PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 1999-10-01 NOTE = "A lookup table that provides the value and explanation for the OBSERVATION_QUALITY keyword in the obs*.tab and the DATA_QUALITY keyword in the rad*.tab files." END_OBJECT = TEXT END 1. Overview This document describes the bit contents of the OBS.QUALITY and RAD.QUALITY keywords, including an explanation for each bit and the code definitions for the bit values. The overall observation quality is affected most by spacecraft and instrument motion; it is determined per observation made and stored in the OBS.QUALITY keyword located in the obs*.tab files. The quality of the data is evaluated per detector and the results are available in the RAD.QUALITY keyword located in the rad*.tab files. Data quality is related to the signal received from the TES instrument and the ground based calibration routines. The remainder of this document is divided into two parts: observation quality characteristics and data quality characteristics. Each quality characteristic is discussed in the order it appears in the keyword, as shown in Table 1. A brief explanation and the code for the bit values is given for each of the ten quality characteristics listed. The quality bit information can be accessed with a "vanilla" data selection command on the field names listed in Table 1. Table 1 - Quality Bit Keywords OBS.QUALITY BIT NO. QUALITY CHARACTERISTIC VANILLA FIELD NAME 1,2 High Gain Antenna Motion HGA_MOTION 3-5 Solar Panel Motion SOLAR_PANEL_MOTION 6 Algor Patch Status ALGOR_PATCH 7 IMC Patch Status IMC_PATCH 8 Momentum Desaturation Status MOMENTUM_DESATURATION 9 Equalization Table Status EQUALIZATION_TABLE RAD.QUALITY BIT NO. QUALITY CHARACTERISTIC VANILLA FIELD NAME 1 Major Phase Inversions MAJOR_PHASE_INVERSION 2 Risk of Algor Phase Inversions ALGOR_RISK 3-5 Calibration Issues CALIBRATION_QUALITY 7,8 Spectrometer Noise SPECTROMETER_NOISE Note that both keywords may contain up to 32 bits; bits currently not assigned to a particular characteristic are reserved for future use. 2. Observation Quality Keyword 2.1 High Gain Antenna Motion The high gain antenna (HGA) was deployed after the end of the aerobraking phase; notable adverse affects of HGA movement appear in TES mapping data starting at ock 1985. Motion of the HGA induces microphonics in TES and appear as noise in the TES data. Higher rates of motion correspond to higher noise levels in the data. For early mapping orbits, ock 1985 to 3588, the HGA was continually in motion, autotracking throughout most of the orbit with a brief rewind period. Starting at ock 3589 the motion of the HGA was restricted to occur only during periods of earth contact. Appendix A.1 contains information on how the HGA motion data is obtained. Disclaimer: this bit should be used with caution as the information source is spacecraft telemetry which is subject to dropped bytes and can not be completely verified. BIT CODE DEFINITION 0 = HGA motion unknown 1 = HGA not moving 2 = HGA moving at 0.05 degree/second (autotrack motion) 3 = HGA moving at 0.51 degree/second (rewind motion) 2.2 Solar Panel Motion Similar to the situation of the HGA, the motion of either one of the two solar panels induces microphonics in the TES instrument that appear as noise in the data. At the start of the mapping phase the solar panels were continuously in motion, autotracking and rewinding to follow the sun as MGS orbited Mars. Starting at ock 3589, orbit rates and motions were altered to reduce the noise affects on TES; under the new sequence the solar panels only move 3 times per orbit and remain stationary during the interim time periods. This "move and hold" pattern will continue until the end of the mission with the exception of expected periods of power constraints which will require continuous solar panel motion to maintain the health of the spacecraft. The amount of noise present in the data due to solar panel motion is an approximately linear function of the rate of panel motion. The bit values reflect the variety of panel motion rates that may be used. For more information on how the solar panel motion and rates are correlated with individual TES observations, see Appendix A.1. Disclaimer: this bit should be used with caution as the information source is spacecraft telemetry which is subject to dropped bytes and can not be completely verified. BIT CODE DEFINITION 0 = panel motion unknown 1 = panels not moving 2 = panels moving at 0.051 degree/second (non-eclipse, autotrack motion) 3 = panels moving at 0.120 degree/second (during eclipse, prior to ock 3589) 4 = panels moving at 0.240 degree/second (during eclipse, starting ock 3589) 5 = panels moving at 0.400 degree/second (used during aerobraking phases) 6 = panels moving & changing between non-eclipse and eclipse rates 7 = not assigned 2.3 Algor patch status Two patches are simultaneously loaded to correct problems in the TES flight software involving the calculation of the sign of the spectral data and the calculation of the location of the zero path difference (ZPD) in the interferogram. Both of these problems are interconnected and can affect the accuracy of the computed spectra. Better data are produced when the Algor flight software patches are onboard, however some data may still be at risk for problems and can be identified from Data Quality bit 2 (see section 3.2). Algor patch 2A modifies the method employed to calculate the