Goembel Instruments: Spacecraft Charge Monitor (SCM)

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Part 2.1: Pre-Phase II Development

Serendipity led to the development of the SCM. While examining data from a satellite that flew in 1981, I noticed a strange phenomenon. The peaks in spectra collected by an electron spectrometer on the spacecraft shifted as the craft moved up and down in altitude. Subsequent communication with other scientists revealed that others knew of the effect and that it was due to the spacecraft floating potential (electrostatic charge) changing as the spacecraft changed in altitude. The work done for the paper, published in the Journal of Geophysical Research, introduced a novel way to measure atmospheric abundances with electron spectroscopy.

At the time I wrote the paper about O/N2 ratios I thought of the 'peak shifting' effect as a mere complication to data analysis. Later on, it became clear that the phenomenon could be exploited to determine spacecraft charge. In fact, after forming my own business to build electron spectrometers for space flight in 1997, I published the a paper in Journal of Spacecraft and Rockets devoted entirely to the determination of charge through electron spectroscopy.

After publishing the Journal of Spacecraft and Rockets paper I learned there was interest in a new spacecraft charge monitor. In fact, I learned that the state of the art in floating potential measurement had changed very little over the previous decades and there was dissatisfaction with the current methods used. Since there might be a market for an electron-spectroscopic charge monitor Goembel Instruments started promoting the development of the SCM by submitting proposals and making presentations.

We are fortunate that satellite data provide a 'proof of concept' for the SCM. The data also reveal that the instrument used to collect the data would make a very poor spacecraft charge monitor. A major obstacle to the use of high energy-resolution electron spectroscopy to measure spacecraft charge was the inherently poor sensitivity of high energy-resolution electron spectrometers. Using the technology available at the time presented a dilemma: either the SCM would need to be very large (not a good thing for space flight instrumentation) or would need to collect data over too long a time to be practical. After spending a lot of time thinking about ways to increase the sensitivity-per-unit-weight of the SCM I invented an entirely new electron spectrometer.

Figure 2.1.01: Plot from an early (1999) ion trajectory simulation of the patented SCM optics

The invention provides the signal strength needed to measure spacecraft charge in seconds rather than minutes with no increase in instrument size.

While promoting the SCM I had a very hard time convincing the industry that Goembel Instruments, with only one employee, would be able to produce flight-quality hardware. A common question I would get was "why should we have you make an instrument instead of … [our usual supplier of electron spectrometers]?" Now part of the answer to that question is that "your usual supplier of electron spectrometers can only supply a huge or weak instrument because they do not have and cannot use my breakthrough design". A patent for my "Large Geometric Factor Charged Particle Spectrometer" was granted in 2004 4.

Goembel Instruments submitted proposals to develop the SCM with little success until three Small Business Innovation Research (SBIR) proposals (Air Force 1999, and NASA 1999 and 2000) received very encouraging reviews, but still no funding. In the meantime, I continued to develop the SCM without funds. I designed all of the parts needed to build what would subsequently be the patented hemispherical electrostatic analyzer - the 'heart' of the SCM.

Figure 2.1.02: Early drawing (circa 2000) of what would become the patented SCM electrostatic analyzer

In 2001 NASA Kennedy Space Center (due in a large part to the efforts of Dr. Carlos Calle) accepted my proposal for Phase I SBIR funds. I proposed to build a laboratory prototype of the SCM that would be capable of collecting electron spectra in the laboratory. Within the six months duration of Phase I, I had not only built a complete working laboratory prototype SCM, but had gone beyond the work originally proposed by testing it in a vacuum chamber equipped with an electron gun at Johns Hopkins University. The total funds for Phase I were $70,000.

Figures 2.1.03 and 2.1.04: SCM sensor head (hemispherical analyzer) and electronics for Phase I

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