HMI Top-level S/C Interface Requirements Summary


1.      Data Continuity and Completeness


·        The HMI data completeness and continuity requirement is to capture 99.99% of the HMI data 95% of the time. In other words, we need very good data integrity over the 90 second observing periods for the Dopplergram and magnetogram observables. We also need these observables for 95% of all observing time in order to meet all the HMI Science objectives.


2.      Spacecraft Pointing and Stability


·        The HMI reference boresight will be maintained to within 200 arc-seconds of sun center through a combination of on-ground alignment, optical bench stability and spacecraft pointing control.


·        The HMI roll knowledge about the X axis needs to be better than 100 arc-sec.  This can be obtained by analysis and correlative data so long as the SDO drift is small and slow, i.e. < 50 arc-sec over the mission life. The nominal roll shall be to maintain a fixed angle with respect to the projection of solar north onto the plane of the sky.


·        The spacecraft will maintain drift of the spacecraft reference boresight relative to the HMI reference boresight to within 14 arc-seconds in the Y and Z axes over a period not less than one week.


·        The spacecraft jitter at the HMI mounting interface to the optical bench shall be less than 5 arc-seconds (3 sigma) over frequencies of 0.02 Hz to 50 Hz in the X, Y and Z axes.


3.      Reference Time


·        The spacecraft on-board time is required to be accurate to 100 ms absolute with respect to ground time, with a goal of being accurate to 10 ms absolute with respect to ground time. 


·        The spacecraft clock provides an absolute reference and long term accuracy. The basic reference clock controlling the HMI observing cycle must maintain a stability of 10^-6 (1 part per million). An internal HMI reference oscillator is planned in order to guarantee our timing requirements.  The HMI clock must be in sync with the spacecraft clock to allow instrument time to be within 100ms of correct absolute time at all times.


4.      Calibration Maneuvers


·        Spacecraft off-point and roll maneuvers are required for HMI calibration and primary science measurements.


·        Off-points are used to determine the instrument flat-field, and require 15 to 20 off-point positions from solar disk center. The expected dwell time at each off-point position is approximately 5 minutes, and maximum offpoint of 12 arc-minutes from disk center.


·        Roll maneuvers are essential to determining the solar shape, requires observations at evenly spaced roll angles over a 360 degree roll. The preferred maneuver has 15 minute dwells every 22.5 degrees (16 steps).


5.      Orbit Knowledge


·        The only orbit information that is critical for HMI calibrations is the spacecraft velocity relative to the Sun. A velocity accuracy of 1 cm/sec for all HMI science purposes.


6.      Thermal Environment


·        HMI needs to cool its CCD sensors to c. -70C with a goal of -90C.  The sink temperature must allow this with an achievable sized radiator.  The Camera Electronics Box and Electronics Box must also use radiative or conductive cooling to maintain nominal operating temperatures.  The HMI optics box will be maintained at a constant temperature and needs to be thermally isolated from the spacecraft.


7.      Field of View


·        HMI needs an unobstructed Sun directed field of view of XXX degree-full angle cone from all points on the observing aperture.


     8.   Mass: (estimates as of 12 March 2003)


Optics Box

< 40 kg

Electronics Box

< 14 kg

Inter-box cable

< 3 kg  (<2 m)


     9.  Power:  (estimates as of 12 March 2003)


Nominal operations:

107.3 w

Peak power

XX w

Eclipse power

XX w

CCD bakeout

22 w

Keep-alive ?

xx w


    10. Telemetry:


Science Data

55 Mbits/sec in 2 redundant channels.


2.5 kbits/sec

Command uplink

2.6 kbits/sec (max)

Diagnostic Data

10 kbits/sec


     11. Mounting Envelope (estimates as of 12 March 2003)


Optics Box

111.4 x 69 x 8.5

Electronics Box

X,Y,Z cm