Wednesday, 23 August 2023
Signals of Opportunity (SoOp) reflectometry is a relatively new approach to microwave remote sensing in which existing, powerful, and non-cooperative satellite transmissions are re-utilized as sources of illumination in bistatic radar. SoOp was first demonstrated using signals from Global Navigation Satellite Systems (GNSS). GNSS reflectometry (GNSS-R) has been studied for 25 years, culminating in the launch of the 8-satellite CYGNSS constellation in 2016. The proposed scientific objective of CYGNSS was high temporal resolution ocean winds observations in the tropics, to improve storm forecasting. Land applications, such as soil moisture, flood inundation and biomass, were subsequently developed. Navigation signals, in general, are advantageous for SoOp due to their use of a pseudo-random noise (PRN) code to enable range-Doppler filtering. GNSS, however, has several limitations due to the low transmitted power, smaller bandwidth, and L-band frequency allocation. Recently, SoOp remote sensing has been demonstrated using other satellite communications signals. P-band (< 500 MHz) is particularly useful due to its penetration depth in soil and vegetation. Furthermore, frequencies in that band are heavily utilized for communications and there are no protections for scientific usage, so SoOp may be the best practical approach to P-band remote sensing. Signals of Opportunity P-band Investigation (SNOOPI), now planned for a February 2024 launch, will demonstrate observation of coherent land reflections from geostationary transmission in two bands, 240-270 and 360-380 MHz. In general, SoOp could enable remote sensing across the entire microwave spectrum, utilizing frequencies allocated to communications and navigation. Performance of SoOp techniques in retrieving specific meteorological variables depends upon the frequency, bandwidth, transmitted power, orbital coverage, and antenna pattern.
In the near future, rapid growth in the use of Low Earth Orbit (LEO) satellite constellations will herald a new era in space technology. Companies such as Starlink, OneWeb and Kuiper, to name a few, are competing to assemble constellations of hundreds to thousands of satellites as a means to providing broadband internet on a global scale. Aptly named “mega-constellations, “ these would illuminate the globe and provide near continuous coverage of locations of interest. There is also resurgent interest in using LEO constellations for Positioning, Navigation and Timing (PNT) purposes, either a dedicated constellations (e.g. Xona Space) or as a complementary service to communications (e.g. Satelles in partnership with Iridium Communications Inc.). Studies have also been conducted on the re-use of communication signals transmitted from LEO for navigation purposes, in a similar manner as SoOp remote sensing.
An extensive survey carried out by Prol etal., 2022 proposed that future LEO PNT constellations have the following characteristics. Carrier frequencies between 5-12 GHz, i.e. between C- band and Ku-band have been suggested best for PNT in LEO to minimize interference with existing systems and path losses. Bandwidths between 10-100 MHz have been recommended for code or code/Doppler based positioning. No specific recommendations on signal modulation were made. Furthermore, signal strength from LEO is expected to be 1000x stronger than GPS signals. These attributes of LEO constellations make them attractive candidates as sources for Signals of Opportunity (SoOp).
This paper will present a survey of present plans for LEO communications and PNT constellations mapping the top-level features of system in the conceptual design and development stages against the predicted performance in SoOp remote sensing of the important meteorological variables of ocean winds, sea surface height and soil moisture. Ocean wind retrievals depends upon the sensitivity of the scattered signal spectrum (usual represented as a delay-Doppler map) to the wave spectrum, sea surface height depends upon a path delay observable (altimetry) and soil moisture depends upon polarimetric reflectivity. Each of these variables has known requirements, in terms of spatial and temporal resolution, and coverage, defined by studies such as the 2017 NASA ESAS Decadal Survey.
Evaluation of LEO satellite signals as potential sources for SoOp requires extracting the following data from public sources; carrier frequency, signal modulation, number of channels, bandwidth and data rate, transmitting power (EIRP), number of transmitters and coverage, orbital information of the constellation, and antenna patterns. Our approach is to gather available data on LEO constellations and then apply first order models to predict the performance, as defined above, on the three representative environmental variables, thus “mapping” parameters defining communication and navigation constellations onto performance metrics related to remote sensing.
For example, signal bandwidth defines the spatial resolution (e.g. size of the iso-range curves) in the case of diffuse scattering. Bandwidth and Signal to Noise Ratio (SNR) define the altimetric precision. The number of satellites and the orbit define the spatial-temporal revisit time and coverage. This study will be conducted for the two classes of LEO constellations: those deployed for communications with opportunistic navigation capabilities, and those dedicated to navigation.
In the near future, rapid growth in the use of Low Earth Orbit (LEO) satellite constellations will herald a new era in space technology. Companies such as Starlink, OneWeb and Kuiper, to name a few, are competing to assemble constellations of hundreds to thousands of satellites as a means to providing broadband internet on a global scale. Aptly named “mega-constellations, “ these would illuminate the globe and provide near continuous coverage of locations of interest. There is also resurgent interest in using LEO constellations for Positioning, Navigation and Timing (PNT) purposes, either a dedicated constellations (e.g. Xona Space) or as a complementary service to communications (e.g. Satelles in partnership with Iridium Communications Inc.). Studies have also been conducted on the re-use of communication signals transmitted from LEO for navigation purposes, in a similar manner as SoOp remote sensing.
An extensive survey carried out by Prol etal., 2022 proposed that future LEO PNT constellations have the following characteristics. Carrier frequencies between 5-12 GHz, i.e. between C- band and Ku-band have been suggested best for PNT in LEO to minimize interference with existing systems and path losses. Bandwidths between 10-100 MHz have been recommended for code or code/Doppler based positioning. No specific recommendations on signal modulation were made. Furthermore, signal strength from LEO is expected to be 1000x stronger than GPS signals. These attributes of LEO constellations make them attractive candidates as sources for Signals of Opportunity (SoOp).
This paper will present a survey of present plans for LEO communications and PNT constellations mapping the top-level features of system in the conceptual design and development stages against the predicted performance in SoOp remote sensing of the important meteorological variables of ocean winds, sea surface height and soil moisture. Ocean wind retrievals depends upon the sensitivity of the scattered signal spectrum (usual represented as a delay-Doppler map) to the wave spectrum, sea surface height depends upon a path delay observable (altimetry) and soil moisture depends upon polarimetric reflectivity. Each of these variables has known requirements, in terms of spatial and temporal resolution, and coverage, defined by studies such as the 2017 NASA ESAS Decadal Survey.
Evaluation of LEO satellite signals as potential sources for SoOp requires extracting the following data from public sources; carrier frequency, signal modulation, number of channels, bandwidth and data rate, transmitting power (EIRP), number of transmitters and coverage, orbital information of the constellation, and antenna patterns. Our approach is to gather available data on LEO constellations and then apply first order models to predict the performance, as defined above, on the three representative environmental variables, thus “mapping” parameters defining communication and navigation constellations onto performance metrics related to remote sensing.
For example, signal bandwidth defines the spatial resolution (e.g. size of the iso-range curves) in the case of diffuse scattering. Bandwidth and Signal to Noise Ratio (SNR) define the altimetric precision. The number of satellites and the orbit define the spatial-temporal revisit time and coverage. This study will be conducted for the two classes of LEO constellations: those deployed for communications with opportunistic navigation capabilities, and those dedicated to navigation.
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