VAISALA PRESSURES SENSORS USED ON PHOENIX AND MSL

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This article is copied from our Basic Report. (Updated 1/19/2017)

Segments from Section 2.2 and 2.4 on Mars Pathfinder (MPF) and Phoenix Pressure Issues.

The range of sensitivity and accuracy of the Vaisala Barocap® and Tavis sensors are crucial. With Mars Phoenix, three Barocap sensors [LL(B1), and RSP1 (B2, B3)] were used.  They had problems associated with a nearby heat source.  Problems were particularly noted when temperatures rose above 0ºC.  According to Taylor et al. (2009) calibration coefficients were also withheld from the Finnish Meteorological Institute (FMI) due to International Traffic in Arms Regulations (ITAR). The 5-12 mbar range of Barocaps was probably due to the data from the Tavis sensors before, but Tavis sensors were limited due to radio occultation pressure experiments (not as accurate as in situ measurements) by the Mariners. Radio occultation results are discussed further in Section 5.

An issue with respect to how fast the dust filters for transducers on landers could have clogged relates to when the air tube was initially exposed to ambient conditions. If open to space all the way down, then air might not rush in so fast; while if the tube were suddenly opened on the surface, more dust might be expected to rush in, even at supersonic speeds. Alvin Seiff, et al. (1997) indicates that for Pathfinder the plan was for atmospheric pressure (and temperature) to be measured during parachute descent from ~8 km to the surface. The air inlet was connected to the flared tube fitting shown in Figure 10B by one meter of 2 mm inside diameter tubing. Dr. Robert Sulliavan (Cornell University) told us (on July 27, 2011) that while 1µ particles on the surface of Mars clump together quickly, larger particles that were easier to move would be lifted on landing. He was not sure about whether they would clog a dust filter as fast. But if MPF suddenly ingested 1µ particles suspended in the air below 8 km right after parachute deployment, the hot air associated with the entry-related heat might cause a problem for the tiny filter.

 
 
2.4 Issues Raised by the FMI. The FMI report by Kahanpää and Polkko (2009) discusses the Vaisala pressure sensor that it designed for use on Phoenix.  It states, "We should find out how the pressure tube is mounted in the spacecraft and if there are additional filters etc."  The one and only filter for the Vaisala transducer is shown on the top of Figure 11A (with its near twin for MSL shown on the bottom of Figure 11A). I challenged the above statement on November 14, 2009, and published a criticism of it on my web site on November 17, 2009.  Kahanpää’s partial response from the FMI to my assertion that, "something stinks" about his request for information on additional filters was a follows:
 

“Your nose smelled also a real issue. The fact that we at FMI did not know how our sensor was mounted in the spacecraft and how many filters there were shows that the exchange of information between NASA and the foreign subcontractors did not work optimally in this mission!” (Kahanpää, personal communication, December 15, 2009).

 In his e-mail of December 15, 2009, Kahanpää made clear that there was no extra filter. However, the confusion in his report highlights another possibility.  As is shown in Figure 11B, the filter is very small (~10 mm2).

Figure 11A above – The top transducer is for Phoenix. Note the tiny dust filter shown under Praw (adapted from Doc. No: FMI_S-PHX-BAR-TN-00 FM-00 Revision 1.0 dated 2009-02-26). The report is entitled The Time Response of the PHOENIX Pressure Sensor). An area of concern for clogging by dust is highlighted. The photo on the right is adapted from http://www.space.fmi.fi/phoenix/?sivu=instrument. The bottom pictures (Figure 11B) are for MSL

       Kahanpää is a critical man to understand. He was the scientist on the REMS Team responsible for publishing pressure data for MSL. As is shown later on Figure 17A, in Section 2.7 and elsewhere in this report, the REMS Team published pressures that varied from 747 hPa to 747 Pa in early September 2012. Annex M to this report details other radical alterations in pressure data published for MSL Year 1. There is cause to ask whether Kahanpää was forced to alter data or whether he published Earth-like pressures to protest what he knew to be deliberate disinformation. 

       Like the Tavis transducers that were used for Vikings and Pathfinder, the Vaisala transducer was exposed to a vacuum on the way from Earth to Mars.  Again, when Phoenix landed, a lot of dust was raised by the retrorocket.  The air pressure outside was supposed to be low, almost as low as outer space.  The flow of air into the transducer therefore should not have been too fast.  However, if the pressure outside was higher than expected, the rate of flow of air and dust into the Phoenix would be faster than planned for, with the result that dust would be rapidly sucked in just like a vacuum cleaner would inhale it.  A tiny filter might well quickly clog with dust so fast (at supersonic speeds) that it would prevent more air from reaching the pressure transducer.

