David D. Morabito

The Cassini May 2000 solar conjunction

Interplanetary spacecraft, which fly in the ecliptic plane, typically encounter solar conjunctions during their main missions. The communications link between an interplanetary spacecraft and Earth is affected by the charged particles that constitute the intervening solar corona and solar wind. As the Sun-Earth-probe (SEP) angle becomes small (usually <3/spl deg/ for X band or 8.43 GHz), the signal suffers increased degradation. The effects on the received signal include time delay and phase fluctuations due to the fluctuating columnar electron density, which in turn cause carrier lock problems and telemetry data loss. Because of these effects, studies of solar corona charged particle effects on spacecraft signals were conducted to determine strategies for optimizing data return during these periods. The first solar conjunction of the Cassini spacecraft occurred between May 8, 2000 (2000/129) and May 18, 2000 (2000/139). During this period, the Cassini spacecraft was within 3.2/spl deg/ of the Sun as seen from Earth with the minimum SEP angle of 0.56/spl deg/ occurring on May 13 (2000/134). This solar conjunction occurred prior to the expected peak of the current solar cycle. Coherent dual-frequency X band (8.43 GHz) and Ka band (32 GHz) data were acquired from 3.2/spl deg/ to near the minimum SEP angle at 0.6/spl deg/ for both ingress and egress. The measurements of amplitude scintillation, spectral broadening and phase scintillation were examined as a function of SEP angle. As expected, these solar effects are significantly less at Ka band than at X band for the same SEP angle. This studys results will be combined with those of other spacecraft solar conjunctions in order to build a statistical database of solar effects as a function of solar elongation angle and phase of the solar cycle. Such studies are useful in the design of telecommunications systems for future spacecraft missions, which may have stringent communication requirements during their solar conjunction phases.

Integrated network architecture for sustained human and robotic exploration

The National Aeronautics and Space Administration (NASA) Exploration Systems Mission Directorate is planning a series of human and robotic missions to the Earths Moon and to Mars. These missions will require telecommunication and navigation services. This paper sets forth presumed requirements for such services and presents strawman lunar and Mars telecommunications network architectures to satisfy the presumed requirements. The paper suggests that a modest ground network would suffice for missions to the near-side of the Moon. A constellation of three Lunar Telecommunications Orbiters connected to a modest ground network could provide continuous redundant links to a polar lunar base and its vicinity. For human and robotic missions to Mars, a pair of areostationary satellites could provide continuous redundant links between a mid-latitude Mars base and Deep Space Network antennas augmented by large arrays of 12-m antennas

Communicating with Mars during periods of solar conjunction

A reliable communications link between Mars and Earth will be required during the initial phase of the human exploration of Mars. The direct communications link can easily be realized during most of the 780-day Earth-Mars synodic period, except when this link encounters increased intervening charged particles during superior solar conjunctions. The effects of solar charged particles are expected to corrupt the data signals to varying degrees. During superior solar conjunctions of interplanetary spacecraft, flight projects routinely scale down or suspend operations by invoking command moratoriums, reducing tracking schedules, and progressively lowering data rates. The actual operations scenarios will vary between flight projects and from conjunction to conjunction. This paper presents results of a study conducted to determine to what extent and by what techniques communications may be maintained throughout Mars-Sun-Earth superior conjunction periods that could occur during early human Mars exploration missions. Using a number of techniques discussed in this paper, it should be possible to maintain some degree of communication throughout all of the superior conjunctions occurring between 2015 and 2026, except for one occurring in 2023, in which actual occultation of the signal source by the Suns disk occurs.

Communications blackout predictions for atmospheric entry of Mars Science Laboratory

The Mars Science Laboratory (MSL) is expected to be a long-range, long-duration science laboratory rover on the Martian surface. MSL will provide a significant milestone that paves the way for future landed missions to Mars. NASA is studying options to launch MSL as early as 2009. There are three elements to the spacecraft; carrier (cruise stage), entry vehicle, and rover. The rover has a UHF proximity link as the primary path for EDL communications and may have an X-band direct-to-Earth link as a back-up. Given the importance of collecting critical event telemetry data during atmospheric entry, it is important to understand the ability of a signal link to be maintained, especially during the period near peak convective heating. The received telemetry during entry (or played back later) allows for the performance of the entry-descent-landing technologies to be assessed. These technologies include guided entry for precision landing, a new sky-crane landing system and powered descent. MSL will undergo an entry profile that may result in a potential communications blackout caused by ionized particles for short periods near peak heating. The vehicle will use UHF and possibly X-band during the entry phase. The purpose of this report is to quantify or bound the likelihood of any such blackout at UHF frequencies (401 MHz) and X-band frequencies (8.4 GHz). Two entry trajectory scenarios were evaluated: a stressful entry trajectory to quantify an upper-bound for any possible blackout period, and a nominal trajectory to quantify likelihood of blackout for such cases.

