203 correction  algorithms  still further. At  some future date,  a rad-hard digital  ASIC  will be obtained with the   algorithms  internally  coded, improving the power consumption, performance, and hardness of  the resulting  systems.  

The system is  designed  to be relatively straightforward to implement  in a customer  design.  The readhead   and code  disk/hub are first mounted,  usually  with pinned  or piloted  interfaces.  Usually  it is not necessary  to  electrically  or optically  center  the code  disk. Then the system  is connected and powered on and four relatively   slow revolutions  of the disk are made while  the encoder  measures the signals.  For limited  angle systems,   such  as Az-El gimbal  systems  often are, four sweeps  of the arc  to be used performs  the same function.  Exact  rotational  speed  and/or  speed stabilit y are not critical for these sweeps.  The preliminary  data is then  processed and the encoder  is calibrated automatically.  The processor  also provides  outputs  that verify  that  the data streams are properly adjusted and merged into  one contiguous data  word.  At this point , the  encoder is  placed in  the normal  operational  mode and no further  calibrat ions will normally  be required.  The  software corrects for  most alignment  issues,  as well as future component  degradation issues  on earth or  in orbit, including degradation of components  with radia tion, temperature shifts and most other  common  issues.  

Figure  3. ARA Space nanoSeries  Remote  3U Electronics Card  

The encoder  can be queried at any time to verify  proper  operational  margins.  The encoder  can even be  recalibrated  on-orbit if required to compensate for unexpected  degradation due to either  natural or strategic  environments.  

The standard customer  interface to the remote card or assembly  is by means  of a synchronous serial   protocol  in which  the customer  supplies  a strobe  to initiate  a reading and a data clock  to shift data out. The   primary  reason for this non-standard protocol,  instead of the more common Synchronous  Serial Interface   with no strobe,  is that many  servo  systems  are sensitive  to the delay  from the time position  data is actually   captur ed to time of receipt  of data.  A second consideration is that the LED primarily  degrades  only when   current  is flowing  through it, so reducing the strobe rate to a minimum,  consistent  with system  objectives,  is a  very good design  practice.  The standard  design  also provides  a shift clock  from the encoder  at the same  frequency  as the clock provided by the customer.  In syste ms with long cable  runs or other  delays,  this  provision  assures robust capture of  the serial data by the customer  electronics, even with lengthy cable  runs.    There are many  advantages   to the BEI ARA nanoSeries  approach.  First is that since  most  encoders will  be very  similar  in basic  design there  is a lot of commonality  of design and analysis,  minimizing the  program -specific   engineering work  required.  Second,  this design has a minimal number  of integrated circuit s, none  of which  are  likely  to become obsolete.  Third  is that critical parts  like LEDs,  photodiode arrays, and ASICs  are stocked at   BEI to minimize lead time.