GPS RADIO OCCULTATION (RO) LIMB-SOUNDING TECHNIQUES HAVE EVOLVED IN PARALLEL WITH THE ADVANCEMENT OF GLOBAL NAVIGATION SATELLITE SYSTEMS (GNSS) FROM A PROOF-OF-CONCEPT TO OPERATIONAL SYSTEMS THAT PROVIDE DATA FOR GLOBAL WEATHER FORECASTING, CLIMATE MONITORING, AND IONOSPHERE STUDIES. THE COSMIC-2 CONSTELLATION WILL UTILIZE THE LATEST ADVANCEMENTS IN MULTI-CONSTELLATION GNSS (MULTI-GNSS) THROUGH ITS TRI-GNSS RECEIVERS TO TRACK OPEN SIGNALS FROM GPS, GLONASS, AND GALILEO SATELLITES. THIS NEW GENERATION OF RO SYSTEMS IS EXPECTED TO INCREASE THE NUMBER OF ATMOSPHERIC AND IONOSPHERIC PROFILES BY AN ORDER OF MAGNITUDE AND TO DRAMATICALLY IMPROVE THEIR MEASUREMENT ACCURACY AND RESOLUTION. THERE ARE, HOWEVER, MANY CHALLENGES REMAINING IN EFFECTIVELY UTILIZING THE ENORMOUS MULTI-GNSS CAPABILITIES TO DETECT, LOCALIZE, AND CHARACTERIZE IRREGULARITIES IN THE IONOSPHERIC PLASMA. THE OBJECTIVES OF THIS PROPOSED RESEARCH ARE: (1) TO DEVELOP ROBUST AND ACCURATE MULTI-GNSS RECEIVER ALGORITHMS TO TRACK STRONG IONOSPHERIC AND LOWER TROPOSPHERIC RO SCINTILLATION SIGNALS; (2) TO DEVELOP DATA-DRIVEN, PHYSICS-BASED METHODOLOGIES TO ACCURATELY LOCALIZE IONOSPHERIC IRREGULARITIES AND SIMULATE RO SCINTILLATION SIGNALS; AND (3) TO PERFORM MIXED-SCALE MULTI-GNSS INTERFEROMETRY TO CHARACTERIZE POLAR IONOSPHERIC IRREGULARITIES WITH UNPRECEDENTED HIGH SPATIAL AND TIME RESOLUTIONS. OBJECTIVE 1 WILL BE ACHIEVED BY EXPLOITING SPACE, TIME, FREQUENCY, AND CONSTELLATION DIVERSITIES OF MULTI-GNSS SIGNALS TO DESIGN ADAPTIVE OPEN LOOP ALGORITHMS TO TRACK RO SIGNALS TRAVERSING IONOSPHERIC IRREGULARITIES AND THE LOWER TROPOSPHERE. WIDEBAND MULTI-GNSS INTERMEDIATE FREQUENCY (IF) DATA COLLECTED FROM OUR HIGH LATITUDE AND EQUATORIAL STATIONS DURING STRONG IONOSPHERIC SCINTILLATION ALONG WITH SIMULATED RO SCINTILLATION SIGNALS, AND WIDEBAND IF RO DATA COLLECTED FROM TROPICAL MOUNTAINTOPS WILL BE USED TO EVALUATE THE PERFORMANCE OF THE ALGORITHMS. OBJECTIVE 2 WILL BE ACCOMPLISHED THROUGH JOINT PROCESSING OF GROUND-BASED AND SPACE-BASED DATA. PHYSICS-BASED BACK-PROPAGATION OF COMPLEX EM FIELDS GENERATED USING COMMON VOLUME MEASUREMENTS OF GROUND-BASED RECEIVER ARRAYS AND SPACE-BASED RO RECEIVERS WILL BE IMPLEMENTED TO LOCALIZE AND ESTIMATE THE IONOSPHERIC IRREGULARITIES. THE RESULTS CAN BE USED TO FURTHER CONSTRAIN THE RO INVERSION ALGORITHM TO OBTAIN MORE ACCURATE IONOSPHERIC PROFILES WHEN IRREGULAR PLASMA STRUCTURES EXIST NEAR THE OCCULTATION AREA. OBJECTIVE 3 WILL BE FULFILLED BY UTILIZING LARGE NETWORKS OF GNSS RECEIVERS AT HIGH LATITUDES AUGMENTED BY LOW EARTH ORBIT (LEO) RO PROFILES FROM COSMIC, CASSIOPE, AND PROSPECTIVE COSMIC-2 TO PERFORM MULTIPLE BASELINE INTERFEROMETRY. THE PROPOSED RESEARCH WILL ADVANCE GNSS RECEIVER AND RO INVERSION TECHNIQUES FOR NEXT-GENERATION LEO RO SYSTEMS AND ENABLE ACCURATE REPRESENTATIONS OF IONOSPHERIC ELECTRON DENSITY PROFILES ESPECIALLY IN AREAS WHERE THERE ARE PLASMA IRREGULARITIES. ACCURATE REPRESENTATIONS OF IONOSPHERIC PROFILES ARE IMPORTANT NOT ONLY FOR IONOSPHERIC STUDIES; THEY ALSO IMPACT THE QUALITY OF CORRESPONDING LOWER ATMOSPHERIC PROFILES, AND AFFECT CHARACTERIZATION OF PERTURBATIONS THAT ARE DRIVEN BY OTHER NATURAL AND MAN-MADE PROCESSES OCCURRING AT THE EARTHS SURFACE INCLUDING, FOR EXAMPLE, EARTHQUAKES, VOLCANIC ERUPTIONS, AND TSUNAMIS. THE PROPOSED ACTIVITIES WILL ALSO DEMONSTRATE THE USEFULNESS OF COMBINING SPACE-BASED AND GROUND-BASED MULTI-GNSS MEASUREMENTS FOR DISTRIBUTED SENSING OF IONOSPHERIC RESPONSES TO SOLAR AND GEOMAGNETIC ACTIVITIES. THE PROPOSED STRATEGIES TO ACHIEVE OUR RESEARCH OBJECTIVES ARE BASED ON EXPLOITATION OF DIVERSITIES OFFERED BY NEW GNSS SIGNALS BEYOND THE CURRENT LEGACY OPERATIONS. THE OUTCOMES WILL SUBSTANTIATE NASAS VISION THAT NEW GNSS SIGNAL STRUCTURES WILL PROVIDE UNPRECEDENTED OPPORTUNITIES FOR REMOTE SENSING OF THE EARTH SYSTEM WITH NEW GROUND-BASED SYSTEMS AND RELATIVELY SIMPLE AND ROBUST SPACE BORNE GNSS RECEIVERS.