ACE Weekly 12/28/2012 - 01/03/2013 All ACE spacecraft subsystems are performing as expected. ======================================================================== Orbit/Attitude: Type Attitude Date 01/03/2013 DOY 003 2013 Thrusters 2R 4R+ 4R- Duration 5:17 min Start 20:07:41z Stop 20:12:58z HGAStart -8.89deg HGAStop +8.99deg SunStart 14.36deg SunStop 16.34deg SpinStart 5.0785rpm SpinStop 5.0796rpm Nutation 0.07deg Firing 27 pulses FuelUsed 0.0906lbs FuelRemain 117.1468lbs FinalSCMass 1351.407lbs The next attitude maneuver is scheduled for Tuesday 01/08/2012. The next station keeping maneuver (SK-64) is scheduled for 01/15/2012. Tracking/range data since the last maneuver (12/28/2012) has only been from one DSN antenna (DSS-27). Without southern hemisphere tracking/range data, there is more uncertainty in the spacecraft ephemeris (but still within the limit for acquisition). The increased uncertainty may also impact station keeping maneuver planning and fuel usage. But as mentioned in the previous weekly report, station keeping fuel usage is only ~10% of the total fuel usage. Nevertheless, we are pushing to obtain at least 1 hour of southern hemisphere tracking/ranging from DSN each week. Normal ephem uncertainty ~2.5 km uncertainty w/o southern range 10+ km limit for acquisition 3,000 km size of L1 orbit ~150,000 km distance to L1 ~1,500,000 km ======================================================================== OCRs: DOY 363 (12/28/2012) MOCR 401 MTASS Attitude Software The MTASS software replaces the old TPOCC/ADS attitude software for generating the daily Attitude/Orbit Reports that are sent to the ACE Science Center. The solutions from the new system are more consistent than the old TPOCC attitude software. The following table shows the error (daily deviation) from the old TPOCC/ADS system and the new MTASS system. TPOCC/ADS MTASS Attitude: 0.5 degrees 0.05 degrees Spin: 0.01 rpm 0.001 rpm During testing, we found that the daily drift due to solar radiation pressure is roughly linear with the spacecraft sun angle and is approximately 1/400 (daily drift ~ sun angle/400). For example, with a sun angle of 20 degrees, the attitude would change ~0.05 degrees per day. DOY 003 (01/03/2013) 1938-1946z & 2235-2242z SIS-052 8 cmds The SIS instrument team performed a 3 hour test with slightly raised bias voltage to determine leakage currents. The instrument team will analyze the data to determine which Matrix strips can be re-enabled if the bias voltage is set to a new value. Note: The third command in this sequence takes 2:43 minutes to complete. The fourth command was sent before that time and was ignored by the instrument. The fourth command was then resent and successfully processed. ======================================================================== Activities: Data Capture: 100% DOY 358-365 2012 ======================================================================== Anomalies: None ======================================================================== Average Sun Angles With Weekly Attitude Maneuvers Dates Avg Sun Avg SEV Sun-SEV (indicates extra s/c tilt) ----------- ------- ------- ---------------------------------- 10/18-10/23 11.0deg 5.1deg 5.9deg 10/23-10/28 9.3deg 3.3deg 6.0deg 10/28-11/06 7.3deg 2.0deg 5.3deg 11/06-11/13 8.4deg 3.8deg 4.6deg 11/13-11/20 10.3deg 6.3deg 4.0deg 11/20-11/27 12.7deg 8.6deg 4.1deg 11/27-12/04 14.9deg 10.1deg 4.8deg 12/04-12/11 16.0deg 10.9deg 5.1deg 12/11-12/18 16.8deg 11.3deg 5.5deg 12/18-12/27 16.5deg 10.7deg 5.8deg 12/27-01/03 16.0deg 9.1deg 6.9deg There are 3 factors that influence the weekly sun angle. They are: 1) The Sun-Earth-Vehicle angle. The farther the spacecraft is from the Sun-Earth line, the larger the spacecraft sun angle will be. This is the biggest influence on the spacecraft sun angle. 2) Spacecraft position in terms of ecliptic plane. When the spacecraft is in the ecliptic plane, the solar orbit is moving the spacecraft attitude directly toward (or away) from the sun. This lowers the average sun angle (11/13-11/20 in above data). When the spacecraft is above or below the Sun-Earth line (10/23-10/28 in above data), the sun angle can be kept at larger values. 3) Spacecraft motion relative to Earth. When the spacecraft is moving slower than the Earth in the solar orbit (bottom half of the current L1 orbit), then the spacecraft's antenna sweeps past Earth faster and sun angle cannot be pushed as far. When the spacecraft is moving faster than the Earth (top half of the current L1 orbit), then the spacecraft's antenna stays pointed at Earth longer and the sun angle can be pushed further. For the next ~2 months, the spacecraft is in the top half of the orbit and the sun angles can be pushed higher. When estimated sun angles were presented to the science team for the SWEPAM proposal, these factors were averaged together to give an *annual* average sun angle. This information is provided here, just to explain the weekly changes in Sun-SEV (i.e. extra s/c tilt). The following is background information that will be included in each weekly report. The project has accepted the SWEPAM team proposal to keep the spacecraft at larger sun angles with weekly attitude maneuvers. The SWEPAM-Ion instrument has a series of channel electron multipliers (CEMs) and larger sun angles allows more responsive CEMs to measure the solar wind. The maximum sun angle follows the Sun-Earth-Vehicle angle (SEV). The SEV angle is determined by the size/shape of the orbit around L1. When the spacecraft antenna is pointed directly towards earth, the spacecraft's sun angle will be equal to the Sun-Earth-Vehicle angle. With weekly maneuvers, the average sun angle can be kept 4-6deg more than the SEV angle. This results in the spacecraft antenna aspect angle being kept between 5 and 9 degrees and never pointing directly back at earth. For reference, the SWEPAM team prefers sun angles above 13 degrees. With the current size of the L1 orbit, the sun angle will be above 13 degrees for ~45% of the time.