JPL Light Weight Balloon Interferometer (LWBI)




Instrument.

The LWBI represents a major upgrade to the JPL MkIV balloon FTIR spectrometer system. We will build a new optical head, containing an interferometer, detectors, translation mechanism, and metrology laser, using newer, lighter-weight technologies and a more mass-conscious approach. The resulting lightweight balloon interferometer (LWBI) will have a performance similar to that of the JPL MkIV, yet will weight just one fifth of the mass. The reduced payload size and mass will make the LWBI much easier to launch, facilitating close co-location with observations from space-borne sensors such as TES and HIRDLS on board AURA.

Easier launch and recovery of the payload will also benefit validation by allowing multiple flights during a single campaign, reducing launch costs, and making flights possible from parts of the world (e.g. the tropics, Antarctica) where large instruments are difficult to launch. The LWBI will be compatible with existing MkIV hardware (suntracker, signal chains, power conditioning, telemetry, tele-command) and software (instrument control, data analysis). Re-use of these existing MkIV components minimizes the cost and development time.

Project Description.

Motivation.

Long-term drift in the response of satellite remote sensors can be difficult to identify and characterize due to the lack of access to the instrument. Uncorrected, this drift may lead to erroneous scientific conclusions. It has long been recognized that the best way to overcome this difficulty is through validation by ground-based or airborne instruments, whose performance can be periodically calibrated. Over the next few years, several new satellites will be launched with infrared remote sensing instruments (ACE, ILAS2, TES, HIRDLS) requiring validation in the 2004-5 time frame. Of particular importance to this proposal are the TES and HIRDLS instruments on board the AURA spacecraft, which will measure a wide range of atmospheric trace gases, including H2O, O3, N2O, CO, CH4, NO, NO2, HNO3, N2O5, ClNO3, CFC-11 and CFC-12. Although aircraft can sample the exact location and time of a satellite footprint, they are limited to altitudes of 10-20 km. Balloon platforms are capable of altitudes up to 42 km, but have difficulty in matching the exact latitude and longitude of a satellite overpass. This is due mainly to the low probability of launching a large balloon at a given date and time. Because flights of large balloons are also costly and time consuming, very few flights can be obtained in a given validation campaign. This results in large statistical uncertainties in the interpretation and comparison of the data.

The main problem with the balloon validation is not the inter-calibration accuracy of the sensors. Rather, it is the difficulty in obtaining adequate co-location of the observations in time and space. Due to gradients in the composition of the atmosphere, small mismatches in the locations of the satellite and "ground-truth" observations can cause differences in the measured gas concentrations which may be misinterpreted as biases, especially if only a few coincidences can be obtained. Although great progress has been achieved in minimizing these affects by means of alternative coordinates systems, such as Equivalent Latitude-Theta space1, there remains a strong argument to achieve close co-location during validation.

If balloon launching could be made sufficiently easy that a validation payload could be launched with a high probability on a given day, and sufficiently inexpensive that a given payload could be flown multiple times during a single campaign, then this would greatly increase the validation value of balloon-borne remote observations.

There is a simple way of both reducing the cost of a balloon flight and improving the launch probability: use a smaller balloon. Smaller balloons can withstand stronger winds during launch, have more options for orientation during launch in confined areas, and can be inflated more quickly. These advantages compound each other to produce a strong inverse relationship between balloon size and launch probability.

A further advantage of small balloons and light payloads is that there are more options for launch services. For large payloads (over 600 kg), the National Scientific Balloon Facility (NSBF) is the only option. But for smaller payloads, the French Centre National d'Etudes Spatiales (CNES) have the capability (and considerable expertise). Payloads below 200 kg can be launched by the Swedish Space Corporation from Esrange at even lower cost. There is a price to be paid for flying a smaller balloon: the payload doesn't float as high unless its mass is reduced substantially. Figure 1 shows the relationship between float altitude and payload mass for various standard types of French balloon. The x-axis is the payload mass in kilograms, and the y-axis is the float altitude expressed in meters. The number that labels each line (e.g. 35) is the balloon volume in thousands of cubic meters. In all cases, the lighter the payload, the higher the balloon float altitude. But what is most interesting is that for large balloons the gradient is smaller than for small balloons. This is because if the balloon already weighs more than the payload, there is little to be gained by reducing the payload mass. On the other hand, if the payload weighs more than the balloon, as is often the case for small balloons destined for lower altitudes, then reducing the payload mass results in a much larger increase in altitude.

