1. GNSS Tropospheric Products
Final conference of the project HUSKROUA/1101/252
GNSS Tropospheric Products
Stepan Savchuk,
project expert,
Dr. Tech. Sci.,
professor of the Department of Geodesy and Astronomy, National University "Lviv Polytechnic" (Lviv, Ukraine)
December 17-19, 2015

2. Степан Савчук Національний університет “Львівська політехніка
РОЗВ’ЯЗУВАННЯ ЗАДАЧ МЕТЕОРОЛОГІЇ ІЗ ЗАСТОСУВАННЯМ ГЛОБАЛЬНИХ СУПУТНИКОВИХ НАВІГАЦІЙНИХ СИСТЕМ (GNSS)
Степан Савчук
Національний університет “Львівська політехніка
Київ, 25 березня 2011

3. Зміст Теоретичні основи GNSS метеорології
Загальні відомості про мережі активних референцних станцій в Україні
01/12/12
MetApps Lec 12

4. GNSS observables Pseudo range (code observation)
the measure of the transit time from satellite to receiver using correlation between received and replicated signal (the time is coded in signal) absolute positioning with accuracy of a few meters.
Carrier phase (phase observation)
the measure of the phase difference btw. received and replicated carrier frequency mm level of relative positioning

5. Non-linear code observation equations

6. GNSS model - error sources vs. parameters
satellites: ephemeris, clocks, differential code/phase biases, ...
receivers: clocks, phase center offsets and variations
differential code/phase biases, …
environment: TROPOSPHERE, IONOSPHERE, multipath,
Earth’s dynamics and crust deformations, …
processing: cycle-slips (initial phases ambiguties)
model deficiencies, …

7. GNSS model - error sources vs. parameters
we introduce: receiver and satellite positions (satellite clocks) and precise models (PCVs, loading effects, mapping functions ...),
we eliminate: ionospheric effect (receiver and satellite clock error),
we estimate: troposphere, ambiguities (receiver clocks)

8. Review of GNSS meteorology
Remote sensing of the atmosphere with the microwave signal of the Global Navigation Satellite Systems (GNSS) has become a well-established field of research with its major application in meteorology.
The primary use of the microwave signals is obviously the positioning of receiving antennas on the Earth.
The following compilation summarizes the today’s GNSS meteorology products that are of interest to the meteorological community and outlines their application.

9. Review of GNSS meteorology
But the waves passing the atmosphere are affected by the concentration of free electrons in the ionosphere and by the air density in the lower stratosphere and the troposphere.

10. Review of GNSS meteorology
There are two major contributions of the atmosphere:
Neutral atmospheric delay composed of hydrostatic component (N2, O2, CO2, trace gases and part of the water vapor contribution) and water vapor component.
Ionospheric delay component due to free electrons. This component is frequency dependent and can be estimated from dual frequency measurements (L1 and L2 frequencies).
These atmospheric influences can be retrieved to a certain degree in the processing of the GNSS data with sophisticated software.
We will focus on the troposphere and discuss the possible parameters to be retrieved from the GNSS data.

11. Concept of ground-based GNSS meteorology

12. GNSS tropospheric products
Tropospheric delay effects in GNSS model is composed from:
a) signal delay (major) + delay due to signal trajectory bending (minor)
b) hydrostatic delay (major) + wet delay (minor)
Standard parameters:
ZTD – zenith total path delay
ZHD/ZWD – zenith hydrostatic/wet path delay
(ZTD = ZHD + ZWD)
STD – slant total path delay
SHD/SWD – slant hydrostatic/wet path delay
(STD = SHD + SWD)
GN – North-South horizontal tropospheric gradients
GE – East-West horizontal tropospheric gradients

13. Troposphere estimation strategies
Network mode
Double-differenced observations  baseline(s)  network
‘Traditional approach’
Availability:
re-processing/post-processing, near real-time
…………………………………………………….
GAMIT/GLOBK Software, Bernese Software
real-time
………………………………………………………
Trimble Atmosphere App
Precise Point Positioning
Raw observations (undifferenced)  autonomous solution!
Alberding GNSS Status Software

14. Zenith total delay - ZTD
The basic tropospheric parameter in present GNSS software is the Zenith Total Delay (ZTD) that describes the signal delay in zenith direction above the receiver. It results from the mapping of the delays to each individual satellite into the zenith direction with appropriate mapping functions. The zenith delay is the combination of all these mapped delays into one parameter. It is therefore an average over all elevation angles and azimuths of the satellites in view and as such a spatial average over a certain part of the atmosphere.

