Calibrations are divided into

o Low level calibrations calibrate the response of the front-end electronics to an input signal. Depending on the function of the electronics in the detector, its response is converted into units of energy or time. Responses are commonly described in terms of their linear, non-linear and saturation properties with respect to the input signal. The baseline noise in the system is also taken into account by comparing the responses with and without the input signal. There are two methods to obtain the low level calibrations from an input signal, either by injecting charge into the preamplifiers via a capacitor with a known value or by triggering on pulses from a beam of particles with known energy. To obtain the response without input signal one can simply generate random software triggers. The specific approach to be used will depend on the peculiarities of the hardware units under test. For example, the relation of scintillation signal to energy depositions for heavy ions relative to showers and minimum ionizing particles in the calorimeter will be calibrated on the ground using a variety of primary beams: electromagnetic showers, protons, and heavy ions. When the LAT is on orbit, some calibrations may be performed on board while others will be done using telemetry data processed on the ground.

o High level calibrations of the instrument use the low level calibrations to measure efficiencies for finding signals from the active detector elements and to provide their relative alignment. To this end, one requires identification of particle tracks via the reconstruction software. Silicon strips in the tracker will be aligned with respect to both the LAT and the observatory (star tracker) coordinate systems. We will calibrate the energy deposition along the calorimeter crystals. To understand the effects due to temperature excursions, some high level calibrations (e.g. alignment) will also be performed during environmental tests on the ground. For on-orbit operations, a dedicated data taking calibration mode may be required. Efficiencies may be monitored on board but calibrated with higher accuracy using telemetry data processed on the ground.

An example of calibrations tasks and when they are performed is shown below - NOW UNDER REVIEW

Task			1 tower		4 towers	LAT - 16 towers
Type	ID	Name	EM	FU	CU	SLAC	NRL Environmental	Spacecraft
ACD High Level Calibration	C1	Detection Efficiency				l	l	l
ACD Low Level Calibration	C2	Veto threshold				l	l	l
ACD Low Level Calibration	C3	High Threshold
ACD Low Level Calibration	C4	Pedestals				l	l	l
ACD Low Level Calibration	C5	Electronic Gain and Linearity				l	l	l
TKR High Level Calibration	C6	SSD Alignment		l	l	l
TKR High Level Calibration	C7	Ladder Alignment		l	l	l	l
TKR High Level Calibration	C8	Tray Alignment		l	l	l	l	l
TKR High Level Calibration	C9	Tower Alignment			l	l
TKR Low Level Calibration	C11	Noisy Channels	l	l	l	l	l	l
TKR Low Level Calibration	C12	Dead Channels	l	l	l	l	l	l
TKR Low Level Calibration	C13	Uniformity of Calibration Signal	l	l	l	l	l	l
TKR Low Level Calibration	C14	Threshold Scans	l	l	l	l	l	l
TKR Low Level Calibration	C15	Time-Over-Threshold Signal	l	l	l	l	l	l
TKR Low Level Calibration	C16	Time-Over-Threshold Count Distribution	l	l	l	l	l	l
CAL High Level Calibration	C17	Light Asymmetry	l	l	l	l
CAL High Level Calibration	C18	Light Attenuation	l	l	l	l
CAL High Level Calibration	C19	Light Yield	l	l	l	l
CAL Low Level Calibration	C20	Scintillation Efficiency		l	l
CAL Low Level Calibration	C21	Pedestals	l	l	l	l	l	l
CAL Low Level Calibration	C22	Electronic Gain	l	l	l	l	l	l
CAL Low Level Calibration	C23	Integral non-linearity	l	l	l	l	l	l
CAL Low Level Calibration	C23	Differential non-linearity		l	l	l	l	l
DAQ Low Level Calibration	C25	Deadtime	l	l	l	l	l	l
DAQ Low Level Calibration	C26	Time Accuracy	l	l	l	l	l	l
Type	ID	Name	EM	FU	CU	SLAC	NRL Environmental	Spacecraft
Task			1 tower		4 towers	LAT - 16 towers