| DESCRIPTION |
Instrument Overview
===================
see [FULCHIGNONIETAL2002]
The Huygens Atmospheric Structure Instrument (HASI) is a multi-sensor package
designed to measure the physical quantities characterising Titan's atmosphere
during the Huygens probe entry, descent and at the surface.
The HASI experiment is divided into four subsystems: the accelerometers (ACC);
the deployable booms system (DBS); the stem (STUB) carrying the temperature
sensors, a Kiel probe pressure sampling inlet, an acoustic sensor and the data
processing unit (DPU). The HASI subsystems, their acronyms, the institutions
responsible for the management (together with the providers) and the elements
included are summarised in Table 1.
TABLE 1 HASI subsystems
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Subsystem (Acronym) Responsible institutions Elements
(Providers)
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Deployable Boom System (DBS) RSSD (LPCE/SSD) PWA electrodes,
Boom Magnetic Actuators,
PWA preamplifiers (HASI-I)
Fixed stem (STUB) UPD (UPD/FMI/IWF) Temperature sensors,
PPI Kiel probe,
acoustic sensor
Accelerometers (ACC) OU-UK (UKC) Four accelerometers
Data Processing Unit (DPU) OG (FMI/IWF/IAA/OG) Electronics boards
Electrical Ground
Support Equipment OG (UPD/OG) EGSE
------------------------------------------------------------------------------
The scientific measurements are performed by four sensor packages: the
accelerometers (ACC), the temperature sensors (TEM), the Pressure Profile
Instrument (PPI) and the Permittivity, Wave and Altimetry package (PWA) (see
Table 2). TEM, PWA and PPI's Kiel probe are mounted externally and ACC is
attached to Huygens' experiment platform at the Probe's centre of gravity in
the entry configuration.
Table 2. HASI sensor packages.
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Sensor package (Acronym) Sensor type Measured parameters
------------------------------------------------------------------------------
Accelerometers (ACC) 3-axes accelerometers Atmospheric deceleration;
(1 X-servo & Descent monitoring;
3 piezoresistive) Response to impact
Pressure Profile Kiel type pressure Atmospheric pressure Instrument
(PPI) probe + capacitive profile
transducers
Temperature (TEM) 2 dual element Atmospheric temperature
Pt thermometers profile
Permittivity, Wave Mutual Impedance Atmospheric electric
and Altimetry (PWA) AC field measurement conductivity;
Wave electric fields &
Lightning.
Relaxation Probe Ion conductivity;
DC electric field.
Acoustic sensor Acoustic noise due to
turbulences or storms.
Radar signal Radar echoes below h=60 km
Processing (FFT)
------------------------------------------------------------------------------
Scientific Objectives
=====================
HASI scientific objectives are:
- Determine the atmospheric pressure and temperature profiles.
- Evaluate the density and molecular weight profiles.
- Determine the atmospheric electric conductivity and charge carrier profiles.
- Investigate ionisation processes.
- Survey wave electric fields and atmospheric lightning; analysis of
quasi-static electric fields leading to storm formation.
- Detect acoustic noise due to turbulence or thunder storms.
- Characterize the roughness, mechanical and electrical properties of the
surface material whatever its phase, solid or liquid.
HASI data will also contribute to the analysis of the atmospheric composition
and to provide information on the surface, whatever its phase: liquid or
solid. HASI will monitor the acceleration experienced by the probe during the
whole descent phase and will provide the only direct measurements of pressure
and temperature through sensors having access to the atmospheric flow.
Electrical measurements will be performed in order to characterise the
electric environment on Titan and to detect effects connected to electrical
processes, such as lightning and thunder. In situ measurements are essential
for the investigation of the atmospheric structure and dynamics. The
estimation of the temperature lapse rate can be used to identify the presence
of condensation and eventually clouds, to distinguish between saturated and
unsaturated, stable and conditionally stable regions. The variations in the
density, pressure and temperature profiles provide information on the
atmospheric stability and stratification, on the presence of winds, thermal
tides, waves and turbulence in the atmosphere. Moreover, the descent profile
can be derived from temperature and pressure data as a function of pressure
and altitude. The return signal of the Huygens altimeter radar is processed by
the HASI electronics, providing an independent estimation of altitude and
spectral analysis of the signal yields information on satellite's surface.
