Tiny Open-Loop Atmospheric Sensing Technique

We will design and fabricate a circuit card capable of retrieving real-time ionospheric and atmospheric refractivity from a low Earth orbit platform. The design must use less than 3 Watts orbit average power and process GPS signals as they rise through the lower troposphere. These data must be accompanied by post-processed orbit data with errors less than 2mm/sec velocity and 20 cm positioning. This design will form the basis of a cube-sat sized 3 Watt radio occultation sensor for LMR-sat.

The TOAST instrument is an open-loop processor of GPS navigation signals. The electronics fits on a single 10 cm square card with RF components and digital components on opposite sides. A dime-sized programmable chip (FPGA) acts as a signal processor for the GPS signals of up to 10 satellites. This FPGA is under the control of a very low-power Linux CPU which handles all of the tracking models for very weak GPS signals transecting the atmosphere. Unlike typical GPS receivers, TOAST tracks without tight phase-locked loop tracking of the received carrier phase. For any given GPS satellite to be observed, TOAST generates a precise 3rd order range and phase model and only updates the FPGA every 1 - 10 seconds. This allows the processor to be loosely coupled with the signal processing to the point where, given sufficient ground to space bandwidth, TOAST can be controlled by a ground-based CPU. However, in this implementation, a Linux CPU will accompany the RF and FPGA logic to provide real-time data.