VEX Parameter Tables

Rev 1.5c Nov 16, 2009

Preface to the wiki version This document is a nearly direct translation from the MS Word version of this document to Twiki wiki format. Along the way a very few typos were corrected and typewriter font has been employed to represent literal text. In addition, a few previously agreed, but undocumented, amendments to the VEX standard (such as LBADR) have been added. To this end, the version number has been bumped from Rev 1.5b to Rev 1.5c. This wiki-fied document will serve as the starting point to the definition of Vex 2.0.

Each VEX primitive $block is allowed to specify only parameters which are defined for that type of $block. The VEX Parameter Tables define in detail these parameters for each type of $block. Each table lists the allowed parameter names and the associated fields. Field numbers in parenthesis are optional. Parameter type '&link' is a VEX linkword as defined in the VEX documentation. If 'units' are specified, the field must have a units label of the proper type; 'units' enclosed in parenthesis are assumed and should not be labeled. Not all parameters need to be specified for a given experiment, only those which are 'relevant'. This listing does not attempt to specify the set of 'relevant' parameters, but is instead left to the assumed expertise of the scheduling-software writer. This listing is not to be regarded as necessarily complete, either with regard to $blocks or the set of native parameters within the $blocks. Many $blocks, particularly the $ xxxx _OBS blocks, are not yet specified. These $blocks will be defined in the near future. Additional parameters will undoubtedly be added as the need arises.

$ANTENNA Block

The $ANTENNA block specifies the detailed characteristics of the antenna itself, including the type of mount and pointing characteristics.

Parameter Field Description Type Allowed values Units Comments
antenna_diam 1 Antenna diameter real > 0 length  
antenna_name 1 Antenna name char     Useful, if needed, to distinguish particular antenna from site (portable antenna, for example)
axis_type 1 Primary axis type char az ¦ ha ¦ x ¦ fixed   Typically,
axis_type=az:el; or
axis_type=ha:dec; or
axis_type=x:yns; (N/S orientation) or
axis_type=x:yew; (E/W orientation), etc...
  (2) Secondary axis type char el ¦ dec ¦ yns ¦ yew   Pairing of parameters 1 and 2 must make sense (see comment immediately above)
  2 axis orientation char   angle 0 deg - north/south orientation;
90 deg - east/west orientation
axis_offset 2 real   length    
antenna_motion 1 Axis type char (axis type)   Must be one antenna_motion= statement for each motion axis.
  2 Slew rate real   ang/time  
  3 Settling time real   time  
pointing_sector 1 Sector ID or
cable-wrap zone ID
&link     May be used to specify pointing sector in $SCHED block
  2 Axis type char (axis type)    
  3 Lower limit real   angle Upper limit > Lower limit
  4 Upper limit real   angle  
  (5) Axis type char (axis type)    
  (6) Lower limit real   angle Upper limit > Lower limit
  (7) Upper limit real   angle  

$BBC Block

The $BBC block connects physical BBC's to the 'logical' BBC's defined in the $FREQ section, and also specifies the connection of the BBC to a 'logical' IF.

Parameter Field Description Type Allowed values Units Comments
BBC_assign 1 Logical BBC 'link' with chan_def= statement in $FREQ block &link      
  2 Physical BBC# int 1-16    
  3 Logical IF 'link' with if_def statement in $IF block &link      

Notes:
  1. All logical BBC's defined in the selected $FREQ 'def' must be present in the selected $BBC 'def', but not necessary vice versa.
  2. The $BBC block is ignored by the correlator.
  3. In the case of digital back ends, the BBCs are not actually physical, but will be numbered as such. In systems with multiple digital back ends, the BBC numbers begin at 1 for each. Details TBD

$CLOCK Block

The $CLOCK section specifies the necessary clock parameters for proper correlation of the data. Normally, this information will be taken from the station logs, transcribed manually or, as a last resort, determined from a fringe search at the correlator. Obviously, the $CLOCK section is needed only by the correlator.

Parameter Field Description Type Allowed values Units Comments
clock_early (1) Starting epoch for validity of this clock model epoch   epoch Model in this statement assumed valid until next specified epoch, if any. May be null of model is valid for entire experiment.
  2 'clock_early' - offset of formatter 1-pps tick with respect to UTC real   time >0 for formatter tick early
  (3) Epoch of origin of clock model epoch   epoch Needed if clock rate or acceleration specified
  (4) Clock rate real   time/time For example, usec/sec
Notes:
  1. In case of 'clock breaks', where multiple sets of clock parameters must be specified for a station during a single experiment, multiple 'clock=' statements may be used, each specifying the starting epoch of validity for the model in the corresponding statement.

$DAS Block

The $DAS block is intended to define the recording hardware present at a station. Because of the many combinations possible, various elements of the hardware are separately specified.

