Uplink Array Research Articles

Analytical expressions for the evaluation of gravity distortion impact on the antenna efficiency applied to large antenna uplink arrays

We present analytical expressions for the evaluation of the antenna efficiency loss due to the gravity distortion on large reflector antennas from 6 to 35 m diameter at X and Ka bands with a lack of usual complexity. These analytical expressions can be used to evaluate the gain reduction versus the maximum deviation of the rim of the reflector due to the gravity distortion extending the model proposed by JPL to other antenna diameter in the range of 6 to 35 m. These expressions were compared with the empirical model for gravity evaluation presented by the Jet Propulsion Laboratory for the DSN antennas obtaining a good match between both models. This is useful to specify the maximum distortion due to the gravity for antenna system design level. For this, a five steps procedure is presented starting with the estimation of the antenna efficiency reduction based on the analytical expressions as a function of the maximum gravity distortion, the antenna size and frequency band and concluding with the estimation of the gain of a single antenna as a function of the elevation angle. The case study is focused on error analyses for large antenna uplink arrays at X and Ka band.

Tropospheric delay statistics measured by two site test interferometers at Goldstone, California

AbstractSite test interferometers (STIs) have been deployed at two locations within the NASA Deep Space Network tracking complex in Goldstone, California. An STI measures the difference of atmospheric delay fluctuations over a distance comparable to the separations of microwave antennas that could be combined as phased arrays for communication and navigation. The purpose of the Goldstone STIs is to assess the suitability of Goldstone as an uplink array site and to statistically characterize atmosphere‐induced phase delay fluctuations for application to future arrays. Each instrument consists of two ~1 m diameter antennas and associated electronics separated by ~200 m. The antennas continuously observe signals emitted by geostationary satellites and produce measurements of the phase difference between the received signals. The two locations at Goldstone are separated by 12.5 km and differ in elevation by 119 m. We find that their delay fluctuations are statistically similar but do not appear as shifted versions of each other, suggesting that the length scale for evolution of the turbulence pattern is shorter than the separation between instruments. We also find that the fluctuations are slightly weaker at the higher altitude site.

Uplink Arraying for Solar System Radar and Radio Science

To improve the uplink capability of the Deep Space Network at X-band frequencies near 7.2 GHz and to potentially prepare for retirement of the aging 70-m antennas, the National Aeronautics and Space Administration (NASA) has sponsored two experimental campaigns at the Jet Propulsion Laboratory (JPL, Pasadena, CA) and one at Harris Corporation (Palm Bay, FL) to show the feasibility of uplink arraying for deep space communication. These three efforts made significant progress in demonstrating the uplink arraying concept and in advancing our understanding of the associated error budget. These efforts focused primarily on demonstrating the feasibility of uplink arraying for communications applications. Uplink arraying is also useful for applications other than routine communications, mainly solar system radar and radio science. Among topics investigated are features and characteristics such as the array bandwidth, atmospheric calibration, array phase and amplitude stability, array blind pointing, and array delay calibration. No insurmountable obstacles are identified for the application of uplink arraying to noncommunications services. However, additional studies are recommended to minimize risk to radar and radio science services.

Uplink array concept demonstration with the EPOXI spacecraft

Uplink array technology is currently being developed for NASA's Deep Space Network (DSN), to provide greater range and data throughput for future NASA missions, including manned missions to Mars and exploratory missions to the outer planets, the Kuiper belt, and beyond. The DSN uplink arrays employ N microwave antennas transmitting at X-band to produce signals that add coherently at the spacecraft, thereby providing a power gain of N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over a single antenna. This gain can be traded off directly for N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> higher data rate at a given distance such as Mars, providing, for example, HD quality video broadcast from earth to a future manned mission, or it can provide a given data-rate for commands and software uploads at a distance N times greater than possible with a single antenna. The uplink arraying concept has been recently demonstrated using the three operations 34-meter antennas of the Apollo complex at Goldstone, CA, which transmitted arrayed signals to the EPOXI spacecraft. Both two-element and three-element uplink arrays were configured, and the theoretical array gains of 6 dB and 9.5 dB, respectively, were demonstrated experimentally. This required initial phasing of the array elements, the generation of accurate frequency predicts to maintain phase from each antenna despite relative velocity components due to earth-rotation and spacecraft trajectory, and monitoring of the ground system phase for possible drifts caused by thermal effects over the 16 km fiber-optic signal distribution network. This provides a description of the equipment and techniques used to demonstrate the uplink arraying concept in a relevant operational environment. Data collected from the EPOXI spacecraft was analyzed to verify array calibration, array gain, and system stability over the entire five hour duration of this experiment.

Uplink Arrays for the Deep Space Network

Deep-space communication and navigation is faced with two challenges in the future: (1) the potential retirement of the largest antennas of NASA's Deep Space Network and (2) an anticipated need for increasing ground system capacity so as to support higher data rates to and from missions operating at remote locations in the solar system, as well as in anticipation of a larger number of simultaneously flying missions. In the transmitting, or uplink, direction, one approach to increasing the effective transmitted power is to array multiple antennas. This is attractive mainly because it promises a lower construction cost than equivalent (large) single antenna systems. In addition, it has the potential for increasing the reliability of the uplink and reducing maintenance costs. This paper introduces the concept of uplink arraying by examining technological challenges and possible solutions to them. Arraying principles are presented and error sources described. The main challenge is to maintain carrier phase alignment among the antennas, and this must be done by periodic calibration. Presently, two calibration methods are being developed at the Jet Propulsion Laboratory as part of an uplink arraying demonstration effort. These methods are briefly discussed.

Phased array of large reflectors for deep-space communication

In this paper the problem of uplink array calibration for deep-space communication is considered. A phased array of many modest-size reflectors antennas is used to drastically improve the uplink effective isotropic radiated power of a ground station. A radar calibration procedure for the array phase distribution is presented using a number of in-orbit targets. Design of optimal orbit and the number of calibration targets is investigated for providing frequent calibration opportunities needed for compensating array elements phase center movements as the array tracks a spacecraft. Array far-field focusing based on the near-filed in-orbit (low Earth orbit (LEO)) calibration targets is also presented and array gain degradation analysis based on the position error of the array elements and in-orbit targets has been carried out. It is shown that errors in the in-orbit targets positions significantly degrade the far-field array gain while the errors in array elements positions are not very important. Analysis of phase errors caused by thermal noise, system instability, and atmospheric effects show insignificant array gain degradation by these factors