This ASCII document explains the amplitude calibration procedure for the S-Band Magellan bistatic radar data (BSR) collected at the Madrid 70-m antenna of the NASA Deep Space Network (DSN). The companion JPG document illustrates the results. SYSTEM TEMPERATURE ================== The system temperature of the S-Band receivers was obtained by comparing noise power with the receiver connected to an ambient load against noise power when the receiver was looking through the antenna at 'cold' sky. The measurements and the derived values for Tsys are summarized in the table below. Times (including START and STOP) are in hours past 0 h UTC on 5 June 1994. Values in columns 'POWER' are relative and should only be compared against other POWER values in the same row. Ambient load physical TEMPERATURE is in degrees C; DERIVED Tsys is in K. Note that a confirmed S-LCP ambient load physical TEMPERATURE was only obtained once -- at the end of the experiments. The S-RCP ambient load physical TEMPERATURE at 13.75 h is suspiciously high; however, a 5 degree error in physical temperature translates into only about a 1.5 percent error in Tsys. BAND RECEIVER TO RECEIVER TO PHYSICAL DERIVED POLN AMBIENT LOAD COLD SKY AMBIENT LOAD Tsys -------------------- -------------------- ------------------ START STOP POWER START STOP POWER TEMPERATURE TIME ---- ------ ------ ------ ------ ------ ------ ----------- ------ ------- SRCP 13.766 13.798 4.3485 13.685 13.754 .31724 26.60 13.75 22.05 SRCP 16.889 16.896 5.3069 16.772 16.863 .38962 20.10 16.92 21.72 SRCP 19.973 19.997 2.8503 19.911 19.966 .24442 21.71 21.60 25.50 SLCP 13.769 13.798 2.9109 13.685 13.754 .21680 24.29 21.60 22.34 SLCP 16.869 16.896 2.9650 16.772 16.863 .24456 24.29 21.60 24.74 SLCP 19.973 19.997 1.8333 19.911 19.966 .15440 24.29 21.60 25.26 Example Calculation: The first S-RCP calculation proceeds as follows. Tsys = Tamb'*Psky/Pamb = (26.60 + 273.15 + 2.54)*(0.31724)/4.3485 = 22.05 where Tamb' sums the power contributions in ambient load configuration 2.50 is a contribution from the maser and 0.04 is from following electronics and 273.15 is the additive correction in converting Celsius to Kelvin The noise power density is then Pn = 1.38 10^-23 * Tsys = 3.0429 10^-22 W/Hz The uncalibrated power density in a signal-free part of the spectrum at approximately 13.75 h can be calculated from data and compared with Pn to obtain a power scale factor. The square root of the scale factor then becomes the multiplier for sample amplitudes which is stored in a GNC file in the form of polynomial coefficients. ATTENUATOR CORRECTIONS ====================== The table below lists the times at which 15 dB attenuation was added or removed in front of the analog to digital converters in the receivers. The attenuation was used to prevent saturation of the 8-bit samplers in the presence of strong carrier signals. The polynomial coefficients in the GNC file were adjusted to compensate for the attenuator changes, making the use of attenuators nearly transparent in PRR and later versions of the files. TIME (seconds past 0 h) S-RCP ATTENUATOR CHANGE S-LCP ATTENUATOR CHANGE ----------------------- ----------------------- ----------------------- 49074 Add 15 dB 49077 Add 15 dB 58585 Rem 15 dB 58600 Rem 15 dB 60182 Add 15 dB 60191 Add 15 dB 69680 Rem 15 dB 69685 Rem 15 dB 71364 Add 15 dB 71550 Add 15 dB ILLUSTRATIONS ============= AMBCALR1.JPG: S-RCP echo power (upper panel) and background noise power density (lower) for the first bistatic radar orbit (end of spacecraft orbit 13032 and beginning of 13033). Noise power density is calculated in a spectral range where there is no known spacecraft or ground signal. The surface echo power is then calculated over the spectral range where the echo is believed to exist by subtracting the noise power from the total power. The zeptowatt (zW) is 10^-21 watt. The vertical green line in the upper panel marks the time when the specular point entered Maxwell Montes; the red vertical line shows approximately when the specular point left Maxwell Montes. The increase in echo power to the left of the vertical red line corresponds to passage of the specular point over the Cleopatra region. In the lower panel the red vertical line shows when the S-RCP attenuation was increased by 15 dB. In other figures, a green vertical line in the lower panel indicates when 15 dB of attenutation was removed. If the general drift of the noise power density points is unaffected by changes in attenuation (as is the case here), the amplitude corrections applied at these times can be judged successful. The coefficients in the GNC file corresponded to the nominal 15 dB changes for S-RCP. AMBCALR2.JPG: Same as AMBCALR1.JPG except for being the second Magellan BSR Orbit (end of spacecraft orbit 13034 and beginning of 13035). The lower panel includes a vertical green line, indicating removal of 15 dB of attenuation (with no visible effect on the noise power density). AMBCALR3.JPG: Same as AMBCALR1.JPG except for being the third Magellan BSR Orbit (end of spacecraft orbit 13036 and beginning of 13037). The lower panel shows a reduction in noise power density immediately after the attenuation increase (red vertical line); but the data points return to the original trajectory after about a minute. The cause for the apparent increase in noise power over 70100-70200 seconds is not known. AMBCALL1.JPG: Same as AMBCALR1.JPG except for being S-LCP data. The noise power density is less stable than in S-RCP, and there is a small discontinuity (about -6 percent) at the introduction of the 15 dB attenuation. The coefficient changes in the GNC file corresponded to 14.5 dB attenuator switching -- a difference of about 12 percent from the nominal value. If the GNC coefficients had been adjusted so that they corrected the sample values by 14.75 dB, there would have been no discontinuity in the S-LCP power density values. The S-LCP noise power density fluctuates 3-5 times this much, however, from unknown causes; so no attempt was made to optimize the S-LCP GNC coefficients beyond the initial offset to 14.5 dB. AMBCALL2.JPG: Same as AMBCALL1.JPG except for being the second BSR orbit. The noise power density jumped about 6 percent and dropped about 6 percent when the attenuation was changed (green and red vertical lines, respectively). AMBCALL3.JPG: Same as AMBCALL1.JPG except for being the third BSR orbit. The noise power density jumped about 6 percent when the attenuation was reduced (vertical green line).