The discovery of large ozone losses in the lower atmosphere has shifted the focus of scientific interest to lower altitudes. A payload weighing 390 lbs (180 kg) would float at 32.5 km altitude on a 35SF balloon (see Figure 1). This balloon (35,000 m3 or ~1 MCF volume) is 25 times smaller in volume than the majority of the balloons that the MkIV instrument has launched on in the past and is four times smaller than the balloons used to launch the OMS payloads during the SOLVE campaign in March 2000. Such a balloon would have a high probability of being launched on any given day, even from a remote site without specialized launch equipment. Alternatively, this same payload could be flown to 27.5 km on a 12 SF balloon.

Background.

Scientific Expertise.

Our group at JPL has extensive experience measuring the atmospheric composition by solar occultation spectrometry and validating satellite remote sensors. The Atmospheric Trace Molecule Occultation Spectroscopy (ATMOS) instrument, designed at JPL, flew four times on the space shuttle between 1985 and 1994. Dr. Toon was a member of the ATMOS Science Cadre and published several papers using ATMOS data.

The JPL MkIV interferometer is a solar absorption FTIR spectrometer built at JPL in 1984 specifically for atmospheric remote sounding2. Toon and Blavier were extensively involved in the assembly and testing of the MkIV instrument, which was used for ground-based observations from various sites including McMurdo, Antarctica, in 19863, Lynn Lake, Manitoba, in 1996, Fairbanks, Alaska, in 19974, Esrange, Sweden, in 1999-20005, and from Southern California. In addition, the MkIV instrument has also flown on the NASA DC-8 aircraft during the polar campaigns of 1987, 1989, and 19926. The MkIV has also flown 13 times on high altitude research balloons since October 1989. Several of these balloon flights were actually in support of validation activities: The four flights in 1992-1994 were part of the UARS validation effort, which resulted in 10 publications featuring MkIV data7-16. The flight in May 1997 from Fairbanks, Alaska, was for the validation of the ADEOS satellite. Seven papers17-23 comparing these MkIV profiles with ILAS observations have been published or have been submitted to the ILAS Validation Special Section of JGR. Figure 2, from Toon et al.18 illustrates the level of agreement between the MkIV and ILAS instruments. The latest two flights in winter 1999/200 from Esrange, Sweden, validated the ozone measurements by the POAM3 instrument24. During the POLARIS campaign in 1997, the MkIV profiles were also extensively inter-compared with in-situ sensors on board the ER-225. In the winter 2002-3 there will be additional MkIV flights from Esrange, Sweden, for validation of the SAGE3 and the ILAS2 instruments.

Further details about the MkIV measurements and field campaigns can be found at http://mark4sun.jpl.nasa.gov. In the remainder of this proposal, there will be many references to the MkIV instrument, and how its capabilities compare with those of the proposed LWBI. It therefore makes sense to summarize the key points about the MkIV instrument, for the benefit of readers unfamiliar with it.

MkIV Instrument Description.

MkIV is similar to the ATMOS instrument, but with the advantage of dual detectors that allow the entire 650-5650 cm-1 (1.8 - 16 mm) spectral domain to be measured simultaneously at high resolution (0.01 cm-1). This broad spectral coverage and high spectral resolution, together with the high signal-to-noise ratio (500:1 for a single scan), allow over 30 different gases to be measured simultaneously in the same airmass including H2O, O3, N2O, CH4, CO, NO, NO2, HNO3, HNO4, N2O5, ClNO3, HCl, HOCl, H2O2, HF, COF2, CF2Cl2 (CFC-12), CFCl3 (CFC-11), CCl4, CHClF2 (HCFC-22), SF6, HCN, OCS, C2H2, C2H6, H2CO, HCOOH, plus many isotopic variants such as HDO and CH3D. This list includes all of the gases that will be measured by the HIRDLS and TES instruments on board the AURA spacecraft.

Measurement Technique.

The MkIV instrument uses the solar occultation technique, whereby solar spectra are measured as the Sun rises or sets, either as a result of the Earth's rotation or as a result of changes in the balloon altitude. By dividing these limb spectra by an "exo-atmospheric" spectrum, derived from high Sun observations, the instrumental and solar features are cancelled, yielding a series of limb transmittance spectra. Since the thermal emission from the Earth's atmosphere can be neglected in comparison with the direct solar irradiance at the wavelengths under study, the radiative transfer is described simply by the Beer-Lambert law. Moreover, the thermal emission from the instrument itself is also negligible compared with the solar irradiance, even with room temperature optics, eliminating the need for in-flight calibrations.

