Ref.:HASI-RP-UPD-105 Issue: 1 Rev.1 Date: 22 March 2005 HASI PPI Data Processing and Calibration Report Institute NAME SIGNATURE DATE Prepared by UPD/CISAS P.Lion, 02/11/2004 Revised by UPD/CISAS F. Ferri 9/12/2004 Approved by FMI G. Colombatti 22/03/2005 Table of Content 1. Acronyms 3 2. Scope of the document 3 3. Applicable and Reference documents 3 4. PPI description 3 4.1. PPI location and accommodation 5 4.2. Measurement principle and data sampling 5 4.3. Operational modes 7 4.4. Telemetry output 7 5. Pre-flight calibration 7 5.1. Static calibration 7 5.2. Dynamic calibration 8 6. In-flight Calibration 8 7. Dynamic corrections on pressure measurements: Total to static pressure 8 8. PPI total error budget 9 9. PPI data processing: Engineering value reconstruction 10 9.1. Raw values extraction 11 9.1.1. NORMAL Session (#A, #B, #C) raw data extraction 11 9.1.2. HEALTH CHECK Session (#G, #H) raw data extraction 11 9.1.3. Housekeeping raw data extraction 11 9.2. Raw values normalisation 12 9.2.1. NORMAL Session (#A, #B, #C) normalisation 12 9.2.2. HEALTH CHECK Session (#G and #H) normalisation 12 9.2.3. Housekeeping data normalisation 12 9.3. Engineering conversion 12 9.3.1. NORMAL Session (#A, #B, #C) engineering conversions 12 9.3.2. HEALTH CHECK Session (#G and #H) engineering conversions 12 9.3.3. Housekeeping engineering conversion 12 10. Scientific conversion of engineering values: Scientific reconstruction values for in flight CheckOuts 12 10.1. NORMAL Session (#A, #B, #C) conversion 12 10.2. Health check Sessions (#G and #H) conversions 13 11. Scientific conversion of engineering values: Scientific reconstruction values for Titan mission 13 11.1. NORMAL Session (#A, #B, #C) conversion 13 11.2. Health check Sessions (#G and #H) conversions 13 12. Higher level products derivation 13 13. Appendixes 13 13.1. Appendix A: PPI channels polling tables 13 13.2. Appendix B : Calibration for in flight CheckOuts 14 13.3. Appendix C : Calibration for Titan mission 15 1. Acronyms C Constant DPU Data Processing Unit HC Health-Check N Normal P Pressure PPI Pressure Profile Instrument R Reference T Temperature 2. Scope of the document Scope of the document is to report on the procedure and results of the calibration of the HASI PPI (Pressure Profile Instrument) subsystem and to present guidelines for data processing and the reconstruction of the pressure measurement. 3. Applicable and Reference documents [AD1] Cassini Mission Huygens Probe Huygens Atmospheric Structure Instrument - HASI PPI Flight Model Acceptance Data Package (PPI FM ADP) E.I.D.P. HASI-FMI-FM-DOC-009 30/03/1995 1 (II/210.B.6) [AD2] HASI Experiment Flight User Manual Document HASI-MA-OG-002 Issue 3, 1 December 1998 (II/196.B.6) [AD3] HASI DPU Software User Requirements Document HASI-SP-OG-004, Issue 7, 7 Sep 1995 (II/179.B.1) [RD1] Makinen, T. Processing the HASI measurements, Advances in Space Research, 17, Issue 11, 219-222, 1996 [RD2] Harri, A. M., B. Fagerstrom, A. Lehto, G. W. Leppelmeier, T. Makinen, R. Pirjola, T. Siikonen and T. Siili, Scientific objectives and implementation of the Pressure Profile Instrument (PPI\HASI) for the Huygens spacecraft, Planetary and Space Science, 46, Issues 9-10, 1383-1392,1998. 