Parameter Field Description Type Allowed values Units Comments
record_transport_type 1 Transport type char Mark3A ¦ Mark4 ¦
VLBA ¦ VLBAG ¦
S2 ¦ K4 ¦ Mark5A
  Additional types may be added. 'S2' is also used for LBADR
  (2) Revision level char     Example: '1.0'
electronics_rack_type 1 Type of electronics rack char Mark3A ¦ Mark4 ¦
VLBA ¦ VLBAG ¦
S2 ¦ K4 ¦ LBA
   
number_drives 1 Number of tape drives (transports) int     Number of physical drives connected to system
headstack 1 Headstack# int 1-4   Specifies correspondence between physical headstack and logical drive. Relevant for MkIIIA, MkIV, VLBA, Mark 5A.
  2 Headstack function char read ¦
write ¦
read/write
   
  3 Drive-number offset with which this headstack is associated int     Added to drive# specified in the station statement in $SCHED block to determine physical drive#. Normally 0 for single-drive systems.
record_density 1 Longitudinal bit density along track int   (bpi) Usually 33,333 or 56,000 (bpi unit is assumed). Used to compute tape speed. Relevant only for MkIII, MkIV, VLBA systems.
tape_length 1 Tape length int   length ¦ time For Mark IIIA, Mark IV, VLBA: actual tape length. For S2, K4: recording time at data rate specified in field 2.
  (2) S2 tape speed char slp ¦ ep (Mbps) Relevant only for S2
  (3) Number of S2 cassettes int 1-8   Must be consistent with total date rate (each S2 cassette records 16 Mbps)
recording_system_ID 1 Recording-system serial number int     Must be unique for each system of a given type within an experiment. This information is often recorded on tape and used at the correlator. See Notes below.
record_transport_name 1 Recording-transport name char     Identifies a particular recording transport. Mostly useful for transportable recording systems.
electronics_rack_ID 1 Electronics rack serial number int     Identifies particular electronics rack
electronics_rack_name 1 Electronics rack name char     Like a serial number, but a name
tape_motion 1 Specifies how tape is to be controlled char start&stop ¦ continuous ¦ adaptive    
  2 Tape early-start time int >=0 time Relevant for start&stop and adaptive motion; specifies how soon tape should start before expected good data.
  (3) Late stop time int   time Relevant for S2 only. Specifies length of time tape should continue running after end of valid data. Typically 0.5 minutes.
  (4) Minimum gap for stopping tape int   time Relevant for S2 only. Specifies minimum valid-data time gap during which tape will be stopped. Typically 3 minutes.
tape_control 1 Define master station in cluster char master   Relevant only in antenna cluster where data from more than one station is recorded on a single recording system. Defines which antenna controls tape motion. Others are assumed to be slaves.
Notes:
  1. The DAS parameters are intended to be sufficient to describe any of the many variants of Mark3A, Mark4, Mark5 and VLBA systems currently deployed. Additional parameters may have to be added in the future as systems evolve.
  2. For Mark IIIA, Mark IV, VLBA and Mark 5A systems, the headstack statement indicates which 'headstack outputs' from the formatter are connected to the recording system(s). There must be one headstack statement for each formatter 'headstack output' to be recorded. Multiple recording systems may be connected to a single formatter.
  3. If multiple headstacks on same drive, each headstack must have a different headstack# (e.g. Mark IV). Systems with multiple simultaneously-recording headstacks on 2 drives (e.g. VLBA with 2 drives) must have two 'headstack=' statements with a different headstack# and logical drive linkword. The headstack #'s must correspond to the headstack #'s in the $TRACKS block.
  4. The recording_system_ID statement identifies the particular DAS used, akin to a serial number, which is usually written in the aux-data field of the recorded data. The recording_system_ID of each DAS within a given type (e.g., Mark4, VLBA, K4, etc) must be different for each DAS within an experiment; the recording_system_ID of a particular DAS is usually assigned at the time of manufacture. The recording_system_ID can be used by the correlator to positively identify the particular DAS upon which a tape was written. The parameter electronics_rack_ID is similar.
  5. If two or more antennas share the same DAS, the $STATION 'defs' for the corresponding antennas must have 'refs' to exactly the same set of $DAS keywords, including particularly the recording_system_ID parameter, except that one (and only one) of the stations must declare itself as the tape-control master with the inclusion of a tape_control=master; statement within the referenced $DAS 'defs' for that station.

$EOP Block

The $EOP block specifies the earth-orientation parameter to be used by the correlator; not needed for scheduling or data-taking.

