GNOMES constellation of PlanetiQ - GNSS-RO commercial weather satellites
PlanetiQ was formed to build, launch and operate the first commercial constellation of GNSS-RO (Global Navigation Satellite System-Radio Occultation) weather satellites, with 20 LEO (Low Earth Orbit) satellites to be deployed. The overall objective is to introduce innovation to satellite weather observations. For this reason, PlanetiQ’s satellites will carry the fourth-generation “Pyxis” radio occultation (RO) sensor. Pyxis builds on the heritage of the gold standard for RO sensors. The PlanetiQ team has developed and built the previous three generations of RO sensors proven on orbit, including 20 flight units for prior, current and upcoming satellite missions. 1) 2)
This state-of-the-art, next-generation RO sensor is smaller, lighter, and consumes less power than prior versions, but has nearly 3x the data collection capability since it will receive signals from all four major GNSS constellations (GPS, GLONASS, Galileo and Beidou). Pyxis is the only GPS-RO sensor in such a small package that is powerful enough to provide more than 10 times the amount of data available from GPS-RO sensors currently on orbit, and to routinely probe down into the lowest layers of the atmosphere where severe weather occurs.
With 20 satellites on orbit, called GNOMES (GNSS Navigation and Occultation Measurement Satellites) PlanetiQ will collect over 50,000 soundings per day or about 400 million data observations, enabling unprecedented improvements in weather forecasting, space weather prediction and climate analytics.
The data will have an average latency of less than 3 minutes, using an existing satellite-based relay system in Geostationary Earth Orbit. This low latency is a dramatic improvement over traditional delivery times and critical to evolving weather and space weather forecasting requirements.
Future instruments planned for PlanetiQ satellites 13-18 include the ATOMMS (Active Temperature, Ozone and Moisture Microwave Spectrometer) and a next-generation microwave radiometer.
ATOMMS, currently funded by NSF (National Science Foundation), will use centimeter and millimeter wavelengths to simultaneously profile temperature, pressure, and water vapor versus altitude. ATOMMS will measure water vapor far more accurately than current sensors, yielding 1% or better accuracy from the lower troposphere into the mesosphere.
After years of delays, PlanetiQ of Golden, CO, founded in 2012, says its constellation of commercial weather satellites will be ready to start launching in early 2020 thanks to $18.7 million in new capital. A redesign that right-sized its satellites while increasing the overall constellation number from 12 to 20 helped make the system more affordable, PlanetiQ Founder and Chairman Chris McCormick said in an interview. 3)
PlanetiQ had previously hoped to launch 12 small satellites in 2016 and 2017, while simultaneously raising capital. That funding took longer than expected to materialize, McCormick said, but now enables the company to accelerate its launch plans. “It’s a little later start than we would have liked, but we are funded, we hired everybody, we bought all our parts, we are putting everything together and getting ready to launch in a few months,” he said.
PlanetiQ went through several design changes with its satellites. Originally designed to weigh over 120 kg, the company slimmed the satellites down to under 20 kg in 2015 when it selected BCT (Blue Canyon Technologies) as its manufacturing partner. McCormick said PlanetiQ realized its satellites had at one point gotten too small to ensure the quality of data the company wanted to achieve. “We were trying to get into slightly smaller spacecraft. That wasn’t working out as well as we would have liked — there was too much risk and not enough signal to noise,” he said.
The company settled on a design that is slightly more than 30 kg, according to McCormick. PlanetiQ is also using a proprietary bus, and will be a microsatellite, not a CubeSat as previously planned.
PlanetiQ’s satellites will measure signals from global navigation satellite systems like the U.S.’s GPS, Russia's GLONASS, Europe’s Galileo and China’s BeiDou, as they pass through the atmosphere to detect weather information — a process known as GNSS radio occultation.