4. PPI description The Pressure Profile Instrument (PPI) includes sensors for measuring the atmospheric pressure during descent and surface phase. The atmospheric flow is conveyed through a Kiel probe, mounted on the STUB stem tip, inside the DPU where the transducers and related electronics are located. The PPI sensors are 6 reference sensors (for housekeeping) and 18 transducers. The transducers are silicon capacitive absolute pressure sensors (Barocap, 8 P), temperature capacitive sensors (Thermocap, 3 T), constant (C) and reference (R) sensors (high stability capacitor, respectively 7C used for housekeeping and 6R used in the pressure measurements). The sensors are organized in three blocks each having eight frequency output channels. The three blocks corresponds to different pressure sensibility range: low pressure 0-400 hPa block 3, sensor 3.7P , 3.8P and 3.3T medium pressure 0-1200 hPa block 1, sensor 1.1P, 1.6P , 1.8P and 1.3T high pressure 0-1600 hPa block2, sensor 2.1P, 2.7P, 2.8P and 2.3T In total there are: 24 frequency channels: 1.1P 1.2R 1.3T 1.4C 1.5R 1.6P 1.7C 1.8P 2.1P 2.2R 2.3T 2.4C 2.5R 2.6C 2.7P 2.8P 3.1C 3.2R 3.3T 3.4C 3.5R 3.6C 3.7P 3.8P and 2 housekeeping voltages (HKV0 and HKV1 for housekeeping). Table 1. PPI characteristics and performance (pressure profile with altitude). ---------------------------------------------------------------------------- Range: 0- 400 hPa low pressure 0-1200 hPa medium pressure 0-1600 hPa high pressure Resolution: <0.04% or +/- 0.005 hPa Absolute accuracy: 1% --------------------------------------------------------------------------- Fig. 1 The PPI subsystem 4.1. PPI location and accommodation The PPI sensor heads and related electronics are located on the PPI board inside HASI DPU. The Kiel-type total pressure Pitot tube inlet (Kiel probe) is mounted at the tip of the STUB stem and a sectioned tube conveys the external pressure to the sensor heads inside Huygens. 4.2. Measurement principle and data sampling The PPI pressure transducer is a variant of the silicon capacitive absolutepressure sensor (Barocap) produced by the Vaisala Co, Helsinki, Finland, for radiosondes flown on stratospheric balloons up to 40 km. The Barocap consists of a very small sensor head with associated transducer electronics. The varying ambient pressure bends a thin silicon diaphragm in the sensor head, causing changes in the head capacitance. That variation is converted into frequency in the PPI electronics. Two types of Barocap, characterised by different thickness of silicon diaphragm, are used. The thinner diaphragm is suitable for 10-3-102 hPa. The thicker diaphragm of the other Barocap completes the required range. In total there are 8 Barocaps.The temperature is measured by Thermocap sensor for compensation. Thermocap technology is similar to that one of Barocap. There are 3 Thermocaps (one for each block) Each Constant sensor is a high stability capacitor (7 constant capacitor in PPI). They are mainly used to check stability and performance of the PPI measurement system.The Reference sensor heads are high stability capacitors (6 Reference in PPI). They provide a fixed capacitance to be used in the pressure measurement in combination with other sensors (ref. Eq. 1).The sensors are grouped in three blocks (Multicap) each composed of eightfrequency output channels. Table 1 Pressure sensor layout Block channel sensor type name 1 1 Medium pressure 1.1P 2 Reference 1.2R 3 Temperature 1.3T 4 Constant 1.4C 5 Reference 1.5R 6 Medium pressure 1.6P 7 Constant 1.7C 8 Medium pressure 1.8P 2 1 High pressure 2.1P 2 Reference 2.2R 3 Temperature 2.3T 4 Constant 2.4C 5 Reference 2.5R 6 Constant 2.6C 7 High pressure 2.7P 8 High pressure 2.8P 3 1 Constant 3.1C 2 Reference 3.2R 3 Temperature 3.3T 4 Constant 3.4C 5 Reference 3.5R 6 Constant 3.6C 7 Low pressure 3.7P 8 Low pressure 3.8P High pressure sensors are capable to measure in the range 0..2000 hPa (e.g. whole pressure profile of Titan) and they are more sensitive and stable in the high part of the range. Low pressure sensors are more sensitive and stable in the low pressure range (e.g. 0..400 hPa). The Medium pressure heads behave as the high pressure heads except that their working range is limited to approximately 1400 hPa. They