GPS and the Ionosphere

Nick PedatellaMarch 5, 2009

ASEN 6090

Outline

● Ionospheric Science – who cares?● GPS for ionospheric science

● Deriving total electron content (TEC)● Ground-based observations● Radio Occultation● GPS contributions to space science

● Ionospheric effect on GPS observations● Scintillation and fading

Ionosphere● Ionized portion of the upper atmosphere● Ionized by solar radiation

(ssl.berkley.edu)

Ionospheric Science & Space Weather● Can disrupt communication

and navigation systems● Influences satellite drag● Enhanced radiation potentially

harmful for humans and satellites

● May disrupt power grid

(telegraph.co.edu)(Denver Post)

Ionospheric Science - Historical Perspective

● Ground-based observations● Ionosondes● Incoherent Scatter Radars● Limited spatial and temporal

resolution (expensive to run)

● Rockets● In-situ satellite

observations

Data CoverageGPS Ionosonde

(Komjathy et al., 2007)

ISR

Ionospheric Science with GPS (1)

● IGS global TEC maps● Generated via Kalman filter● Fit to minimized influence of

receiver distribution

● Provide vertical TEC every 2h on a 2.5 x 5 deg lat x long grid

● Difficulty capturing finer temporal and spatial features.

Ionospheric Science with GPS (2)● Raw observations improve temporal resolution● Can fill in gaps with other observations

(Pedatella et al., 2009)

Ionospheric Science with GPS (3)● Biggest drawback is lack of vertical information● Tomography provides some information on

vertical structure but still limited

GPS Radio Occultation (1)● Radio occultation (RO) originally used to study

planetary atmospheres● Occultation receivers onboard: CHAMP, SAC-C,

GRACE, COSMIC● Provide good vertical resolution – meters to kilometers● GPS is also fundamental for providing precise orbits

necessary for RO

(ucar.edu)

GPS Radio Occultation (2)● Primary advantage of GPS RO is spatial coverage

● Significant impact on assimilative space weather models

● Biggest limitation is temporal

Ground GPS Coverage COSMIC Coverage

(Komjathy et al., 2007)

GPS Radio Occultation (3)

● Electron density profiles are generally in good agreement with collocated independent measurement sources

● Can have significant errors in the profiles that are associated with the inversion process● Assume spherical symmetry

in the inversion

(Lei et al, 2007)

Major Contributions to Space Sciences (1)

● Spatial and temporal resolution permits many new discoveries

(Tsugawa et al., 2006)(Foster et al., 2002)

● Improved real-time monitoring of the ionosphere● Important for space weather forecasting

Major Contributions to Space Sciences (2)

Ionospheric Effects on GPS - Scintillation

● Rapid amplitude and phase fluctuations● Not necessarily associated with solar activity

(Groves, 2004)

(http://gps.ece.cornell.edu)

Ionospheric Effects on GPS

(http://gps.ece.cornell.edu)

Tracking LevelL2

C/N

o (d

B-H

z)

Frequency Dependent Fading

Frequency Dependent Fading

(L1)

(L2)

(http://gps.ece.cornell.edu)

Frequency Dependent Fading

(http://gps.ece.cornell.edu)

Summary● GPS offers significant advantages over other

ionospheric observations● Enhanced spatial resolution of measurements● Real time, near-global observations● Significantly improves global assimilative models

● Limited by:● Limited/no vertical resolution● No coverage over oceans● Partly solved by GPS RO

● Solar activity and scintillation influences receiver tracking and result in loss of lock

Questions?