Edgar H. Satorius

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.

Fixed-point implementation of adaptive digital filters

This paper presents some preliminary results on the behavior of adaptive digital filters when implemented with fixed-point arithmetic. In particular, adaptive prediction error filters configured in both tapped-delay line and lattice forms are examined and their performance is compared in terms of the number of bits required to reach a given steady-state error level. The lattice structures are of special interest since, at least in the non-adaptive case, they are known to have well-behaved numerical properties (as compared with a tapped-delay line realization) when implemented with fixed-point arithmetic. The results of our study indicate that this is also true in the adaptive case. Ideally, this study should ultimately lead to efficient implementations (e.g., in VLSI) of adaptive digital filters.

Analysis of a Rain Compensation Algorithm for K/Ka-Band Communications

A rain compensation algorithm (RCA) has been developed for use in the advanced communications technology satellite (ACTS) mobile terminal (AMT) system. The basic goal of the RCA is to control the transmitted data rates (9.6, 4.8 or 2.4 kbps) in the forward and return links so that a 3 dB link margin is maintained at the highest possible transmitted data rate. In this paper, analyses of both theoretical and practical issues relating to the RCA are presented. In addition, sample simulations of a one-dimensional version of the RCA at the MT are presented which illustrate typical RCA performance using both simulated and recorded pilot fade field data. It is found that with suitable post-processing, the RCA can provide reasonable (conservative) data rate estimates without making excessive data rate changes, i.e., data rate change fluctuations. It is anticipated that the results presented here will not only be useful for the eventual operation of the RCA, but more generally will be useful in the design and operation of other rain compensation techniques for K/Ka-band communication systems.

Adaptive noise cancelling of a sinusoidal interference using a lattice structure

Lattice structures have recently been proposed for application to such areas as adaptive linear prediction, adaptive noise cancelling, and adaptive equalization. In this paper, we will consider the suppression of a sinusoidal interference by the use of an adaptive lattice structure implemented as a noise canceller. The performance of the adaptive lattice will be analyzed and compared with that of an LMS transversal adaptive noise canceller. Experimental results will also be presented and discussed.

Adaptive modulation and coding techniques in MUOS fading/scintillation environments

In this paper, adaptive modulation and coding structures are presented that are appropriate for the MUOS environment. The proposed approach adaptively controls the modulation format, FEC coding rate, transmission power and even possibly bandwidth spreading gain in response to changing channel conditions in an attempt to optimize throughput while maintaining acceptable error rate performance. Being able to estimate and then predict the channel is vital to the system performance. Channel prediction algorithms; have been developed and the advantages of the adaptive modulation/coding techniques are presented together with implementation issues.

Enhancement of advanced range telemetry (ARTM) channels via blind equalization

The Joint Services Advanced Range Telemetry (ARTM) Program at Edwards Air Force Base has been evaluating FQPSK-B for possible upgrades to the existing telemetry equipment. It has been found in the wideband channel sounding experiments sponsored by ARTM that the in-flight fading channel can be modeled as a 3-ray multipath channel. Delay spread for a typical in-flight channel is in the order of 300 nanoseconds. Furthermore, the pre-flight channel is characterized by much more severe multipath, in which the delay spread is in the order of microseconds covering one or more symbols when the FQPSK-B transceiver operates at a rate of millions of symbols per second. This adverse channel condition inevitably causes tremendous distortion in the received signals due to severe inter-symbol interference (ISI) from the multipath. This paper provides an assessment of the potential ability of blind equalization to reduce the FQPSK-B system susceptibility to degradation caused by dynamic frequency selective fading in the aeronautical telemetry environment. In particular, a blind equalizer applique that can be inserted prior to the demodulator without knowledge of the received signal such as carrier frequency, symbol timing and sequence, etc, is proposed. Since it is desired that the equalizer applique operate independently of the carrier frequency and given that the modulation of interest is constant envelope (PCM-FM or FQPSK-B), we have selected the constant modulus algorithm (CMA) cost function for implementation. Extensive tests on both simulated and recorded FQPSK-B data transmitted over different ARTM channels have been conducted and the blind equalizer structure has shown substantial improvements, even on the difficult ARTM pre-flight channels. The CMA adapts the equalizer coefficients to minimize the deviation of the output envelope from an arbitrary constant level. This paper depicts the pre-flight and in-flight channel conditions using time and spectral domain measurement. It quantifies the benefit of the blind CMA tapped delay line equalizer. Due to the extensive signal processing requirements associated with the very high sampling rate (100 MHz) of the FQPSK-B system, hardware implementation complexity is very high. Complexity reduction issues regarding the implementation of the CMA using field programmable gate array (FPGA) are also presented.

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.

A communications system testbed

We describe a testbed that is being developed for the evaluation of demodulation algorithms in communication systems. This testbed is intended to provide a research tool in the area of signal demodulation and is built around easily programmable, general purpose and digital signal processors. A variety of analog and digital modulations can be evaluated in real and non-real-time scenarios and under different channel conditions ranging from intersymbol interference to modulated or unmodulated co-channel interference. Testbed architectural and design details are presented along with preliminary results of our algorithm development and testing program.

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.

A FREQUENCY OFFSET ESTIMATION AND COMPENSATION ALGORITHM FOR K/Ka‐BAND COMMUNICATIONS

This paper presents a new open-loop technique for estimating and correcting Doppler frequency shift in K/Ka-band communication systems with special reference to the advanced communications technology satellite (ACTS) mobile terminal (AMT) modem, which utilizes square-wave pulse-shaped, binary differential phase shift-keyed (DPSK) modulation. The novelty of this estimation scheme is that it exploits the Doppler-induced phase shift over a fraction of a symbol interval to provide an estimate of the Doppler offset, without requiring symbol synchronization. Furthermore, by utilizing time-differential detection (delay-and-multiply), the proposed technique can tolerate much larger frequency offsets than existing open- or closed-loop techniques. Analytical results are provided for the variance of the above estimator and the error probability performance of the AMT is evaluated in the presence of the Doppler correction. Practical design considerations are also discussed, including a method for modifying the front end, digital bandlimiting filter in such a way that Doppler bias effects in the new estimator are eliminated. Simulation results reveal that, in general, performance improves with increasing data rates, i.e., the new frequency offset estimation/compensation algorithm induces a degradation from ideal of approximately 1 dB at a 6 dB energy per data symbol (bit) and a 2.4 kbps data rate. However, there is no appreciable degradation when the data rate is increased to 9.6 or 19.2 kbps.