Unfortunately, GNSS occultations are limited by their L-band wavelengths which were chosen to minimize interaction with the atmosphere and are therefore suboptimal for atmospheric sounding. Here we report on progress toward a satellite to satellite occultation system operating at cm & mm wavelengths near the absorption lines of water vapor (22 and 183 GHz) and ozone (165 and 184 GHz). This system, called the Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS), combines key features of GNSS RO and MLS, together with several additional features. This system will profile temperature and water vapor simultaneously, unlike GPS RO, and has much wider dynamic range that enables profiling water vapor from the surface to the mesopause. Over most of this altitude range, the random uncertainty of the water vapor and temperature profiles are <3% and 0.4 C respectively and the absolute uncertainties are still smaller. Vertical resolution is 100 m or better. The system both sees clouds and sees through them. Performance in cloudy conditions will generally be within a factor of 2 of clear sky performance
With funding from NSF, we developed a prototype ATOMMS instrument and demonstrated some of the ATOMMS features including the ability to measure water vapor to 1% in optical depths up to 17 (7 orders of magnitude reduction in signal power).
With the orbiting ATOMMS system, the number of occultations scales as the number of satellites squared such that 12 and 32 satellite constellations will produce approximately 2,000 and 15,000 ATOMMS occultations per day respectively. Each ATOMMS satellite will also carry GNSS RO receivers, and will produce approximately 34,000 and 90,000 daily GNSS occultations respectively as well.
We envision initiating this orbiting ATOMMS capability with a small constellation of 4 to 6 satellites that would focus on profiling the remote polar regions via 230 to 520 daily ATOMMS occultations over the poles. Subsequently the number of satellites in the constellation would be increased to provide increasingly dense coverage across the globe, that increases as the square of the number of ATOMMS satellites.
The ATOMMS satellites are ESPA class satellites. Our estimates indicate that the cost will be approximately $5M per satellite and instrument, including launch. A 32 ATOMMS satellite constellation would cost approximately $160Mn.