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Background: DSS

Observations on the GBT are scheduled using the Dynamic Scheduling System (DSS). The GBT Data Sender queries this system for details about upcoming observations, and sends that data to the OSSDB via the REST API.

Note that while the DSS contains scheduled observations, observers are not constrained to following those stated intentions, and could potentially  change their observation during the session.

GitLab Repository

The GitLab repository with this code can be found here: https://gitlab.nrao.edu/nms-evla/nrao-oss/gbt_data_sender

Sphinx Documentation

The full documentation for this project can be found here: http://planck.gb.nrao.edu:9438/index.html

Open issues

• Duplication of data entries – currently the Data Sender is not checking to see if it is sending a scheduled observation already in the OSSDB, creating the potential for duplicate entries.

The DSS Database

observations are heavily affected by weather, among other factors. So, in order to maximize efficiency, GBT observations are scheduled only 24-48 hours in advance. The schedule consists of a series of Periods: contiguous blocks of time that a given set of Receivers are being used to observe a portion of a given Proposal. The public schedule can be viewed here.

The DSS is currently  the sole source of information for the GBT Data Sender. 

Note that while each scheduled Period DSS is linked to an explicit set of Receivers, observers are not technically constrained to that set: they can observe using any available Receiver.

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To learn more about the DSS, please see the following resources:

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wiki

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The

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GBO ODS Exporter

The exporter consists of a Django command in the nell app, which pulls values from the DSS database and pushed them to the OSS database. The read and write URLs for the database are stored in a configuration file with restricted permissions.relevant component of the DSS is a web application named nell, which is written in Python using the Django web framework. The Exporter is implemented as a Django management command, ods_export.py . It runs via a cron job, once every 5 minutes. The flow is quite simple:

1. Check NRAO ODS to see if there is already data for the next-scheduled Period. If so, bail out
2. Inspect the next-scheduled Period for the relevant values
3. Send values to ODS API via HTTP POST

DSS Information

For a given looktime defined as now + 30 minutes, we first identify the observing period (look_period) and receiver (period_rx) which correspond to that time. 

look_period = Period.objects.filter(start__gte=self.looktime).order_by("start").first()
period_rx = Period_Receiver.objects.filter(period=self.look_period.id).first()

Site Data

The following information is sent as a JSON object to be a site_data entry in the OSS database.

obs_id

string

look_period.session.name

site_id

string

"gbt"

site_lat_deg

float

38.433129

site_lon_deg

float

-79.839839

site_el_m

float

824.0

src_id

string

look_period.session.target.source

src_ra_deg_j2000

float

look_period.session.target.vertical

src_dec_deg_j2000

float

look_period.session.target.horizontal

src_radius_deg

float

0.0

trk_rate_ra_deg_per_sec

float

0.0

trk_rate_dec_deg_per_sec

float

0.0

freq_lower_hz

float

period_rx.receiver.freq_low * 1E9

freq_upper_hz

float

period_rx.receiver.freq_hi * 1E9

src_start_utc

timestamp

look_period.start

src_end_utc

timestamp

look_period.start + look_period.duration

status

string

look_period.state.name

notes

string

look_period.session.observing_type.type

Limitations

The current version of the GBO Data Sender has a few severe limitations.

The primary limitation is inherent to the current design: the DSS lacks the level of granularity needed to accurately reflect GBT position at a given time. There is only one location provided for a Period, but Periods can be many hours long, and the GBT is not actually restricted to any positional offset from this location value.

There are a few other limitations that could be resolved with slight modifications to the current script:

1. The Period derivation has the potential to skip over a Period in the case where the next-scheduled Period is less than 30 minutes away when the Exporter runs
2. The Exporter selects an arbitrary Receiver for the next-scheduled Period; it does not consider all Receivers