sign of the spectral data by computing the phase for more frequencies, thus improving the phase determined for the output spectra. TES PROM flight software relies upon the symmetry of the interferogram, characteristic to TES I, to calculated ZPD. The TES II interferogram is notably asymmetric and another method must be used to calculate ZPD; this alternate calculation is accomplished with Patch 2B. BIT CODE DEFINITION 0 = Algor flight software patch not onboard TES 1 = Algor flight software patch onboard TES 2.4 IMC patch status This bit applies to TES data collected while using Image Motion Compensation (IMC) (see obs*.fmt, IMC_COUNT keyword). The IMC software patch was used to control the direction of steps taken for motion compensation as related to the spacecraft reference frame. For aerobrake orbits, imc moving in the forward direction (bit value 0) will compensate for the spacecraft orbital motion; for mapping orbits, imc moving in the reverse direction (bit value 1) will compensate for the spacecraft orbital motion. BIT CODE DEFINITION 0 = imc moving in forward direction - IMC patch not onboard 1 = imc moving in reverse direction - IMC patch onboard 2.5 Momentum Desaturation status Normal spacecraft operations include routine firing of mono propellant thrusters for a duration of about 3 minutes to adjust the angular momentum of the spacecraft. Any change in spacecraft motion has the potential of introducing noise into the TES data. The amount of noise contributed by momentum desaturation has not been established at this time. Disclaimer: this bit should be used with caution as the information source is spacecraft telemetry which is subject to dropped bytes and can not be completely verified. BIT CODE DEFINITION 0 = autonomous angular momentum desaturation not occurring on spacecraft 1 = autonomous angular momentum desaturation occurring on spacecraft 2.6 Equalization tables status These tables do not affect the quality of the data. The purpose of the equalization tables is to improve the data compression ratio. TES PROM flight software resets the equalization table values to default values after every cold or warm reset. To avoid this, equalization table edits and the equalization reset patch (2C) are loaded simultaneously with the equalization tables. Further information regarding the Equalization Tables is available in "TES Software Specification Document, Instrument Flight Software" [Hughes, SBRC, 1991]. At the time of this writing, the equalization tables have only been used during aerobraking. When in use, the entropy bits were also reset from their default values: for Detector Mask 7, spec_entropy= 4 and reference_det= 2. BIT CODE DEFINITION 0 = equalization tables not onboard TES 1 = equalization tables onboard TES 3. Data Quality Keyword 3.1 Major Phase Inversions Spectra with major phase flips or other grossly inaccurate features due to lost bits, incorrect zpd determination, or excessive "ringing" are identified in this bit. These are major problems with the spectra and possible minor phase flips are not detected here. Appendix A.2.1 contains more information regarding how this bit is identified. BIT CODE DEFINITION 0 = data does not contain major phase inversions 1 = data does contain phase inversions 3.2 Risk of Algor Phase Inversions Spectra with the possibility of inaccurate minor phase flips due to algor problems are assigned a value of 1 in this bit. These flips may not actually be present or recognizable in the calibrated radiance spectra, but careful inspection should be performed before using this data. Appendix A.2.2 contains more information regarding how algor phase inversions are identified. Disclaimer: this bit should be used with caution as the potential for phase inversions has been identified and verified to the best of our ability, but some "low risk" data may actually contain phase inversions. BIT CODE DEFINITION 0 = data at low risk of algor phase inversion 1 = data at high risk of algor phase inversion 3.4 Calibration Issues These bits are currently undefined and reserved for future use. 3.5 Spectrometer Noise The value of this bit is a representation of the noise level in the data due to the performance of the spectrometer over time. To completely characterize the noise levels in a particular observation, this bit should be used in conjunction with other quality bits related to noise inducing factors, such as HGA or solar panel motion. Spectrometer noise is calculated from the standard deviation of 10-ick space observations made at least once a day expressly for this purpose. Appendix A.3 contains more information regarding how the instrument noise is calculated for this bit. The bit value 0 is used for all aerobraking orbits and for mapping orbits where the necessary space observations are not available. BIT CODE DEFINITION 0 = instrument noise not calculated 1 = instrument noise at nominal levels 2 = instrument noise at anomalously high levels 3 = not assigned A. Appendices A.1 Determining High Gain Antenna and Solar Panel Motion The high gain antenna (HGA) motion is determined from the values encoded in the spacecraft telemetry channels. Channel F-0621 defines the HGA Gimbal Drive Electronics (GDE) elevation motor status as "moving" or "not moving" at specific times, sampled at regular intervals. Similarly, channel F-0622 defines the HGA GDE elevation motor direction as "forward", corresponding to autotrack