IRAC Instrument Handbook - IRSA - California Institute of Technology

Figure 5.3: Diagram of the wrapping of negative values due to truncation of the sign bit.
5.1.6 Wraparound Correction: IRACWRAPDET AND IRACWRAPCORR
IRAC Instrument Handbook
IRAC suffers from two kinds of “wraparound" errors, wherein DN values are actually multi-valued. That
is, a given DN actually corresponds to more than one possible flux level.
IRACWRAPDET (sign truncation wraparound)
As a means of data compression, IRAC discards the sign bit of its data before transmission to the ground.
This creates an ambiguity in that negative numbers appear in the raw data as very large positive numbers.
However, by design the detector reaches physical saturation before “electronic” (A/D) saturation (Figure
5.3). That is, the maximum physical values the detectors ever have are around 45,000 DN for the InSb
arrays and 60,000 DN for the Si:As arrays, which are less than the maximum 16-bit value of 65,535.
IRAC uses the 2s-complement storage system for negative numbers. In this system negative numbers are
denoted by setting the sign bit and then complementing (i.e., flipping) all the remaining bits. For example,
in 2s-complement storage, −1 is represented by 65534 in unsigned integer form. Therefore, values higher
than the maximum saturation levels must be “wrapped" negative numbers. For each array a set of
maximum values has been chosen such that no pixel will be erroneously identified as wrapped. The
module then flags any pixels lying in the “wraparound" DN region. Observers are strongly cautioned to
check for possible saturation problems by examining the structure in their data. If a user finds that any
Pipeline Processing 73 Level 1 (BCD) Pipeline