Parameter Field Description Type Allowed values Units Comments
TAI-UTC 1 Ephemeris TAI-UTC real   time Normally fixed for entire experiment
A1-TAI 1 Ephemeris A1-TAI real   time Normally fixed for entire experiment
eop_ref_epoch 1 Epoch of first 'EOP point' epoch   epoch  
num_eop_points 1 Number EOP points int     Number of points over which interpolation is done
eop_interval 1 Time space of EOP points real   time Typically 24 hrs
ut1-utc 1 - num_eop_points Time series of ut1-utc values real   time Must be num_eop_points values in this statement. Units specification may be omitted after first field.
x_wobble 1 - num_eop_points Time series of x-pole values real   angle Must be num_eop_points values in this statement. Units specification may be omitted after first field.
y_wobble 1 - num_eop_points Time series of y-pole values real   angle Must be num_eop_points values in this statement. Units specification may be omitted after first field.

$EXPER Block

The $EXPER block contains general information useful for the success of the VLBI administrative process.

Parameter Field Description Type Allowed values Units Comments
exper_name 1 Experiment name char     Typically will be the standard 6-char experiment designator (example. RDWPS1)
exper_description 1 Experiment description char      
exper_nominal_start 1 Epoch of nominal experiment start epoch      
exper_nominal_stop 1 Epoch of nominal experiment end epoch      
PI_name 1 PI name char      
PI_email 1 PI e-mail address char      
contact_name 1 Contact name char      
contact_email 1 Contact e-mail address char      
scheduler_name 1 Scheduler name char      
scheduler_email 1 Scheduler e-mail address char      
target_correlator 1 Target correlator char VLBA ¦
VSOP ¦
JIVE ¦
Haystack ¦ etc.
  Others may be added.

$FREQ Block

The $FREQ block describes the signal and sampling characteristics the channels recorded on the tape, where a 'channel' is defined as a single USB or LSB output from a BBC. This includes such information as total RF sky frequency, sideband, channel bandwidth, sampling rate and bits/sample. The $FREQ block does not attempt to describe the recording mode, since the same set of channels may be recorded in different recording modes (or on different equipment) at different stations. Each frequency channel is defined by a chan_def= statement with at least 8 fields in each statement. The capability of specifying frequency switching is also built into the chan_def= statement by assigning each frequency channel to one or more numbered 'states'. The switching_cycle= statement then specifies the length of time spent in each of the states.

Parameter Field Description Type Allowed values Units Comments
chan_def 1 'Band_ID': RF band name &link     Link to selected $SOURCE 'def' where it may be used to describe source-structure characteristics for scheduling purposes. May be omitted if not relevant.
  2 RF sky frequency at 0Hz in the BBC output real   freq This frequency, in combination with the LO frequency specified in the $IF block allows computation of the BBC LO frequency for this channel.
  3 Net sideband of this BBC channel char U ¦ L   Note that this will be opposite the labeling on the BBC itself if the IF going into the BBC is net LSB
  4 BBC Channel bandwidth real   freq  
  5 'Chan_ID': Logical channel name &link     Must be different for each channel. Used as link to selected $TRACKS 'def'.
  6 'BBC_ID': Logical BBC name &link     Link to selected $BBC 'def where connection to physical BBC is made.
  7 'Phase-cal_ID': Logical phase-cal name &link     Link to selected $PHASE_CAL 'def' block to specify details of phase-cal tone(s). Null specifies no phase-cal to be detected.
  (8,9,...) Frequency-switched state numbers in which this channel is active int     Used only with frequency switching. Specified state numbers are separated by colons. States must be ordered 1,2,...
switching_cycle 1 Phasing of frequency-switch cycle char wrt_obs_start ¦ wrt_min_mark   Relevant only if frequency-switching being used. Timing of switching cycle begins according to this specification.
  2 State 1 period real   time Interval over which state 1 is active
  3... State 2 period, etc real   time Interval over which state 2 is active, etc.
sample_rate 1 Sample frequency real   sample rate Ignored for S2.

Notes:
  1. There must be one chan_def= statement for each BBC channel to be recorded.
  2. If one or more stations in an experiment observe different sets of frequency channels, there must be a separate 'def' for each different set of channels.
  3. A resolved link must exist for every specified 'linkword' in each chan_def= statement.

$HEAD_POS Block

The $HEAD_POS block defines the headstack positioning as a function of 'headstack-position reference number' for Mark IIIA, Mark IV, and VLBA systems; the $HEAD_POS block is irrelevant for other types of recording systems.. One headstack_pos= statement is required for each potential headstack position (or set of positions if multiple simultaneous recording headstacks).

Parameter Field Description Type Allowed values Units Comments
headstack_pos 1 Position reference number int >0    
  2 Headstack 1 position int range of headstack motion length Allowed range depends on system, but is typically -1000 to +1000 um.
  (3) Headstack 2 position int range of headstack motion length Required only if headstack 2 is being used.
  (4) Headstack 3 position int range of headstack motion length Required only if headstack 3 is being used.
  (5) Headstack 4 position int range of headstack motion length Required only if headstack 4 is being used.

$IF Block

The $IF block defines the IF bands used in the observations and is linked to the $BBC block (which specifies the detailed BBC-to-IF connections). An if_def= statement must be defined for each of the IF 'links' specified in the selected $BBC 'def'.

