Satellite Constellations and Radio Astronomy

Adam (N4ADK)
Apr 19, 2019 · 8 min read

In the San Augustine Plains of central New Mexico, 27 radio telescopes stand tall, operating nearly 24 hours, 7 days a week capturing extremely weak signals emitted from all over the Universe. This flat and vast land, once a seabed, sits at an altitude of 7,000 feet and is surrounded by 360 degrees of mountains. Despite the ideal conditions of this location, “listening” to these faint radio emissions is becoming increasingly difficult as the Earth becomes “noisier” in the same direction in which these dish antennas are pointed, the sky. At ground level, the National Radio Astronomy Observatory’s (NRAO) Very Large Array keeps a continuous live monitoring station dedicated for receiving Wi-Fi and Bluetooth radio transmissions for visitors’ cellphones and other electronic devices. Besides their inherent interference, these signals can also emit multiples of their intended frequency, causing interference in many places at once (due to poor engineering[1]). When “radio noisy” visitors drive into the VLA area, they are spotted on the monitor. The visitor center receives an email alert to remind the visitors to turn off their cell phones. If the visitors fail to respect the RF-quiet rules after being asked to turn off their cellphones, a portable spectrum analyzer and feed horn antenna can be used to find the direction of the RFI sources.

The spectrum analyzer displays frequency on the horizontal axis and amplitude, or signal strength, on the vertical axis. The Very Large Array differs from Green Bank Observatory in that it does not possess any Radio Quiet Zone[2] boundary. Nonetheless, the consequences of such RFI (radio frequency interference) may completely destroy a scientist’s observation data, or potentially mislead with invalid data points (that’s $5000/hour of American tax payer dollars!)

The radio spectrum consists of frequencies 3 kHz to 300 GHz, and this large area is regulated by an international body known as the International Telecommunication Union (ITU). Frequency bands are allocated accordingly to the different services (radar, amateur radio, television, telephone, etc.). What distinguishes radio astronomers from everyone else who uses the radio spectrum is that radio astronomers are passive users. While others are transmitting on their assigned frequencies, using the spectrum actively, radio astronomers are listening wherever the deep space emissions may lie, such as the microwave remanence of the Big Bang. Today, the radio environment is becoming saturated with signals, and companies are in a constant battle to purchase a piece of this precious spectrum. While the ITU does help in the protection of small slivers of radio spectrum for radio astronomers, sharing portions of the spectrum is not an option as other signals can easily overwhelm the receiver electronics of the scientists. This incredible sensitivity is an unfortunate truth that must compete alongside commercial interests (ex. 5G).

While mitigating terrestrial interference (cellphones, mobile wireless networks, vehicles, etc.) for radio astronomy telescopes is a day job in and of itself[3], orbiting transmitters present significantly more danger. Satellites in the path of a highly sensitive radio telescope have been a concern as early as 1982 with the launch of Russian (previously Soviet) GLONASS satellites. The problem with GLONASS was its main transmit frequency (1612 MHz) overlapping with the spectral line of the hydroxyl (OH) radical. Basically, spectral lines are what is shown when light (or radio emission) is projected on a molecule. This can be either an absorption or an emission of frequencies. Radio astronomers observing at this spectral line were out of luck as soon as one of these satellites cleared the horizon in its orbit. The frequencies in use were eventually removed to accommodate, but a few decades later Motorola laid plans to launch an array of 66 Personal Communication System (PCS) satellites into low earth orbit with downlinks in the 1621.25 to 1626.5 MHz band. This was the Iridium constellation that turned radio astronomers’ heads, and their radio telescopes away.

The original fleet of Iridium satellites in 1998 was in a non-geostationary orbit with the intention of providing seamless voice and data communication all over the world. It wasn’t that Iridium satellites’ downlink frequencies directly transmitted in radio astronomy bands, but it was the sidebands of the signals that overlapped into observing territory that caused the problem. Iridium failed to abide by an ITU-R Recommendation with their downlink signals causing interference of up to 30 dB above the levels deemed harmful. Thirty decibels, the units for power gain, represent a logarithmic ratio between the output and input power; This means Iridium was transmitting signals 1000 times stronger than they should have been. Being only a recommendation, ITU-R Recommendation RA.769 was not mandatory for them to follow. To cope with this, engineers at the Very Large Array in New Mexico spent much time and effort developing “Iridium filters” that blocked a portion of the band that received the interference. Still, according to one RF Engineer at the VLA, radio astronomers did not seem to like using this feature very much. RFI removal exists in both an active and post-processing form, but it could only help things minimally. The entire problem was solved when Iridium declared bankruptcy in late 2000 and the satellites were scheduled to be de-orbited.

