522 
meters square). Sources of geometric errors in the 
instrument, spacecraft and ground processing system that can 
be ignored when correcting MSS have to be considered when 
correcting TM. "The design of the TM itself creates a 
complete new set of error sources not present (or. not 
requiring correction) with MSS. These include: 
Scan mirror nonlinearity 
Scan line corrector nonlinearity 
Scan repeatability 
Scan gap 
Timing and channel delays 
Band to band alignment 
Bi-directional scan alignment. 
The problem of geometrically correcting TM data is 
further compounded by structural disturbances (jitter) in the 
form of low amplitude motion caused by mechanical resonances 
produced by the interaction of the TM, MSS and High Gain 
Antenna. The effect is significant and requires correction 
on a scan basis. Identification of this effect had a major 
impact on the program since it required the mounting of a 
3-axis Attitude Displacement Sensor (ADS) on the TM 
instrument, the additional use of the 32 Kbps link to 
transmit ADS data to the ground and a partial redesign of the 
telemetry handling and image data processing systems. 
The second major performance requirement, that of 
correcting the radiometry to within + 1 quantum level, has 
not been difficult to meet, even though TM is quantized to 8 
bits and MSS to only 6 bits. Methodology for calibrating and 
correcting TM radiometry is essentially identical to 
established MSS techniques. TM has all solid state detectors 
with significantly improved signal-to-noise performance 
compared to the less stable photomultiplier tubes in three of 
the MSS bands. Post Yaunch analysis of TM data nas, in+fact, 
led the Landsat Project Science Office to characterize the 
instrument as the most sensitive and best calibrated 
radiometer ever flown for land remote sensing. Post launch 
analysis of Landsat-4 and 5 data has revealed radiometric 
effects in TM not present in MSS that contribute to minor 
systematic radiometric variability. These effects are being 
analyzed and are fundamentally understood. Recommendations 
are being formulated for implementation of calibration and 
correction methodologies addressing this variability. 
Clearly, the design approach for the TM Image 
Processing System (TIPS) was strongly influenced by the 
stringent geometric and, to a lesser degree, radiometric 
performance requirements. There were, however, other 
principal contributors that are briefly summarized below. 
Collectively they defined the need for new processes, special 
purpose hardware, new correction techniques, flexibility, and 
new processing control and data management techniques. 
Funding and schedule constraints demanded use of available 
technology.