Parameter Field Description Type Allowed values Units Comments
if_def 1 'IF_ID' link word &link     One if_def= statement must be present for each separate IF.
  2 Physical IF name char     System dependent. Used to create procedures to select proper IF. See Notes.
  3 Polarization char R ¦ L    
  4 Total effective LO of IF (just before signal enters BBC) real   freq Positive number
  5 Net sideband of IF char U ¦ L    
  (6) Phase-cal frequency interval real   freq Typically 1 or 5 MHz. Null or omission indicates no phase-cal.
  (7) Phase-cal base frequency real   freq Usually 0, in which case may be null or omitted.

Notes:
  1. The 'total effective LO' is used in conjunction with the total sky frequency specified for each channel in the $FREQ block to calculate the local-oscillator setting in each individual BBC.
  2. The 'Physical IF Name' is a system-dependent designation specifying which IF is selected. For the VLBA system, IF's A, B, C, and D may each be selected with either a 'Normal' or 'External' input, leading to designations AN, AE, BN, BE, CN, CE, DN, and DE. For the Mark III system, IF's 1, 2, and 3 may each be selected with either a 'Normal' or 'Alternate' input, leading to the designations 1N, 1A, 2N, 2A, 3N, 3A.

$PASS_ORDER Block

The $PASS_ORDER block defines pass and group ordering relevant for Mark IIIA, Mark IV, VLBA and S2 systems. For Mark IIIA, Mark IV and VLBA system, each pass is defined by a two-part field composed of a numeric 'headstack-position reference' (defined in the selected $HEAD_POS 'def') followed by an alphabetic 'subpass identifier' (defined in the selected $TRACKS 'def'), e.g., 2A. For S2, the S2_group_order= statement defines the order of usage of tape groups.

Parameter Field Description Type Allowed values Units Comments
pass_order 1 First pass ID char     e.g., 1A. First pass assumed to be in 'forward' direction. For S2, specified group#.
  2 Second pass ID char     e.g., 2A
  ... etc        
S2_group_order 1 First group number int     Specifies order in which S2 groups are to be recorded.
  2 Second group number int      
  ... etc        

Notes:
  1. The number of fields present in the pass_order= or S2_group_order= statements specifies the number of tape passes or groups. Number of cassettes per group is defined by recording mode specified in $TRACKS section. S2 groups are numbered starting at 0.
  2. First pass is assumed to be in the forward-tape-motion direction (Mark IIIA, Mark IV, VLBA).

$PHASE_CAL_DETECT Block

The $PHASE_CAL_DETECT block is used to specify the phase-cal tones to be detected at the observing station.

Parameter Field Description Type Allowed values Units Comments
phase_cal_detect 1 pcal_ID &link     Links to a chan_def= statement in selected $FREQ def.
  2 Tone number (from DC edge of BBC output) int     Tone number of first tone to be detected. See tone-number definition in Notes.
  (3) Tone number int     Tone number of second tone to be detected
  ... etc        

Note:
  1. The actual phase-cal frequencies are determined by the LO frequencies specified in the $IF and $FREQ blocks.
  2. The phase-cal frequency spacing are specified in the $IF block.
  3. Tone number 1 is defined as first tone above 0Hz in the BBC output channel, etc. Phase-cal detection will be done on the set of specified 'tone#'s, which are listed in order of preference in case more tones are specified than can be detected by the hardware. The tones are numbered positively from the low (DC) edge of the BBC output band, with tone number 1 being the first tone above DC. Tones may also be specified as negative numbers corresponding to their position from the nominal band edge, with tone number -1 being the first tone below nominal band edge. A tone number of 0 specifies state counting, rather than phase-cal detection, should take place.

$PROCEDURES Block

The $PROCEDURES block specifies parameters relevant to various procedures at an observing station. Timing parameters are to be used as constraints to the scheduling program. The set of timing parameters listed is for the NASA 'sked' program. Other scheduling programs may use these and/or other parameters.

Parameter Field Description Type Allowed values Units Comments
tape_change 1 Tape-change time real   time Required parameter
headstack_motion 1 Time to complete headstack motion real   time Required parameter
new_source_command 1 Time to initiate pointing to new source real   time Required parameter
new_tape_setup 1 Time to setup system for new tape real   time Required parameter
setup_always 1 Setup system for each observation char on ¦ off   Optional procedure
  2 Time to setup system real   time  
parity_check 1 Do parity check char on ¦ off   Optional procedure
  2 Time needed to do parity check real   time  
tape_prepass 1 Do tape prepass char on ¦ off   Optional procedure
  2 Time needed to do tape prepass real   time  
preob_cal 1 Pre-observation calibration char on ¦ off   Optional calibration procedure
  2 Time needed for procedure real   time  
  3 Procedure name char      
midob_cal 1 Mod-observation calibration char on ¦ off   Optional calibration procedure
  2 Time needed for procedure real   time  
  3 Procedure name char      
postob_cal 1 Post-observation calibration char on ¦ off   Optional calibration procedure
  2 Time need for procedure real   time  
  3 Procedure name char      
procedure_name_prefix 1 Specify standard procedure library char     Specifies that a 'standard' procedure library is to be used.

