# Analysis of Maize Sowing Periods and Cycle Phases Using Sentinel 1&2 Data Synergy
## 2 Materials and Methods
### Identification of Crop Emergence and Sowing Periods  
The backscattering and NDVI signals were analyzed during the maize growing season. The sensitivity of the VH/VV signal was exploited to identify the emergence periods of maize (EP), i.e., the time at which the germinating plant emerges from the ground and starts to grow. According to the NDVI series, the initial growing phase develops during May and June. Even if the NDVI signal suggests some fields are already sowed during April, the analysis of backscattering signal was performed from May, to avoid the uncertainty related to the fields still cultivated with winter crops in April. The IPR daily signal was compared to the NDVI for two growing stages: the initial and growing phase (May-June-July) and the middle-final phase (August-September-October). The two phases were defined according to the mean NDVI values. From May to July the NDVI increases because of the cover development, reaching the maximum between July and August. In the last three months, the NDVI gradually decreases because of the chromatic change of maize leaves, up to rapid and strong variations indicating net changes corresponding to the harvesting times. The EP periods were identified through an approach based on the analysis of the IPR signal. The bare soil period was identified for each field, according to the NDVI data, and the IPR backscattering signal was calculated for this period to identify the noise from moisture and roughness variability in bare soil. For each field, the noise induced by bare soil was removed from the IPR signal for the complete growing season, as described in Equation (5):  
\[IPR_{f}=IPR_{GS}-\frac{\sum_{hp}IPR}{lbp} \tag{5}\]  
where \(IPR\) is the normalized VH/VV backscattering (non-dimensional), \(IPR_{f}\) is the filtered \(IPR\) obtained by removing the backscattering from pure bare soil, \(IPR_{GS}\) is the IPR backscattering during the growing season (\(GS\)), and \(lbp\) is the length of the period before plant emergence [days]. The filtered normalized backscattering, expressed as PolarimetricRatio Index (IPR\({}_{l}\)) should have values close to zero until the plants start to grow out from the ground. The EP was assigned to each field according to the time in which the IPR\({}_{l}\) signal exceeded the 0.1 threshold. This cut-off value was fixed according to the uncertainty associated with the radar signal [48]. The maize sowing periods (SP) were inferred from the EP, considering the daily air temperature and the soil moisture conditions in the days before the emergence, as previously done by [PERSON] et al. [16]. According to the method described by [PERSON] et al. [49], the days needed by the planted seeds to emerge from the ground were computed calculating the daily Growing Degree Units (GDU) [\({}^{\circ}\)C]. This indicator, also named Heat Units, describes the relation between crop development and the daily temperature. The heat accumulation, calculated considering the daily minimum and maximum temperatures, is used to measure the time required by crops to reach specific development phases. Several authors provide estimations of the total GDU required by the maize plants to emerge through the soil: [PERSON] and [PERSON] [50] calculated a GDU requirement of 69.4 \({}^{\circ}\)C while [PERSON] et al. [51] suggested a range between 50 and 66.7 \({}^{\circ}\)C. For maize cultivated in the Eastern part of Po Valley (North-East Italy), a GDU requirement of 61 \({}^{\circ}\)C was proposed by [PERSON] et al. [52]. In the present work, a mean GDU requirement of 62.9 \({}^{\circ}\)C was assumed as reference value for maize to reach the emerging phase, to be consistent with general recommended values and specific GDU estimated for Northern Italy.