Impact of the ionosphere on an L-band space based radar

We have quantified the impact that the ionosphere would have on a L-band interferometric synthetic aperture radar (SAR) mission using a combination of simulation, modeling, Global Positioning System (GPS) data collected during the last solar maximum, and existing spaceborne SAR data. Using the Jet Propulsion Laboratory's Global Ionospheric Maps (GIM) total electron content (TEC) estimates derived from the worldwide array of GPS stations, we determined that the sun synchronous orbit which would minimize TEC at the time of imaging has dawn and dusk equator crossings. Such an orbit also avoids the equatorial post-sunset irregularities. We used the GIM data to examine the day-to-day variability in the background ionosphere and to quantify the impact of the background ionosphere on single pass SAR performance. With the exception of Faraday rotation related effects on single polarization systems, degradation due to the background ionosphere can be avoided if a reasonable model for the ionosphere is used during processing. Our studies reveal that Faraday rotation angles rarely exceeded the 10° threshold that impacts biomass retrieval and that repeat pass interferometric SAR decorrelation due to variations in the background ionosphere causing variable Faraday rotations is a negligible effect. Even a dawn-dusk orbit will not avoid high latitude ionospheric irregularities. We evaluated the strength of the ionospheric irregularities using GPS scintillation data collected at Fairbanks, Alaska and modeled the impact of these irregularities on azimuth resolution, azimuth displacement, peak sidelobe ratio (PSLR), and integrated sidelobe ratio (ISLR). Our examination of ionospheric artifacts in InSAR data has revealed that the artifacts occur primarily in the polar cap data, not auroral zone data as was previously thought.