Notes:
  1. The procedure_name_prefix parameter is intended to allow the specification of frequently-used procedure libraries that individual stations may have honed to their particular requirements. For instance, the geodesy community frequently uses the same station setup over and over again (e.g., so-called 'SX2C' setup). The specification of the 'standard procedures' for a station relieves the requirement of creating a new set of procedures for an experiment.

$ROLL Block

The $ROLL block defines the barrel-rolling sequence that may be used in VLBA and Mark4 recording systems. It is intentionally defined in a very general way, but for the most part will probably be confined to a few 'canned' modes. The roll sequence is specified with a roll= statement for each track participating in the barrel roll, plus statements defining the roll period and reinitialization interval.

Parameter Field Description Type Allowed values Units Comments
roll 1 Roll on/off char on ¦ off   Optional. See Notes.
roll_def 1 Headstack # int 1-4    
  2 Home track int track #   Track# that would be written in the absence of barrel-roll.
  3 Step 0 destination track       Track to which home track is written when barrel-roll is initialized (step 0)
  4 Step 1 destination track int track # - Track to which home-track is 'switched' on first increment of barrel-roll.
  5 Step 2 destination track int track # - Track to which home-track is 'switched' on second increment of barrel-roll.
  etc          
  n+2 Last-step destination track in n-step sequence int track # - Track to which home-track is 'switched' on last step of barrel-roll. Returns to Step 0 as next step.
roll_inc_period 1 Roll increment period in frames int   (frames)  
roll_reinit_period 1 Roll-sequence reinitialization period in seconds (at recording) real any time Fixed at 2 sec for VLBA. Mark4 can be specified.

Notes:
  1. Barrel-roll is confined to tracks 2-33 within a given headstack. For cases of barrel-roll using multiple headstacks, the roll sequence definition must include all headstacks.
  2. Barrel-roll is applied in the formatter as the last step before the data are written to tape.
  3. System tracks do not participate in barrel-roll.
  4. The number of fields in the roll= statements defines number of positions in the roll sequence. All roll= statements must specify the same number of positions in the roll sequence.
  5. Note that all track# references elsewhere in the VEX file are to the 'home track#'.
  6. roll=off; is default. Presence of any roll_def= statements implies roll=on; .

$SCHEDULING_PARAMS Block

The $SCHEDULING_PARAMS block specifies various parameters needed for the scheduling program. Since each scheduling program may have its own unique set of parameters, the $SCHEDULING_PARAMS block is specified strictly as a literal block which must be parsed and interpreted by the relevant scheduling program. The parameters listed are some of those for the current version of the NASA 'sked' program. Other scheduling programs may use these and/or other parameters.

Note: These parameters are examples only!
Parameter Field Description Type Allowed values Units Comments
sched_program 1 Scheduling program name char      
  2 Revision char      
default_scan_length 1 Default scan length     time  
lookahead 1 Lookahead for source rise/set     time  
min_scan_length 1 Minimum scan length     time  
minimum_between_scans 1 Minimum time between scans     time  
modular_scan_length 1 Schedule on minute marks     time  
max_display_width_col 1 Display screen width     (columns)  
confirm 1 Confirm new scans   on ¦ off    
mutual_vis 1 Force all stations to see source, or allow subnet   all ¦ subnet    
low_SNR_reject 1 Reject stations if SNR too low   auto ¦ man   Primarily for geodesy
variable_scan_length 1 Use SNR calculation to set scan length   on ¦ off   Primarily for geodesy
min_sun_angle 1 Min angle between source and sun     angle  
tape_usage_sync 1 Synchronize tape usage   on ¦ off    
sked_optimize 1 Type or optimization for auto-scheduling   sky_coverage ¦ covariance   Primarily for geodesy
window 1 Sliding window for optimization     time Primarily for geodesy
maximize_num_obs 1 Maximum total # of observations   on ¦ off   Primarily for geodesy
minimize_idle 1 Minimize idle time between scans   on ¦ off   Primarily for geodesy
minimize_slew 1 Minimize antenna slew time   on ¦ off   Primarily for geodesy

$SEFD Block

The optional $SEFD block allows the sensitivity of each IF to be modeled and used for a crude calculation of expected SNR when used with specified observing modes an d scan-length times. For geodesy, these calculations can be used to automatically adjust scan times for minimum-acceptable SNR in order to densify the schedule as much as possible. The particular SEFD model to be used can be specified, along with the model parameters.

