The Focal Plane Assemblies (FPAs) design includes 36 distinct
spectral bands broken into four separate FPAs: Visible (VIS),
Near Infrared (NIR), Short- and Mid-Wave Infrared (SWIR/MWIR),
and Long-Wave Infrared (LWIR). Each FPA focuses light onto
a certain section of detector pixels, which are relatively
large – ranging from 135µm to 540 µm square.
The large number and variety of detector pixels are what make
the wide variety of MODIS data possible. When light hits a
detector pixel, it will generate a distinct signal depending
on the type of light it is sensitive to. The signals that
the pixels generate are what scientists process and study
to learn about Earth’s land surfaces, water surfaces,
and atmosphere.
Technical Description
Two custom PIN photo-voltaic (PV) silicon hybrids cover the
VIS and NIR FPAs (0.4 µm to 1.0 µm), an HgCdTe
PV detector hybrid is used on the SWIR/MWIR FPA (1.2 µm
to 4.5 µm) (Fig. 1), and another on the LWIR FPA (bands
up to 10 µm) (Fig. 2, 3, 4). The LWIR FPA also includes
a six-band photo-conductive (PC) detector for the wavelengths
beyond 10 µm. The detectors are grouped into two ambient
focal planes for SWIR/MWIR and LWIR.
Each detector pixel contains an array of indium bumps to
increase the interconnection reliability and to supply mechanical
support. The MODIS viewing path is 10 km along track at nadir,
and 25 km along track at ±55 degrees. This means that
there are 10 detector elements along track for each of the
1 km bands, 20 for each of the 500-m bands, and 40 for the
250-m bands. Two of MODIS’ bands – 13 and 14 –
each have dual arrays of 10 elements along track. The outputs
of these arrays are summed in the scan direction, which is
called Time Delay Integration, or TDI. The detectors are mated
to readout integrated circuits (ROICs), which provide signal
preamplification. The signals are then multiplexed and sent
off-chip via 1 to 3 output lines per ROIC. Readout circuit
design features include redundant bias, auto clock, shift
registers, and an output amplifier to improve reliability
and minimize single-point failures. Capacitive transimpedance
amplifier (CTIA) readout-unit cell preamplifiers provide customized
gains for each of the multiple bands within a single readout.
Three of the FPA configurations – VIS (Fig. 5), NIR
(Fig. 6), and SWIR/MWIR – support multiple frame rates
on the same readout. Another readout design feature is an
input calibration circuit in which controlled amounts of charge
are injected into the input node.
Each FPA has a high degree of commonality in the packaging
design, which allows for common input-output interfaces between
the FPAs and the system electronics. To facilitate the packaging
design, a common PV hybrid is used across the VIS through
LWIR FPAs, which necessitates the use of silicon PIN hybrids
for the VIS and NIR FPAs (rather than a monolithic design).
Each hybrid is mounted on an identical motherboard and connects
to the system electronics with an identical cable. This significantly
reduces the complexity of downstream system warm electronics
and allows for packaging to build up generic motherboard/cable
assemblies that can be used for any of the four FPAs. A common
filter bezel to which the filter mask and the filters are
mounted further simplifies the design and manufacturing.
