PDS_VERSION_ID       = PDS3
LABEL_REVISION_NOTE  = "2022-01-01"
RECORD_TYPE          = STREAM

OBJECT               = INSTRUMENT
  INSTRUMENT_HOST_ID = JNO
  INSTRUMENT_ID      = SRU

  OBJECT             = INSTRUMENT_INFORMATION
    INSTRUMENT_NAME  = "STELLAR REFERENCE UNIT"
    INSTRUMENT_TYPE  = "CAMERA"
    INSTRUMENT_DESC  = 
"


Instrument Overview
===================

The Juno Stellar Reference Unit (SRU) is a custom built star sensor 
manufactured by Selex Galileo (now Leonardo Finmeccanica) in Florence, Italy. 
The SRU provides inertial attitude information to Juno's attitude 
determination software by producing vector measurements of star-like objects 
observed within its field of view (St. Pierre et al., 2012). The SRU 
optical heads are mounted on the forward deck of the Juno spacecraft. Only 
one SRU is operated at any given time. SRU-1 is the nominal operational unit, 
and SRU-2 provides cold spare redundancy. The SRU may also be operated in a
snapshot mode which allows collection of full or partial images by the 
silicon charge coupled device (CCD) focal plane array.

Juno's SRU Team operates the SRU as a broadband visible (450-1100 nm) 
science imager for the purpose of studying low-light features and phenomena 
of the Jovian system, such as Jupiter's faint dust ring and auroral 
emissions. SRU images of lightning on Jupiter's dark side (Becker et al., 
2020) support Juno's study of Jupiter's atmospheric dynamics and 
composition, and images of Jupiter's moons under low illumination 
conditions support Juno's satellite science.

The Radiation Monitoring Investigation (RMI) of the Juno mission was designed 
to characterize the external Jovian radiation environment using the 
Stellar Reference Unit (SRU) and other instruments on the spacecraft (Becker 
et al., 2017). Juno's RMI (a component of the SRU Team) measures noise from
penetrating electrons within SRU images collected at Jupiter specifically 
for this purpose. Noise signals are created by ionization events within the 
active regions of the SRU focal plane array pixels as penetrating charged 
particles pass through the material. The objective is to characterize 
Jupiter's >10 MeV electron environment within regions of the jovian 
magnetosphere where little to no in situ high energy electron data have 
previously been collected.


Detailed instrument description
===============================

The Juno Stellar Reference Unit (SRU) is a low-light, broadband visible 
(450-1100 nm) imager with no filters. The camera has a 29.924 mm focal 
length, spatial resolution of 0.57 mrad per pixel, and a 16.4 deg (square) 
field of view. The collecting area of the optics is 4.155 cm2. The SRU 
detector is a frame transfer silicon CCD with a 512x512 pixel useful 
imaging region, a 512x512 pixel light shielded storage region, 17 micron 
pixel pitch, and a 200,000 electron full well capacity (Becker et al., 
2005). The camera gain is 15.47 signal electrons per output analog-to-digital 
data number (DN). The throughput of the optical system, QT (the CCD 
quantum efficiency (Q) multiplied by the optics transmission (T)), is 
provided as a function of wavelength in Appendix C of the Software Interface 
Specification (SIS). 

Because the SRU must perform the star measurement function within the 
extremely harsh radiation environment of Jupiter's magnetosphere its optical 
head is very heavily shielded in order to reduce noise from penetrating 
ionizing radiation. The CCD registers impacts by penetrating charged particles 
as elevated noise signals within a cluster of pixels local to each 'hit.' The 
noise signal electrons are created by ionization events within the active 
depletion and diffusion regions of the pixels as charged particles pass 
through the silicon. Given the heavy shielding of the SRU optical head, only 
external electrons >1 MeV are energetic enough to generate noise in the CCD 
with significant probability, either as an impacting primary electron of 
reduced energy, or by virtue of a secondary particle generated during the 
transit into the optical head materials. Although there is a probability for 
external 1 to 10 MeV electrons to generate noise in the SRU CCD, penetrating 
>10-MeV electrons should be the dominant contributors to CCD noise events in 
the Juno SRU at Jupiter. This is primarily due to the spectral hardness of the 
Jovian electron distribution, such that penetrating >10-MeV electrons will be 
the dominant contributors to CCD noise events (Becker et al., 2017). 

As Juno is a spin-stabilized spacecraft, the SRU CCD is nominally operated 
using time delay integration (TDI) to compensate for the motion of the ~two 
revolutions per minute (RPM) spacecraft spin. Without TDI, the spacecraft spin 
causes the scene to move along the CCD column direction. When operated using 
TDI, the rows of the image are shifted during the exposure time at an angular 
rate consistent with the spacecraft spin. A typical spin rate of 2 RPM (12 
degrees per second) results in a 2.7 ms integration dwell time between shifts. 
This integration time is calculated by the SRU based on the angular rate 
information provided by the spacecraft. During TDI exposure, the first rows of 
the image are shifted into the optically shielded storage region and are no 
longer exposed to light; however, the pixels in the storage region continue to 
be exposed to impacts from penetrating radiation. The number of TDI shifts is 
a function of the desired exposure time. Post-exposure, the transfer of the 
image to the storage region occurs over a period of (526 - Number of TDI 
shifts during exposure) x 2 usec. Pixels are then read out at a rate of 1 us 
per pixel; the time to read out a full row of the CCD image is approximately 
0.51 ms. The fact that the CCD is read out in a row-wise fashion following 
transfer into the storage region creates an effective range of exposures to 
penetrators on a single frame which adds to the measurement capability.  For 
example, this range is 1:25 for 10 ms exposures given the ~250 ms readout 
time for a full frame image. 

When operated in a 'no-TDI' mode, the pixels in the image region are exposed 
to photons without undergoing any row shifts during the exposure time. This 
causes the scene to smear along the CCD column direction at a rate equal to 
the spacecraft angular spin rate. Frame transfer and readout occurs in the 
same fashion as with TDI images. 

The measurement dynamic range of the SRU's particle detection algorithm was 
determined to be 200 - 10 million counts cm^-2 CCD s^-1. Measured count rates 
agreed with simulated expectations to within 20%. See Becker et al. (2017) for 
further details. 

See the Software Interface Specification (SIS) Stellar Reference Unit 
Standard Data Products (Daubar et al. 2022) for more information.

"
  END_OBJECT = INSTRUMENT_INFORMATION

  OBJECT             = INSTRUMENT_REFERENCE_INFO
    REFERENCE_KEY_ID = "BECKERETAL2005"
  END_OBJECT         = INSTRUMENT_REFERENCE_INFO

  OBJECT             = INSTRUMENT_REFERENCE_INFO
    REFERENCE_KEY_ID = "BECKERETAL2017"
  END_OBJECT         = INSTRUMENT_REFERENCE_INFO

  OBJECT             = INSTRUMENT_REFERENCE_INFO
    REFERENCE_KEY_ID = "DAUBARETAL2022"
  END_OBJECT         = INSTRUMENT_REFERENCE_INFO

  OBJECT             = INSTRUMENT_REFERENCE_INFO
    REFERENCE_KEY_ID = "STPIERREETAL2012"
  END_OBJECT         = INSTRUMENT_REFERENCE_INFO

END_OBJECT = INSTRUMENT

END