FLAG DICTIONARY FOR SPACE RADIO TELESCOPE LOG FILES Version 2.0 J. S. Ulvestad, Y. Murata, and A. B. Wiercigroch February 19, 1996 1. INTRODUCTION --------------- This document provides a dictionary of flags to be supplied along with a correlator input file for a space radio telescope. It is meant to accompany the document "Alternative Log Input to the VLBA Correlator for Space VLBI Observations," by J. D. Romney. It is intended that these flag entries be used by anyone analyzing the VLBI data for scientific purposes. They also may be used by the correlator for information purposes, if desired. The format of the FLAG entry given in the correlator log specification is given below: yydddhhmmssxx/FLAG/,, Note that the definitions of flag conditions provided by tracking stations are intended to be common for all different tracking stations. However, it will probably be the case that not all tracking stations can provide all possible flags; each station will supply the subset of flags that it is able to determine. 2. SEVERITY-CODE ---------------- The log specification defines a zero severity code to indicate that a previously flagged condition has ended. We add a further definition of the severity code. For data flagged by either the satellite control group or the tracking stations, a severity code of "1" would indicate data that are possibly bad, while a severity code of "2" would indicate data that are definitely bad. Other severity codes might be used in correlator output files to identify data that are flagged bad at the correlator, but such codes are not needed in a specification of input logs to the correlator. 3. CONDITION-CODE ----------------- The condition codes should be grouped according to the mission element identifying flagged data. Condition codes numbered from 0 to 99 should indicate data flagged on the basis of spacecraft parameters. Condition codes numbered from 100 to 199 should indicate data flagged bad on the basis of tracking-station parameters. Each condition code would have a single severity code associated with it; i.e., a single condition code can be used either to identify useless data or questionable data, but not both. Different gradations of data "uselessness" from the same cause have different condition codes. 4. EXPLANATION -------------- As a default, each condition code and severity code is to be accompanied by a nominal explanation of the reason for the flag. These explanations are cryptic, and can be replaced by other entries should more specific information be available. Therefore, any software which is used to search FLAG records should search by the condition code and/or severity code, not by the textual explanation. 5. DICTIONARY ------------- The flag dictionary follows. For each entry in the dictionary, the condition code ("CC"), severity code ("SC"), and default explanation are given. Additional explanations for each item are given following the dictionary. 5.1 SPACECRAFT FLAGS -------------------- CC SC Explanation -- -- ----------- 0 2 Phase lock off 1 2 In eclipse 2 2 Battery insufficient 3 2 Observing mode incorrect 4 2 Observation unit non-functional 5 2 Off source 6 1 Pointing questionable 7 1 Extraordinary thermal conditions 8 1 High Tsys 9 1 Poor phase stability 10 1 Low voltage on spacecraft component 11 1 Phase-calibration on by accident 12 1 Link parameters wrong 13 1 Sampler level incorrect 14 1 Low-quality orbit 15 1 Status unknown 16 2 Miscellaneous 17 1 Miscellaneous 5.2 TRACKING STATION FLAGS -------------------------- CC SC Explanation -- -- ----------- 100 2 Antenna not tracking spacecraft 101 1 Antenna tracking spacecraft poorly 102 1 Low link SNR 103 2 Tracking receiver out of lock 104 2 Data demodulator out of lock 105 1 Excessive synchronization errors 106 2 Transmitter off when expected to be on 107 2 VLBI recorder not recording data 108 2 Recorder operating when not observing 109 1 Poor recording quality 110 1 Phase residuals too high 111 1 Poor link coherence 112 1 Pre-pass test failed 113 1 Phase calibration tones not detected when expected 114 1 Spurious signal found by autocorrelator 115 1 NRT correlator fringe-search failed 116 1 Poor coherence in NRT correlation 117 2 Miscellaneous 118 1 Miscellaneous 5.3 ADDITIONAL COMMENTS ON SPACECRAFT FLAGS ------------------------------------------- Most of the above entries in the spacecraft flag dictionary are fairly self-explanatory. This section gives a few more details. CC SC Description -- -- ----------- 0 2 The phase-lock loop on the spacecraft is not operational. 1 2 Normally, observations are not scheduled during eclipse, and the phase-link will be turned off. However, it is possible that a tracking station still will attempt to operate and will produce a log for such a period. Therefore, the eclipse flag is included for completeness. 2 2 Normally, observations will not be made when the battery is insufficient. However, it is conceivable that a link will be scheduled, but only part of it will be executed because of a low power condition on the spacecraft. 3 2 The spacecraft may be commanded to observe in the wrong mode, which may be inconsistent with all the ground radio telescopes. 4 2 Any of the components of the spacecraft observing system may be non-functional. For instance, the 22-GHz receiver may be broken, or the on-board formatter may be non-operational. This flag would cover any of the observing-system components. 5 2 For VSOP, this flag indicates that the pointing is in error by more than 1.0 degree. 6 1 For VSOP, the pointing accuracy is reported as being better than 0.01 degree, between 0.01 and 1.0 degree, or greater than 1.0 degree. This flag indicates that the pointing is known to be in error by between 0.01 and 1.0 degree. 