Parameter Field Description Type Allowed values Units Comments
sefd_model 1 SEFD model name char Shaffer ¦ ?   Models may be added
sefd 1 IF_ID 'linkword' &link     Link to 'IF_ID' in selected $IF 'def'
  2 Zenith SEFD real   flux-density  
  3, ... Model parameters        

$SITE Block

The $SITE block describes the location of an antenna and may be either earth-based or earth-orbiting. Horizon masks for earth-based sites may be specified as an aid in scheduling.

Parameter Field Description Type Allowed values Units Comments
site_type 1 Type of site char fixed ¦ earth_orbit    
site_name 1 Full site name char <=16chars    
site_ID 1 Standardized 2-char site name char 2 chars    
site_position 1 x real   length  
  2 y real   length  
  3 z real   length  
site_position_epoch 1 Epoch of site_position epoch   epoch  
site_position_ref 1 Reference for site position char      
site_velocity 1 x-velocity real   speed  
  2 y-velocity= real   speed  
  3 z-velocity= real   speed  
horizon_map_az 1..n List of azimuth values corresponding to values in horizon_map_el real   angle Units specification may be omitted after first field.
horizon_map_el 1..n List of elevation limits at azimuths specified in horizon_map_az real   angle Units specification may be omitted after first field.
zen_atmos 1 Zenith atmosphere added delay real   time Typically on order of 7 nsec
ocean_load_vert 1 Ocean-loading vertical amplitude real   length  
  2 Phase real   phase  
ocean_load_horiz 1 Ocean-loading horiz amplitude real   length  
  2 Phase real   phase  
occupation_code 1 4-char occupation code char     Primarily used for geodetic experiments
inclination 1 Earth-orbit parameter real   angle  
eccentricity 1 Earth-orbit parameter real   -  
arg_perigee 1 Earth-orbit parameter real   angle  
ascending_node 1 Earth-orbit parameter real   angle  
mean_anomaly 1 Earth-orbit parameter real   angle  
semi-major_axis 1 Earth-orbit parameter real   length  
mean_motion 1 Earth-orbit parameter real   -  
orbit_epoch 1 Earth-orbit epoch epoch   epoch  

$SOURCE Block

The $SOURCE block defines the sources to be observed and specifies their relevant characteristics, particularly position. A single source is defined in each 'def' block. A crude source model may be specified for each observed 'Band_ID' specified in the $FREQ block for purposes of auto-scheduling (primarily geodesy).

Parameter Field Description Type Allowed values Units Comments
source_type 1 Generic source type char star ¦ earth_satellite   Specifies coordinate system in which position of object will be specified. Other types may be added.
  2 Experiment source type char target ¦ calibrator ¦ dummy   dummy may be declared if the source if specified for pointing purposes only. Station field system may use this information.
source_name 1 Source name char <=16 char   Typically same as 'def' label name (e.g., 3C273B)

For 'source_type=star':
Parameter Field Description Type Allowed values Units Comments
IAU_name 1 Standard IAU source ID char 9-char   Example: 0102-0304
source_position_ref 1 Origin of source position char <=16-char   For traceability of source position
ra 1 Right-ascension RA   RA Example: 01h02m03.456s
dec 1 Declination dec   dec Example: -03d04'05.678"
ref_coord_frame 1 Source-position reference frame char B1950 ¦ J2000    
ra_rate 1 RA proper motion real   ang rate Typically asec/yr
dec_rate 1 Declination proper motion real   ang rate Typically asec/yr
source_position_epoch 1 Epoch of stated position epoch   epoch Needed only if non-zero ra_rate or dec_rate
source_model 1 Component number int     One source_model= statement for each major source component for each 'Band_ID' link to the selected $FREQ 'def'
  2 'Band_ID' linkword to selected $FREQ 'def' &link      
  3 Component flux-density real   flux-density  
  4 Component major axis real   angle Angle subtended on sky
  5 Component axis ratio real      
  6 Component position angle real   angle  
  7 Component RA offset wrt specified source position real   angle  
  8 Component dec offset wrt specified source position real   angle  

For 'source_type=earth_satellite':
Parameter Field Description Type Allowed values Units Comments
inclination 1 Orbit inclination real   angle  
eccentricity 1 Orbit eccentricity real     Unitless
arg_perigee 1 Argument of perigee real   angle  
ascending_node 1 Longitude of ascending node real   angle  
mean_anomaly 1 Orbit mean anomaly real   angle  
semi-major_axis 1 Orbit semi-major axis real   length  
mean_motion 1 Orbit mean motion real      
orbit_epoch 1 Epoch of stated orbit epoch   epoch  

$TAPELOG_OBS

The $TAPELOG_OBS block contains information about the media (disks or tapes currently) that were used during the experiment. This table is normally appended after the completion of an experiment. Each station can have multiple media listed which can in principle overlap in time if parallel recording occurred.