The brightness and stability of the Sun confer a high inherent accuracy and sensitivity to atmospheric profiles obtained by the solar occultation technique. Calibration is trivial since emission from the instrument and atmosphere are both negligible compared with that from the Sun. The solar absorption technique is also much less sensitive to non-LTE effects since it measures absorption from molecules residing in their ground state (unlike thermal techniques which measure emission from molecules in an excited state).

If a close co-incidence is achieved, the high resolution spectra produced by a balloon-borne FTIR spectrometer may also be compared directly with those from the space-borne sensor, after accounting for differences in their spectral and vertical resolutions. This allows a determination of whether differences in the retrieved vmr profiles arise from differences in the measured spectra or inconsistencies in the retrieval algorithm, providing a means of diagnosing the cause of any discrepancies. Since the MkIV (and the proposed LWBI) covers the entire spectral region from 1.8 to 15.5 microns, any space-borne mid-IR remote sensor could be validated in this manner.

The LightWeight Balloon Interferometer (LWBI).

Description.

The LWBI will offer all of the advantages of the MkIV, at a fraction of the mass. MkIV was built in an era when the focus of scientific interest was the upper stratosphere26-30. The very large balloons required to achieve altitudes greater than 40 km (weighing more than 8000 lbs including parachute, flight train, and helium), made it meaningless to try to make a 400 lbs instrument lighter. So there was never any serious effort made to lightweight the MkIV: reducing its mass by 20% (saving 80 lbs) would only reduce the total mass by 1%, increasing the float altitude by just 70 m. In the absence of mass constraints, the MkIV instrument was therefore built much larger and heavier than necessary. For example, it has the capability to operate with a 200 cm optical path difference (OPD). Although this is useful for laboratory and ground-based observations, for ballooning the optimium OPD is in the 50 cm range. The MkIV OPD is therefore oversized by a factor of ~4, causing the instrument to be a lot larger and hence heavier than a new instrument designed solely for ballooning.

Table 1. Mass (lbs) breakdowns for the existing MkIV interferometer and the LWBI.
Sub-System MkIV LWBI
Optical Head 460 92
Suntracker 52 52
Data Recorder 10 10
Electronics 46 46
Cables 30 10
Cooling System 20 0
Batteries 146 55
Gondola 266 75
Azimuth Drive 98 0
Insulation 70 10
Balancing Mass 20 05
TM/TC 31 31
TOTAL 1249 396

The MkIV instrument was also designed to operate at a pressure of one atmosphere. This made it easier to accomodate the He:Ne reference laser, which has a high voltage power supply, the translation mechanism scan motor, and the signal chain electronics, which would overheat at low pressure. But building the MkIV inside a pressure vessel greatly increased the total mass. The proposed LWBI would have a solid-state reference laser (Nd:YAG). Not only is this much smaller and lighter than the He:Ne lasers, but more importantly, its low voltage power supply would operate at low pressure, eliminating the need for a pressure vessel. The LWBI would also have a much lower power, vacuum-operable motor.

A novel feature of the new payload is the absence of an azimuth drive. Currently, an azimuth drive is used to keep the MkIV suntracker continuously pointed toward the Sun, so that the MkIV instrument can take data without interruption. In the proposed LWBI, the gondola will be free to rotate and so the solar image will be periodically be blocked by a stainless steel cable (75,000 psi yield strength) that supports the gondola. Since this cable will be 6 mm (1/4") in diameter (sufficient to hold a 200 kg payload during a 10g acceleration without yielding) and will be located 600 mm from the suntracker, it will only subtend 10 mrad and will therefore block the 20 mm entrance pupil for just 0.6% of the time.

When a drop in the solar intensity does occur, that part of the interferogram will be cut out and replaced by an average of the neighboring interferograms. This procedure is successfully used to correct ground-based MkIV interferograms when the solar beam is momentarily interrupted, e.g. by insects or birds. It works well provided that the interruptions don't occur exactly at ZPD or in the same place in successive interferograms. Since 99.4% of the data will be unaffected by the cable, the probability of it causing a serious data loss is small.