For a more detailed description of HASI scientific return see
[FULCHIGNONIETAL1997; FULCHIGNONIETAL2002]
Pressure, temperature and density profiles
------------------------------------------
Entry phase
-----------
HASI will be the only instrument operating during the entry phase. Information
on density, pressure and temperature in Titan's atmosphere, from an altitude
of about 2000 km down to about 190-170 km, during the high-speed entry phase,
relies primarily on the data collected by HASI's 3-axis accelerometer. The
atmosphere's density profile, rho(z), is proportional to the acceleration
along the flight path -a_s, through the equation:
rho(z) = - (2 m a_s) / (C_D A V_r^2) (1)
The vehicle mass m, the aerodynamical drag coefficient C_D and Huygens' cross
sectional area (A) are constants known from ground tests. V_r = V_i + V_atm
where V_i is the Probe's velocity in the inertial frame, V_atm represents the
contributions of the wind and atmospheric corotation, and V_r is the vehicle
velocity relative to the ambient atmosphere.
Descent phase
-------------
Following the maximum deceleration near 270 km, the descent phase begins in
the stratosphere at 170 km altitude when the Huygens pilot chute is deployed
and the thermal shield jettisoned. HASI booms are deployed and the sensors are
directly exposed to the ambient atmospheric flow.
During the parachute descent, nominally beginning at 170 km, pressure and
temperature will be measured directly by pressure and temperature sensors with
access to the unperturbed field outside of Huygens' boundary layer. Both the
measured temperature and pressure, T_meas and p_meas, could need dynamic
corrections:
T_corr = T_meas - (r V_r^2) / (2 c_p) (2)
p_corr = p_meas - (rho V_r^2) / 2 (3)
where c_p is the specific heat constant pressure, V_r^2/2c_p is the
temperature increment from conversion of the kinetic energy to thermal energy
in the gas flow approaching the sensor, and r is the recovery factor (related
to the fraction of thermal energy actually experienced by temperature sensors
in laboratory calibrations). This relation are valid considering a ideal gas,
for real gas refer to PPI calibration folder.
Sensor Overview
=============================
Accelerometers (ACC)
--------------------
The Accelerometer subsystem (ACC) is placed at the centre of mass of the
descent module of the Probe. It consists of one highly sensitive single axis
accelerometer (X-servo) and three piezoresistive accelometer (X, Y, Z piezo),
their conditioning electronics and two temperature sensors (Temp1 and Temp2)
used for thermal compensation. The X-servo accelerometer output is amplified
providing two channels (HIGH and LOW gain).
Each channel has a switchable range (fine and coarse resolution) which is set
autonomously (FINE range set at startup and swithched on COARSE prior
saturation or anyhow after Tdata (T0+10s)). X-servo selection is performed
autonomously by checking the measured value against a setable threshold.
7 channels and relevant sampling:
- Xservo LOW gain at 100 Hz
- Xservo HIGH gain at 100 Hz
- Xpiezo at 50 Hz
- Ypiezo at 50 Hz
- Zpiezo at 50 Hz
- Temp 1 (Tservo) at 1.5625 Hz
- Temp 2 (Tpiezo) at 1.5625 Hz
Values are arithmetically averaged to produce lower sampling rates.
Table 3. ACC characteristics and performance
----------------------------------------------------------------------------
X-axis servo accelerometer (along Probe path)
High resolution setting
Range: 2-20 mg
Resolution: 1-10 ug
Low resolution setting
Range: 1.85-18.5 g
Resolution: 0.9-9 mg
Relative accuracy: 1 % of full scale
X/Y/Z-axis piezoresistive accelerometers
Range: +- 20 g
Resolution: +- 15 mg
----------------------------------------------------------------------------
Temperature sensors (TEM)
-------------------------
HASI temperature sensors are two redundant dual element platinum resistance
thermometers (TEM) mounted on the STUB in order to be appropriatelly located
and oriented with respect to the gas flow during the measurements.
Each TEM has a primary sensor (fine, F) directly exposed to the air flow and
a secondary sensor (coarse, C) which is annealed in glass of the supporting
frame and is used as spare unit in case of damage on the primary sensor.
Temperature measurement is performed by monitoring the resistance of TEM
sensors; the resistance of each TEM sensor is measured by a four wire
configuration.
Temperature measurements can be performed in HIGH and LOW resolution range
(60-110K for HIGH and 100-330K for LOW resolution ) by switching HIGH and
LOW gain channel. The range selection is performed by HASI S/W calculating
the rough resistor value and comparing against a setable threshold.
