  The Sun's magnetic field is strongly structured over a broad range of scales.
The magnetic spatial power spectral analysis provides a powerful tool to
understand the various scales of magnetic fields and their interaction with
plasma motion. We aim to investigate the power spectra using spherical harmonic
decomposition of high-resolution SOHO/MDI and SDO/HMI synoptic magnetograms
covering three consecutive solar cycle minima in a series of papers. As the
first of the series, we calibrate and analyze the power spectra based on
co-temporal SDO/HMI and SOHO/MDI data in this paper. For the first time, we
find that the calibration factor $r$ between SOHO/MDI and SDO/HMI varies with
the spatial scale $l$ of the magnetic field, where $l$ is the degree of a
spherical harmonics. The calibration factor satisfies $r(l)=\sqrt{-0.021
l^{0.64}+2} \quad(5<\mathrm{l}\leq539)$. With the calibration function, most
contemporaneous SOHO/MDI and SDO/HMI magnetograms show consistent power spectra
from about 8 Mm to the global scales over about 3 orders of magnitudes.
Moreover, magnetic power spectra from SOHO/MDI and SDO/HMI maps show
peaks/knees at $l\approx120$ corresponding to the typical supergranular scale
(about 35 Mm) constrained from direct velocimetric measurements. This study
paves the way for investigating the solar-cycle dependence of supergranulation
and magnetic power spectra in subsequent studies.
