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?