are mainly used to check the long term stability of the PPI. The variable capacitance (P, T) and the constant capacitance (C, R) are transformed into a frequency variation by means of a stable oscillator (each block has a dedicated oscillator).The pressure and temperature measurements consist of three samplings/measurements:the frequency period of the selected sensor head and the frequency periods of the two Reference channels relevant to the sensor. Then the three samplings/measurements are combined together to provide the Y value: Y=(S-R1(S))/(R2(S)-R1(S)) Eq 1 where: S is the period measurement of the sensor output frequency R1(S) is the period measurement of the 1st Reference channel of the block containing the sensor S. R1 is the reference channel of low frequency (high capacitance) R2(S) is the measurement of the 2nd Reference channel of highest frequency (lowest capacitance) Y is the compressed pressure data in the range [-1,+1] The two PPI housekeeping voltages correspond respectively to the monitoring of the oscillators and multiplexer supply voltage (PPI HKV0) and PPI power supply voltage (PPI HKV1). Pressure measurement is organized in a sequence of NORMAL and HEALTH-CHECK session as in the following 7-session basic cycle: [HC+N+N+N+HC+NN]. NORMAL session includes three different types (session A = LOW, session B = MEDIUM, session C = HIGH). Each is defined as a sequence {YSh} of 36 YSi statistical pressure data at a regular rate of 2 each 2.4s (2 items at 0.42 Hz). Two YSi data are obtained from 16 raw frequency measurements (periods). Each NORMAL session data is composed by the arithmetical average of five YSi values and their variance HEALTH-CHECK session includes two different types (G and H). Each is defined as a sequence {Fh} of 37 frequency channel raw data (either sensor or reference) acquired every 5.17 s (7.16 Hz). Each Fi frequency raw data is the time corresponding to the measured frequency period. Each HEALTH-CHECK session data set is composed by 37 Fi values 4.3. Operational modes HASI starts to sample PPI sensors at the beginning of the descent phase, starting from T0+10s (=Tdata) when the sensors are still under the front shield (front shield jettisoning at T0+32.5s) in order to get data during the transitional phase helping to connect entry and descent profiles. PPI measurement is organized in sessions (ref 4.2) and divided in three phases: Phase session sequence starting time (DDB time) LOW pressure phase G A A A H A A Tdata=T0+10s MEDIUM Pressure phase G B B B H B B Tmid=T0+75min HIGH pressure phase G C C C H C C Thigh=T0+105min The switching to different phases/modes is triggered by HASI timeline (predefined Tmid, Thigh). PPI will keep going sampling, in high pressure mode, till the loss of the link. The PPI channels polling tables for normal and health check session are reported in appendix A. 4.4. Telemetry output PPI telemetry data are organized in telemetry (TM) packets per sessions plus a data format containing housekeeping voltages: Data format format name content #64 PPI session A YSi statistical data of NORMAL session A (couples at 0.42 Hz) #65 PPI session B YSi statistical data of NORMAL session B (couples at 0.42 Hz) #66 PPI session C YSi statistical data of NORMAL session C (couples at 0.42 Hz) #70 PPI HC session G raw data Fi of HEALTH CHECK session G (at 6.7 Hz) #71 PPI HC session H raw data Fi of HEALTH CHECK session H (at 6.7 Hz) #72 PPI HKV housekeeping voltages 1 & 2 (HKV0 & HKV1) (averages over 64s, 32 sums at 0.5 Hz) Each TM packet is time stamped with mission time (=native time) when the first data is written in Time relevant to each data value is derived from the packet time stamp and the PPI session sampling rate and scheme. 