Bernese GPS Software Version 5.0 - Bernese GNSS Software

11.4 Troposphere Modeling in the Bernese GPS Software
As we saw in the previous section an a priori model requires some input parameters (pressure,
temperature, and humidity). These values are usually derived from a standard model
atmosphere. You may alternatively introduce measured meteo values for some (or all) stations
(option “Meteorological data” in GPSEST). The corresponding file description may be
found in Sections 11.5.2 and 22. For stations without explicitly introduced values, the standard
model is used automatically. Programs CODSPP and MAUPRP always use a standard
atmosphere model.
Usually the tropospheric refraction is not taken into account by an a priori model alone,
but always together with estimating additional parameters. Although it is possible to use
only an a priori model it is by no means recommended if high accuracy is aimed for.
Let us clarify this statement by having a look at the implications of small biases in ground
meteorological data (pressure, temperature, humidity) on the estimated station heights.
Table 11.2, together with Eqn. (11.1), gives an impression of the sensitivity of the estimated
station height (independent of the baseline length) on biases in surface meteorological
measurements for different atmospheric conditions. We see, e.g., that in a hot and
humid environment (last line in Table 11.2) an error of only 1% in the relative humidity will
induce a bias of 4 mm in the tropospheric zenith delay, which will in turn produce (using
Eqn. (11.1)) a height bias of more than one centimeter! It is common knowledge that it is
virtually impossible to measure the relative humidity to that accuracy (let alone using a
standard atmosphere model); moreover the measured humidity is usually not representative
for the entire environment of a site. This is why experience tells that estimation of troposphere
parameters is a necessity if high accuracy is required and if only ground met data are
available. Similar remarks are true for temperature measurements. It should be possible,
on the other hand, to measure surface pressure to the accuracy level necessary (0.1 mm) to
render pressure-induced height errors harmless. You should always keep in mind the orders
of magnitude reflected in Table 11.2 when using ground meteorological data. Our conclusion
is, that only if you are able to provide meteorological values stemming from Water Vapor
Radiometers you have a chance to get around the estimation of tropospheric zenith delays.
There is one exception to that rule: if you are working in a rapid static mode with short
observation times (less than 1 hour), you may be best advised not to estimate troposphere
parameters but to use only an a priori model together with the standard atmosphere.
Table 11.2: Tropospheric zenith delay as a function of temperature T, pressure P, and
relative humidity H.
T P H
∂∆̺
∂T ∂∆̺
∂P ∂∆̺
oC mbar % mm/ oC mm/mbar
∂H
mm/%
0o 1000 50 3 2 0.6
30o 1000 50 14 2 4
0o 1000 100 5 2 0.6
30o 1000 100 27 2 4
Bernese GPS Software Version 5.0 Page 245