Today, radio astronomy faces a new front of enormous satellite constellations, the big three being: SpaceX’s Starlink, OneWeb, and IridiumNEXT. The SpaceX Starlink satellite constellation aims to launch around 12,000 satellites to serve the purpose of a space-based Internet system. The OneWeb constellation’s end plan is to have almost 3,000 satellites in orbit to also serve the purpose of an Internet service. Iridium NEXT, like the original constellation, is a telecommunications satellite constellation consisting of 66 satellites. Of the three, Starlink obviously grabs the most attention and instills the most fear for obvious reasons. Harvey Liszt, astronomer and spectrum manager for the NRAO, reached out to FCC Chairman Ajit Pai in February 2018 to express concern over SpaceX’s constellation.

“SpaceX, which plans to use the 10.7–12.7 GHz band for its downlink, has not yet fulfilled its obligations under US131. Coordination between SpaceX and the AUI observatories (together with NSF) trailed off inconclusively around the middle of 2017 after a tentative and rather preliminary treatment of radio astronomy’s concerns and the manner in which SpaceX planned to address them.”

Footnote US131 of FCC’s Title 47 states that, if transmitting in the 10.7–11.7 GHz band, coordination between non-geostationary satellite orbit licensees and certain radio astronomy observatories is necessary to “achieve a mutually acceptable agreement”. While post-observation RFI excision does exist in some forms, current technology does not yet allow for this when dealing with continuous RFI sources such as 5G and satellite downlinks. This could change in the future, but for now the lower frequency range of 1 to 20 GHz is in bad shape when it comes to collecting clean data. Given this situation, observation frequency in radio astronomy has been trending higher and higher. The Very Large Array currently observes from 1 to 50 GHz. NRAO’s latest project, ngVLA (Next Generation VLA), proposing to begin collecting science in the late 2020s, would have a frequency range of 1.2 to 116 GHz! At the same time space constellations are also making use of these higher frequencies, specifically with ISLs (Inter-Satellite Links). Different from the downlink frequencies (spacecraft to ground), these ISL frequencies will operate in the V-band (40–75 GHz) and provide just as much harm as the downlinks. On November 19th 2018, the FCC granted SpaceX an authorization to transmit in V-band.

Five months after the NRAO’s letter to the FCC, Liszt sent another similar letter, this time regarding the OneWeb constellation. Liszt explains that awareness around the 10.7–11.7 GHz band is crucial because of radio astronomy’s passive band of 10.68–10.7 GHz. Passive bands prohibit any radio emissions in protection of radio astronomy observation, like spectral lines (water, OH molecule, etc.). However, within these passive bands are many bands that merely urge spectrum users to “take all practicable dispositions to protect the radio astronomy service from harmful interference”. The 10.68–10.7 GHz band, however, is not one of these bands where leniency exists, so it is understandable that the NRAO wishes for a “robust demonstration that they [OneWeb] can, in the aggregate, fully protect radio astronomy from their unwanted emissions into the passive band.”

Liszt also points out an incompatibility of OneWeb’s proposed ground level power flux density with ITU-R Recommendation RA 769. Essentially, the detrimental threshold for interference to radio astronomy operating around these frequencies is some “58 dB smaller” (or quieter) than what OneWeb’s filings state. The ITU-R recommendations give active services detailed calculations on what maximum levels of data loss are acceptable for radio astronomer observers. While allowing up to 5% data loss is considered generous of radio astronomers, Liszt states, “unwanted emissions into a passive service band would be a terrible precedent.” Still, the Iridium NEXT constellation was developed without any prior consultation with radio astronomy agencies. Instead of discussing with radio astronomers, they chose to operate under the assumption that they will cease transmission “during periods of observation” as recommended. Iridium requests that astronomers provide them with a 3-day notice to implement RFI reduction for the specified observatories.

When it comes to RFI for radio telescopes, an array will always be less affected than a single beam; the more antennas and the bigger the baseline, the less impact RFI will have (see A. R. Thompson). It won’t be surprising to see a continual development of arrayed radio telescope systems in the future. It seems almost equally as likely for these future radio telescopes to be in orbit around Earth to escape the terrestrial radio clutter or even on the far side of the moon (China Chang’e 4 low frequency research). Or, perhaps we will see more petitions for intentional RQZs (Radio Quiet Zones) like Green Bank Observatory or in the strict case of China’s FAST radio telescope, mandatory relocation of local residents. Either way, with or without cooperation from commercial entities, electromagnetic spectrum managers have a lot of work on their hands. When considering the continuation of high-level science provided by radio astronomy, a new challenge for the world’s engineers arises. The challenge to accommodate for the desire to carry out these large commercial orbital missions and study what’s beyond Earth.

[1] Known as harmonics, these are signals that are multiples of a desired frequency that are expected to be minimized by the design engineer.

[2] The National Radio Quiet Zone (NRQZ) is a large area of land in the United States designated as a radio quiet zone, in which radio transmissions are heavily restricted by law to facilitate scientific research and military intelligence.


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Adam (N4ADK)

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Amateur radio operator, photographer, bass player from Richmond, VA.

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