Polly Estabrook

Critical spacecraft-to-Earth communications for Mars Exploration Rover (MER) entry, descent and landing

For planetary lander missions, the most challenging phase of the spacecraft to ground communications is during the entry, descent, and landing (EDL). As each 2003 Mars Exploration Rover (MER) enters the Martian atmosphere, it slows dramatically. The extreme acceleration and jerk cause extreme Doppler dynamics on the X-band signal received on Earth. When the vehicle slows sufficiently, the parachute is deployed, causing almost a step in deceleration. After parachute deployment, the lander is lowered beneath the parachute on a bridle. The swinging motion of the lander imparts high Doppler dynamics on the signal and causes the received signal strength to vary widely, due to changing antenna pointing angles. All this time, the vehicle transmits important health and status information that is especially critical if the landing is not successful. Even using the largest Deep Space Network antennas, the weak signal and high dynamics render it impossible to conduct reliable phase coherent communications. Therefore, a specialized form of frequency-shift-keying must be used. This paper describes the EDL scenario, the signal conditions, the methods used to detect and frequency-track the carrier and to detect the data modulation, and the resulting performance estimates.

A 20/30 GHz personal access satellite system design

A strawman design for a personal access satellite system (PASS) to provide low-bit-rate voice and data services to users in the contiguous United States (CONUS) in the early 2000s using the 20/30 GHz band is presented. As currently envisaged, the PASS system links suppliers throughput CONUS with users in 142 fixed spot beams of 0.35 degrees beamwidth in one satellite hop; user-to-user communication requires two satellite hops. Through the use of time-division multiple access (TDMA) on the forward link and frequency-division multiple access (FDMA) on the return link, this system will support 2000 duplex voice channels at 4.8 kb/s with a satellite of moderate size and weight. For the user terminal, four antenna designs that trade-off compact size, cost, and electrical performance are described. Last, an experimentation plan taking advantage of planned Ka-band satellite capabilities is outlined.<<ETX>>

Direct-to-Earth communications with Mars Science Laboratory during Entry, Descent, and Landing

Mars Science Laboratory (MSL) undergoes extreme heating and acceleration during Entry, Descent, and Landing (EDL) on Mars. Unknown dynamics lead to large Doppler shifts, making communication challenging. During EDL, a special form of Multiple Frequency Shift Keying (MFSK) communication is used for Direct-To-Earth (DTE) communication. The X-band signal is received by the Deep Space Network (DSN) at the Canberra Deep Space Communication complex, then down-converted, digitized, and recorded by open-loop Radio Science Receivers (RSR), and decoded in real-time by the EDL Data Analysis (EDA) System. The EDA uses lock states with configurable Fast Fourier Transforms to acquire and track the signal. RSR configuration and channel allocation is shown. Testing prior to EDL is discussed including software simulations, test bed runs with MSL flight hardware, and the in-flight end-to-end test. EDA configuration parameters and signal dynamics during pre-entry, entry, and parachute deployment are analyzed. RSR and EDA performance during MSL EDL is evaluated, including performance using a single 70-meter DSN antenna and an array of two 34-meter DSN antennas as a back up to the 70-meter antenna.

ACTS aeronautical experiments

A description of the two aeronautical mobile satellite experiments utilizing NASAs Advanced Communications Technology Satellite (ACTS) is presented. The low bit rate experiment is principally a Ka-band technology demonstration of a prototype 4.8 Kbps aeronautic mobile terminal employing three experimental active electronically steered arrays. The high bit rate experiment can demonstrate a 64 Kbps to 384 Kbps satellite link between a ground terminal and an aircraft.

Carrier Recovery Enhancement for Maximum-Likelihood Doppler Shift Estimation in Mars Exploration Missions

One of the most crucial stages of the Mars exploration missions is the entry, descent, and landing (EDL) phase. During EDL, maintaining reliable communication from the spacecraft to Earth is extremely important for the success of future missions, especially in case of mission failure. EDL is characterized by very deep accelerations, caused by friction, parachute deployment and rocket firing among others. These dynamics cause a severe Doppler shift on the carrier communications link to Earth. Methods have been proposed to estimate the Doppler shift based on Maximum Likelihood. So far these methods have proved successful, but it is expected that the next Mars mission, known as the Mars Science Laboratory, will suffer from higher dynamics and lower SNR. Thus, improving the existing estimation methods becomes a necessity. We propose a Maximum Likelihood approach that takes into account the power in the data tones to enhance carrier recovery, and improve the estimation performance by up to 3 dB. Simulations are performed using real data obtained during the EDL stage of the Mars Exploration Rover B (MERB) mission.

X-Band Choke Ring Horn Telecom Antenna for Interference Mitigation on NASA’s SWOT Mission

A low-gain antenna (LGA) is designed for high cross-polarization discrimination (XPD) and low backward radiation within the 8.025-8.4-GHz frequency band to mitigate cross-polarization and multipath interference given the spacecraft layout constraints. The X-band choke ring horn was optimized, fabricated, and measured. The antenna gain remains higher than 2.5 dBi for angles between 0° and 60° off-boresight. The XPD is higher than 15 dB from 0° to 40° and higher than 20 dB from 40° to 60° off-boresight. The calculated and measured data are in excellent agreement.