Operational Status:
- requestable, ready for deployment
General Description:
- The Purdue GPS multistatic and occultation instrument for atmospheric, land, and ocean remote sensing (GIZMOS) is an instrument to measure the water vapor distribution in the troposphere by inversion of the bending angle profile produced during occultation of a GPS satellite by the limb of the Earth. The instrument also uses GPS signals reflected from land or ocean to determine properties of those surfaces (soil moisture and surface roughness, respectively). Occultation samples represent distances of ca. 300 km and so are complementary to the single-point measurements obtained by conventional soundings. The instrument is a passive remote-sensing system that records the full bandwidth (intermediate frequency) of the GPS signals and stores the measurements on hard disks for subsequent analyses.
Measurement Characteristics: Soundings are of refractive index, so must be interpreted in terms of the combined effects of temperature and humidity on refractive index. Properties average over distances that represent of order 100 km horizontally, so are different in character from radiosonde soundings. Flights must be planned with known locations of GPS satellites to benefit from occultation during the flight. For a summary of the theory underlying sounding by occultation, see the article by Kursinski et al., 1997, referenced in the "Documentation" section at the end of this page.
- The following diagram illustrates how the "bending angle" changes as the aircraft and satellite move and how a profile of bending angles are measured. The refractivity obtained by inversion of the bending angle measurements then provides information on the water vapor profile (if temperature is known) or of temperature (where water vapor is negligible).
- Overall estimate of uncertainty:
- Response time:
- Precision:
- Other measurement characteristics (comments on signal/noise, bias limits, etc):
- The following diagram illustrates how the "bending angle" changes as the aircraft and satellite move and how a profile of bending angles are measured. The refractivity obtained by inversion of the bending angle measurements then provides information on the water vapor profile (if temperature is known) or of temperature (where water vapor is negligible).