Observing the Moon at Microwave Frequencies Using a Large-Diameter Deep Space Network Antenna

The Moon radiates energy at infrared and microwave wavelengths, in addition to reflecting sunlight at optical wavelengths. As a result, an antenna pointed at or near the Moon will result in an increase in system operating noise temperature, which needs to be accounted for in RF telecommunications, radio science or radiometric link calculations. The NASA Deep Space Network (DSN) may use its large-diameter antennas in future lunar robotic or human missions, and thus it is important to understand the nature of this temperature increase as a function of observing frequency, lunar phase, and angular position of the antenna beam on the lunar disk. This paper reports on a comprehensive lunar noise temperature measurement campaign and associated theoretical treatment for a 34-m diameter Deep Space Network antenna observing an extended source such as the Moon. A set of measurements over a wide range of lunar phase angles was acquired at DSS-13, a 34-m diameter beam waveguide antenna (BWG) located at Goldstone, California at 2.3 GHz (S-band), 8.4 GHz (X-band) and 32 GHz (Ka-band). For validation purposes, independent predictions of noise temperature increase were derived using a physical optics characterization of the 34-m diameter antenna gain patterns and Apollo model-based brightness temperature maps of the Moon as input. The model-based predictions of noise temperature increase were compared with the measurements at all three frequencies. In addition, a methodology is presented that relates noise temperature increase due to the Moon to disk-centered or disk-averaged brightness temperature of the Moon at the microwave frequencies of interest. Comparisons were made between the measurements and models in the domain of lunar disk-centered and disk-averaged brightness temperatures. It is anticipated that the measurements and associated theoretical development will be useful in developing telecommunications strategies for future high-rate Ka-band communications where large diameter DSN antennas will be required.

Link design and planning for Mars Reconnaissance Orbiter (MRO) Ka-band (32 GHz) telecom demonstration

NASA is planning a Ka-band (32 GHz) engineering telemetry demonstration with Mars Reconnaissance Orbiter (MRO). Capabilities of Ka-band for use with deep space mission are demonstrated using the link optimization algorithms and weather forecasting. Furthermore, based on the performance of previous deep space missions with Ka-band downlink capabilities, experiment plans are developed for telemetry operations during superior solar conjunction. A general overview of the demonstration is given followed by a description of the experiment planning during cruise, the primary science mission and superior conjunction. As part of the primary science mission planning the expected data return for various data optimization methods is calculated. These results indicate that, given MROs data rates, a link optimized to use of at most two data rates, subject to a minimum availability of 90%, performs almost as well as a link with no limits on the number of data rates with the same minimum availability requirement. Furthermore, the effects of forecasting on these link design algorithms are discussed

The characterization of a 34-meter beam-waveguide antenna at Ka band (32.0 GHz) and X band (8.4 GHz)

New antennas for the NASA Deep Space Network (DSN) have been built to replace the aging antennas of older designs for deep-space communications. These new antennas incorporate a new dual-shape design as well as a beam waveguide (BWG), which utilize a series of additional secondary mirrors to relocate the focal point into a stationary room below the main reflector. The advantages of using such a design include increased isolation of the feed package from outside environmental factors, such as moisture, wind, and temperature changes; and ease of access to the equipment for maintenance, troubleshooting and repair purposes. This article reports on the performance of a beam waveguide antenna at X-band and Ka-band microwave frequencies. The Ka-band antenna performance experiment (KaAP) antenna-efficiency measurements presented in this article were acquired at the Goldstone DSS-13 research and development (R&D) beam-waveguide antenna between December, 1993, and November, 1995. The measured antenna efficiency and ground-station figure-of-merit (gain divided by operating system noise temperature) as a function of elevation angle and their uncertainties are presented. Also described are the station configuration, the measurement technique, the modeling used in the analysis processing, and the historical evolution of the DSS-13 Ka-band antenna-efficiency measurements as progressive improvements and configuration changes were implemented.

MRO Ka-band Demonstration: Cruise Phase Lessons Learned

NASAs mars reconnaissance orbiter (MRO) was launched on August 12, 2005 and was inserted into Mars orbit successfully on March 10, 2006. From September 2005 through January 2006, the Ka-band signal from the spacecraft was tracked several times by NASAs deep space network (DSN). During these tracks a number of Ka-band functions for both the spacecraft and the ground systems were tested. These tests were performed to assess the readiness of the ground system and spacecraft to support Ka-band demonstration activities during MROs primary science phase (PSP). As a result of these tests a number of important lessons were learned regarding the performance of the Ka-band link and the performance of the ground equipment. This paper presents the performance of the ground antenna pointing, delta differential one-way ranging (DeltaDOR) performance and the telemetry performance of the Ka-band link. These results indicate that the extra effort for pointing calibration of the ground antennas at Ka-band is extremely useful; Ka-band shows potential for improved DeltaDOR performance and the models for Ka-band performance of the ground station microwave noise temperature are very accurate.

Operational demonstration of ka-band telecommunications for the mays reconnaissance orbiter

The objectives of the operational demonstration are to verify that the anticipated benefits of the higher carrier frequency can actually be realized under realistic operating conditions, and if possible to provide a significant enhancement to scientific data return.

Deep Space C3: High Power Uplinks

The uplink transmitters of the Deep Space Network (DSN) perform three key functions in support of space missions: navigation, command uplink, and emergency recovery. The transmitters range in frequency from S‐band to Ka‐band, and range in RF transmit power from 200W to 400kW. Future improvements to the uplink transmitters will focus on higher frequency transmitters for high data rate communications, high power X‐band uplinks for emergency recovery, and/or in‐phase uplink arraying for either application.