Atmospheric Sensing Using GNSS RO - GNSS Remote Sensing: Theory, Methods and Applications - page 148

LEO and geostationary satellites (Ho et al. 2010b ). The radiosonde soundings have
long been deems as the ground truth for the upper-air in-situ observations. However,
the evolving instrumentation upgrade over the past decades as well as the various
brands of radiosonde units used at different countries raise the concerns about how
accurate the radiosondes are for long-term climate monitoring. The highly accurate
and precise RO measurements provide a unique opportunity to assess the quality of
radiosonde data. The diurnal bias due to radiative effects and the difference among
various radiosonde brands have been revealed after being compared with the near
coincident RO soundings (e.g., Kuo et al. 2005 ;Heetal. 2009 ).

6.6 Future Application of Radio Occultation

6.6.1 Future GNSS and GNSS RO Missions

According to Table 5.2 , the current RO sounding number ( 3,000 per day),
produced mainly by COSMIC and GRAS and a couple of other LEO satellites, is
still a relative small sample in regional scale. Most of the current available LEO RO
satellites only track the GPS satellites. The current number of RO daily soundings
( 1 daily profile per 4 ı 4 ı grid) significantly limits the impact of RO observations
on the regional weather prediction such as tropical cyclone and severe weather
and thunderstorm prediction, since the average spacing of about 400 km cannot
adequately resolve the storm structures and their environment.
A COSMIC follow-on constellation is now being planned by the US and Taiwan
and will produce five to ten times as many RO soundings as COSMIC. The new
RO receiver (Tri-G) developed by NASA JPL will take advantage of the expanding
GNSS transmitters and will be able to track different GNSS signals, including the
GPS, GLONASS, Galileo and Compass/Beidou. Each of these GNSSs will consist
of 24-30 transmitting satellites when fully operated. This will allow a significant
increase in the number of possible RO soundings obtained by an individual LEO RO
receiver. It is expected that COSMIC-II will provide over 10,000 daily soundings
(i.e., 1 profile per 2 ı grid) (Anthes 2011 ). Such dense global sampling will lead to
a major, positive impact on regional severe weather and tropical cyclone prediction,
climate monitoring as well as weather, climate and ionospheric research.

6.6.2 Airborne and Mountain-Top GNSS RO

While the GNSS radio occultation measurement from the space has the advantage of
global coverage (e.g., one LEO receiver provides 500 global distributed soundings
per day), the sampling in a particular region is still relatively sparse without a large
number of LEO receivers. Therefore with the current number of LEO RO satellites,