PlanetiQ is on its fourth-generation sensor, which will be able to measure all the way down to the surface. That level of detail will distinguish PlanetiQ from its competitors, he said. “We are trying to get five-to-seven-day forecasts as good as one-day forecasts now,” he said.
PlanetiQ received a $3.5 million contract from the National Oceanic and Atmospheric Administration last September as part of a CWDP (Commercial Weather Data Pilot) program. Competitors Spire Global and GeoOptics, both of which have satellites in orbit, also received NOAA pilot contracts.
The U.S. Air Force is also spending money on commercial weather data, having allocated $7 million of a $20 million budget for such data as of January.
Figure 1: Geometry of a typical GNSS-RO event and resulting data products derived from the technique (image credit: PlanetiQ)
The GNOMES experimental microsatellites, with a mass of ~30 kg, are being manufactured by BCT (Blue Canyon Technologies) of Boulder, CO, using the XB bus. The spacecraft are three-axis stabilized and nadir following.
Communications: Each GNOMES microsatellite will carry a single X-band transmitter to downlink data and conduct telemetry, tracking, and command (space-to-Earth). This transmitter is the SDR-X model supplied by BCT (Blue Canyon Technologies), with transmission characteristics as provided in Table 1.
The X-band and S-band antennas are designed and supplied by Haigh-Farr Inc. Both are nearly hemispherical in their gain patterns and are nadir-pointed. For both antennas, the gain is generally constant and varies between 0 and 5 dBi over Earth coverage angles.
A link budget can be formed from the transmitter characteristics shown in Table 1and the expected X-band antenna coverage. The PDF (Power Flux Density) at the maximum gain (5 dB) is calculated to be -117 dB (W/m2) over the total bandwidth of the transmitter at the worst case injection altitude of 530 km and -119 dB(W/m2) at the nominal operational altitude of 650 km. Therefore, the largest PFD resulting from the X-band transmitter on the GNOMES-1will be -130 dB(W/m2·MHz) at the subsatellite point from 530 km, a value that is well below the recommendation given by the ITU (International Telecommunication Union).
The ITU also recommends the following limits of PFD from space stations as received at the Earth’s surface. These limits relate to the PFD obtained only under free-space path loss conditions and a 4 kHz bandwidth.
Table 2: ITU PFD limits at the Earth’s surface
Propulsion: The on-board ion propulsion system will allow for precise orbit insertion after launch vehicle separation, with sufficient fuel to lower the satellite perigee and accelerate de-orbit after end of mission. Further description of re-entry disposal, orbital debris mitigation plan.
Figure 2: Artist's rendition of the first PlanetiQ satellite GNOMES-1 (GNSS Navigation and Occultation Measurement Satellite-1) in orbit (image credit: PlanetiQ)
Launch: The GNOMES-1 microsatellite of PlanetiQ was launched as a passenger payload on 30 August 2020 (23:19 UTC) on a SpaceX Falcon-9 vehicle from SLC-40 (Space Launch Complex-40) of the Cape Canaveral Air Force Station, Florida. The SAOCOM-1B satellite of CONAE (Argentina) is the primary mission on this flight. 4)
Orbit: Sun-synchronous near-circular orbit (frozen dawn/dusk orbit), altitude = 619.6 km, inclination = 97.86º, period = 97.1 minutes, repeat cycle of 16 days (8 days for the constellation), LTAN (Local Time on Ascending Node) at 6:00 hours.
• Tyvak 0172 is a small spacecraft built by Tyvak Nanosatellite Systems. Details about its mission have not been disclosed by SpaceX or Tyvak.
• GNOMES-1: A radio occultation microsatellite for PlanetiQ (of Golden CO, USA) was launched with SAOCOM-1B. The GNOMES-1 (GNSS Navigation and Occultation Measurement Satellite-1) with a mass of ~ 30 kg is the first of a planned fleet of around 20 microsatellites, being developed by PlanetiQ, to collect radio occultation data by measuring the effects of the atmosphere on signals broadcast by GPS, GLONASS, Galileo and BeiDou navigation satellites. The information can yield data on atmospheric conditions useful in weather forecasts.