5. Pre-flight calibration 5.1. Static calibration The static calibration for pressure sensors has been performed in two phases, the first one at high pressures (19 points . 0,100...1800mbar) and the second allow pressures (0, 5, 10, 15, 20, 30, 40, 65, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, atmosphere) both at temperatures between -45 and +55 deg C (10 deg C steps). For temperature sensor calibration 13 temperature points between -45 and 75 deg C (10 deg C steps). For pressure sensors the primary calibration has been conducted at room temperature, furthermore long term stability has been verified and possible drifts have been observed in two pressure areas (in vacuum below 0.5 mbar and at atmospheric pressure , 970..1030 mbar).Further tests have been conducted to verify sensitivity to acceleration, sensor hysteresis (not significant) effect, long tubing and leakage.A discontinuity in the behaviour oh the low pressure Multicap 3 has been observed at high temperatures (above 55 deg C). 5.2. Dynamic calibration The purpose is to verify PPI behaviour in dynamic conditions, with rising pressure during descent phase, and investigate possible problems as: _ High hysteresis _ Long time constant (of sensors or tubing) _ Effect of temperature gradient caused by "adiabatic process". The results have shown that the deviation of any PPI pressure sensor does not increase more than 0,1% of the actual pressure (in the range 100-1000mbar). 6. In-flight Calibration During the cruise phase HASI experiment and the Huygens probe have been switched on regularly for performing in-flight CheckOut (CO). These COs have been performed approximately every 6 months since launch, to test the probe and its subsystems during simulated entry, descent and surface proximity phases and also to upload SW patches.PPI have been subjected to a test sequence during each CO, monitoring the pressure (and PPI board temperature) conditions inside Huygens. In space conditions (zero-g and vacuum) only temperature conditions varied due to the heating of the experiment platform due to CDMS working. It is not possible to perform an in flight calibration since there is no a more accurate reference sensor. The only purpose was to check the status of the subsystem and sensors and eventually monitoring any drift and/or ageing effects. 7. Dynamic corrections on pressure measurements: Total to static pressure Pressure measurements are relevant to total values and have to be corrected taking into account the dynamic conditions [RD2]: Eq. 4---See Figure 2 The dynamic correction of the measured pressure profiles will be carried with an iterative method as from [RD2] and in [Fulchignoni et al 1997]. 8. PPI total error budget Figure 3 Note: the errors are worstcase values (usually at pressure near 400mbar) as from [AD1] *) the complete operating range for PPI is 0...1800mbar for pressures < 100mbar use accuracy at 100mbar for pressures < 1000mbar use accuracy at 1000mbar Uncertainty The major component of the total error is systematic. The confidence limit for PPI is 1% or 1 hPa (whichever is better/smaller). 9. PPI data processing: Engineering value reconstruction In every packet for every data format are contained: Figure 4 The packet data field layout are: For df #64,65,66 and for data format #70 and #71 See Figure 5 For data format #72 See Figure 6 9.1. Raw values extraction The first step is the extraction of the sub-fields from the packet for each data format. 