Radio Occultation From GPS/MET to COSMIC

Background: Global Positioning System (GPS) Satellites Low-Earth Orbit (LEO) Satellites A GPS receiver in LEO can track GPS radio signals that are refracted in the atmosphere GPS Satellite LEO Orbit Atmosphere Radio Signal LEO Satellite

Occultation Geometry Occultation geometry  During an GPS occultation a LEO ‘sees’ the GPS rise or set behind Earth limb while the signal slices through the atmosphere Occultation geometry The GPS receiver on the LEO observes the change in the delay of the signal path between the GPS SV and LEO This change in the delay includes the effect of the atmosphere which delays and bends the signal

Signals Abundant Glonass Galileo --------------- 60–90 sources GPS Glonass Galileo --------------- 60–90 sources in space T. Yunck, JPL

Unique Attractions of GPS Radio Occultation 1. High accuracy: Averaged profiles to < 0.1 K 2. Assured long-term stability 3. All-weather operation 4. Global 3D coverage: stratopause to surface 5. Vertical resolution: ~100 m in lower trop 6. Independent height & pressure/temp data 7. Compact, low-power, low-cost sensor T. Yunck, JPL

Radio Occultation Mission Overview JPL + Stanford Use Radio Occultation (RO) to Explore planetary atmospheres COSMIC Operational Demonstration 2006-2011 METOP (COSMIC II) Operations RO Missions UCAR manages GPS-MET RO Mission - Proof of Concept 1995 - 1997 CHAMP and SAC-C Missions Improved Proof of Concept 2000 - 2005

COSMIC at a Glance Constellation Observing System for Meteorology Ionosphere and Climate (ROCSAT-3) 6 Satellites launched in late 2005 Orbits: alt=800km, Inc=72deg, ecc=0 Weather + Space Weather data Global observations of: Pressure, Temperature, Humidity Refractivity TEC, Ionospheric Electron Density Ionospheric Scintillation Demonstrate quasi-operational GPS limb sounding with global coverage in near-real time Climate Monitoring Geodetic Research

COSMIC Status

Payloads GPS Occultation receiver Tiny Ionosphere Photometer (TIP) High-resolution (1 Hz) absolute total electron content (TEC) to all GPS satellites in view at all times (useful for global ionospheric tomography and assimilation into space weather models) Occultation TEC and derived electron density profiles (1 Hz below the satellite altitude and 50 Hz below ~140 km), in-situ electron density Scintillation parameters for the GPS transmitter–LEO receiver links Data products available within 15 - 120 minutes of on-orbit collection Tiny Ionosphere Photometer (TIP) Nadir intensity on the night-side (along the sub-satellite track) from radiative recombination emission at 1356 Å Derived F layer peak density Location and intensity of ionospheric anomalies (Auroral Oval) Tri-band Beacon (TBB) Phase and amplitude of radio signals at 150, 400, and 1067 MHz transmitted from the COSMIC satellites and received by chains of ground receivers. TEC between transmitter and receivers Scintillation parameters for LEO transmitter - receiver links

6 Satellite COSMIC Microsat Constellation COSMIC System 6 Satellite COSMIC Microsat Constellation S band S band GPS s/c L band S band Taiwan OPS R.O. RT Data E/S (Fairbanks) R.O. RT Data E/S (Kiruna) RT Fiducal Network TT&C TT&C NSPO MOC, MCC, SCC, FDF T1 T1 Payload Commands and All Real-Time Data Products LAN Real Time CDAAC (Boulder) S/C Telemetry C W B T1 TACC vBNS STARTAP Tanet, I2 Internet U.S. Universities & Mission Teams NESDIS Other Users Other Customers University Science Centers Operational Centers

NESDIS CDAAC Getting COSMIC Results to Weather Centers NCEP Input Data NESDIS CDAAC ECMWF CWB GTS BUFR Files WMO standard 1 file / sounding UKMO JMA Canada Met. This system is currently under development by UCAR, NESDIS, + UKMO Data available to weather centers within < 180 minutes of on-orbit collection