The fundamental goal of the Pyxis-RO instrument and GNOMES mission is to provide as many high-quality GNSS occultations as possible to maximize its impact on NWP (Numerical Weather Prediction) forecasts, and space weather and climate predictions. To accomplish this task, PlanetiQ must:
• Demonstrate a high signal-to-noise ratio (SNR) (~ 2000 V/V) GNSS-RO instrument to provide all-weather atmospheric data from the surface of the planet to the top of the ionosphere.
• Demonstrate the ability to detect and track GNSS signals during super refraction conditions, and to derive data from the lower troposphere in all cases and conditions.
• Deliver high impact/high value data to NWP centers and demonstrate forecast accuracy improvements.
The Pyxis instrument onboard each PlanetiQ satellite tracks dual-frequency signals from GPS, Galileo, GLONASS, and BeiDou GNSS satellites. Pyxis tracks both rising and setting occultations to double the numbers of soundings. With our design for a future operational system, the GNOMES aim to track with a 100% duty cycle, each and every orbit, as all operational weather satellites do.
Daily occultation counts are dependent upon the number of functional GNSS satellites on orbit as well as the signals that they broadcast. Although the GPS constellation is fully populated, PlanetiQ collects signals only from the Block IIR-M and more recent GPS blocks, which broadcast the L2C signal. We also collect data from all 24 GLONASS satellites, and all the available Galileo and BeiDou satellites (Table 3) for the signal sources from each constellation). All signals collected are freely available to civil users.
The Pyxis-RO instruments are designed to receive the publicly available GNSS signals from the GPS (L1 at 1575.42 MHz and L2 at 1227.6 MHz), Galileo (E1 at 1575.42and E5 at 1207.14MHz), GLONASS (FDMA signals centered at L1 at 1602 MHz and L2 at 1246 MHz), and BeiDou (B1 at 1561.1 MHz5and B2 at 1207.14 MHz) constellations, as shown in Table 3.Dual-frequency measurements from each occulting satellite are necessary to resolve the ionospheric contribution to the signal path delay. Additionally, the reception of multiple GNSS signals will allow PlanetiQ to cross-check and validate the accuracy of its data observations.
Observations of GNSS signal Doppler frequency and carrier phase amplitude will be collected and stored on-board before periodic transfer to the ground for further processing. Linking the observations to post-processed orbital geometry information will identify the bending angle unique to each occultation event, which is then translated to a vertical refractivity profile at a given location. The use of post-processed orbits also ensures that the weather products are not derived from spoofed or inaccurate signals, especially from the foreign GNSS satellites.
PlanetiQ satellites will rely on ground stations from KSAT and Atlas Space Operations to link to its satellites. The atmospheric soundings measured by the GNOMES will be downlinked via commercial ground station networks. PlanetiQ is in negotiations with Kongsberg Satellite Services (KSAT) for use of their ground station network as specified in Table 4. For S-band uplink and X-band downlink, KSAT supplies a network of 3.7 meter antenna dishes. PlanetiQ also plans to use ground stations of various sizes (3.4 m to 9.1 m) from ATLAS Space Operations, specified in Table 5, for X- and S-band communications.
The ground stations shown in Table 4 and Table 5 represent a superset of possible ground stations under consideration from KSAT and ATLAS. The ultimate set of ground stations will depend on the injection inclination of GNOMES-1: a sun-synchronous launch will dictate the use of the polar ground stations at Svalbard, Fairbanks, Troll, Hartebeesthoek, Punta Arenas, and eventually McMurdo, while a lower inclined launch will employ stations at Hartebeesthoek, Longovilo, Chitose, Harmon, and Tahiti. Combinations of the ground stations shown in Table 4 and Table 5 allow for at least 27 opportunities for data transfer per day for each of the GNOMES.
