Atmospheric Sensing Using GNSS RO - GNSS Remote Sensing: Theory, Methods and Applications - page 131

an approach can be implemented within the existing flight receiver hardware that
applied CL tracking through software modification (Sokolovskiy 2001 ). The OL
tracking was successfully implemented for the first time by JPL in the SAC-C RO
receiver in 2005, and significantly led to a much higher percentage of RO soundings
to penetrate deep into the lower troposphere (Sokolovskiy et al. 2006b ). OL tracking
is later being routinely applied on the RO receivers aboard COSMIC (Sokolovskiy
et al. 2009 ;Aoetal. 2009 ) and most of the follow-on RO missions as shown in
Table 5.2 in Chap. 5 . Besides eliminating the large tracking errors and improving
the ability to probe deeper into the lower troposphere, OL tracking enables the
acquisition of rising occultations, thereby doubling the number of occultations from
the same instrument.

6.2.4 Calibration Errors

After the POD processing, the satellite orbits and velocities can be precisely
determined. The precision of RO GNSS transmitter and LEO receiver satellite obits
can be within meter and several centimeter levels, respectively (Bertiger et al. 1994 ).
Such position errors primarily affect the altitudes of the retrieved bending angle
and refractivity profiles. The altitude error is in the order of 10 cm for typical
occultation geometry and is generally negligible (Kursinski et al. 1997 ).
The satellite velocity errors, especially the component along the ray path directly
affects the atmospheric Doppler measurement and so the atmospheric property
retrievals. Given the LEO receiver velocity errors of 0.05 (mm s 1 ) (Bertiger and
Wu 1996 ), the corresponding root-mean-square (RMS) errors in geopotential height
and temperature is 10 m and 0.3 K near 50 km altitude. The errors decrease rapidly
at lower altitudes and should not have significant impact at lower altitudes.
In general, the differencing procedure is necessary to removes the clock errors
(especially for LEO receiver clock) that could affect the signal phase measurements.
The differencing technique allows the unclassified receiver to achieve high accuracy
clock solutions even with SA modulation on, which allows the usage of much
smaller size, power and less expensive LEO receiver. The downside is the additional
thermal and local multipath noise on each differencing link, which, however, can be
significantly reduced with higher SNR measurements.
When applying zero differencing (e.g., no differencing), clock errors could leak
into the phase measurements and so the retrievals. Beyerle et al. ( 2005 ) demonstrate
the zero-difference processing can produce highly accurate excess phase data with
GRACE-B (equipped with an ultra-stable-oscillator) occultation measurements. On
the other hand, when the ground reference station is used in double differencing,
the troposphere related errors could add to the clock errors and result in retrieval
errors. Such tropospheric noise induced errors is generally small compared with
other sources of noise (Treuheft and Lanyi 1987 ).