Geoscience Reference
In-Depth Information
Ionospheric Parameters from Dual-Frequency Measurements
Although the initial aim of the space-borne altimeters is the accurate measurement of
the sea surface height, the two separate operational frequencies give the opportunity
to obtain information about TEC along the ray path as well. The primary sensor of
both Topex/Poseidon and Jason-1 as well as Jason-2 is the NASA Radar Altimeter,
operating at 13.6GHz (Ku-band) and 5.3GHz (C-band), simultaneously (Fu et al.
1994
). Similar to GNSS, the ionospheric effect on the altimetry measurements is
proportional to the TEC along the ray path and inversely proportional to the square
of the altimeter frequency. At the Ku-band, the sensitivity of the range delay to
the TEC is 2.2mm/TECU. Thus, the range at this signal can be over-estimated by
2-40 cm due to the ionosphere (Brunini et al.
2005
). According to Imel (
1994
), the
precision of the Ku-band range delay correction in one-second data averages is about
5 TECU or 1.1 cm. In fact, the precision of the satellite altimetry derived TEC is a
more complex issue, since it is also affected by non-ionospheric systematic effects. A
systematic error which might bias the TEC estimates due to its frequency dependence
is the so-called Sea State Bias (SSB) (Chelton et al.
2001
).
The ionospheric range delay
dR
derived from the altimeter measurements at the
two frequencies is directly provided in mm, and has to be transformed into TECU. It
has to be noted, that in the case of satellite altimetry derived TECnomapping function
is needed, since the measurements are carried out normal to the sea surface and thus,
the ray path is assumed vertical. Consequently, the transformation formula is:
f
Ku
10
−
3
VTEC
alt
=−
dR
·
[
TECU
]
,
(99)
10
16
40
.
31
·
with
f
Ku
being the Ku-band carrier frequency in Hz.
Theoretically, the TEC values obtained by satellite altimetry are expected to be
lower than the ones coming from GNSS, since unlike GNSS the altimetry satellites
do not sample the topside ionosphere due to their lower orbit altitude. However,
several studies have demonstrated that Topex/Poseidon and Jason-1 systematically
overestimate the VTEC by about 3-4 TECU compared to the values delivered by
GNSS; e.g. Brunini et al. (
2005
), and Todorova (
2008
).
4.3.3 Estimating TEC from LEO Satellite Data
Low Earth Orbit (LEO) satellites operate at orbital altitudes between 260 and
∼
3500 km. Among their different scientific objectives, the global sounding of the
vertical layers of the neutral atmosphere and the ionosphere is of great importance.
Some of these missions carry dual-frequency GPS receivers onboard, which makes
them capable of remote sensing the atmosphere using the Radio Occultation (RO)
technique. The RO technique is based on detecting the change in a radio signal pass-
ing through the neutral atmosphere and the ionosphere. As a radio signal travels
through the atmosphere, it bends depending on the gradient of refractivity normal to
