Station Real Time Software

Initialization

MCB RESET

All Monitor Control Bus (MCB) devices are reset during initialization.

TIC

Station timing task, TIC, generates semaphores to synronize the real-time tasks.

LOGWRITER

All offline logs are writen by LOGWRITER task. All other realtime tasks write to a message queue. This message queue is read by the LOGWRITER task and written to files in 15 minute blocks.

DISPATCH

Task DISPATCH reads the default command file (start.cmd), which sets all station hardware in the defualt state upon reboot.

STATUS

Task STATUS generates an ascii summary of the station state for diagnostic purposes.

Realtime Tasks

DISPATCH

The station control task, DISPATCH is spawned by initialized. The DISPATCH task sets the station hardware into the desired state, based on the command file inputs. The desired station state read by the MONCHK task.

DISPATCH exits upon reaching the end of the input command file. The station operators restart the DISPATCH task using the SCREEN program.

MONCHK

System status is monitored by the MONCHK task. This task periodically calls a set of "monchk functions" which query the station hardware and software. Monitor data are logged at pre-defined intervals. The monitor data are compared with the desired state of the station hardware. If hardware values fall outside of predefined limits or are not equal to the desired state, anomalies are raised and recorded in the station log. The MONCHK task may spawn REACQUIRE and EMERGENCY in response to unusual circumcstances.

REACQUIRE

If the satellite signal is lost for a signifcant time, the satellite downlink signal must be reaquired and the formatter clock reset. At the time of setting the formatter clock, the decoder clock is read and the value recorded.

EMERGENCY

During high winds, power outages or other serious anomalous conditions, an EMERGENCY task is spawned. This task notifies the station operators and takes appropriate action.

GEOMETRY

The GEOMETRY task reads the orbit file and maintains a circular buffer of orbit file entries. The orbit file entries read into the system always bracket the current time.

POINT

The antenna motion is controlled by the POINT task which commands antenna motion. If satellite trajectories are followed, the POINT task interpolates between entries from the orbit file. If astronomical objects are followed, then the azimuth and elevation of the source is followed based on calculations internal to POINT.

TWT

The Two-Way-Timing task uses the orbit file data maintained by GEOMETRY to calculate parameters for the TWT module to follow the uplink and downlink frequency predictions. Once a second, the TWT task performs a cubic interpolation of the orbit. The differences between the predicted and measured downlink are recorded in a phase residual file. This file is written directly to the station internal drive. After the pass, the file is copied to the offline system and deleted from the internal drive.

SCREENS

Many tasks are spawned, at low priority, to support the operator interface package, called SCREENS. Many SCREENS are typically used by the operator, and each SCREEN will cause the SCREEN control program to spawn two tasks. One task will take the asyncronous operator input, and the other task will continuously display the hardware values on the terminal. Many SCREENS (such as TAPE support) were taken unmodified from the VLBA software.

SAMPLE Tasks

Some hardware devices respond too slowly or too un-reliably to allow direct communication between device and time critical task. Sample tasks, like SAMPLEGPS continually pole the hardware device at some frequent rate. The values are saved and immediately returned to any other TASK require the information.

SAMPLE GPS

The station absolute time is calibrated relative to GPS measurements of the offset between station 1 Hz and GPS 1 Hz. The True Time GPS module returns the GPS time of the station 1 Hz.

SAMPLE UPS

Some critical modules are kept on UPS power backup in order to maintain station timing. The SAMPLEUPS task priodically reads the UPS state, returning whether the station is on UPS backup and if a power glitch has occured.

SAMPLE RTM

The Round Trip Measurement task measures the variation in the fiber optical cable length between the maser source in the Interferometer basement and the antenna vertex rack. The module which measures the round trip time is located at the Interferometer, and the SAMPLERTM task uses "RPC" calls to the realtime computer called "sitetime", to retrieve the maser status.

SAMPLE FMT

The calibration tones are extracted from the downlink data by the VLBA formatter module. The tone extraction must be timed relative to the satellite 1 Hz pulse, and this timing is performed by the SAMPLEFMT task. The calibration tone data are used by the CALTONES SCREEN and raw calibration tone counts are logged.

Command Files

Orbit Files

SCREENs