Projecting SAA-Free Times

Below is a periodically updated (every week or so) list of the projected start and end times of the SAA-free periods of the HST orbit as determined from the SOGS scheduling software for each SAA model.


Projected SAA-Free Start/End Times for each SAA-Model from Current Extrapolated Orbit File:
SAA Model 02 (FGS guiding) SAA Model 05 (FGS exposures) SAA Model 23 (NICMOS)
SAA Model 24 (STIS/CCD) SAA Model 25 (STIS/MAMA)
SAA Model 27 (ACS/SBC) SAA Model 28 (ACS/CCD)
SAA Model 29 (WFC3/UVIS) SAA Model 30 (WFC3/IR)
SAA Model 31 (COS NUV/MAMA) SAA Model 32 (COS FUV/XDL)


The 'flip' side of this would be the times when the telescope is in the SAA-impacted orbits. The table below gives the times when the telescope is projected to first enter the SAA and then the time which it last exits the SAA.


Projected Contiguous SAA-impacted Start/End Times for each SAA-Model from Current Extrapolated Orbit File:
SAA Model 02 (FGS guiding) SAA Model 05 (FGS exposures) SAA Model 23 (NICMOS)
SAA Model 24 (STIS/CCD) SAA Model 25 (STIS/MAMA)
SAA Model 27 (ACS/SBC) SAA Model 28 (ACS/CCD)
SAA Model 29 (WFC3/UVIS) SAA Model 30 (WFC3/IR)
SAA Model 31 (COS NUV/MAMA) SAA Model 32 (COS FUV/XDL)

This data should be understood in the proper context. Changes to the ephemeris occur, and estimates of the SAA crossings become increasingly uncertain the further into the future the projection is made. Typically, the uncertainty is a function of the error in the nodal regression rate, with added uncertainty due to HST's in-track position. These two uncertainties can be considered in their effect independently (although, in fact, they are not and they are both manifestations of precession error):

The in-track uncertainty can directly be thought of as influenced by the variable rate of atmospheric drag on the telescope. The graph below shows the relationship between constant amounts of error in the drag-rate, in-track error, and the duration between when the ephemeris was generated and when the projection is to be made for.

Note the log-log plot above expresses something fundamental about the in-track uncertainty: in-track error grows quadratically in time. If the uncertainty is 'x' at some time 'T', then at time '2*T', the uncertainty will be 4 times larger.

The HST extrapolated orbit file contains (currently) no altitude decay rate, so the in-track error for the times corresponds directly with the contours in the plot. For the 10-week orbit file, the events do have a decay rate applied, so the contours in the plot would correspond to the error in the decay rate used to fit and produce the orbit file.

The decay rate is variable and not predictable beyond approximate levels. The decay rate is a function of the ballistic profile of HST (which changes each orbit) and the level of solar activity and its history. Generally, decay rates are highest during solar-maximum, and least during solar minimum. The decay rate uncertainty is a function of the projection time interval. In the plot below, the altitude decay rate history of HST is plotted from launch to the current time-frame with 49-day and 1-year rolling averages. To get an estimate of the solar activity level in some future year, subtract the approximate duration of the solar cycle (11.4 years) and compare with the plot. However, each solar cycle is different in intensity and duration (and even period), so such projections are only approximate.