The Field System (FS) acts as the controller for VLBI data acquisition hardware. The current version of FS, FS 9.1.5, runs on a Linux PC and it connects to hardware via MAT and/or MCB serial bus(es).
The FS can be used to alter hardware settings and monitor hardware status by using SNAP commands. SNAP is a line-based command language which gives access to low-level hardware features of a Mark III, VLBA, or Mark IV data acquisition rack. Individual SNAP commands can be entered interactively, they can be grouped into named SNAP procedures (in '.prc' procedure files), and they can be put in time-controlled sequences in schedule '.snp' files. All SNAP command responses are shown in FS start-up window and also logged in a '.log' file. The most common way to use the FS for VLBI observations is to run '.snp' schedule files as described in section run-sched .
A typical FS usage session would consist of the following:
fs
schedule=vt002,#1 -- start /usr2/sched/vt002.snp at line #1
help=schedule -- get a help window describing 'schedule' command
form -- get status report of formatter
terminate -- stop the FS
Arguments to SNAP commands follow a leading equals sign and multiple parameters are separated by commas.
If you need to set up the FS for the first time, please read the section first-setup first, before continuing with the following sections which assume that the FS computer has already been set up.
To stay updated with the latest development news about the FS, please make sure that at least somebody at your institute is subscribing the 'pcfs@bootes.gsfc.nasa.gov' FS mailing list. (Send a free-form request to join to 'pcfs-request@bootes.gsfc.nasa.gov'.) This is not a high-volume mailing list, but information posted here is not sent to any other VLBI-related mailing list and thus all FS users should be receiving it.
If you have questions about, suggestions for, or problems with the FS you can contact 'Ari.Mujunen@hut.fi' and/or Ed Himwich at 'weh@vega.gsfc.nasa.gov'.
An elaborate set of documents exists for FS9---they currently span three volumes. (If you do not have the set of documentation binders, please contact Nancy Vandenberg at 'nrv@bootes.gsfc.nasa.gov'.) Volume 1 is intended to support VLBI observing operations while Volume 2 contains reference material about MAT and MCB protocols, FS internal architecture, and computer-related issues. Volume 3 is actually a supplement to volumes 1 and 2: it contains new computer-related material introduced when FS was upgraded from FS8 (VENIX) to FS9 (Linux). If your supplement binder still contains the manuals "Operators' Reference" and "Experiment Operations" (as they were shipped in this way) may we suggest that you move these to their original location in Volume 1 and discard the old FS8 versions of these manuals.
The following main categories of information are present in Volume 1:
Volume 2 (and the supplementary third volume) contain the following:
After you have performed the first-time setup steps described in section first-setup , it is still useful to check the following a few days before a session:
if your site has multiple antennas with different diameters, slew speeds, axis types, and limits. Antenna position is not defined here but in 'location.ctl'.
hardly ever needs to change except when you need to move MAT or MCB cables from one RS232 port to another.
lists receiver RF frequency, rack type, and tape drive parameters. VLBA racks can change from 'vlba' to 'vlbag' (MkIII compatible sampler wiring/connectors) and receiver RF frequency changes when changing receivers, but other parameters should stay relatively unchanged.
defines site mane and code and antenna location. May change if your site has multiple antennas.
the rate parameter may need updating if the FS PC clock lags or gains significantly. Please see section setcl .
To prepass tapes you can use the SNAP command 'schedule=prepass,#1'. The default schedule 'prepass.snp' calls the station default SNAP procedure 'prepass' and after it unloads the tape. Invoking the schedule instead of just the SNAP procedure has the added benefit that all prepass runs will be logged into '/usr2/log/prepass.log'. If you use the 'label=xxxxxxxx,xxxx' command instead of 'cont' after threading in the tape, you will automatically get a permanent record of which tapes you have prepassed and when.
Running schedules is described in "Experiment Operations" manual section 4.0. Section 4.5 lists typical problems during schedules and describes corrective actions.
Setting up the FS PC and Linux is covered in "Computer Reference" manual and additional information can be found in "Computer Reference Appendices". Each version of FS comes with its own setup notes, for example a file 'fs91up.txt' which outlines the differences of setting this particular version of FS when compared to the general instructions presented in sections 4.5 and 4.6 of "Computer Reference".
The following files at directory '/usr2/control' set the characteristics of a FS installation:
antenna diameter, slew speeds, axis type, and limits.
controls antenna calibration programs (ACP), lists useful pointing sources.
assigns MAT, MCB, antenna, and GPIB Linux device names.
tape drive characteristics, receiver RF frequency, data acquisition rack type, recorder type, etc.
narrow track head positioner calibration, LVDT voltage to head position scaling factors, inchworm motor speeds. Can be calibrated using instructions in "Narrow Track Calibration" manual.
GPIB addresses of two-character GPIB device mnemonics. Mainly used for cable delay counter unit 'CA'.
defines station name and one-letter code and antenna coordinates.
address assignments of MAT devices, device mnemonics (two characters) to hexadecimal addresses.
address assignments of MCB devices, two-character device mnemonics to two-character hexadecimal identifier. Identifiers are dynamically assigned and downloaded at FS startup time by the procedure 'vlbainit', called from automatic 'initi'. Whenever a given MCB module is powered off/on, the address information must be downloaded again with a corresponding 'bbc01=addr', 'rec=addr', or 'form=addr' SNAP command.
pointing model coefficients are kept here.
control file for 'xtrac' pointing measurement post-processing program.
defines the 32 A/D converted monitoring voltage channels of NASA S/X CDP receiver.
defines the temperature sense diode characteristic voltage-to-kelvins curve.
defines defaults to 'drudg', for example bar code label printer type.
enumerates station-specific SNAP commands.
enumerates site-specific field system programs that should be automatically started at 'fs' startup, except 'antcn' antenna communications.
definitions for interfacing a spectrum analyzer via GPIB to FS.
keeps FS PC clock characteristic drift rate (seconds/day), please see below section setcl .
Please see "Control Files" manual for a detailed description of contents of control files.
When the FS is started, the standard 'initi' SNAP procedure which is automatically run at every startup runs the program 'setcl' to synchronize FS PC time to formatter time. 'setcl' does not actually change time in FS PC, it merely measures the offset between PC and formatter clocks.
Whenever the FS needs the current UT time, the FS PC clock is corrected with the measured offset plus a rate estimate. For FS time to be accurate, this rate estimate must accurately describe how many seconds a day the PC clock lags or gains. 'setcl' can be used to measure clock drift rate using the command 'sy=run setcl rate'. For this to work correctly you need to:
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