        With a clogged filter, pressure at the Barocap pressure sensor head would stay pegged at a low pressure reading. If there was a higher pressure on the outer side of the dust clog, it could not be felt on the inner side where the Barocap resided. This could explain the confusion by Kahanpää & Polkko and why they asked in their report about more filters being present. Even if the FMI team eventually received the needed information about relocation of heat sources, corrections to the pressure indicated at the Barocap pressure sensor head would not reflect what the true pressure was on the other side of the dust clog. 

One difference between the Vikings and both Pathfinder and Phoenix is that the latter two landers did not include Radioisotope Thermoelectric Generator (RTG) heaters. Therefore, it would be expected that as the sun grew lower on the horizon and temperatures dropped, pressure would go down steadily. In looking at data for Phoenix derived from Nelli et al., 2009, this is exactly what happened (see Figure 12A).  The pressure fell in a nearly linear fashion.  

Figure 12A is extracted from graphs produced by Nelli et al. (2009).3 Their graphs included projections made from a General Circulation Model (GCM) with values hypothesized for 3 am, 9 am, 3 pm and 9 pm local time at Phoenix. We added Ls and data about day length for clarity. Phoenix landed in the Martian arctic in late spring. There was no sunset until Ls 121.1 on its 96th sol on September 1, 2008. By the time the mission ended there were about 16.7 hours of sun light each day.

Figure 12A - Pressure and Temperatures Recorded by Phoenix (adapted from Nelli et al, 2009). Ls and day length data has been added to the top graph.

The pressure data appears to be sol averaged, while the temperatures are not.  But what kind of pressure drop would be expected if the average temperature dropped from 195K to 180 K, with a starting pressure of 8.5 mbar? The answer is about 7.85 mbar. The actual pressure at the end of the series shown on the graph is about 7.4 mbar, which is better than a 94% match with the prediction based on Gay-Lussac’s Law and a clogged pressure tube. However, when Phoenix landed on Mars on May 25, 2008, it was not yet summer.  The summer solstice occurred on June 24, 2008. By that time there was no change in the temperatures evident on Figure 12A, but pressure was running about 8.2 mbar. Using the same temperatures as above with an entering argument of 8.2 mbar the projected pressure would be 7.57 mbar. That is an agreement of 97.78%.

Unlike pressure calculations based on an inverse of normal temperature and pressure relationships that factor in RTG heat becoming available to Viking transducers, on Phoenix there was no RTG. If there was no heater, pressures would be expected to fall directly with the fall in ambient pressures. This happened, but there were indeed four heaters that were turned off just before the lander died (http://www.nasa.gov/mission_pages/phoenix/news/phoenix-20081028.html). The third one operated the Surface Stereo Imager –and the meteorological suite of instruments. It was thought that electronics that operate the meteorological instruments should generate enough heat on their own to keep most of those instruments and the camera functioning. This sounds like there was no need to pump heat into the pressure transducer. If so, there may indeed have been slow cooling of the air trapped behind   the clogged dust filter, with no timed heat pumps to cause pressure spikes seen with the Vikings and MSL.  

    There was nothing to keep Phoenix alive once it got too cold. Its death supposedly came when ice built up on and broke the solar arrays (http://www.nasa.gov/mission_pages/phoenix/news/phx20100524.html#.VNpJky6LWlI)).

With respect to Phoenix design, Kahanpää & Polkko repeatedly mentioned funding problems, although the meteorology package for Phoenix cost $37,000,000.  Not only was an anemometer unfunded, but a way to change the dust filter was also left off the shopping list. Indeed it is unclear if anyone conducted tests to see to how much dust was required to clog the filters, or if such tests were conducted, what size dust particles, and what density of dust particles were involved.  

Kahanpää & Polkko (2009) stated that the Mars Science Laboratory (MSL), launched in 2011, is a $2 billion cornerstone mission and is therefore handled in a different way than the $454 million dollar scout mission Phoenix. The actual cost of MSL was $2.5 billion. However, MSL’s FMI-built sensors (delivered in 2008, see http://space.fmi.fi/solar.htm) are in the 0.01 to 11.5 mbar range (see http://www.spaceflight101.com/msl-rems-instrument-information.html), still too low (the REMS Team reported a mean pressure of 11.49 mbar for Sol 370). I discussed this problem with Dr. Ashwin Vasavada, JPL’s Deputy Director of the MSL, but the inadequate transducer was apparently sent anyway.