7 1 The spacecraft may have just come out of eclipse, or the Sun may be shining at a 90-degree angle and causing large thermal gradients. This may give rise to known problems with pointing or system temperature that can be identified in other flags. This flag also would cover high physical temperatures of a variety of on-board components. 8 1 A high system temperature is reported for the observing frequency. This may or may not be associated with extraordinary thermal conditions on-board. 9 1 The on-board phase-lock loop may be performing poorly, or the phase of an individual local oscillator may not be completely stable. 10 1 Readback of monitor data from some particular spacecraft component may indicate a lower-than-nominal voltage, causing some doubt about the performance of that component. 11 1 The VSOP default is to leave phase-cal tones on during continuum observations and off during spectral line observations. It is possible that they might be on by accident during line observations, or during some continuum observation when it was specifically desired that they be off. 12 1 Link parameters may be incorrect. For example, the automatic-gain-control on the spacecraft may be too high. 13 1 The level of the on-board sampler may be set too high or too low, causing lower signal-to-noise than would be expected for correct 2-bit or 1-bit sampling. 14 1 The spacecraft orbit may be known to be of low quality, because of inadequate geometrical diversity in tracking, noisy Doppler data, temporary outages of the link, and so on. This information cannot be supplied by the individual tracking station, so we have included it with the spacecraft flags because it must be inserted in the log by the spacecraft control group. 15 1 There may be occasions when inadequate information is available to definitively determine the spacecraft status, so this flag simply provides a means of making that notation. 16 2 This flag covers any spacecraft condition not included in the above definitions that causes data to be unusable. 17 1 This flag covers any spacecraft condition not included in the above definitions that causes data quality to be questionable. 5.4 ADDITIONAL COMMENTS ON TRACKING-STATION FLAGS ------------------------------------------------- CC SC Description -- -- ----------- 100 2 The ground antenna may not be tracking the spacecraft when it is scheduled to do so. 101 1 The ground antenna may be tracking the spacecraft poorly, as defined by the tracking station. For example, this may give rise to high bit-error rates or high phase residuals. 102 1 The signal level measured by the tracking station indicates that the link SNR may be low, so the bit error rate may be too high. This may happen in the event of poor tracking or a low transmitted power by the spacecraft, but it is included with the tracking station flags because this is where the measurement of the link level takes place. 103 2 The tracking receiver at the ground station may be out of lock. 104 2 The data demodulator at the ground station may not be demodulating the downlink data successfully. 105 1 Excessive synchronization errors may have been detected in the downlink signal. Definition of "excessive" in percentage terms must be done after some experience is acquired. Presumably, it might be possible to include the actual percentage of errors in the explanation field. 106 2 The ground-tracking station transmitter is not operating when it is scheduled or commanded to do so. 107 2 The VLBI recorder is not recording data when it is scheduled to do so. There often will be tracking passes that are meant only for acquiring Doppler data, when no recording is scheduled; no flag should be included during these situations. In those cases, no log will be sent to the correlator (although we assume that a log will still be sent to the Science Operations Group). 108 2 The recorder may be operating when the spacecraft is not scheduled to be observing an astronomical source. This flag will prevent time-wasting by correlation during such intervals. It may be that this flag should be inserted by the spacecraft control group rather than by the tracking stations. 109 1 The tracking station may have reason to expect poor recording quality, because of results of a pre-pass test, a read-after-write test, a known or suspected problem with a particular head or data track, an error in head positioning, or other similar causes. 110 1 Phase residuals on the two-way link may be measured to be higher than some threshold. This might be due to a phase-lock loop problem on the spacecraft, or a poor predicted orbit. 111 1 A poor coherence may be measured on the two-way link. This may or may not be simultaneous with high phase residuals. It is possible to have poor coherence with low residuals, or good coherence with high (but linear in time) residuals. 112 1 Some pre-pass test may have failed in a way that makes the recorded data or the station performance suspect. 113 1 The tracking station may not detect phase-calibration tones in the downlink data when such tones are expected to be on. 114 1 An autocorrelator (presently planned only for the Usuda station) may detect a spurious signal. 115 1 A near-real-time correlator (presently planned only for Usuda) may fail to detect fringes when fringes are expected. This may be due to a lower SNR than detected or to a poor spacecraft orbit. 116 1 Fringes are detected in a near-real-time correlation, but with poor coherence. In many (most?) cases, this will be due to use of a relatively poor orbit or to performing the correlation without using a time correction table. 117 2 This flag covers any tracking condition not included in the above definitions that causes data to be unusable. 118 1 This flag covers any tracking condition not included in the above definitions that causes data quality to be questionable.