Figure
5. Gravity anomaly in the Amazonas basin using GRACE and GRACE-FO
monthly solutions.
According to Földváry (2015), the annual amplitude is underestimated due
to the averaging over the sampling period by 1.15%, thus the effect of
the sampling error exceeds this phenomenon. Nevertheless, the GRACE and
GRACE-FO models notably scatter from the annual characteristics (c.f.
Figure 5, where also a best fit annual and semiannual curve is
presented). The deviations of the detrended and unbiased GRACE/GRACE-FO
monthly gravity anomalies from an annual and semiannual periodic curve
is 35.58% of the magnitude of the signal, in average. This deviation
consists of non-periodic signal, modelling errors (such as errors of the
atmospheric correction, leakage of signal from outside of the basin,
errors of smoothing and de-correlation filtering) and observation errors
(integrating such errors as system-noise error in the KBR range-rate
measurements, accelerometer error, errors of the ultra-stable
oscillator, and orbit errors) as well (Swenson et al., 2003).
Considering that the errors of GRACE are believed to be largely free of
annual components (Wahr et al., 2006), the estimated annual component on
Figure 5 can be considered to be purely due to the annual variations of
the total water storage, consequently, it can be considered to be a real
signal, while the error content is included in the non-periodic terms.
Accordingly, the unmodelled non-periodic components and observation
errors can be handled independently from the annual component, and the
later component can be described by an error of 2.49% of the signal
content due to the sampling and an error of 1.15% of the signal content
due to the averaging.
Example in the space domain: the Hungarian Gravimetric
Network
In order to indicate the relevance of the errors contaminated by the
sampling, a case study of the comparison of two epochs of the Hungarian
Gravimetric Network (MGH) is provided. The first epoch of the network
was dated to the 1950s, labelled as MGH-50 (Facsinay-Szilárd, 1956),
while the second one is dated to 2000, referred to as MGH-2000 (Csapó,
2000). The distribution of the gravimetric stations is shown in Figure 6
for the MGH-50 and the MGH-2000 networks.