The Doppler Centroid (DC) algorithms estimate the centre frequency of the Doppler spectrum of the data, related to the azimuth beam centre. The DC locates the azimuth signal energy in the azimuth frequency domain. It is needed so that the signal energy in the Doppler spectrum can be correctly captured by the azimuth compression filter, providing the best signal-to-noise ratio and azimuth resolution.

The DC varies in both range and azimuth. The range variation depends on the satellite attitude and how closely the illuminated footprint on the ground follows an iso-Doppler line on the ground as a function of range. The azimuth variation is caused by slow changes in satellite attitude as a function of time.

The DC is estimated at different ranges in the data and a polynomial function of range is fitted to the measurements. The DC is updated in successive azimuth blocks.

Doppler Centroid Estimation

The Doppler is composed of a fine DC frequency (the fractional part), which is ambiguous to within the Pulse Repetition Frequency (PRF) and a second absolute component, which is an integer multiple of the azimuth sampling rate.

The DC is expressed as:

The DC estimation algorithm includes the following steps:

1. Absolute DC calculation from orbit and attitude.
2. Fine DC estimation.
3. Weighting factors calculation.
4. Fine DC estimate unwrapping.
5. Absolute DC estimation.

For TOPSAR data, the data must be demodulated in the azimuth direction before applying the phase-based correlation DC estimator.