Abstract
High precision Global Positioning System (GPS) receivers, with the advantages of all-weather work and low cost, are now widely used to routinely monitor precipitable water (PW) vapor. They are so successful that the progressive phasing out of the costly and sparse in situ radio soundings (RS) is now a certainty. Nevertheless, the sub-daily to annual monitoring of high levels of the PW by GPS receivers in the tropics and the equatorial area still needs to be asserted in terms of metrology accuracy. This is the subject of this paper, which focuses on a tropical site located in mid-ocean (Tahiti). The metrology assessment was divided into two steps. Firstly, a GPS internal assessment, with an in-house processing based on the Bernese GNSS Software Version 5.2 and a comparison with the Center for Orbit Determination in Europe (CODE) products. Secondly, an external assessment, with a comparison with RS PW estimates. In contrast with previous works that only used PW estimates from the Integrated Global Radiosonde Archive (IGRA) website, we estimated the RS PW from the balloon raw data. This is especially important in tropical areas, where IGRA estimates only consider balloon measurements taken below approximately 5500 m. We show that, in our case, this threshold is one of the main sources of bias between GPS and RS estimates, and that the formula used to translate the GPS zenith wet delays (ZWD) to PW estimates also needs to be revisited for high level water vapor contents in the atmosphere.
Highlights
Atmospheric water vapor plays an important role in atmospheric processes including atmospheric radiation balance, water cycles, the transfer of energy, and the formation of clouds and precipitation [1,2,3]
We did a cross-check between our Global Positioning System (GPS)-derived estimates with the radio soundings data estimates from a nearby site, and checked the internal consistency of the radio soundings estimates and of the GPS data estimates, in contrast to many studies that assume that the radio sounding data taken from the Integrated Global Radiosonde Archive (IGRA) archive are “the” reference
We checked the internal consistency of the modeling of the zenithal total delay (ZTD) and zenith wet delays (ZWD) estimates from our GPS data processing with respect to the Center for Orbit Determination in Europe (CODE) products and the Saastamoinen model
Summary
Atmospheric water vapor plays an important role in atmospheric processes including atmospheric radiation balance, water cycles, the transfer of energy, and the formation of clouds and precipitation [1,2,3]. If we have an on-site good estimate of both the PW from an external source (essentially radio soundings, but this can be absorption lines of water vapor in the atmosphere or radiometric observations), and ZWD from a collocated GPS receiver, Π and Tm can be directly derived. We have a collocated weather station giving us Ts, we can obtain by linear regression over a time series of Tm and Ts site-dependent values of the slope and intercept This is basically the approach we applied in this paper, as we had the luxury to have a radio sounding station close to the Geodesy Observatory of Tahiti (OGT), with an International GNSS Service (IGS)-grade GPS receiver with weather data.
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