Kamal Oudrhiri

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.

Radio Science measurements in presence of data on optical and RF links

Radio Science (RS) experiments currently rely on unmodulated CW RF signal carrier for spectral purity and maximized signal-to-noise ratio. This requires missions to carefully schedule them away from periods of high rate telemetry. In the era of optical communications, current systems experience the same problem. In this paper, we derive a data processing architecture that will yield high-accuracy RS or link science type of information on the ground from communication signals readily transmitted from space assets, in a broad range of communication frequencies, from RF to optical. This technique is intended to save power, bandwidth and scheduling demands on the spacecraft. Our proposed technical approach is applicable to a broad suite of modulations (phase and/or intensity), of receiver types (coherent and/or non- coherent), and carrier frequencies (e.g., microwave, optical), thus providing an architectural improvement to present state-of-the-art communication systems utilized by NASA as well as to future systems. This method is an additional module to the existing communication receiver architecture, and does not require modifications to their operation. We propose a practical system that approaches the ultimate theoretical performance for estimating the amplitude, phase, and frequency variations due to the changes in the planet atmosphere. For optical links with intensity modulated laser transmission or phase modulated CW laser communications the proposed optical receiver provides both data detection and signals required to extract RS data such as amplitude, phase and frequency due to planetary atmospheric changes. We can extract the same information required for RS data by using differential methods of encoding and at the optical receiver a local laser, a phase shifter, and an array of photon detectors are used.

Radio science measurements with suppressed carrier

Radio Science started when it became apparent with early deep space missions that occultations by planetary atmospheres would affect the quality of radio communications. Since then the atmospheric properties and other aspects of planetary science, solar science, and fundamental physics were studied by scientists. Radio Science data was always extracted from a received pure residual carrier (without data modulation). For some missions, it is very desirable to obtain Radio Science data from a suppressed carrier modulation. In this paper we propose a method to extract Radio Science data when a coded suppressed carrier modulation is used in deep space communications. The type of modulation can be BPSK, QPSK, OQPSK, MPSK or even GMSK. However we concentrate mostly on BPSK modulation. The proposed method for suppressed carrier simply tries to wipe out data that acts as an interference for Radio Science measurements. In order to measure the estimation errors in amplitude and phase of the Radio Science data we use the Cramer-Rao bound (CRB). The CRB for suppressed carrier modulation with non-ideal data wiping is then compared with residual carrier modulation under the same noise condition. The method of derivation of the CRB for non-ideal data wiping is an innovative method that is presented here. Some numerical results are provided for a coded system.

In situ measurements of USO performance in space using the twin GRAIL spacecraft

Ultra-stable oscillators (USO) are flown on a variety of different science missions to provide stable timing and/or navigation. Typically, their performance is measured at the part in 1013 level before launch and can only be verified in space via measurements that must propagate through, and potentially be degraded by, the Earths atmosphere. To date, two missions are able to demonstrate USO performance in space, without atmospheric limitations, using twin spacecraft: Gravity Recovery and Climate Experiment (GRACE) and Gravity Recovery and Interior Laboratory (GRAIL). With the recent launch of GRAIL, a NASA mission to map out the gravity field of the Moon, the clock and timing community has access to microwave tracking data from a pair of satellites flying independent USOs. In addition, since GRAIL took a circuitous three-month journey to the Moon, there was a 30-minute opportunity to obtain USO versus USO data when the spacecraft were not in orbit around the Earth or the Moon. This paper presents GRAIL Allan deviation data obtained during this payload-checkout in September 2011 and during the Science Phase in March 2012, and also analyzes data from GRACE using these new methods.

Limitations on the use of the power-law form of S/sub y/(f) to compute Allan variance

An exact solution to the well-known integral transform that relates the spectral density of the instantaneous fractional frequency deviation, S/sub y/(f), to the Allan variance, /spl sigma//sub y//sup 2/ (/spl tau/), is presented for the case of a power-law representation of S/sub y/(f). The approximate solution to this integral transform, which is found throughout the literature, is also derived. A graphical convergence analysis is presented, showing the range of applicability of the approximate solution. The results reinforce the use of the approximate solution, which converges quickly to the exact solution under virtually all reasonable measurement conditions.

Communications during critical mission operations: preparing for InSight’s landing on Mars

Radio communications with deep space missions are often taken for granted due to the impressively successful records since, for decades, the technology and infrastructure have been developed for ground and flight systems to optimize telemetry and commanding. During mission-critical events such as the entry, descent, and landing of a spacecraft on the surface of Mars, the signals level and frequency dynamics vary significantly and typically exceed the threshold of the budgeted links. The challenge is increased when spacecraft shed antennas with heat shields and other hardware during those risky few minutes. We have in the past successfully received signals on Earth during critical events even ones not intended for ground reception. These included the UHF signal transmitted by Curiosity to Marsorbiting assets. Since NASAs Deep Space Network does not operate in the UHF band, large radio telescopes around the world are utilized. The Australian CSIRO Parkes Radio Telescope supported the Curiosity UHF signal reception and DSN receivers, tools, and expertise were used in the process. In preparation for the InSight missions landing on Mars in 2016, preparations are underway to support the UHF communications. This paper presents communication scenarios with radio telescopes, and the DSN receiver and tools. It also discusses the usefulness of the real-time information content for better response time by the mission team towards successful mission operations.

