Chip-Scale Atomic Clock [4].

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... dependency on the ground station to make the navigation measurements, especially in applications like servicing groups of small satellites on a lunar mission. The Colorado Nanosat Atomic Clock Testbed (CONTACT) graduate project team at the University of Colorado Boulder is developing an experiment to fly a Microchip (formerly Microsemi) CSAC ( Fig. 1) onboard MAXWELL to quantify its performance on a representative CubeSat platform. During this experiment, the onboard NovAtel OEM729 GPS receiver calculations are performed using this CSAC as an external reference input. The CSAC experiment is conducted for a minimum of five days, recording position, clock bias, pseudorange, phase, ...

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... the GPS receiver and the MAXWELL ADCS is crucial to the ADCS operations and the CSAC experiment. Fig. 4 shows a block diagram of where the CSAC and GPS receiver fit into the MAXWELL system. The MAXWELL team was able to fabricate an ADCS board that hosts the GPS receiver unit. The MAXWELL cardstack, with the ADCS and GPS receiver is shown in Fig. 10. The successful hardware mounting was followed by establishing the UART communication between the ADCS and the GPS receiver, by observing the communication over serial channels. The ADCS-GPS setup is tested by using the ADCS to request information from the receiver's communications port using commands from the NovAtel documentation ...

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... profile over which the GPS receiver-CSAC hardware will be tested. Since MAXWELL is in a LEO with a period of about 90 minutes, an initial test profile was generated with linearly changing temperature. To allow for understanding of the behavior at temperature extremes, dwells were added at 0 • C and 40 • C. A plot of this profile can be seen in Fig. 11. Although this linearly changing profile provides a good first step, it is not a perfect representation of temperature profiles expected in LEO. This is because the day and eclipse phases of the satellite will not be changing in a linear manner, and will depend on the orbit specifics, perturbations, drift in orbit over time and several ...

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... with the satellite and the antenna, the GPS constellation is also added to the model to allow for a coverage analysis. Fig. 12 shows a snapshot of this simulation, where the simulated MAXWELL orbit is seen in a green color, very close to Earth. The image also shows the GPS constellation in a side view and a top-down view. After adding all the required components to the STK simulation, the GPS constellation can be assigned to the simulated MAXWELL satellite to ...

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... . 13 shows the visibility of the zenith-pointing and the Sun-pointing modes, both using an 80 degree half-angle antenna FOV. The STK Coverage tool provides us with the GPS satellite visibility counts and DOP calculations for each scenario. The DOP information for zenith-pointing and Sun-pointing modes (80 degree half-angle FOV) over a 24 ...

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... the zenith-pointing and the Sun-pointing modes, both using an 80 degree half-angle antenna FOV. The STK Coverage tool provides us with the GPS satellite visibility counts and DOP calculations for each scenario. The DOP information for zenith-pointing and Sun-pointing modes (80 degree half-angle FOV) over a 24 hour period show dramatic differences (Fig. 14). ...

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... expected, with a wide FOV zenith-pointing antenna, the simulation presents continuous solutions and low TDOP magnitudes (below 2) (Fig. 14). Whereas in the Sun-pointing mode, the DOP values are much higher and show periods of discontinuity because of insufficient GPS satellite visibility. These results confirm that a zenithpointing configuration is preferable for the experiment. We also used these results to approximate the estimated bias to see the ADEV in the different ...

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... was used to develop a comparison with the experimental results that were recorded with the rooftop GPS antenna drop (using internal clock, Rubidium and CSAC driving the GPS receiver -Subsection IV.A). This method of calculating the bias was used with the antenna placed on the satellite in different pointing configurations. As defined in Fig. 14, the satellite-mounted configurations included a Sun-pointing antenna and a zenith-pointing antenna. These configurations were simulated with 80 degree and 110 degree half angle cone FOVs, to observe any significant differences in the data collected using the different FOVs. A ground station antenna, located in Boulder, was also ...

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... calculate a more realistic error approximation for the bias calculations, rather than using a one meter error. The parameters used for this calculation would be errors associated with being in space, ionospheric errors, tracking errors, multipath errors, etc. This would provide us with a more accurate predictive ADEV plot than what is seen in Fig. ...