Table 4: KSAT ground station locations (uplink and downlink)
PlanetiQ and its subcontracted ground station suppliers will seek the appropriate licenses for all ground stations deemed necessary for the chosen orbit for GNOMES-1 under the agreements with the ground station providers. PlanetiQ may subscribe to other ground stations within the KSAT and ATLAS networks, as needed, to decrease latency and avoid transmission overlap with other missions and will amend this application and coordinate all such operations, as necessary.
Ground Station Access
To facilitate with spectrum use coordination, the technical capabilities of the GNOMES system is described here. The GNOMES carry the SDR-X X-band transmitter/S-band receiver, produced by BCT. The radios are software defined, and have an adjustable data rate up to a maximum 25 Mbit/s. PlanetiQ plans to use 10 Mbit/s for nominal operations on-board the GNOMES. During a station pass, the GNOMES must downlink the accumulated atmospheric measurements from the previous fraction of an orbit while in range of the ground station. With the broad distribution of ground stations around the Earth, the GNOMES will have frequent telecommunications opportunities. GNOMES-1 is designed to downlink roughly half an orbit’s worth of scientific data and satellite telemetry (165 MB) at each pass. The time to downlink 165 MB of data given a 10 Mbit/s data rate is approximately 140 seconds. Including an estimated 10 seconds for framing overhead, 150 seconds is needed at each pass to downlink the necessary data.
A simulation of GNOMES-1 in its possible anticipated orbits was conducted using the orbital software package Systems Toolkit (STK) from Analytical Graphics, Inc. The length of time of the line-of-sight radio access between GNOMES-1 and the set of ground stations most applicable for its particular orbit (equatorial for 37° inclined, polar for sun-synchronous orbits) was recorded for one year’s worth of orbits at the nominal operational altitude of 650 km. An elevation mask of 5 degrees or more was be imposed at each of the ground stations to limit data transfer to elevations above the surrounding foliage and structures, as well as to ensure our link budget supports our data rates.
The distributions of pass times for each grouping of stations are shown in Figure 3 for GNOMES-1. The majority of GNOMES-1 pass times for data transfer are over 400 seconds for the ground stations for either polar or equatorial orbits. Less than 5% of passes are less than 150 seconds for either sets of ground stations, at any orbit inclination.
With a data downlink rate of 10 Mbit/s, there is temporal flexibility within the satellite-station pass for data downlink for the majority of passes. The ground stations within the KSAT and ATLAS networks, shown in Table 4 and Table 5, also give geographic diversity for downlink opportunities. The GNOMES also have on-board data storage sufficient for multiple passes without downlink. For every pass, there is time for data transmission, plus additional time and storage to mitigate against concurrent transmission with other high priority missions.
PlanetiQ plans to actively engage in pre-coordination and coordination activities with other S-band/X-band spectrum users to avoid potential interference during transmission. The scheduling of ground station use for either uplink or downlink transmissions will be coordinated by KSAT and ATLAS for each pass.
Utility of Data
Improvement to Numerical Weather Prediction: Current weather forecasts from the NWP centers are based on data that is insufficient in both quality and quantity. Much of the current data is produced by orbiting infrared cameras and microwave instruments. These infrared cameras cannot penetrate through clouds (which regularly cover 70% of the Earth), and microwave instruments have poor vertical resolution, the inability to discern tropospheric boundary layer conditions, and are ineffective over land. PlanetiQ aims to fix this shortfall of global atmospheric data with the demonstration of our Pyxis-RO instrument on-board the two GNOMES platforms. The experiment will be capable of collecting global measurements of the atmosphere through all possible conditions from the Earth’s surface, through the boundary layer, the troposphere, and finally, up through the stratosphere.
