The NOAA GOES system supports the DCS by relaying transmissions from Data Collection Platforms (DCPs) to earth stations. There are thousands of DCPs in the western hemisphere and their transmissions are relayed through the two GOES spacecraft to earth stations located in many countries. The uplink, earth-to-space, spectrum used by the NOAA DCS is currently assigned between 401.7 Mega-Hertz (MHz) and 402.0 MHz. The actual transponder on the GOES spacecraft supports a UHF uplink between 401.7 MHz and 402.4 MHz, to provide potential international DCS program support.
The RF spectrum between 401 MHz and 403 MHz is allocated for use by several applications. Some applications are considered “primary” applications. This means that they are given priority use of the spectrum and cannot be interfered with. Other applications are considered “secondary” applications. These applications must not cause interference to primary applications and must also tolerate any interference from a primary application. Government use of the 2 MHz between 401 MHz and 403 MHz is considered primary for meteorological satellite and earth exploration satellite applications. These government applications must only transmit from earth-to-space. The NOAA DCS application fits this application description. Non-government use of the same 2 MHz also supports meteorological satellite and earth exploration satellite applications from earth-to-space, but only as secondary applications. This means these commercial applications cannot interfere with government applications like the NOAA DCS application. There is a third type of allocation permitted between 401 MHz and 403 MHz. This third allocation is the “shared use” allocation that both government and non-government entities may use. In this third allocation, the use of meteorological radiosondes is permitted as a primary application. In addition, in this allocation, but only between 401 MHz and 402 MHz, space operations from satellites may be transmitted, from space to earth, as a primary application.
The satellite industry has recently started using these allocations at 401-403 MHz. Two companies in particular that combined have almost 200 satellites currently on orbit using this spectrum are Planet Labs and Spire. Planet Labs launched 88 of their Dove satellites in February 2017, helping India set the record for the most satellites launched from a single rocket. These satellites joined over 60 Doves already operating on orbit. Planet Labs positions their satellites in various, 3-axis stabilized, inclined orbits with their plan to photograph the entire earth once per day at 3-5 meter resolution. Planet Labs satellites are licensed to use 60 kHz centered at 401.3 MHz, to transmit command and telemetry data down to the earth as each space craft flies over Planet Labs earth stations. The spacecraft antenna has a half-power beam width of 70 degrees and transmits an Effective Isotropic Radiated Power (EIRP) of 1 Watt. A different frequency (at 450 MHz) is used to uplink command and telemetry. Imagery from Planet Labs satellites is transmitted to the earth at 8 Giga-Hertz (GHz.)
Spire has 33 Lemur-2 satellites, also in various, 3-axis stabilized, inclined orbits. Spire’s Lemur-2s monitor shipping by receiving Automatic Identification System (AIS) transmissions from maritime vessels. The Lemur 2 satellites also collect various weather data. Spire satellites use 15 kHz channels in a 200 kHz wide assignment from 402.6 MHz to 402.8 MHz for command and telemetry data. Spire requested, and was granted, a waiver to transmit and receive from the spacecraft in the 200 kHz wide assignment. There are 13 United States Spire earth stations and they are each configured to transmit up into space with 1400 Watts EIRP across a 60 degree antenna half-power beamwidth. Spire has indicated the next generation Lemur-3 satellites will only use the 402-403 MHz band for backup command and telemetry.
While Planet Labs and Spire are the two most visible satellite companies deploying large constellations that are using the 401-403 MHz spectrum, there are other satellite companies with similar or even larger constellation deployment plans and some of them may decide to use the spectrum at 401-403 MHz. As the aggregate number of transmitters in space grows the potential for adjacent channel interference to DCS grows. A satellite frequency assignment on top of the DCS spectrum may cause co-channel interference to the DCS system as well. If the satellite allocation involves primary space operations transmitting from space to earth, DCS users may have to share the spectrum as co-primary users.
In order to address the potential spectrum challenges created by this new era of satellite technology, the members of the DCS user community must become good stewards of the UHF spectrum at 401-403 MHz. To be good spectrum stewards involves regulatory, technical, and community efforts. The DCS users must monitor and respond to satellite applications before the Federal Communications Commission that have the potential to impact DCS operations. In addition, the technical aspects of acceptable and unacceptable interference to the DCS system must be studied and understood. Also, to ensure the DCS user community is properly informed, the users must share the information they gather, regarding spectrum issues, with each other and the whole NOAA GOES user community.
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