth of 64 MHz for that subband, leading  to a channel separation of 64 MHz/1152=  55.56 kHz.

Again, while the SRCT understands the various hardware constraints on correlator configuration, it does not distinguish between configurations which are available as Shared Risk Observing (SRO), and those which are only available through the Resident Shared Risk Observing (RSRO) program. If in doubt, please consult with NRAO staff.