On December 9, 2012 at http://davidaroffman.com/custom3_45.html we published a prediction that maximum pressure published for MSL would occur around January 31, 2013. Initially our estimate of the date was only off by 2 days, but our 9.45 to 9.5 mbar estimate was higher than the 9.25 mbar published by the REMS Team. But on July 3, 2013 REMS changed all its data. Our estimate was then listed as off by 19 days, but the new pressure was 9.4 mbar, quite close to our 9.45 to 9.5 figure. Our slightly off eye-balled prediction was only  based on our beliefs that the REMS Team would extrapolate (politically expedient) results from pressure curves seen by Viking I and 2 (see Figure 12B), making sure to keep all their invented data points between those of Viking 1 and Viking 2 because MSL’s altitude was between those two probes.  Sure enough when we called attention to four MSL pressures that were above the curve in August and September 2012 (see the red hexagon on Figure 12B and Table 3); JPL dropped them back to match the curve when they revised their data on July 3. Likewise, after a pressure of 11.49 mbar was reported for MSL sol 370 and we called JPL about it, the next sol (371) pressure was back down to 8.65 mbar. By March, 2014 JPL/the REMS Team altered the pressure for Sol 370 too and rolled it back to only 8.65 mbar (865 Pa).

Figure 12B: Except for Sol 370 the black MSL pressure curve is suspiciously too close to the Viking 2 curve (usually) above it and the Viking 1 curve always below it.

TABLE 3 – Pressures revised by JPL/REMS after we highlighted them or published them in earlier versions of our Report

Date

MSL Sol

Ls

Initial Pressure Reported

Pressure for the previous

sol

Final Pressure Reported after JPL Revisions

Aug 25, 2012

19

160.4

785 Pa

 

719 Pa– then changed to N/A

Aug 27, 2012

21

161.4

790 Pa

N/A

741 Pa

Sept 1 to Sept 5, 1012

26

164

 742 to 747 hPa       74200 to 74700 (Pa)

743 Pa

 

Sep 12, 2012 (This date later changed to 9/11/2012)

36

169.5

799 Pa

749 Pa

750 Pa

Sep 16, 2012 (date later altered)

39

172.3

804 Pa

750 Pa

753 Pa - then changed to 751 Pa  

Date

MSL Sol

Ls

Initial Pressure Reported

Pressure for the previous sol

Final Pressure Reported after JPL Revisions

Sep 16, 2012 (date later altered)

39

172.3

804 Pa

750 Pa

753 Pa - then changed to 751 Pa 

 

Oct 3, 2012

Series alteration starts here and goes to 10/12/2012

57

181

779 Pa

770 Pa

769 – Pa. Note the steady progression without reversals that were seen between 10/3/2012 and 10/12/2012 in initial results. This series looks very fudged.

Oct 4, 2012

58

182

779 Pa

 

769 Pa

Oct 5, 2012

59

183

781 Pa

 

771 Pa

Oct 6, 2012

60

183

785 Pa

 

772 Pa

Oct 7, 2012

61

184

779 Pa

 

772 Pa

Oct 8, 2012

62

184

782 Pa

 

774 Pa

Oct 9, 2012

63

185

786 Pa

 

775 Pa

Oct 10, 2012

64

186

785 Pa

 

776 Pa

Oct 11, 2012

65

186

785 Pa

 

777 Pa

Oct 12, 2012

66

187

781 Pa

 

778 Pa

Nov 11, 2012

95

204

815.53 Pa

822.43 Pa

822 Pa

Dec 8, 2012

121

221

865.4 Pa

867.5 Pa

869

Feb 19, 2013

192

267

940 Pa – a high until now. Pressures had been declining since a high of 925 Pa in late January 2013.

921

N/A

Feb 22, 2013

195

269

886 Pa – quite a large drop

Last 2 reports were 940 Pa on Feb 19 and 921 Pa on Feb 18, 2012

N/A

Feb 27, 2013

200

272

937 Pa

917 Pa

N/A

May 2, 2013

262

311

900 Pa

868.05 Pa

N/A

Aug 21, 2013

370

9

1,149 Pa

865 Pa

865 Pa

Aug 27, 2014

731

185

754 Pa

771 Pa

771 Pa

Oct 11, 2014

775

211

823 Pa

838 Pa

838 Pa

Nov 10, 2015

1160

66

1177 Pa

898 Pa

899 Pa

Nov 12, 2015

1161

66

1200 Pa

899 Pa (revised)

898 Pa

April 2, 2016

1300

131

945 Pa

753 Pa

752 Pa

April 3, 2016

1301

131

1154 Pa

753 Pa (2 sols earlier, 751 Pa on Sol 1302

752 Pa

Oct 17, 2016

1492

242

921 Pa

906 Pa

910 Pa

Oct 23, 2016

1498

243

897 Pa

906 Pa

907 Pa

Oct 27, 2016

1502

249

928 Pa

903 Pa

907 Pa

Jan 10, 2017

1575

296

860 Pa

868 Pa

871 Pa

Table 3 shows some (not all) of how JPL/REMS altered off the curve data for August and September 2012 and August 2013 and on through at least January 12, 2017 after we either brought the deviations up to JPL Public Relations Director Guy Webster, or published on our davidaroffman.com and marscorrect.com websites.

 

 

Figure 12C: Pressures originally published by the REMS Team and JPL that were more than 7 Pa off the expected curves were altered to fit the curve.