The Pyxis-RO mission will demonstrate the highest capability GNSS-RO atmospheric sounder to date. Its observations of precise temperature, water vapor, atmospheric pressures, and wind data will be sent to and processed by the NWP centers to create more accurate and timely weather forecasts. High frequency sampling of the occultation signal allows for improved vertical resolution, as dense as 100 meters (see Figure 4 for ray path geometry). The higher quality measurements of the atmosphere’s current conditions collected by the Pyxis-RO sensor will enable even better forecasting ability by the NWP centers on a global scale.
Additionally, the Pyxis-RO instrument will assess the state of the ionosphere by obtaining the total electron count (TEC), electron density profiles (EDP), and scintillation characteristics (S4) through dual-frequency atmospheric sounding. These measurements are important to ionospheric models used to monitor space weather and ionospheric conditions affecting communication and navigation signals.
Receipt of Foreign GNSS Signals: The Pyxis-RO receivers are designed to detect dual-frequency signals from the four major GNSS constellations (GPS, GLONASS, Galileo, and BeiDou). However, any concern over the use of foreign GNSS signals is unwarranted, as any possible deliberate falsification or “spoofing” of foreign GNSS signals will be detected by the GNOMES and known well before PlanetiQ releases any weather data products, described in the following section.
The GNSS satellites are ideal transmitters, as their on-board atomic frequency standards provide a precise and accurate reference for the carrier waves collected by the Pyxis-RO instrument. The high accuracy of these signals makes it possible to derive key characteristics of the atmosphere. The addition of signals from foreign GNSS constellations provides a greater number of sources from which to obtain viable occultation measurements.
The characteristics of the lowest layers of the atmosphere are found by calculation of the navigation signal’s unique bending angle, which is obtained via open-loop tracking. During open loop tracking, the occultations are found by differencing the measured amplitude and Doppler of the carrier phase from accurate models based on geometry and observed atmospheric conditions. This requires resolving the relative velocity between the GNOMES and GNSS satellites (where a satellite in LEO has a typical velocity of 7 km/sec and a GNSS satellite at 26,000 km altitude is approximately 3 km/sec) to 0.15 to 0.2 mm/sec. 5) This level of orbital accuracy requires post-processed orbits and clock performance for the GNSS satellites, as well as fine-tuned orbital and clock information for the GNOMES in LEO. On-board scheduling and orbit determination for the GNOMES are performed by a navigation engine using only GPS observations. The GNSS orbit and clock data used to derive the atmospheric characteristics will be obtained from reputable post-processing facilities, such as NASA Jet Propulsion Laboratory (JPL) or University Corporation for Atmospheric Research (UCAR). Any false observations will be detected well before atmospheric products are made.
Typical GNSS spoofing is performed by recreating false GNSS signals from a local transmitter. However, a spaceborne receiver makes this scenario highly unlikely. Falsifying the satellite ephemeris/almanac message could also possibly cause receiver issues; however, because the Pyxis-RO instrument is directed to collect Doppler and phase measurements from values obtained directly from the particular satellite’s ephemeris, a false ephemeris will lead to no measurements.
PlanetiQ will not be the only RO data provider to use foreign GNSS signals, as the NOAA led RO mission, COSMIC-2/FORMOSAT-7, is equipped with the Tri-GNSS (TriG) receiver built by NASA JPL and intends to collect signals originating from GPS, GLONASS and Galileo for its occultations. 6) NOAA has recently awarded contracts for commercially obtained space-based radio occultation data with no limitations on the signal source. In fact, they specifically state the need for GNSS-RO measurements. The RFP language for the contract from NOAA contemplates that data would be provided from foreign GNSS satellites. The Pyxis-RO instrument, and subsequent data processing, will be able to validate the foreign GNSS signals as viable sources for atmospheric sensing.
Both government-lead and commercial RO missions contribute to numerical weather prediction and forecasts, and are desired in near real-time. However, post-processing of the GNSS orbits and clocks will always be necessary to derive the atmospheric characteristics, and any spoofing will be detected in this post-processing. Therefore, the weather products obtained via RO are resistant to false signals.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).