The University of Calgary’s “Enhanced Polar Outflow Probe (ePOP) Radio Receiver Instrument (RRI)” on the CASSIOPE satellite was able to detect several ARRL Field Day stations on June 28. The RRI listened on 80 and 40 meter segments. Virginia Tech graduate researcher Nathaniel Frissell, W2NAF, said that during the first 25 seconds of 7 MHz reception, he and his team aurally decoded and identified 23 stations, most in Illinois, Wisconsin, and Indiana, before the signals “abruptly disappeared.” He said very few signals were detected on 80 meters.
“This experiment was designed to simply test the feasibility of conducting HF Amateur Radio-satellite ionosphere and propagation studies,” Frissell told ARRL. “These results show that this is feasible, and that it is possible to detect interesting geophysical features.” The others involved in the analyzing the results were Gareth Perry of the University of Calgary; Ethan Miller, K8GU, of Johns Hopkins University’s Applied Physics Lab; Magdalina Moses, KM4EGE, of Virginia Tech, and CW Skimmer developer Alex Shovkoplyas, VE3NEA.
The sudden disappearance of signals on 40 meters, Frissell said, suggests that CASSIOPE was passing over regions of differing ionospheric electron densities. “The plasma frequency of the ionosphere is directly proportional to the square root of the electron density,” he explained. “Signals transmitted from Earth and vertically incident on the ionosphere will be reflected back to Earth at the altitude where the plasma frequency matches the transmitted frequency. A satellite flying above this layer will be shielded from the signals below.” The ePOP experiment on CASSIOPE is a suite of eight instruments that study the outflow of plasma from the ionosphere into near-Earth geospace.
Frissell has documented the group’s results in a presentation, “ePOP RRI Observations of Amateur Radio Transmissions.”
Frissell said that at the time of the satellite’s pass, the peak plasma frequency was 6.950 MHz at roughly 290 km altitude, as measured by the Millstone Hill ionosonde in Westford, Massachusetts. “If the conditions were similar to what CASSIOPE was experiencing at its location, it would be able to hear the 7 MHz signal but not the 3.5 MHz signals,” he said. “This is, in fact, what we observe.”
He said the 7 MHz signals abruptly disappeared once CASSIOPE reached 42° N latitude. “We believe it is likely the satellite was above an ionospheric layer that had a plasma frequency greater than 7 MHz, thereby shielding the satellite from the ground transmissions,” he said. “Unfortunately, ionosonde data near this point in the trajectory is not currently available.” He said an International Reference Ionosphere 2000 (IRI-2000) model run tended to support the plasma frequency-shielding hypothesis. He and his fellow researchers plan to follow up with more thorough modeling and analysis.
The researchers were able to record signals appearing within a 30 kHz band segment on 40 meters (7010-7040 kHz) in a special .wav file that required CW Skimmer multi-channel CW decoding and analysis software to decipher and identify individual stations. Frissell said CW Skimmer detected more than 23 signals, but after the results were checked manually, it was determined that the software was unable to accurately identify some individual stations. “CW Skimmer automated detection had difficulty in this case because of the flutter present in the signals observed by the satellite.” Frissell said. He explained that flutter is fast, chaotic amplitude and frequency modulation of the CW carrier. (An audio file of Field Day participant WR9Y, extracted via CW Skimmer, provides an idea of what the RRI actually was hearing.)
“In conclusion,” Frissell said, “we believe this was a successful experiment that provides an interesting view of a possible plasma density transition region, as well as a basis for designing future HF Amateur Radio-satellite ionospheric experiments.”
CASSIOPE (CAScade Smallsat and IOnospheric Polar Explorer) is a Canadian-designed and built satellite. It has two primary tasks — a telecommunications technology demonstration, and a science mission — the latter carried out by the ePOP. Funded by the Canadian Space Agency, CASSIOPE is operated by a community of Canadian and US scientists from more than a dozen institutions. ePOP operations are headquartered at the University of Calgary.