SCM in Detail

Goembel Instruments has been developing the Spacecraft Charge Monitor ever its conception, as published in the January, 1998 issue of the Journal of Spacecraft and Rockets. The article, "Instrument for measuring spacecraft potential" detailed how spacecraft charge could be monitored in a novel way. The method is based on electron spectroscopy. We are fortunate to have data from an instrument that provided actual satellite data that confirms the technique would work.

The method is dependent on a phenomenon known as "photoionization." When sunlight strikes the Earth's upper atmosphere (at about 100-150 miles altitude) the molecules of nitrogen and oxygen present there eject electrons that are travelling at only a few speeds. The speed of the electron is measured in "eV" - electron Volts, the kinetic energy of the electron (energy due to the mass and speed of the electron). It turns out that a very significant portion of the electrons produced in photoionizaton travel at four well defined energies between 20-30 eV; 22.2, 23.9, 25.2, and 27.2 eV. In the diagram below we see that the sunlit atmosphere of the Earth produces electrons between 20-30 eV that travel away from the lower altitudes where they are produced (~150-250 km) to higher altitudes where they can be detected by spacecraft (the term "24 eV electrons" that appears in the illustration is shorthand for the 22.2, 23.9, 25.2, and 27.2 eV electrons that are produced).

24 eV Electrons

The basis of spacecraft charge measurement by the SCM is the measurement of the energy of electrons produced by the photoionization nitrogen and oxygen in the upper atmosphere. The SCM's reference potential is spacecraft ground. Spacecraft charging is appearent if we see a shift in the energy location of the peaks that would (on an uncharged spacecraft) appear at 22.2, 23.9, 25.2, and 27.2 eV. If it appears that the electrons have been slowed down as they approach the spacecraft, it is because the spacecraft is charged negatively and the negative charge of the spacecraft repulses the negatively charged electrons. For instance, if the spacecraft has a negative floating potential of ten volts relative to the space plasma where the photoelectrons were produced, the four photoionization peaks will appear at 12.2, 13.9, 15.2, and 17.2 eV. By the same mechanism, a positively charged spacecraft will accelerate the electrons to higher speeds and we will see the peaks shifted to higher energies. One of the outstanding features of this method for the determination of spacecraft floating potential is that there is a one-to-one correlation between the shift in the location of the photoionization peaks and the spacecraft floating potential. Thus, the data analysis for floating potential promises to be direct and accurate. The diagram below illustrates the one-to-one correlation between spacecraft floating potential and a shift in the energy location of the photoionization peaks in an electron energy spectrum.

Peak shift analysis

In fact, none of the methods now used to determine spacecraft floating potential are as straightforward or as accurate as the electron spectroscopic method that will be used by the Goembel Instruments device.

Instruments that are designed to measure the speed of electrons are called electron spectrometers (they measure the energy distribution of electrons). We are fortunate to have data that confirm spacecraft floating potential can be measured with an electron spectrometer. The data came from a series of satellites that flew in the 1970s. One instrument that was mounted on the satellites was a very specialized electron spectrometer, one that measured very low speed (i.e. 1-100 eV) electrons well. The SCM will be able to detect the same low speed electrons, but with a 60 times higher rate of electron detection due to the patented Goembel Instruments design. The higher rate of electron detection will allow the instrument to collect the data needed to determine floating potential in a few seconds, rather than in a few minutes. The difference in time needed to collect data, seconds versus minutes, makes the Goembel Instruments device very appealing to those who would like to measure spacecraft floating potential. The SCM is needed because the novel spectroscopic method it uses offers an order of magnitude increase in the accuracy of spacecraft charge sensing, it is easily accommodated on a spacecraft, and it is available at a low cost.