PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "Steve Noll, 1998-10-28; Robert Tolson, 1999-11-22" OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = MGS INSTRUMENT_ID = ACCEL OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ACCELEROMETER" INSTRUMENT_TYPE = "ACCELEROMETER" INSTRUMENT_DESC = " Instrument Overview =================== An accelerometer is an instrument that measures the acceleration of the case of the sensor due to external forces. All accelerometers have a 'proof mass' and it is the tendency of the proof mass to move relative to the case that is a measure of the acceleration of the case. Early accelerometers produced output that was directly related to acceleration; but modern sensors integrate the internally measured signal, to reduce noise, and the output is proportional to the change in velocity over the integration time. In high precision accelerometers, like those on MGS, the proof mass is an electronically floating magnetic body. The electromagnetic field is varied to keep the proof mass stationary relative to the case. The voltage required to accomplish this is proportional to the acceleration. The accelerometers on MGS are sensitive to acceleration of the center of mass (c.m.) of the s/c, pseudo-accelerations (i.e., centrifugal) due to rigid motion of the s/c about the c.m., and differences in gravitational force at the proof mass and the c.m. of the s/c (gravity gradient). The MGS Inertial Measurement Unit (IMU) contains four accelerometers. The principal accelerometer used in the aerobraking analysis is the z-axis accelerometer. This accelerometer is located at approximately (-0.44, -0.38, 0.72) m relative to the center of mass. The accelerometers are Sundstrand QA1200-AA08 model Q-Flex and continuously integrate acceleration to obtain velocity data. The instrument is sampled every 0.1 seconds. The data are recorded in instrument counts or quantized velocity increments equivalent to 0.332 mm/s per count. The QA1200 bias has a specified temperature sensitivity of 10 mg/K or approximately 0.3 counts/K. The temperature of the IMU assembly is actively controlled. IMU telemetered temperatures are quantized at 0.12 K and typical changes during an entire aerobraking pass are between two quantized values. Consequently, only a single, constant bias is determined for each pass through the atmosphere. Scientific Objectives ===================== Accelerometer data were used to characterize the nature of the atmosphere, to determine the effect of the atmosphere on each orbit, and to predict the effect of the atmosphere on future orbits. Calibration =========== The instrument was calibrated on each orbit to determine drift in instrument bias. Bias is determined by monitoring the accelerometer instrument during periods of inactivity before and after entering the atmosphere. The bias acceleration is then estimated over the entire pass by trending the data from the pre- and post-atmospheric entry periods. The pre- and post-atmospheric periods were defined by instrument turn-on and turn-off times and a lower limit on the altitude of data used for calibration, typically 200 km. Operational Considerations ========================== The instrument readings are affected by changes in temperature. The instrument is mounted in the inertial measurement unit (IMU). The temperature of the IMU box is monitored by two temperature sensors, one inside the box and one on the box housing, and controlled by a single heater. The quantum of temperature measurement inside the IMU is 0.117 deg C and the highest temperature variation was 2 quanta over a period of 4 seconds. To calculate the effect of this temperature variation on accelerometer measurements, the acclerometer manufacturer's estimate of 0.3 counts of acceleration change per deg C of temperature change was used. Therefore, a 2 quanta temperature variation over 4 seconds resulted in an acceleration change of 5.8 * 10^-6 m/s^2. When compared to the acceleration values, the value of acceleration due to temperature change was negligible for altitudes within 30 km of periapsis. The temperature did not exhibit any increase or decrease that would indicate atmospheric heating was affecting the accelerometers inside the IMU. IMU temperature data, as telemetered during the first 201 orbits, yielded changes of less than 0.25 degrees C over an aerobraking pass. Operational Modes ================= The data from the accelerometer are passed to the telemetry deck during an aerobraking pass from the time the s/c reaches aerobraking orientation until the s/c returns to nominal orbit attitude. Measured Parameters =================== An accelerometer is an instrument that measures the change in velocity of the spacecraft over a time interval by measuring the fluctuations of voltage to a magnetically floating assembly. The assembly is essentially a simple magnetic mass floating in a magnetically charged ring. When the spacecraft accelerates, the position of the mass inside the ring is maintained by varying the voltage. By recording the change in voltage to the ring and knowing the physical properties of the mass, the acceleration of the spacecraft can be calculated. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "CANCROETAL1998" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "TOLSONETAL1999A" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END