Eliminating the azimuth drive not only saves its own mass (98 lbs), but also saves the mass of the batteries (33 lbs) that power it, and allows a lighter gondola structure. The total mass savings resulting from the elimination of the azimuth drive is ~150 lbs. Removing the azimuth drive also eliminates a single-point failure and reduces the number of people and equipment needed during field campaigns.

Proposed Work Plan.

The MkIV balloon instrument consists of several sub-systems (suntracker, optical head, signal chains, azimuth drive, power conditioning, telemetry, tele-command). By far, the largest and heaviest part of the current MkIV balloon payload is the optical head (see Table 1). By building a new optical head that would be five times lighter than the current MkIV's, and eliminating the azimuth drive, while retaining all the other sub-systems, the total balloon payload mass could be reduced by a factor three. The LWBI would interface to the existing MkIV suntracker, power regulators, control electronics, signal chains, and telemetry systems, allowing convenient and inexpensive ground-testing.

The layout of the LWBI is illustrated in Figure 4. It will be the same basic optical configuration as the MkIV instrument: a double-passed configuration, single-sided interferograms, and dual detectors. The LWBI will weigh less by virtue of:

1) Using a smaller beam diameter (20 mm instead of 30 mm).

2) Operating at ambient pressure (eliminating the pressure vessel).

3) Having a maximum OPD of 50 cm (rather than 200 cm)

4) Using a solid-state Nd:YAG reference laser (instead of He:Ne)

5) No azimuth drive

6) A more mass-conscious design

Although the LWBI beam diameter is only 20 mm (versus 30 mm for the MkIV), the loss of signal-to-noise ratio is not a concern. Firstly, the LWBI will be photon noise limited and therefore the loss of signal will be partially offset by a reduction in the noise level. Secondly, for most gases, systematic errors in the MkIV spectra (e.g. zero level offsets) limit the accuracy of the retrievals, and these will be improved in the LWBI by the use of more linear detectors (PV HgCdTe). The LWBI will also benefit from several recent developments that have occurred at JPL:

1) Layout of the LWBI optical head High-linearity, DC-coupled signal chains that were built in 1999 for the SOLVE campaign

2) New lightweight, low-power electronics that were developed in 2002 for the SOLVE2 campaign

3) A lightweight translation mechanism that is currently being developed at JPL and ASI (Alliance SpaceSystems Inc, Pasadena) under a PIDDP proposal ("The Planetary Atmosphere Occultation Spectrometer", PI: G. C. Toon) that is funded for the period 2002-2004.

4) A Nd:YAG solid-state reference laser, developed at JPL for use in the TES instrument.

5) Extensive design work into light-weighting FTIR spectrometers that was performed at JPL (using internal funds) in support of the MARVEL Mars Scout proposal (PI: Mark Allen, JPL), which was one of four proposals recently selected to go to Step 2.

These developments, together with the existing hardware, software, and expertise residing within the MkIV group, make it possible to develop a working new instrument in a cost-effective manner and in a short schedule.

Schedule.

The schedule is driven by the desire for the LWBI to be ready to begin balloon validation activities in Spring 2005. Since the AURA spacecraft is likely to be launched in 2004, this will be timely for validation of TES and HIRDLS. Due to the long procurement time of certain items (beam-splitter, compensator), it will not be possible to complete the task in less than two years.Note that balloon flights are beyond the scope of this proposal. The work described here only includes design, fabrication, and testing of the LWBI.

Management Approach.

Dr. Geoffrey Toon, the Principal Investigator, will hold overall responsibility for the work performed, including the definition of the instrument requirements necessary to achieve good science and validation quality.

Dr. Jean-Francois Blavier, the MkIV cognizant engineer, will be responsible for leading the design effort for the LWBI, ensuring its compatibility with the existing MkIV hardware, instrument control software, and data acquisition systems.

Dr. Bhaswar Sen will be responsible for the processing and analysis of data acquired by the LWBI.

Much of the fabrication of the interferometer will be performed by Mr. Ron Howe, a member of the JPL Balloon Support Group who has extensive experience with the design and fabrication of balloon-flight hardware. None of the work on the LWBI will in any way jeopardize the readiness of the MkIV instrument, or prevent us from flying the MkIV instrument again in the future. We are simply building a new optical head that interface with the remainder of the MkIV balloon spectrometer systems in exactly the same manner as the existing MkIV optical head.


Photos taken Oct 10, 2005 at JPL showing the LWBI base plate which was designed and manufactured by ASI(Alliance Space Systems inc).



Photos taken in July 3, 2007 of the almost completed LWBI



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