System for Observing Dynamic Atmospheric Phenomena
- Tuesday, 01 April 2014
System has drastically increased transmitter capability at much reduced cost.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Existing observing systems are inadequate to measure a variety of dynamic atmospheric processes. Ground-based or airborne systems do not observe over sufficiently large regions to capture the context or time history of many phenomena. Space-based systems do not observe a specific region over sufficient duration with sufficient spatial resolution to capture the essential features of the phenomena. Radar systems are limited in what they can observe because of the need for scattering sources to be present in the observed volume.
A system that coordinates ground- and space-based measurements solves these problems. Arrays of ground-based instruments and low-Earth-orbiting satellite constellations with specific orbital geometries will sample sufficiently large regions with sufficient resolution to observe continuously dynamical processes in the atmosphere that occur over hours.
The system consists of ground-based transmitters with frequency agility to create a signal that will propagate through the atmosphere with variable refractive index or variable absorption (depends on signal frequency), and polar orbiting satellites in a constellation geometry carrying software-defined radios or radiometers to receive the transmitted signal from the ground-based array.
The ground-based transmitters carry omnidirectional antennas oriented vertically to transmit signals to the satellites flying overhead. The satellites carry software-defined radio receivers that can be reconfigured in flight or programmed in advance to coordinate with the ground-based transmitters. The transmitter array will broadcast range-encoded signals that are demodulated by the satellite receivers. The satellite receivers will also continuously track carrier phase of the transmitted signal.
The satellite constellation is configured as a “staggered string of pearls.” The design is meant to sequentially fly over the ground-based array for periods of a few hours, with relatively small gaps between contacts. This permits “staring” at the atmospheric region containing the ground-based array for periods of a few hours.
This system defines a new mission architecture trade space that makes it possible to study dynamic atmospheric phenomena as they evolve over hours. The key insights are coordinated ground and space observations, the necessary constellation orbital geometry, and a tomographic retrieval approach applied to the atmosphere.
Another advantage of the system is drastically increased transmitter capability at much reduced cost. The transmitters are on the ground, where power is cheap and reliability requirements are reduced compared to spaceborne transmitters. The space segment consists of receivers only, which are much less expensive to engineer for space and require less power than transmitters.
The picture can guide the reader to understand the innovation better. The differences between what is shown and the innovation are as follows:
- The innovation is focusing on the atmospheric, not ionospheric, atmosphere component. The atmosphere rests directly above the ground-based assets (shown as receivers in the figure), starting at zero altitude. The atmospheric geometry provides greater geometric diversity than occurs in the ionosphere, producing improved vertical resolution.
- The fundamental data types here will be delay of radio signals and absorption of radio, infrared or visible signals, or possibly a combination of these.
- The ground-based assets here will likely be transmitters, but could be receivers, transmitters, or both.
- The satellites may be spaced in time or be grouped so that they do not all lie in a plane. That configuration will permit observations of up to hours duration over regions where the ground-based assets are deployed.