4 sensors:
- TEM1 fine (F1)
- TEM1 coarse (C1)
- TEM2 fine (F2)
- TEM2 coarse (C2)
Table 4. TEM characteristics and performance
----------------------------------------------------------------------------
Range (60-110K)
Resolution 0.02K
FINE absolute accuracy 0.5K
COARSE absolute accuracy 0.8K
Range (90-330K)
Resolution 0.07K
FINE absolute accuracy 2K
COARSE absolute accuracy 2K
----------------------------------------------------------------------------
Pressure Profile Instrument (PPI)
---------------------------------
The Pressure Profile Instrument 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 tip, inside the DPU where the
transducers and related electronics are located. The PPI sensors are 6
reference sensors and 18 transducers. The transducers are silicon capacitive
absolute pressure sensors (Barocap, 8), temperature capacitive sensors
(Thermocap, 3) and constant sensors (high stability capacitor, 7, used for
housekeeping) 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 P3.7 , P3.8 and T3.3
medium pressure 0-1200 hPa block 1, sensor P1.1, P1.6 , P1.8 and T1.3
high pressure 0-1600 hPa block2, sensor P2.1, P2.7, P2.8 p and T2.3
In total there are:
24 frequency channels:
1.1P 2.1P 3.1C
1.2R 2.2R 3.2R
1.3T 2.3T 3.3T
1.4C 2.4C 3.4C
1.5R 2.5R 3.5R
1.6P 2.6C 3.7P
1.7C 2.7P 3.8P
1.8P 2.8P
and 2 housekeeping voltages (HKV0 and HKV1 for pressure measurement
reconstruction).
Table 5. PPI characteristics and performance
----------------------------------------------------------------------------
Range: 0-1600 hPa
Resolution: 0.01hPa
Absolute accuracy: 1%
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Permittivity, Wave and Altimetry (PWA) sensors
----------------------------------------------
The PWA sensors are 6 electrodes and an acoustic transducer. The electrodes
placed on the deployable booms (DBS) form a quadrupolar probe consisting of a
pair of mutual impedance transmitter (TX) and receiver (RX) to measure
atmospheric electric conductivity due to free electrons and detect wave
emission. The pair of electrodes placed at the tip of the booms are relaxation
probe for meauring ion electric conductivity and quasi-static electric field.
The acoustic sensor (ACU) is mounted on the STUB detect sound waves to
correlate with acoustic noise, turbulence and meteorological events.
The radar return signals of the Huygens Proximity Sensor, containing
information on surface properties and altitude, are processed also by HASI.
The radar signal is converted to 10KHz and filtered by the Radar Altimeter
Extension (RAE) board and passed to the PWA A/D converter and signal
processor. The radar input signals in HASI/PWA are the blanking signal and the
analogue intermediate frequency signal (echo signal). The PWA signal processor
performs FFT, digital integration and data packetising and controls the data
acquisition. The spectrum and altitude information of the radar return signal
are added to the HASI data stream.
Data products:
PWA-ACU
PWA-RP
PWA-MI
PWA-ACDC
PWA-RAE
The mutual impedance is measured by applying sine wave pulses of 0.2-20 V at
fixed frequencies (from 100 Hz up to 6 kHz after impact) on two TX electrodes.
The modulus and phase of the impedance are computed after analysis of the RX
electrodes' signal (with intelligent choice of TX driver range and RX
amplifier gain). The signal received by the dipoles is sampled at a rate of
12800 Hz; a statistical analysis and a Fast Fourier Transform can be performed
every 20 ms, yielding a 50 Hz spectral resolution in a 6.4 kHz bandwidth. The
receiving dipole and signal analyser will detect wave emissions in the
atmosphere. It is also intended to detect quasi-static electric fields with
amplitudes of up to several V/m using the two relaxation sensors.
The conductivity due to positive and possibly negative ions will be measured
in parallel with the relaxation sensors. Potentials will be applied between
the descent module and the sensors for 1 s every minute by closing switches;
the sensors and the vehicle return independently to their equilibrium
potentials when the switches open.
Measuring these potentials as a function of time (64 words in 59 s) will
confirm or disprove the existence of free electrons and yield the ion
conductivity. The relaxation electrodes are grounded at the end of the
measurement sequence. A microphone is mounted on the STUB for detecting
acoustic events.
