• Geometry and Observation Volumes
  • ​Ground
  • Low Earth-Orbiting
• Method
• Common-Volume Results
• Conclusions and Future work

Ground-based

• CSU GPS Lab high-rate GNSS data collection sites
  • e.g. Poker Flat, Alaska; Jicimarca, Peru
• receiver networks
  • CORS, MONITOR, etc.

Low earth-orbiting (LEO) satellite-based

• Radio Occultation (RO) missions:
  • CHAMP, COSMIC

GNSS Data

We will use Alaska and Peru to exemplify high and low-latitude geometries

Assumptions

• SGP4 for COSMIC
  • accuracy generally better than 1km
• SP3 for GNSS
  • ​polynomial interpolation of precise orbit

• model signal propagation paths as straight line segments

• no restriction on visibility aboard COSMIC platform 

Ground-based

Higher orbital inclination of GLONASS satellites results in better coverage at the poles.

Ground observations

GLONASS

GPS

Alaska

Peru

ionosphere piercing point for various IPP heights

RO Tangent Point

orbital altitude: between 700-800 km

orbital inclination: 72 degrees

occultation tangent point (TP)

We use "occultation tangent point" as proxy for RO observation volume.

COSMIC Tangent Point Distribuion

2014 March-May 90-day scatter of COSMIC-GPS occultation tangent points.  Fringes of occultations appear at 72, 45, and 21 degrees due to orbital inclination and satellite geometry.

Image from UCAR showing 24-hour COSMIC occultation tangent point occurence.

http://www.cosmic.ucar.edu/

COSMIC (LEO-based)

horizontal TP speed proportional to

vertical TP speed proportional to

Vertical TP Distribution

90-day scatter and histogram of COSMIC-GPS independent (>100km spatial separation) occultation tangent points.  Fringes result from satellite geometry.  Large percentage of high-altitude occurrences due to TP speed induced by satellite motion.

TP Azimuth by Latitdue

Geometry

Common-Volume

Method

1. compute "3D intersections" for all ray-path pairs
2. screen results to find valid common-volume observations

example with GPS only

3D Line-Segment Intersection

• point-of-interest (POI)
  • midpoint b/w points of closest approach
• proximity
  • distance b/w points of closest approach

"the points of closest approach between two line segments"

*must handle special case where point of closest approch is on segment endpoint.

Mask/Filters

• GPS 1 (for RX) elevation > threshold (5 degrees)
• closest approach of LEO ray-path to Earth surface > 2 km altitude

• proximity < threshold (100 km)
• POI altitude < threshold (1500 km)

Results

GPS

GLO

Temporal occurrences of common-volume observations (gray/black) along with COSMIC satellite elevation (color).

Results

GPS

Sharp spikes in common-volume occurrences correspond to COSMIC ray-path passing near ground receiver location.*

*This was verified for several cases using custom 3D common-volume visualization

Results

GPS

GLO

Alaska

Peru

COSMIC 1

images originally published at www.cosmic.ucar.edu

Occultation occurrences over 24 hours for COSMIC and COSMIC-2

COSMIC 2

Results

Alaska

Peru

COSMIC 2

Conclusions

• High-latitude ground receivers see periodic common-volume occurrence corresponding to COSMIC orbital period
• Low-latitude see a-periodic occurrences
• Low-altitude common-volume POIs occur near the ground receiver
• There are "poleward deficits" of common-volume occurrences at low-elevation for low-latitude and mid-to-high elevation for high-latitude
• Sharp spikes in our common-volume metric correspond to ray-path passes close to a receiver

COSMIC 2 fixes this

Motivation and Future Work

Where can we have improved imaging resolution in the ionosphere

Adaptive-mesh tomographic imaging of ionosphere electron density

Ionosphere Tomography

Acknowledgements

This research was supported by the Air Force Research Laboratory and NASA.

References

• TS Kelso et al. Validation of sgp4 and is-gps-200d against gps precision ephemerides. 2007

•  "COSMIC-2." COSMIC 2. UCAR, n.d. http://www.cosmic.ucar.edu/cosmic2. 02 Jan. 2016.

• Chen-Joe Fong et. al. Formosat-3/COSMIC spacecraft constellation system, mission results, and prospect for follow-on mission. 2009.