9.1.1. NORMAL Session (#A, #B, #C) raw data extraction In data format #64 packets the first 54 words are divided in 36 subfields of 24 bit each.Each datum of 24bit is obtained from the subfields Y2a,Y1a,Y0a. The first 8 bits are the variance and the other 16bits (Y value) are compressed pressure or temperature reading. The sensor reading sequence is reported in [RD2] and in Appendix A. Y_RawVal bit 15-0 Var_RawVal bit 23-16 The following 2 words are divided in 16 bit subfields and are reference value of R1 and R2. R_RawVal bit 15-0 The same extraction procedure is used for data formats #65 (Session #B) and #66 (Session #C) (for sensor reading sequences Appendix A). 9.1.2. HEALTH CHECK Session (#G, #H) raw data extraction In data format 70 packets are contained 56 words divided in 37 statistics subfields of 24 bit each. Each datum of 24bit is obtained by a sequence F2a, F1a, F0a. The sensor reading sequence (pressure, temperature and reference) is reported in Appendix A. Y_HC_RawVal bit 23-0 the same extraction procedure is used for data formats 71 (Session #H), the sensor reading sequence is in Appendix A. 9.1.3. Housekeeping raw data extraction In data format 72 packets are contained 56 words for raw housekeeping voltage values. HK_RawVal bit 15-0 9.2. Raw values normalisation Raw values for each sensor are normalised as follows. 9.2.1. NORMAL Session (#A, #B, #C) normalization Y_Norm_Val= RawVal Var__Norm_Val= RawVal R_ Norm_Val = 1/ R_RawVal 9.2.2. HEALTH CHECK Session (#G and #H) normalization Using Eq. 1(formula 3 page 105 HASI-MA -OG-002 issue 3 [AD2]): Y_HC_Norm_Val= (S-R1)/(R2-R1) in each block R1 and R2 are the reference channel readings for in each block (e.g. R1.5, R1.2 for block #1). 9.2.3. Housekeeping data normalization HK_Norm_Val = RawVal 9.3. Engineering conversion For every sensor reported above the normalised value with sign is converted in engineering units. 9.3.1. NORMAL Session (#A, #B, #C) engineering conversions Y_Eng_Val= 2-15 * P_Norm_Val Var_Eng_Val= 2-18 * Var_Norm_Val R_ Eng_Val = 4,5*106* R_Norm_Val 9.3.2. HEALTH CHECK Session (#G and #H) engineering conversions Y_HC_Eng_Val= 4,5*106*210*Y_HC_Norm_Val 9.3.3. Housekeeping engineering conversion HK_Norm_Val = 10*HK_Norm_Val/(216-1) 10. Scientific conversion of engineering values: Scientific reconstruction values for in flight CheckOuts 10.1. NORMAL Session (#A, #B, #C) conversion T_Val =[(1/(a- Y_Eng_Val))-b]/c T_Val_K=T_Val+ 273.16 Eq.2 P_Val= ((1/(A-Y_Eng_Val))+(K4 Y_Eng_Val4)+(K3* Y_Eng_Val3)+(K2* Y_Eng_Val2)- (O+TFFSET*T_Val)/(G+TGAIN*T_Val) Eq. 3 where T is expressed in Kelvin [K] and P in [hPa] The calibration coefficients used in the above formulas are reported in Appendix B. 10.2. Health check Sessions (#G and #H) conversions Same formulas for Nominal Sessions (#A,#B,#C) 11. Scientific conversion of engineering values: Scientific reconstruction values for Titan mission 11.1. NORMAL Session (#A, #B, #C) conversion See Figure 7 for Eqns 4 & 5 The calibration coefficients used in the above formulas are reported in Appendix C. 11.2. Health check Sessions (#G and #H) conversions Same formulas for Nominal Sessions (#A,#B,#C) 12. Higher level products derivation Starting from pressure measurements, ambient pressure, velocity and altitude profiles are retrieved through an iterative process (refer to [RD1, RD3]) using real gas equation, temperature (as measured by HASI TEM) and atmospheric mixing ratios (as provided by GCMS). 13. Appendixes 13.1. Appendix A: PPI channels polling tables PPI Normal Session#A, #B, #C and Health Check Session #G, #H tables Table 1 PPI session tables sensor sequences (See Fig 8 & 9 ) P1.6 and P2.1 sensors measure the "housekeeping pressure" inside the DPU box. 13.2. Appendix B : Calibration for in flight CheckOuts Scientific conversion coefficients (See Figure 10) In ASCII format [Sensor channel, a, b ,c] 1.3,0.95520,4.34272,0.0239358 2.3,0.94665,3.35377,0.0182452 3.3,0.92365,2.52352,0.0137031 Table 2 PPI temperature conversion coefficients (See Figure 