Testing for EMC on a spacecraft isn’t unlike testing for terrestrial equipment except for the fact that spacecrafts live in a different electromagnetic environment.
Here on Earth, every piece of electronic equipment must contend with its electromagnetic environment. Systems such as nearby equipment, broadcast signals, and the like mean that you either need to test in a shielded enclosure or find some remote area. Your equipment must play nice with everything around it. In space, however, many of these sources of outside interference aren’t present. When putting a satellite into space, do you need to perform the same emissions and susceptibility tests as you would for an earthbound system?
That’s the question that EMC engineers at NASA face. EMC standards, which NASA uses as a basis for EMC testing, were written with an emphasis on earthbound equipment. In space, the primary sources of interference are systems on the same spacecraft. John McCloskey, chief EMC engineer at NASA’s Goddard Space Flight Center (GSFC), has spent the bulk of his career testing the James Webb Space Telescope (JWST), the replacement for the Hubble Space Telescope. JWST is expected to launch in 2021 from French Guiana atop an Ariane 5 rocket.
The JWST (Figure 1) will have a mirror consisting of hexagon-shaped reflective panels that will unfold once in orbit. A sun shield, about the size of a tennis court, will enable the telescope to operate at the cryogenic temperatures required to achieve the necessary sensitivity. Behind the telescope’s primary mirror assembly is the Integrated Science Instrument Module (ISIM), which includes four science instruments. McCloskey and others are responsible for ensuring that the JWST’s electronic systems don’t interfere with the satellite’s radio receivers, nor with each other.
Figure 1: James Webb Space Telescope entering thermal vacuum chamber for testing. (Source: NASA)
Because satellites spend their operational lifetimes in space, it’s the on-board systems that have the greatest potential to interfere with the satellite’s receivers. There is, however, no specific EMI standard for spacecraft. NASA EMC engineers use limits and test procedures based on MIL-STD-461G. If spacecraft radio receivers don’t get hit with the full electromagnetic spectrum in space, do you need to test for emissions outside those frequencies used by the receivers on the platform? McCloskey says no, so why spend money for unnecessary testing? If JWST’s radio receivers aren’t in use prior to deployment in space, then there should be no reason to test for radiated emissions and susceptibility outside the frequencies of the intentional receivers. Tests for interference among the JWST’s subsystems focus on crosstalk from cables.
“You must, of course, test for emissions levels within the RF bands of the intentional receivers,” said McCloskey. “But emissions outside those receiver bands are generally not a significant factor.”
Another issue arises as to whether emissions testing should be performed at the integrated instrument or spacecraft level versus testing at the unit level. Testing at the integrated instrument or spacecraft level requires more people. McCloskey estimates daily testing costs to be at least 10 times higher than at the unit level, wherein each “box” is tested separately. Testing at the unit level lets engineers diagnose and fix any potential problems when they are much less costly to address than at higher levels of integration. Figure 2 shows the EMC team at GSFC, where engineers test the unit level and instrument level.
NASA engineers at Goddard Space Flight Center performed EMI tests on the JWST using a chamber inside a clean room. (Source: NASA)
McCloskey prefers to perform unit tests in which each unit operates in a configuration that brings out the highest emissions. “We want to make each unit an emissions culprit,” he said. “That might require two or three tests for the highest emissions at different frequencies.” EMC engineers consult with designers to find those conditions. Such conditions might, for example, occur when the main computer sends commands to a motor or when it’s sending significant amounts of data. “It’s OK to have test-only modes such as those for moving motors or states of high data intensity to get worst-case conditions.”
JWST unit-level testing for radiated emissions covered the frequency range of the S-band receiver: 2.0898521 GHz to 2.0916521 GHz. In addition, tests were needed to ensure that the JWST electronics that must operate during the launch sequence wouldn’t interfere with the UHF uplink from mission control to the Launch Vehicle Command Destruct receiver — 420 MHz to 480 MHz at 35 dBµV/m. Outside of that band, testing was still performed to ensure that units would not interfere with each other and that cables and enclosures were properly constructed.
Unit-level emissions tests are performed with several EMI antennas that cover different frequency ranges, all at 1 m from the EUT. Table 1 lists those antennas.
Table 1: JWST radiated emissions frequencies and antennas.
Like any other electronic device, the units in the JWST must pass radiated susceptibility tests, which are more time-consuming that emissions tests. Because all JWST equipment was tested to 18 GHz at unit level, it was sufficient to test the ISIM to only 8 GHz once integrated to the instrument level.
Figure 3: A double-ridged waveguide horn covers frequencies from 1 GHz to 18 GHz. (Source: ETS-Lindgren)
JWST also has no intentional transmitters operating below 1 GHz. Nevertheless, engineers chose to test down to 30 MHz so that they could characterize sensitivity of the detector systems in the science instruments at those frequencies.
EMC in space isn’t all about emissions interfering with radios. Non-RF devices such as computers and scientific instruments must properly operate with the spacecraft. Conducted emissions and susceptibility tests let engineers test how units and systems can interfere with each other. Crosstalk among cables can cause signals to couple into unintended systems. Here, the problem is common-mode current, which McCloskey measures with current probes. For susceptibility tests, engineers inject common-mode current and look for errors that it might cause.
Engineers also look for worst-case situations between offenders and sensitive cables. In such situations, distance matters. To determine if there is a potential problem, engineers calculate the field strength from any offenders at the locations of sensitive cables. Emissions levels not only have to meet standards but be low enough to not disrupt spacecraft functions.
In addition to testing for radiated/conducted emissions and susceptibility, engineers must ensure that power quality at the system level doesn’t disrupt JWST functions. A custom line-impedance stabilization network (LISN) helps to control line impedances on the power bus from the batteries to the electronic equipment.2, 3 When equipment such as motors turn on, current surges combine with line impedance to produce voltage dips. Connecting an oscilloscope to the power network lets engineers view the power bus in both the time and frequency domains.
From an EMC perspective, space places unique conditions on electronics, some of which are easier to contend with than Earth-based equipment. In space, the ambient electromagnetic environment is dominated by on-board equipment.