Station Real Time Software

The station real time comtrol computer is a single MVME 147 computer running the VxWorks real-time operating system. The computer configuration and programming style is similar to that of the VLBA antenna control system. Many of the software modules used for controlling the VLBA antennas and hardware are used in the OVLBI system.


The initialization function for the tracking station runs usually only at system reboot. During reboot of the system, several critical tasks are spawned and hardware reset functions are preformed:
All Monitor Control Bus (MCB) devices are reset during initialization.
Station timing task, TIC, generates semaphores to synronize the real-time tasks.
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.
Task DISPATCH reads the default command file (start.cmd), which sets all station hardware in the defualt state upon reboot.
Task STATUS generates an ascii summary of the station state for diagnostic purposes.

Realtime Tasks

A large number of tasks are spawned by INITIALIZE and by the operator interface (SCREENS). The realtime tasks are described below:
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.

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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

The realtime control task, DISPATCH, sets the station state based on input command files. These command files are ascii files that are readable (and editable). The top level files are generated by the offline program, SCHEDCO. The top level command file "includes" several lower level command files that setup all station hardware. Numerous command files have been written to set the hardware in special test modes.

Command files are also used to place the system in a mode for making pointing observations of astronomcical raio sources.

Orbit Files

The orbit files are simple ascii tables time tagged by date and time. The GEOMETRY task reads the orbit prediction data and makes it available to the POINT and TWT tasks.


The operator interface to the tracking station control computer is the same as that for the VLBA, the SCREEN package. Little design documentation is available for the package, but since the GBES version has a different layout that the VLBA version, a GBES SCREENs users guide is available.

Return to the Station Software Guide. The National Radio Astronomy Observatory (NRAO) is a facillity of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
Last update: 97 July 1