Processing GFO SDRs into NOAA IGDRs: 04/01/00 --------------------------------------------- 1) Reformat 744-byte header + 256-byte SDR data records -> 64-byte IGDR record. 2) Edit records: altimeter must be fine-tracking; skip zero-filled records. (Edit out record if Quality Word #1, bit 2 or bit 3 is on). 3) Adjust the time base of the SDR to the 1-sec. average IGDR: a) "Ratio" is defined as the length of each on-board clock "tick", in microseconds. This value is on the order of one, with a nominal value of 1 - 80 ppm = 0.99992 microsec/tick. Since the ground-based time-tagging unit for GFO was installed at NAVSOC in June '99, the value of ratio is constantly monitored, providing accurate SDR time stamps. b) Shift from time of first height to time of midframe (1-sec. average): utc_mid_frame = utc_h1 + [(5.5-1)/10] * 0.98 * ratio c) Before accurate SDR time available: Correct 10 Hz timing, per MacArthur & Kilgus, to remove ~46.55 msec bias: utc_final = utc_mid_frame + .0758988 + (0.049 - .0783488) * ratio d) When accurate SDR time available: utc_final = utc_mid_frame - time_bias_init * ratio time_bias_init = -49.001 msec (header record item #18). 4) Compute 1-second averages and sigmas. For range, use iterative outlier removal about linear fit to 10 Hz data; for SWH & AGC use simple averages. 5) Apply instrumental corrections to 1-second averages: a) Range: h_corr = h_unc - h_fm + h_swh_att - h_cal + h_cog - h_init (D19) (D46) (H12) (H17) (H16) "D19" is SDR data record item # 19, "H12" is header record item #12, etc. h_cal = 0 mm; h_cog = 292 mm; h_init = 20.815 m. b) SWH: none applied. c) AGC: agc_corr = agc_unc + a_temp + a_ht + a_swh_att - a_cal (D43*) (D44) (D45) (H13) a_cal = 0 dB. * In the April, 2000 reprocessing a_init is no longer subtracted from agc_corr, and corrected values of a_temp are computed using the latest temperature correction coefficients as a function of the measured receiver temperature: a_temp = k_t0 + k_t1*rcvr_temp + k_t2*rcvr_temp**2 k_t0 = -5.5301; k_t1 = 0.1323; k_t2 = 0.0 d) Compute sigma-0 from the 1-second average AGC: sigma-0 = agc_corr - a_init (H19) a_init = 31.86 dB. 6) Combine range with SLR orbit: SSH = Orbit-h_corr. Lat, Lon defined by orbit. The reference ellipsoid is the same as for Topex/Poseidon: ae = 6378136.3 m; f = 1/298.257 . 7) Add geophysical corrections to the IGDR: a) OSUMSS95 mean sea surface (formerly OSU91A geoid). b) CSR_3.0 ocean+load tide. c) Topex/Poseidon (TIDPOT) solid earth tide. d) NCEP operational weather model wet & dry tropo. e) NVAP climatological wet tropo. f) IRI95 iono. g) land/ocean & deep-ocean flag bits.