Parameters in this table are:
Parameter Field Description Type Allowed values Units Comments
VSN 1 Recorder number int      
  2 Volume Serial Number (VSN) char     8 character string, conventionally all capital
  3 Start time epoch   epoch Epoch when this disk/tape started
  4 Stop time epoch   epoch Epoch when this disk/tape stopped

$TRACKS Block

The $TRACKS block defines the various multiplex (fan-in and fan-out) modes that can be used to record data on the Mark3A, Mark4, VLBA and LBA DAS's. In cases where a mode uses fewer than the full number of heads in a single pass, alphabetical 'sub-passes' are defined (tape passes with the headstacks in a fixed position). For purposes of multiplex definitions, the sample data from each channel are separated into a 'sign' bitstream and (for 2-bit sampling) a 'magnitude' bitstream. The fan-out modes (single bitstream to 1, 2 or 4 tracks) are defined with a set of fanout_def= statements, one such statement for each bitstream and subpass, which defines the destination tracks and bit ordering among the tracks. In this way a complete definition of the multiplex format is specified. The 'ChanID' linkword in each fanout_def= statement connects a particular bitstream to the selected 'def' in the $FREQ block. The fan-in modes (1, 2 or 4 bitstreams to a single track) are defined by a set of fanin_def= statements. Each such statement defines the bitstreams written to a single track in a specified subpass. The set of 'ChanID' linkwords in each fanin_def= statement connects the particular bitstreams to the selected 'def' in the $FREQ block. and specifies their multiplex order on the track. Within each fanout_def= or fanin_def= statement is a field which specifies the 'sub-pass' to which it applies. A 'subpass' is defined as a single tape pass for which the headstack(s) are held at a fixed position. Typically, for example, 16 of 32 tracks may be written in a single tape pass; for this case there are 2 sub-passes with a given headstack position. By convention, the subpasses are labeled A, B, C, ....,etc. Note that, except for the VLBA_trnsprt_sys_trk= statement, all references to 'track numbers' in the $TRACKS block are more properly labeled as 'home tracks' since barrel-rolling in the formatter (Mark 4 and VLBA) and track switching within the recorder (VLBA only) may lead to modified physical track assignments. Normally, the actual physical track numbers (pre-barrel-rolled) correspond identically to 'home track' numbers.

Parameter Field Description Type Allowed values Units Comments
fanout_def 1 Sub-pass ID char single char   One fanout_def= statement required for each bitstream. By convention, subpass_ID uses characters A, B, C, etc. Null for Mark 5A and LBA.
  2 'Chan_ID' linkword char -   Link to 'Chan_ID' in selected $FREQ def
  3 Sign or magnitude bitstream char sign ¦ mag   Fields 2 and 3 uniquely define a single bitstream
  4 Headstack number int 1-4    
  5 First multiplex track int track#   Track # within headstack
  (6) Second multiplex track int track#   Required for fanout 1-to-2 or 1-to-4
  (7) Third multiplex track int track#   Required for fanout 1-to-4
  (8) Fourth multiplex track int track#   Required for fanout 1-to-4. Number of fields specifies fan-out ratio.
fanin_def 1 Sub-pass number int single char - By convention, uses characters A,B,C,...etc.
  2 Headstack number int 1-4    
  3 Track number int track#    
  4 'Chan_ID' linkword for multiplex bitstream 1 char     Link to 'Chan_ID' in selected $FREQ def
  5 sign or magnitude bitstream (of 'Chan_ID') char sign ¦ mag   Fields 4 and 5 uniquely define the bitstream which occupies bit position 1 on the specified track.
  (6) 'ChanID' linkword for multiplex bitstream 2 char     Fields 6 and 7 required if fanin is 2-to-1 or 4-to-1
  (7) sign or magnitude bitstream char sign ¦ mag   Fields 6 and 7 uniquely define the bitstream which occupies bit position 2 on the specified track.
  (8) 'ChanID' linkword for multiplex bitstream 3 char -   Fields 8 and 9 required if fanin is 4-to-1
  (9) sign or magnitude bitstream char sign ¦ mag   Fields 8 and 9 uniquely define a bitstream which occupies bit position 3 on the specified track.
  (10) 'ChanID' linkword for multiplex bitstream 4 char     Fields 10 and 11 required if fanin is 4-to-1
  (11) sign or magnitude bitstream char sign ¦ mag   Fields 10 and 11 uniquely define a bitstream which occupies bit position 4 on the specified track.
track_frame_format 1 Frame format on tape track char Mark3A ¦ Mark4 ¦ VLBA   Mark3A and Mark4 are slightly different data-replacement formats. VLBA is non-data-replacement format. VLBA can write Mark3A format.
data_modulation 1 Pseudo-random data modulation char on ¦ off   Default is 'off'.
VLBA_frmtr_sys_trk 1 Formatter 'system' track# to be written with specified data int 0 ¦ 1 ¦ 34 ¦ 35   Applicable to VLBA formatter only. Specifies data to be written to a particular 'system' track formatter outputs.
  2 Data type to be written to system track specified in Field 1 char xtk_parity ¦ duplicate   xtk_parity if cross-track parity to be written; duplicate is this track is to duplicate one of the normal data tracks.
  3A If Field 2 is xtk_parity: First track# of contiguous set of tracks covered int 2 ¦ 10 ¦ 18 ¦ 26   Limited to specified set of 'first track #'s'
  4A If Field 2 is xtk_parity: #tracks covered by cross-parity int 8 ¦ 16    
  3B If Field 2 is duplicate: 'home track#' of data to be written to specified 'system' track# int 2-33   Will always be 'home track' data; is not barrel-rolled
VLBA_trnsprt_sys_trk 1 Physical 'system' track (head#) to be written as a duplicate of specified formatter output track (recorder input track).. int 0 ¦ 1 ¦ 34 ¦ 35   This is a duplication within the transport itself, so includes all barrel-roll
  2 Formatter output track (recorder input track) to be duplicated int 2-33    
S2_recording_mode 1 Recording mode ID char See Notes   Example: '32x4-2'
S2_data_source 1A Define S2 data source char Mark4_formatter ¦ VLBA_BBC_1-4 ¦ VLBA_BBC_5-8   Relevant for S2 only
  1B Define LBADR bit encoding char VLBA ¦ VSOP   Relevant for LBADR only. VSOP encoding is mag/sign, VLBA encoding is offset binary
  2 Define BBCx selection from Mark 4 formatter &link     For Mark IV only: Link to 'BBC_ID' in $FREQ. See Notes.
  3 Define BBCy selection from Mark 4 formatter &link     For Mark IV only: Link to 'BBC_ID' in $FREQ. See Notes.