11) In ASCII format [Sensor channel, A, K4, K3, K2, G, O, TGAIN, TOFFSET] 1.1,0.70933,0.576038,0.420474,0.222446,-0.0016989,3.11431,0.000000191,-0.000278 1.6,0.70944,0.913535,0.322299,0.243633,-0.0015446,3.20610,0.000000176,-0.000291 1.8,0.70900,0.509454,0.421665,0.227899,-0.0017922,3.11988,0.000000210,-0.000274 2.1,0.69483,0.260947,0.346241,0.207277,-0.0012719,2.91446,0.000000146,-0.000167 2.7,0.69070,0.369996,0.377318,0.211959,-0.0011483,2.88523,0.000000131,-0.000201 2.8,0.67356,0.352843,0.390846,0.222883,-0.0011488,2.87117,0.000000121,-0.000173 3.7,0.88774,-0.487971,0.080418,0.028051,0.0084094,1.16132,-0.000001127,-0.000274 3.8,0.88500,-0.130457,-0.286945,0.172239,0.0097982,1.23710,-0.000000834,-0.000392 Table 3 PPI pressure conversion coefficients Formulas: Y_Val = (S-R1)/(R2-R1) Eq.1 T_Val =[(1/(a- Y_Eng_Val))-b]/c T_Val_K = T_Val + 273.16 Eq.2 P_Val= ((1/(A-Y_Eng_Val))+(K4 Y_Eng_Val4)+(K3* Y_Eng_Val3)+(K2* Y_Eng_Val2)- (O+TFFSET*T_Val)/(G+TGAIN*T_Val) Eq. 3 where T is expressed in Kelvin [K] and P in [hPa] 13.3. Appendix C : Calibration for Titan mission Scientific conversion coefficients (See Figure 12 for formatted version) In ASCII format [Sensor channel, a, b ,c] 1.3,0.95520,4.34272,0.0239358 2.3,0.94665,3.35377,0.0182452 3.3,0.92365,2.52352,0.0137031 Table 4 PPI pressure conversion coefficients (See figure 13 for formatted version) In ASCII format [Sensor channel, Y_off,A, a11,a12,a13,a21,a22,a23,a31,a32,a33] 1.1, -0.00007, 0.719, -657.4292279, -0.100342031, 0.000326826, 0, 0, 0, 0, 0, 0 1.6, -0.00007, 0.714, -686.2551263, -0.109695236, 0.000333509, 0, 0, 0, 0, 0, 0 1.8, -0.00007, 0.726, -669.4615346, -0.101388028, 0.000319535, 0, 0, 0, 0, 0, 0 2.1, -0.00003, 0.716, -966.4281555, -0.133100727, -0.000019593, 0, 0, 0, 0, 0, 0 2.7, -0.00016, 0.712, -1080.174483, -0.155553526, 0.000344837, 0, 0, 0, 0, 0, 0 2.8, -0.00016, 0.695, -1079.884751, -0.155847787, -0.000062807, 0, 0, 0, 0, 0, 0 3.7, -0.0003, 0.809, 88.06750069, 0.018178688, -0.000014784, -3.802727572, -0.001704142, 0.000002594, 0.077164611, 0.000059704, 0 3.8, -0.0003, 0.805, 73.98128576, 0.010673543, 0.000006087, -3.097715374, -0.001015383, 0.00000088, 0.059014787, 0.000032967, 0 Table 5 PPI pressure conversion coefficients Table 6 PPI pressure Correction Coefficients (See Slide 14 for formatted version) In ASCII format [Sensor channel, K11,K12,K13,K14,K21,K22,K23,K24,K31,K32,K33,K34] 1.1, 1917.589245, 101.0844221, 0, -190.2248322, 0.085946109, 0.089560648, 0, 0, -0.000389231, 0, 0, 0 1.6, 2124.845234, 45.34485761, 24.09251419, -493.6641469, 0.090065834, 0.119673487, 0, 0, -0.000380453, 0, 0, 0 1.8, 1875.879196, 159.5436089, 59.17257105, -46.5313299, 0.097258901, 0.081547068, 0, 0, -0.000380038, 0, 0, 0 2.1, 2509.297639, 256.2924743, 132.7041323, 44.98558513, 0.189549766, 0.122206777, 0.076148017, 0.001893328, 0, 0.000599493, 0.001312938, 0.001760405 2.7, 2768.424329, 303.534838, 168.0515145, 68.96209852, 0.189656425, 0.163047384, 0.126348058, 0, -0.000782556, 0, 0, 0 2.8, 2760.141431, 316.1517516, 178.497679, 71.01049135, 0.203270515, 0.174390885, 0.162238146, 0.097758547, 0.000040707, 0.000776008, 0.001218297, 0.001811214 3.7, -107.2712543, 30.39331205, 32.0691027, 26.69205533, 0.006594045, 0, 0, 0, 0.000105811, 0, 0, 0 3.8, -98.17188197, 29.34880543, 31.14502493, 25.26979811, 0.022303389, 0, 0, 0, 0.000083481, 0, 0, 0 Table 7 PPI pressure conversion coefficients Formulas: Y_Val = (S-R1)/(R2-R1) Eq.1 Y_c=Y_Val + Y_off Eq. 2 T_Val =[(1/(a- Y_Eng_Val))-b]/c Eq. 3 P_Val= Sum[i=1:3]Sum[j=1:3](aij*T_Val(j-1))*(A-Y_c)^(-i) + Sum[i=1:3]Sum[j=1:4](Kij*T_Val(i-1))*(Y_c)^(j-1) Eq. 4 where T is expressed in Celsius [deg C] and P in [hPa] P.S. Formulas and coefficients in ASCII format as to be included in the CALIBRATION PDS directory