Summary Radio occultation is a new and promising remote sensing technique Technique was demonstrated and now COSMIC aims to: Improve data quality in lower troposphere - new technology Increase number of soundings Show impact in operational models - work at NCEP, UKMO, ECMF COSMIC launch is on schedule for December of 2005 COSMIC “operational demonstration” - should be followed by continuous operational missions

RO provides best results between 8-30 km (effects of moisture and ionosphere are negligible). Is capable of resolving the structure of the tropopause and gravity waves above the tropopause. “dry temperature” computed from refractivity assuming no water vapor Figure from the paper by Nishida et al., J. Met. Soc. Japan, 78(6), p.693, 2000.

Result Improvements at UCAR Most recent RO processing Statistics A comprehensive reprocessing of all CHAMP, SAC-C and GPS/MET data has been completed using CDAAC 1.01 software New

RO - ECMWF Comparison Data Poor regions Data Rich regions Comparison of RO profiles with ECMWF profiles in data rich (most land masses) and data poor regions (mostly oceanic regions). The better agreement from 5-30 km indicates that superior ECMWF performance in data rich regions

Observed Atmospheric Volume L~300 km Z~1 km Top: Schematic depiction of tubular volume over which atmosphere contributes information to a single occultation phase (ray) measurement. The intensity of shading in the tube represents the relative weighting of atmospheric properties that contribute to the value retrieved at the center of the tube. For typical atmospheric structures, L and Z are approximately 300 and 1 km respectively. Bottom: Typical along-track weighting function for a single radio occultation measurement (from Melbourne et al, 1994). Most of the information is contributed by a mesoscale atmospheric volume centered at the ray tangent point.

Processing Overview Processing of profiles takes < 15 minutes after data reception

Radio occultation processing overview R. Kursinski

Deriving Bending Angles from Doppler The projection of satellite orbital motion along signal ray-path produces a Doppler shift at both the transmitter and the receiver After correction for relativistic effects, the Doppler shift, fd, of the transmitter frequency, fT, is given as where: c is the speed of light and the other variables are defined in the figure with VTr and VTq representing the radial and azimuthal components of the transmitting spacecraft velocity. a vT

Ionospheric calibration Is performed by linear combination of L1 and L2 bending angles at the same impact parameter (by accounting for the separation of ray tangent points). bending angle impact parameter Effect of the small-scale ionospheric irregularities with scales comparable to ray separation is not eliminated by the linear combination, thus resulting in the residual noise on the ionospheric-free bending angle.

Optimization of the observation bending angle The magnitude of the residual noise can be very different for different occultations, but it almost does not depend on height for a given occultation. Above a certain height, climatology provides better estimate of the atmospheric state than RO observation. The observed bending angle is optimally weighted with climatology. This does not improve the value of the bending angle at large heights, but results in reduction of error propagation downward after the Abel inversion. where The weighting function is calculated individually for each occultation.

Define the refractional radius x=nr, where n=1+N*10-6 Now we have a profile of refractivity as a function of height

Atmospheric refractivity N=(n-1)*10-6 Ionospheric term dominates above 70 km Hydrostatic (dry) wet terms dominates at lower altitudes Wet term becomes important in troposphere (> 240 k) and Can be 30% of refractivity in tropics Liquid water and other aerosols are generally ignored

Deriving Pressure Temperature Humidity After converting GPS Doppler phase => a(a) => n(r) and removing effects of ionosphere, from we have a profile of dry refractivity for altitudes from ~150 km down to the 240K level in the troposphere. We use the hydrostatic equation, dP = -g r dz = -g nd md dz to derive a vertical profile of pressure versus altitude over this altitude interval. If we start high enough P(ztop) =0 with negligible error Given P(z) and nd(z), we can solve for T(z) over this altitude interval using the equation of state (ideal gas law): T(z) = P(z) / (nd(z) R) Below the 240k level we need additional information (usually temperature from a weather model) to obtain water vapor pressure and humidity.