Notes:
  1. Reference to Mark IV Memo 230 (aka VLBA Acquisition Memo 393) may help to clarify the details of multiplex and signal switching in the Mark IV and VLBA systems.
  2. The actual fanin/fanout ratio is implied by the number of subfields in the fanout_def or fanin_def statements, as indicated in the above table.
  3. For a fanin/fanout ratio of 1-to-1 (i.e. one bitstream to one track), either fanout_def or fanin_def statements may be used.
  4. Cross-track parity is computed after barrel-rolling as the last step before writing to tape.
  5. The 'S2_recording_mode' parameter specifies the recording mode to which the S2 system is to be set. The available modes are documented in 'S2-RT User's Manual, Version 3.0 (162)', October 1996, ISTS-SGL-TR96-033, available at ftp://s2.sgl.ists.ca. The selected mode defines the number of recording 'groups' and recorded inputs. The S2_group_order parameter in the $PASS_ORDER section specifies the order in which the groups are to be recorded.
  6. The 'S2_data_source' parameter specifies the origin of the sampled data recorded by the S2 recording system. Typical data sources are either through the so-called 'phase-cal' outputs of the Mark IV formatter or direct sampler outputs from the VLBA system. This keyword was reused with a different meaning for electronics_rack_type=LBA.

Mark IV formatter : the sampled data from the USB and LSB outputs of each of two selected BBC's can be directed to the 'phase-cal output'. The data available to the S2 are unformatted 2-bit samples at 32 Msamples/sec, regardless of the sample rate chosen for output to the Mark IV recording system. If we designate the two selected Mark IV BBC's as BBCx and BBCy, the Canadian VIA (VLBI Interface Adapter) implements a fixed mapping to S2 inputs as follows:
S2 Input IN0 IN1 IN2 IN3 IN4 IN5 IN6 IN7
Mk4 bit stream Lx/sign Lx/mag Ux/sign Ux/mag Ly/sign Ly/mag Uy/sign Uy/mag

VLBA samplers : the input to the Canadian VIA box may be taken either from BBC's 1-4 or BBC's 5-8, depending on the physical connector to which it is attached. The mapping within the VIA is fixed, as follows:
S2 Input IN0 IN1 IN2 IN3 IN4 IN5 IN6 IN7 IN8 IN9 IN10 IN11 IN12 IN13 IN14 IN15
BBC1-4 U1/s U1/m L1/s L1/m U3/s U3/m L3/s L3/m U2/s U2/m L2/s L2/m U4/s U4/m L4/s L4/m
BBC5-8 U5/s U5/m L5/s L5/m U7/s U7/m L7/s L7/m U6/s U6/m L6/s L6/m U8/s U8/m L8/s L8/m
The information in the above tables is taken from the 'VLBI System Interface Adapter (VIA) User's Manual, Ver. 1.3', ISTS/SGL, October 25, 1996, available at ftp://s2.sgl.ists.ca.
Topic revision: r5 - 2010-08-24, CormacReynolds
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