Table 6. PWA characteristics and performance
----------------------------------------------------------------------------
Mutual Impedance
Electron conductivities, ground conductivity
Conductivity range: 10^-11 - 10^-7 /Ohm m
Relative permittivity range: 1-100
Time resolution: 2-3 s
AC field measurement
Natural wave phenomena, e.g. lightning
Threshold: > 2 uV/m/Hz
Dynamic range: 80 dB
Frequency range: 0-10 kHz
DC field
Atmospheric electricity, Schumann resonances
Threshold: 1 mV/m
Range: 1 mV/m - 30 V/m
Dynamic range: 16 bit
Relaxation probe
Ion-electron conductivities
Conductivity range: 10^-15 - 10^-11 /Ohm m
Accuracy time constant: 0.1 s
Time resolution: 1 min
Acoustic
Natural acoustic phenomena, e.g. thunder, rain, hail, surface waves
Threshold: 10 mPa
Dynamic range: 90 dB
Frequency range: 0-6 kHz
Resolution, amplitude: 3%
frequency: 50 Hz
time: 1 s
----------------------------------------------------------------------------
Altimetry
Surface structure, roughness
Maximum range: 60 km
Range resolution: 40 m @ 24 km
S0 minimum: -10 dB @ 60 km
S0 accuracy: 1.5 dB
----------------------------------------------------------------------------
Operational modes and measurements
==================================
HASI mission phases
--------------------
ENTRY
from higher than 1270 km (~1900 km) to 170 km;
Tacc=Probe-ON +21min 30s to T0 (expected at 28 min)
DESCENT
from Tdata=T0+10s to surface
DESCENT 1st state from Tdata to TdataH=T0+2min 30s
DESCENT 2nd state from TdataH to Tradar=T0+32min
DESCENT 3rd state from Tradar to last km (~T0+2h12min)
IMPACT state from last km to surface (~T0+2h14min)
SURFACE state from ACC impact detection to link loss
Table 7. HASI timeline, probe events and related dynamic conditions during
mission (expected)
------------------------------------------------------------------------------
Time Mission altitude Vertical accel.
(min) time (km) velocity (m/s2)
(m/s)
COAST T0-22d 5mn Tsep Probe separation from Orbiter
T0-18 mn Tp Probe power-up and CDMS activities
T0-10 mn Thasi HASI ON (17:46 preT0)
T0-8 mn ~1900 Tacc ACC sampling start
ENTRY 0.03 ~T0-5 mn 1260.8 6145.28 0.61 Tentry
2.7 401.082 6114.32 11.031
3.37 229.334 3404.9 120.78 max acc
4.13 170.24 638.54 16.886
4.42 162.648 429.37 8.3181
DESCENT end of entry phase
0.03 162.063 307.91 24.196 T0
descent device deployment begins
T0+00.25s Pilot chute deployed and inflation
T0+2.5s Back cover release,
main chute deployment & inflation
T0+10s 157.3 192.6 12.363 Tdata, T&p sampling;
0.5 T0+32.5s 157.452 99.63 2.469 Front shield jettison
1 T0+1mn DISR cover jettison
1.05 154.904 67.42 0.598 Td1
1st BOOM release attempt start
1.55 153.002 62.06 0.255 Td1w
1st BOOM release attempt end
2.2 150.658 59.72 0.113 Td2
2nd BOOM release attempt start
2.5 149.775 59.08 0.086 TdataH
PWA sampling (mode A) start
T0+15mn 114.733 36.61 0.02 Main chute jettison,
stabiliser deployment & inflation
25.78 74.906 44.58 0.049
32.12 61.372 27.49 0.028 Tradar
RADAR sampling, PWA mode C
48.45 42.103 14.71 0.007 tropopause
75.12 24.262 8.76 0.002 Tpmed
Medium p sampling start
105.1 11.034 6.29 0.001 Tphigh
High p sampling start
PWA mode D
134.5 1.025 5.22 0 last km IMPACT mode
SURFACE 138.1 0 0 0 Tloss
Loss of radio link
Table 7.B HASI timeline, probe events and related altitude during mission at
Titan (as measured by HASI)
------------------------------------------------------------------------------
UTC Mission Time Mission Time Event h [km]
(SPC Time) [ms]
HASI ON
9:01:17.372 4:19:44.500 15584500 first ACC pck 2787.013 T0-9min
(after resets)
9:07:46.128 4:26:13.000 15973000 ACC range coarse 639.142
9:09:07.407 4:27:34.535 16054535 max acc 240.941 T0-73.340s
9:10:13.997 4:28:41.500 16121500 S0 162.420
