using speed of sound measurements to constrain the huygens probe descent profile
DESCRIPTION
Using speed of sound measurements to constrain the Huygens Probe descent profile. H. Svedhem, J-P. Lebreton ESA/RSSD, NL J. Zarnecki, B. Hati Open University, UK. John Tyndall’s atmospheric experiment 1875. - PowerPoint PPT PresentationTRANSCRIPT
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Using speed of sound measurements to constrain the Huygens Probe descent profile
H. Svedhem, J-P. LebretonESA/RSSD, NL
J. Zarnecki, B. HatiOpen University, UK
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Planetary Probe Atmospheric Entry Workshop, Lisbon, 6-9 October 2003
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ILEWGJohn Tyndall’s atmospheric
experiment 1875
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About 100 years later Jean-Pierre Lebreton proposes to fly an acoustic sensor to Titan
• We now talk about miniaturisation…..
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The Acoustic Properties Investigation (API) of the Surface Science Package on Huygens
• API has two sets of sensors and one card of electronics incorporated in to the SSP Top Hat and electronics box– API-S, (sounder) is a monostatic SODAR for detection of
atmospheric precipitation during the descent, surface characterisation during the last phase of the descent and detection of sea depth in case of landing in a liquid.
– API-V, (velocity) will measure the speed of sound across a 15 cm long path during the descent from an altitude of about 50 km down to the surface, and in the liquid in case of landing in a liquid.
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API-S
• The API-S in principle works as a conventional SODAR.• The return signal is proportional to the number density of
the scattering particles in the scattering volume and to the particle diameter to the 6th power. (Rayleigh scattering)
• For both volume scattering and surface scattering the signal is inversely proportional to the square of the distance.
• The similarities to Radars are striking. By coincidence the wavelength of the API-S and the probe altimeter are both about 2 cm. Comparisons will be useful.
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API-S Modes 1/2
• Atmospheric sounding mode, >7km. Search for hydrometeors and turbulence. Pulse length 10 ms. Binned samples for the closes 50 m are stored each 2 seconds.
• Surface proximity mode,7km>h>1km. Pulse length 10 ms. Search for surface return AND hydrometeors. Binned samples at higher resolution around the surface bin each 3 seconds.
• Near surface mode. h<1km. Pulse length 2 ms. Search for surface structure and topography. Binned samples at highest resolution around the surface bin each second.
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API-S Modes 2/2
• Surface mode. After impact, search for depth of liquid. Pulse length 10 ms. Send one pulse, listen for 10 s. Binned data around the maximum return.
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API-V mode
• One mode only at h<60 km. Sensor A transmits a pulse and start a 4 Mhz counter, sensor B receives the pulse and stops the counter. Immediately afterwards the sequence is repeated in the reverse direction. Both data are stored. Frequency of measurement is 1 s.
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API-S reflectivity factors, Z
Hydrometeor condition Precipitation rate Z[mm/h] [mm6m-3]
Cloud 0.001 to 1Fog 0.01 to 1Drizzle 1 10Light Rain 1 200Heavy Rain 25 33000Light Snow 1 1000Heavy Snow 10 40000
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API-S Performance
-40
-30
-20
-10
0
10
20
30
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
log Z [mm6m-3]
Echo
leve
l [dB
, ref
20
uPa]
d=50 md=10 mNoise level
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Performance API-S
• Reflectivity factors on earth are known, (previous graph). Titan situation is hard to estimate.
• Garry (1996)estimated, based on data from Toon et al (1988) that the reflectivity factors at Titan are too low to be detected by API-S. The method for these calculations was however unconventional and seem to give too pessimistic results.
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API-V pressure sensitivity
0.01
0.1
1
10
100
0 200 400 600 800 1000 1200 1400
Pressure [mBar]
Rec
eive
r out
put [
mV]
N=10N=1
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Error analysis
• M = ·R · T · c-2
M/M=((T/T)2+ (2c/c)2)1/2
• For the gasses at the temperatures we have c is about 200 m/s, we get with 250 ns resolution and 15 cm path, 2c/c 7 ·10-4. T 0.1 K which at 100 K gives T/T = 10-3. The contributions are thus of the same order of magnitude.
M/M 1.2 ·10-3 .
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Empirical approach, gas
• In stead of calculating the mean molecular weigh one may go directly to find mixing ratios from calibrated measurements.
• The mixing ratio will be accurate to better than 1 % for binary gasses. The number is dependent on which species are involved.
• This will work well for binary mixtures but is difficult for mixtures of three or more gasses (or liquids)
• For mixtures of three or more components a test of the expected sound speed can give useful constraints to measurements by other instruments.
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Empirical approach, liquid
• For the liquids the sound speeds are typically ten times higher while the resolution remains 250 ns. Hence the c/c is the dominating error.
• The spread in sound speed is larger and therefore a precision in the mixing ratio similar to that of gasses will be achieved, i.e. about 1% for binary mixtures.
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API-V in Nitrogen gas
150
170
190
210
230
250
270
290
310
-190 -170 -150 -130 -110 -90 -70 -50Temperature [deg C]
Soun
d sp
eed
[m/s
]
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API-V in 100% Methane gas
200
250
300
350
400
450
500
-200 -150 -100 -50 0 50Temperature [deg C]
Soun
d sp
eed
[m/s
]
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API-V in 100% Ethane gas
245
250
255
260
265
270
275
280
285
290
-100 -80 -60 -40 -20 0Temperature [deg C]
Soun
d sp
eed
[m/s
]
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API-V in 100% Ethane liquid
1100
1300
1500
1700
1900
2100
-190 -170 -150 -130 -110 -90 -70 -50Temperature [deg C]
Soun
d sp
eed
[m/s
]
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API-V in 100% Methane liquid
1320
1340
1360
1380
1400
1420
1440
1460
1480
1500
-180 -175 -170 -165 -160Temperature [deg C]
Soud
spe
ed [m
/s]
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What about the miniaturisation, did it work
out?
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