Introduction The purpose of the Pierre Auger Observatory is to study high energy cosmic rays from space. Cosmic rays are charged particles that interact high in the Earth's atmosphere creating secondary particles. These secondary particles interact in the atmosphere and create even more particles which then interact, and so on. This cascade of particles is known as a cosmic ray shower. Due to the high occurrence of low-energy cosmic rays, they have become relatively well understood in the 80 years since the discovery of cosmic rays. Cosmic rays at higher energies are much rarer. Cosmic ray showers with energies greater than 1019 eV only strike the Earth with a rate of 1 per square kilometer per centry. Cosmic rays with these high energies are a big mystery. We don't know what creates them or how they could be created. Scientists in the Pierre Auger Observatory Collaboration study these cosmic rays by closely analyzing the parameters of the shower that impacts the Earth's surface. We hope that by studying these very high energy cosmic rays, we can find evidence about their origins.
The Pierre Auger Observatory will consist of two giant detector arrays, each covering 3000 square kilometers. One will be constructed in the Northern Hemisphere, and one is already being built in the Southern Hemisphere. Each will consist of 1600 water Cherenkov particle detectors ("surface detector stations") in a triangular array with a 1.5 km detector spacing. Arround the surface detector array will be 4 atmospheric fluorescence detectors that detect light from cosmic ray shower particles ionizing nitrogen gas in the atmosphere. See Figure 1 below for a map of the Southern Hemisphere Pierre Auger Observatory. The objective of the Pierre Auger Observatory is to measure the arrival direction, energy, and mass composition of cosmic ray air showers above 1019 eV. 
Figure 1: The Pierre Auger Observatory Southern Hemisphere Site near Malargüe, Argentina. The surface detector stations are shown in red. The flourescence detectors are labeled in yellow with radial range lines drawn in green. Image courtesy of The Pierre Auger Observatory international site.
Auger at Case
In order to determine the energy and arrival direction of cosmic ray showers, scientists reconstruct the shower from data collected by the detectors. Reconstruction from surface detector data relies heavily on the ability to distinguish the order in which the stations were triggered by an atmospheric shower. To do this, the surface detector stations must be able to accurately determine the timing of each event. Global Positioning System (GPS) receivers are installed on each of the surface detector stations for this purpose. The arrival time of a particle is measured by a GPS receiver relative to a reference receiver. Each GPS receiver has some offset in nanoseconds from the reference receiver. For accurate shower reconstruction, the GPS receivers must have a stable timing offset of less than 25 nanoseconds. At Case Western Reserve University, we have calibrated the offsets of the GPS receivers sent to the Southern site in Argentina. We have also done advanced GPS quality assurance tests to ensure that the GPS receivers installed in Argentina are stable and reliable.
We have also tested GPS antennas to make sure they receive satellite signals before they are shipped to Argentina. Currently, we are testing micro tank power control boards (mTPCBs) and shipping them to Argentina.
In addition to hardware testing, we have also done basic surface detector reconstructions to check the stability of offsets and the effect of offsets on reconstruction. This approach starts with the data and works back to check if the results comply with our measured offsets. So far we have been using relatively unsophisticated plane fitting methods, but we are beginning to get into curved shower reconstruction and Monte Carlo simulation of cosmic ray showers. The purpose of this is to check if we can spot possible problem GPS receivers in the field through data analysis, and to check that offsets seem stable after installation in the field.
GPS Calibration
GPS calibration is performed by finding the time difference, in nanoseconds, of a GPS receiver's one second pulse with a one second pulse from a reference reciever. This difference is averaged over a long period of time to determine both the average deviation from the pulse, known as the calibration offset for each GPS reciever, and the variance in the difference. These numbers, along with other tests, help to determine if a reciever is fit to be sent to Argentina to be used in the experiment. Figure 2, below, illustrates the distribution of room temperature offsets for the 1958 tested receivers.
Figure 2: Histogram of rack offsets from 1958 GPS receivers
The receivers are tested in two different settings: at room temperature, and in an oven where temperature is varied from +70 degrees Celsius to -10 degrees Celsius in a plan laid out by the environmental stress screening and burn-in procedure. The oven test runs for 24 hours to ensure the GPS receivers are operational under continuous daily thermal cycling in the field environment and to determine the variance of the calibration offset due to thermal variation in the receivers. During the burn-in procedure GPS units are tested against thermal failure.
We have tested 1958 GPS receivers, and only 88 have failed due to the following reasons: offset larger than 25 nanoseconds, offset fluctuation of greater than 1 standard deviation, defective hardware, poor communication with the reference receiver, and difference in the offsets for rack and oven testing of greater than 1 standard deviation. Antennas are tested outside to make sure they make contact with satellites. We have tested and shipped 1700 GPS receivers and antennas to Argentina.
We are working on a database that will make detailed GPS testing and calibration information available to Pierre Auger Observatory collaboration members.
Micro Tank Power Control Board (mTPCB) Testing
We test mTPCBs in a similar fashion to GPS receivers. We spray them with humiseal to protect them from humidity, then test them at room temperature to make sure they work, and then we oven test them for 24 hours with temperature cycling from +70 Celsius to -10 Celsius. As of June 22, 2005, we have shipped 300 mTPCBs to Argentina, and have 87 more waiting to be shipped. We will receive another 800 mTPCBs this summer which we will test and ship.
Reconstruction
We have used simple plane-fit reconstructions to reconstruct surface detector data from cosmic ray showers in order to ensure that GPS timing is well understood within the detector array. See Figure 3 for an example of a plane fit.
Figure 3: This is a plot of data from event number 01008777 which occurred in September of 2004. Stars represent surface detector stations plotted with respect to their Easting and Northing positions in meters on x and y axes, and their particle detection time on the z axis. The pink star is the surface detector station with the strongest signal, red stars are surface detector stations that were used to fit the plane, and blue stars are surface detector stations that recorded a signal but were not part of the fit because they failed our event selection criteria.
We have been using timing residuals (the perpendicular distance of each detector from the plane fit in nanoseconds) to determine if GPS offsets are significant in shower reconstruction, if they are stable over time, and if they are being applied correctly to the data. Plane fits are not very accurate, so we are moving into curved reconstructions and monte carlo simulations of showers to further examine the impact of GPS offsets.
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