PC-STEER

Refers to the computer which actually drives the telescope. It presently consists of a PC running a program written in Turbo Pascal under DOS. It runs in slave mode and interfaces to the outside world via an HPIB interface responding to commands sent via the HPIB bus.

PC-STEER understands 22 ASCII type commands (cf. HartRAO Antenna Control System, page 20) and returns a status string everytime it is addressed in listen mode (idem. page 23).

PC-STEER also has a graphical interface with basic user interaction via the keyboard.

This system is working today and is used during VLBI observations and the Rhodes mapping program. However in order to replace HP 1000s a number of mandatory improvements must be made :

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radiometer digital voltmeter (DVM) readings be acquired synchronously with telescope coordinates,

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the hardware interface's known design deficiencies (power supply is instable, A/D conversion is done a long distance from the power supply etc.) have to be solved,

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UTC time as generated by the station clock must be read by PC-STEER and used to synchronize and set the computer time,

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replace the HPIB interface with Ethernet,

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apply time-of-day dependent hour angle (HA) and declination (DEC) error correction as on HP 1000, this is implemented as a table of offsets stored as a 2D function of UT and day of year (eventually to be replaced by a model),

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convert PC-STEER from Turbo Pascal running under DOS to C running under Linux.

Q: A FUNDAMENTAL QUESTION TO BE ASKED IS DOES PC-STEER HAVE TO BE CONVERTED TO LINUX AND C FOR THE FIRST PHASE OF THE NCCS OR WOULD IT BE POSSIBLE TO MAKE ONLY MINOR MODIFICATIONS TO PC-STEER AND CONCENTRATE ON THOSE PARTS OF THE SYSTEM WHICH DO NOT EXIST IN THE BEGINNING ?

A: THE ANSWER TO THIS WILL DEPEND LARGELY ON THE TECHNICAL FEASABILITY OF RUNNING PC-STEER UNDER LINUX AND THE HUMAN WORKLOAD INVOLVED IN DOING SO, A COMPREHENSIVE SET OF BENCHMARKS AND A DETAILED ANALYSIS ARE REQUIRED TO DETERMINE THIS.

In addition to the necessary changes the following changes are desirable but not mandatory (cf.A Proposed Configuration for the New Control Computer System (NCSS)) :

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control tilt and focus of subreflector (currently analog input and outputs),

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read angles from a second DEC encoder (using multiplexed encoder inputs) to improve reliability of DEC encoder readings,

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apply independent error maps to the two DEC encoders (note the error maps do not exist yet),

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check quality of data from encoders in low and high speed,

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apply refraction correction separately from the structural error map (formula available from VLBI field system),