SOFTWARE REQUIRMENTS FOR THE GREEN BANK OVLBI EARTH STATION

		      Larry R. D'Addario
			11 January 1991


General Requirements

	A set of programs running in a dedicated control computer at 
the earth station is required to perform the following main 
functions.

1.  Automatically control and monitor all equipment at the station.  
It must do this by reading one or more *control files* and writing 
one or more *log files* while communicating with the station 
hardware in real time.  One of the control files will be the 
*schedule file*, which will contain the starting and stopping times 
of each assigned tracking pass and the equipment setup parameters to 
be used (e.g., recording mode).  Another file will contain 
time-specific geometric data, such as the predicted orbit of the 
satellite.  Monitoring data will be collected from the hardware on a 
predetermined schedule and analyzed to detect any equipment faults;
selected data will be written to a log file.  Equipment control 
includes commanding the antenna motion to track the satellite, with 
all necessary geometrical calculations.

2.  Provide automatic protection for the antenna and other major 
hardware.  This involves stowing the antenna in the event of very 
strong winds, and shutting down some electronics in the event of
high temperatures or other severe faults. 

NOTE:  Once the appropriate programs are started, operation should 
continue without human intervention until the end of the schedule 
file, unless a severe fault or disturbance is detected.

3.  Provide displays for a human operator and allow manual control.
The operator interface must be runnable either locally, at the 
station computer's console, or remotely via local- or wide-area
networks.  The operator should be able to examine current monitor
data, review the log files, override the automatic functions, select
which control files to use, and transfer files to and from remote
computers.  Operator monitoring and file transfers must not
interfere with the automatic operation of the station, which
proceeds simultaneously. 

4.  Analyze some of the monitor data to determine certain 
scientifically important parameters, and put these in a form (TBD) 
needed by other elements of the mission.  This includes 
interpreting the round-trip timing measurements for both orbit 
determination and correlator timing.  It also includes extracting 
spacecraft receiver data from the wideband data stream and 
re-formatting it as required.  Programs for these functions may run 
off-line, not in real time.

5.  Provide programs for converting files received from other 
elements of the mission into earth station control files, if 
necessary.  This includes schedule information and predicted orbit 
data.  Such programs may be run off-line.

NOTE:  Tasks 4 and 5 involve defining interfaces to other 
organizations both inside and outside the NRAO.  These include the 
VLBA project, NASA/JPL, and each of the foreign space agencies.


Constraints

1.  The interfaces should have the "look and feel" of a VLBA station 
as much as possible.  This includes the operator displays and 
commands; in particular, devices that have nearly identical 
counterparts at VLBA stations (e.g., recorders) should be seen 
through identical operator interfaces.  In addition, the schedule 
and log files should follow VLBA formats whenever possible.

2.  All real-time communication with the station hardware will be 
via the VLBA monitor and control bus (MCB), which is described in 
NRAO Specification A55001D001.

3.  Communication with locations outside the station will be via the 
Green Bank ethernet LAN.  Gateways to wide-area networks are 
available.

4.  Software will be written in the C language whenever possible, 
since this language has become a de facto NRAO standard.


Options

1.  The VLBA stations have a control computer that uses Motorola 
processors (680x0) in a VME bus architecture, running under the 
VxWorks operating system.  This same hardware and operating system 
might be adopted for the earth station.

2.  A hardware platform based on Intel 80x86-series processors in an
IBM PC architecture might be adopted.  An appropriate operating 
system that provides good support for real-time operations and 
networking would need to be selected.  Use of MS-DOS does not appear 
to be feasible.

3.  The VLBA control software has been written as numerous C 
modules, and includes considerable code for controlling and 
monitoring VLBA station hardware over the MCB, for providing 
operator displays, and for following a schedule file.  Regardless of 
the platform and operating system adopted for the earth station, 
much of this code can be adopted.

4.  Interferometrics, Inc., is a commercial producer of VLBA 
recording equipment and is engaged in other businesses involving 
VLBI.  They have announced a project to produce a system of VLBI 
station control software within the PC architecture that will 
support both Mark III and VLBA recording equipment.  By virtue of a 
license agreement, NRAO will have the right to use this code.  
Especially if option 2 above is adopted, we should collaborate with 
Interferometrics in choosing a platform and operating system, and we 
should consider using as much of their code as possible.