motion, or "reverse", corresponding to rewind motion. The rate values were obtained from personal communication with spacecraft engineers (Stuart Spath, Lockheed Martin Astronautics, August 1999) as they are not recorded in the spacecraft telemetry. Combining the rate and motion information with the time of each TES observation gives the HGA motion status recorded in the first two bits of the OBSERVATION_QUALITY keyword. The solar panel motion status is determined in a similar manner. The motion of each solar panel, SAM and SAP, is defined independently in separate spacecraft telemetry channels, F-0801 and F-0821 respectively. In each channel the elevation motor status is given as "moving" or "not moving" at specific times. Again the rate of solar panel motion is not recorded in the spacecraft telemetry and was obtained through communication with the spacecraft engineers. The solar panel rate varies over the course of the mission, and also throughout the course of an orbit. During a single orbit, two rates are used: a faster rate during solar eclipse periods, and a slower rate for autotrack motion during non-eclipse periods. From the point of view of the spacecraft, the time of solar eclipse varies by orbit and must be obtained from the heliocentric surface occultation beginning (SOCCSB) and end (SOCCSE) time entries in the Orbit Propagation and Timing Geometry (OPTG) file. To fully assign the solar panel bit value in the quality word, the telemetry motion information, the known rates, and the solar eclipse times must be combined with each TES observation and associated time. Because the solar panels communicate telemetry only at specific time intervals which may or may not correspond with each TES observation time, some logical interpolation was applied to determine the value of these bits. For example, TES observations obtained at a time that falls between a telemetry record showing motion during an eclipse period and a record showing motion during a non-eclipse period (or vise versa) would be tagged with the bit value 5 corresponding to panel motion during a transition period. If the panel is known to be moving, but the rate can not be logically determined, the observation is tagged with the bit value 6. A.2 Determining Phase Inversions A.2.1 Major phase inversions and other grossly inaccurate spectra An algorithm detects major phase flips or other grossly inaccurate features due to lost bits, incorrect zpd determination, or excessive "ringing". These are spectral problems that are clearly identified when the spectrum is plotted, but may not be noticed otherwise. The algorithm checks for these problem spectra using two methods: specific thresholds and derivatives. The threshold checks that uncalibrated radiance values are within specific thresholds in several wavelength regions: 200-220cm-1 (value range -10 to 3); 645-680cm-1 (value range -80 to 1); and 1610-1650cm-1 (value range -12 to 7). If any spectral channel lies outside this range of values, the spectrum is determined to be bad and a value of 1 is assigned. The derivative check takes the derivative of the spectrum from 200-530cm-1 and 800-1200cm-1. If the absolute value of any derivative throughout this range is >15, then the spectrum is assigned a value of 1 indicating a problem with the spectrum. A.2.2 Algor phase inversions Algor phase inversions are due to low temperature contrast between the sensor and the target, and it occurs because the phase of the spectrum is interpolated between a number of points in the spectrum. In spectral regions where the measured voltage is near 0, it becomes impossible to interpolate this value exactly. This only occurs at the shorter wavelengths (< 850 cm-1) and has not been observed at longer wavelengths. The algorithm that checks for possible minor phase flips due to algor problems is fairly strait forward. The uncalibrated radiance spectrum is scanned between 850 and 1400 cm-1 for values with an absolute value of less than 1. This is an arbitrary threshold where the phase flips have been known to occur. Where the entire range is either above 1 or below -1, the phase flips are assumed to not be present and the spectrum is assigned a quality value of 0. If any spectral sample in the spectral range inspected is within these bounds, then the spectrum is assigned a bit value of 1 indicating that the likelihood of minor phase flips is probable. A.3 Determining Spectrometer Noise The spectrometer noise recorded here is a representation of the results from a study to monitor the health of the instrument over the course of the mission. For this study, 10-ick space observations are routinely collected at least once a day. Because movement of the HGA and solar panels induces increased noise in the spectrometer, 10-ick space observations collected during these times are used only when no others are available. This strategy has been in effect since ock 3589. The raw radiance for the 10-ick set is averaged together and the standard deviation is calculated from 3 selected wavelength ranges: 300-400 cm-1, 900-1000 cm-1, and 1500-1600 cm-1. Finally, the average value of the standard deviation in these three ranges is used to define the value of this bit. Standard deviations of 0.00 to 0.28 are considered nominal levels of spectrometer noise from the space observations utilized; standard deviation values above this range are tagged as "anomalously high" levels.