(CASU detection 10 m/s2) ACCXservo=9.645m/s2
9:10:20.747 4:28:47.875 16127875 T0 (PDD firing) 160.098 T0=S0+6.375s
9:10:20.997 0:00:00.250 250 Pilot chute deployment 159.995 T0+0.25s
& inflation
9:10:22.032 0:00:01.285 1285 Pilot chute ACC peak 159.888 T0+1.285s
9:10:23.247 0:00:02.500 2500 Back cover release, 159.745 T0+2.5s
main chute deploym. & inflation
9:10:25.232 0:00:04.485 4485 Main chute ACC peak 159.531 T0+4.485s
9:10:30.747 0:00:10.000 10000 Tdata 158.904 T0+10s
9:10:53.247 0:00:32.500 32500 frontshield jettison 156.421 T0+32.5s
9:11:20.747 0:01:00.000 60000 1st BOOM release attempt start 153.361 Td1
MCA sequence
9:11:22.747 0:01:02.750 62750 1st BOOM release attempt end 153.040 Td1w
9:11:22.747 0:02:20.000 140000 2nd BOOM release attempt start 147.106 Td2
9:12:40.747 0:02:22.750 142750 2nd BOOM release attempt end 146.960
9:12:50.747 0:02:30:000 150000 PWA mode A 146.559 TdataH=T0+2.5min
9:25:20.747 0:15:00.000 900000 Main chute jettison, 112.893 T0+15min
stabiliser deployment & inflation
9:42:50.747 0:32:30.00 1950000 RADAR sampling,PWA mode C 60.992 Trad=T0+32.5m
10:25:27.747 1:15:07.200 4507200 PPI Medium p mode 26.050 Tpmed=T0+1:15:00
(session B)
10.55.26.747 1:45:06.000 6306000 PPI High p mode 13.301 Tphigh=T0+1:45:00
(session C)
11:16:22.998 2:05:57.749 7757749 PWA mode D 5.286 DDB 7 km*
11:34:56.997 2:24:36.250 8676250 last km 0.872 last km*
11:38:10.578 2:27:49.840 8869840 Impact detection 0 Timpact
12:10:20.747 3.00.00.000 10800000 autoreset/ last HASI data SW reset
*correspondent UTC time derived from Huygens HK parameter S2013E_H3B
ACC modes
--------------------
For the acceleration there are two types of data: 'raw' data and statistics
data (obtained from average of 100 Hz sampling, on-board processing).
Channels readouts are summed in order to get the following sampling rate:
ACC Xservo 3.125 Hz in ENTRY; from Tacc till T0+10 s
4.167 Hz till T0+32 min
1.754 Hz last km (~132 min) Impact detection
and in Surface state
ACC X, Y, Z piezo 1.6129 Hz in ENTRY and last km
and in Surface state
Statistics 0.1 Hz always, except in impact detection
ACC Servo & piezo Temperature 0.097 Hz always, except in impact detection
Impact trace (0.5 s before & 5.5 s after impact)
X piezo 200 Hz
Y piezo 200 Hz transmitted after Timpact
Z piezo 200 Hz
p.s. NO ACC data are transmitted during impact phase
TEM modes
--------------------
All 4 TEM sensors are sampled every 5 s. The measurement sequence is the
following:
F1,C1, F2, C2.
Sampling rate:
1 Temperature point every 1.25s (0.8 Hz)
but same sensor sampled every 5s (0.2Hz)
IMPACT STATE only F1 and F2 are sampled
1 Temperature point every 1.25s (0.8 Hz)
but same sensor sampled every 2.5s (0.4Hz)
PPI modes
--------------------
DDB time HASI time PPI normal session
T0+10s Tdata A Low pressure
T0+75' Tpmed B Medium p sampling start
T0+105' Tphigh C High p sampling start
Two pressure values every 2.3 s are produced.
PPI channels polling tables for normal and health check session are reported
in PPI calibration report.
PWA modes
--------------------
Phase DDB time HASI time altitude range PWA mode measurements
Descent_2 T0+2.5' Tdatah ~60 km
|
| REFERENCES |
Fulchignoni, M., F. Angrilli, G. Bianchini, A. Bar-Nun, M.A. Barucci, W.
Borucki, M. Coradini, A. Coustenis, F. Ferri, R.J. Grard, M. Hamelin, A.M.
Harri, G.W. Leppelmeier, J.J. Lopez-Moreno, J.A.M. McDonnell, C. McKay, F.H.
Neubauer, A. Pedersen, G. Picardi, V. Pirronello, R. Pirjola, R. Rodrigo, C.
Schwingenschuh, A. Seiff, H. Svedhem, E, Thrane, V. Vanzani, G. Visconti, J.
Zarnecki, The Huygens Atmospheric Structure Instrument (HASI), ESA SP 1177,
163-176, 1997
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