Atmospheric Sensing Using GNSS RO - GNSS Remote Sensing: Theory, Methods and Applications - page 129

troposphere with a maximum of 1 % near the surface; the error decrease sharply
upward and reach the minimum of 0.2 % at around 20 km; and then increases
again to 1 % at 40 km (Kursinski et al. 1997 ).
As described in Chap. 5 , the derivation of atmospheric properties from GNSS
RO observations involves in RO raw amplitude and phase measurements (e.g., Eq.
5.17 in Chap. 5 ) along with multiple retrieval steps that are affected by various
error sources. Individual error sources can be grouped into three major categories:
measurement errors, calibration errors and retrieval errors. Here we briefly discuss
the effects of these errors.

6.2.3 Measurement Errors

During an occultation event, the principal observables are the RO signal phase and
amplitude, which could be affected by the measurement errors including the thermal
noise, the clock instability, local multipath and receiver tracking errors.

6.2.3.1 Thermal Noise

The thermal noise is generally random and un-correlated for any two occultations
and is caused by the finite GNSS signal strength and the receiver amplifier noise.
The thermal noise can produce random phase errors. Because thermal errors
vary rapidly with height, they could contribute significantly to refractivity and
temperature error at high altitudes but produce little pressure error (Kursinski et al.
1997 ).

6.2.3.2 Clock Instability

The signal phase is also affected by the phase stability of the clocks of the GPS
occultation and reference satellites as well as the LEO and possibly ground reference
receivers. The clock errors can largely be removed through differencing technique
in the retrieval process. However the residual clock errors could still be important
at high altitudes but should not dominate retrieval error in general (Kursinski
et al. 1997 ).

6.2.3.3 Local Multipath

The widely used broad-beam GPS receiver antennas significantly simplify GPS
instrumentation but are sensitive to the local multipath caused by the multiple
signals arriving at the antenna after scattering off structures near the antenna. The
local multipath could create slowly varying phase errors, which could potentially