ALOS Data Users Handbook

ALOS Data Users HandbookIn SRU, the electric charge accumulated at each element of the CCD is transmitted to the transmission register ofthe CCD as a trigger of LineSync from ELU. The completion of electric charge transmission (Φ TG falling ofCCD) is defined as the actual observation time "tn" of each line.After the completion of the electric charge, data are forwarded from ASP to ELU and stored in the buffer duringthe period by the next LineSync.ELU takes time data and ADS data in the internal buffer as a trigger of LineSync. The time is defined as Tn.ELU takes the starting points of the 1PPS reference time (every even second) T GPS (total 40 bits) acquired byU-SDB I/F at the timing of the 1PPS reference signal.Since the resolution of the 1PPS reference time is 1 second, in order to obtain the resolution which candiscriminate the PRISM imaging cycle (0.370 millisecond), it drives a 19-bit self-running counter insidePRI-ELU using the 1MPPS reference signal, and generates the time of 1LSB = 2 microseconds.The self-running counter is reset at the timing of each 1PPS reference signal and counts up again.PRI-ELU inputs the count value of the self-running counter until adding an imaging time data to an observationsignal and the 1PPS reference time (T GPS ) into AUX data as time data (T n ). T n is given as follows:It is a total of 29 bits of the high-order 16 bits (16 μsec resolution; 0 to approx. 1 sec) of the 19-bit self-runningcounter (1LSB = 2μsec) and the low-order 13 bits (1 second resolution; 0 to 8,191sec) of GPS seconds (24 bits)of T GPS .The relation between real imaging time (t n ) and (T n ) is as follows:tnΔTδT= T + ΔTndelay3σp− p= ΔTdelay+ δTDET / ASP3σp− p− ΔT: Indefinite errorELU: Fixed biasThe elements of the fixed bias are given in Table 3.4-4 and the elements of indefinite error are given in Table3.4-5.3-35