Radio science from an optical communications signal

NASA is currently developing the capability to deploy deep space optical communications links. This creates the opportunity to utilize the optical link to obtain range, Doppler, and signal intensity estimates. These may, in turn, be used to complement or extend the capabilities of current radio science. In this paper we illustrate the achievable parameter estimation errors in estimating range, Doppler, and received signal intensity of a one-way non-coherent optical link. We provide a joint estimation algorithm with performance close to the Cram`er-Rao bound. We draw comparisons to range estimates based on a coherent radio frequency signal, illustrating that large gains in either precision or observation time are possible with an optical link.

An innovative direct measurement of the GRAIL absolute timing of Science Data

The Gravity Recovery and Interior Laboratory (GRAIL), a NASA Discovery mission, twin spacecraft were launched on 10 September 2012 and were inserted into lunar orbit on 31 December 2011 and 01 January 2012. The objective of the mission was to measure a high-resolution lunar gravity field using inter-spacecraft range measurements in order to investigate the interior structure of the Moon from crust to core. The first step in the lunar gravity field determination process involved correcting for general relativity, measurement noise, biases and relative & absolute timing. Three independent clocks participated in the process and needed to be correlated after the fact. Measuring the absolute time tags for the GRAIL mission data turned out to be a challenging task primarily because of limited periods when such measurements could be conducted. Unlike the Gravity Recovery and Climate Experiment (GRACE), where absolute timing measurements are available using the GPS system, no absolute timing measurements were available on the far side of the Moon or when there were no DSN coverage periods. During the early cruise phase, it was determined that a direct absolute timing measurement of each spacecraft Lunar Gravity Ranging System (LGRS) clock could be directly observed by using a DSN station to eavesdrop on the Time Transfer System (TTS) S-band inter-satellite ranging signal. By detecting the TTS system directly on earth, the LGRS clock can be correlated directly to Universal Time Coordinated (UTC) because the TTS and LGRS use the same clock to time-tag their measurements. This paper describes the end-to-end preparation process by building and installing a dedicated hardware at Goldstone station DSS-24, selecting favorable lunar orbit geometries, real time signal detection and post processing, and finally how the absolute timing is used in the overall construction of lunar gravity fields.

GRAIL USOs; Another in-flight quartz radiation experiment

The Gravity Recovery and Interior Laboratory (GRAIL) mission successfully ended in December, 2012 after an extended science phase of over 280 days mapping the gravitation field gradient of the Moon with a precision of better than 50 E-6 gs over the entire lunar surface. The mission was performed by two tandem flying spacecraft, both of which carried a Microwave Dual One-way Ranging (DOWR) instrument that together formed a highly precise relative measure of the distance between the two spacecraft as they orbited the Moon. The precision of the radio link maintained by the DOWR instruments was derived from the frequency stability of the ultra-stable oscillators (USOs) on-board each spacecraft. The opportunity to observe the USOs frequency throughout the GRAIL mission provides a record of not only the intrinsic performance of the oscillators, but their behavior during exposure to space conditions. We describe the effect to the frequency of each GRAIL USO, A and B, resulting from the March 7th, 2012 X5.4 level solar flare, just several days into GRAILs science collection phase. We discuss the impact of this radiation exposure, and the asymmetric behavior of two USOs coincidentally perturbed by the same space-weather event.

Testing theories of relativity with ranging via dual Radio links

The BepiColombo mission to Mercury has Radio Science objectives that explore planetary gravity and interior structure as well as tests of aspects of the general theory of relativity. Both classes of science objectives rely on high precision Doppler and ranging at dual wavelengths, X-band and Ka-band. In particular, the relativistic tests require ranging between a ground station and the spacecraft to 15 cm (+/− 5 cm), which is approximately one order of magnitude better than current capabilities at X-band. A new scheme has been developed to reach this accuracy via three techniques: (1) utilizing the simultaneous propagation of two links modulated with ranging codes in order to calibrate the dispersive media, (2) recording the uplink reference tone in the downlink data stream to provide accurate calibration, and (3) employing open-loop receivers with user-developed post-processing algorithms to produce the final data products. This paper12 will present the proposed instrumentation being prototyped, the encountered challenges and their solutions, as well as the expected results and their implications to the scientific investigations of BepiColombo and future missions.