15. Zenith total delay - ZTD
A priori atmospheric values for SULP Station (Lviv) Geodetic height(m) =
Met source UFL VMF1G JB Dry Zen UFL Wet Zen GP25 Dry Map VMF1 Wet Map VMF1
Yr Doy Hr Mn Sec PRN Azimuth Elevation Total Zen, m Total Slant, m
Total Slant = Slant total delay (STD)

16. Comparison of zenith total delays from GNSS and radiosonde measurements

17. Zenith total delays from GAMIT Software
………………………..
more than 150 stations Ukraine

18. Comparison of zenith total delays from different GNSS processing algorithms

19. Zenith total delay - ZTD
The ZTD is traditionally either separated into a dry and wet part, called the Zenith Dry Delay (ZDD) and the Zenith Wet Delay (ZWD), or into a hydrostatic and non-hydrostatic (also termed: wet) part. The hydrostatic and dry part are much larger in amplitude but less varying in time. They are of the order of 2.30 m at sea level. The non-hydrostatic or wet part is more variable but smaller in amplitude, typically 0.0–0.40 m.

20. Zenith Dry Delay (ZDD) and Zenith Wet Delay (ZWD)
A priori atmospheric values for SULP Station (Lviv) Geodetic height(m) =
Met source UFL VMF1G JB Dry Zen UFL Wet Zen GP25 Dry Map VMF1 Wet Map VMF1
Yr Doy Hr Mn Sec PRN Azimuth Elevation Total Zen,m Dry Zen,m Wet Zen,m
The hydrostatic delay can be inferred from surface pressure without any correction for the water vapor included therein.
The dry delay additionally needs measurements of ground water vapor pressure.

21. Effect of the Neutral Atmosphere on GPS Measurements
Depending on the research groups, the one or the other formulation is used. Note that GNSS software packages model the dry or hydrostatic delay with the corresponding mapping function as a priori given delays, whereas the wet or non-hydrostatic part is estimated.

22. Integrated Water Vapor (IWV) and Precipitable Water Vapor (PWV)
Integrated Water Vapor (IWV) [kg/m2] and Precipitable Water Vapor (PWV) [mm] are two equivalent terms and denote a parameter describing the column integrated water vapor in the atmosphere. They are not considered a GPS observable as such, but can be derived from ZTD, ground pressure and mean atmospheric temperature. Mean atmospheric temperature can be approximately inferred from surface temperature or determined from NWP models. Compared to water vapor radiometer measurements as reference method, show that GPS can recover PWV with an Root Mean Square (RMS) uncertainty of 1.0–1.5 mm.

23. Comparison of Integrated Water Vapor from different GNSS processing algorithms

24. GNSS-meteorology in Europe

25. Conclusions
The ZTD from GNSS processing is a broadly accepted descriptor of the integrated atmospheric state. Its assimilation into NWP models as ZTD has become a very common application of GNSS meteorology. As such, it is treated like any other measurement of the atmosphere. It is bias-corrected and gets some uncertainty assigned in the form of a standard deviation. The quantification of the standard deviation of GNSS ZTDs requires accurate reference measurements. The radiosonde is usually considered to be the reference of choice.

26. Conclusions
The systematic part of the GNSS ZTD uncertainty amounts to a few millimeters of station-specific mean offset and 1–2 mm annual fluctuation. The mean offset and the annual systematic fluctuation are likely to contain radiosonde contributions of millimeter magnitude. The random measurement uncertainties of GNSS ZTDs are determined to be 2.5 mm–3.5 mm in winter and 3.5–5.0 mm in summer.

27. Suggestions to maintain, extend, upgrade Space emergency system
Establishment of a real time monitoring and warning system in the region Hungary, Slovakia, Romania and Ukraine by means of exploitation of satellite technologies
to maintain, extend, upgrade Space emergency system
upgrading for Multi-GNSS (Galileo)
jointing E-GVAP
utilizing Space emergency system data in meteorology