Zenith Tropospheric Delay	Precipitable Water Vapor

The map images above show the real-time ZTD (left) and PWV (right) over the Japanese islands. It is processed at Nippon GPS Solutions (NGS), Tokyo, Japan, with RTNet software. PWV is mapped from forecast pressures from Clock for Weather (CFW) GPS enhanced forecast system. True real-time processing of tropospheric delay and other GPS estimates such as coordinate and inospheric delays are possible to be provided with RTNet software with real-time.

The image above shows 30 minute gradient of ZTD (ZTD difference between current time and 30 minutes ago). Researches using temporal gradient of ZTD have been perfomed using ZTD difference of 30 min., 1 hour, and 6 hours, for example. RTNet makes it possible to serve such ZTD gradient with higher temporal resolutions and shorter latency.

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The figure above shows ZTD forecast updated every 6 hours in Clock for Weather (CFW) forecast system. Most closest timing plot for the ZTD plot in the top panels is displayed.

GPS ZTD estimates in the map domain can be converted into PWV by using CFW forecast.

An evaluation of pressure forecast in one month of January, 2009, indicated that forecast pressure has precision within 2 hPa upto 1.5 day forecasts and precision of temperature forecast is within 2.5 deg. Celcius. Thesse facts suggest that mapped PWV using the CFW forecast have precision of 0.4 mm in PWV unit in the worst case. Historical researches on precision of GPS derived PWV have shown that GPS PWV has precision of about 1.5 mm (standard value) in all weather conditions, and the error of 0.4 mm by using CFW is much smaller than the standard value.

Please inquire of NGS about real-time ZTD processing system and CFW-based PWV mapping system in the domain.