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Mars Atmospheric Entry Integrated Navigation with Partial Intermittent Measurements 时间:2017-09-30 11:55 来源:未知作者:admin 点击: 次 Abstract:Signal degradation suffered by the vehicle is a combination brownout and blackout during Mars atmospheric entry. The communications brownout means that signal fades and blackout means that the signal is lost completely. The communications brownout and blackout periods are analyzed and predicted with an altitude and velocity profiles. In the brownout period, the range measurements between the vehicle and the orbiters are modeled as intermittent measurements with the radio signal arrival probabilities, which are distributed as a Rayleigh distribution of the electron number density around the entry vehicle. A new integrated navigation strategy during the Mars atmospheric entry phase is proposed to consider the probabilities of the radio measurements in the communications brownout and blackout periods under the IMU/beacon scenario based on the information filter with intermittent measurements. Numerical navigation simulations are designed to show the performance of the proposed navigation strategy under the integrated navigation scenario.
1. Introduction
On 5 August 2012, the Mars Science Laboratory’s (MSL) Curiosity rover successfully landed on Mars after suffering the Seven Minutes of Terror, in which the MSL entry, descent, and landing (EDL) system brings the velocity of the vehicle from about 5.9 km/s to 0.75 m/s. Especially, during Mars entry phase, the vehicle experienced the most rugged aerodynamic environment, which includes high temperature and pressure, peak heating (peak temperatures of up to 2090°C), and peak deceleration (out at 15g), and the signal between the vehicle and the orbiters can fade or suffer from total loss to make the ultrahigh frequency (UHF) relay links from the MSL to the orbiters suffer a period of about 70 s of degradation [1, 2]. The Mercury, Gemini, and Apollo spacecraft entering the Earth’s atmosphere and the Mars Pathfinder entering Mars’ atmosphere all endured communications blackouts, and the lasting time is from 30 seconds to several minutes [3]. During communications blackout period, it is difficult to perform real-time communication between the vehicle and the orbiter to provide the range measurements in time for the navigation system. So, it is necessary to predict trends or likelihood of the signal degradation and design appropriate navigation technology and strategy.
The communications blackout is caused by reflecting or absorbing the electromagnetic at some frequencies by the sheath of ionized atmospheric gases around the spacecraft when the spacecraft enters into a planetary atmosphere with a hypersonic velocity. Whether or not the communications blackout occurs depends on the electron number density (END), the critical electron number density (CEND) at the link frequency, such as UHF. If the electron number density exceeds the corresponding critical electron number density, the communications blackout may happen under some frequencies. In order to analyze and forecast any possible blackout problem, two aerothermodynamic analysis programs, which include the Jet Propulsion Laboratory (JPL) Normal Shock and Chemical Equilibrium Program (also called the Horton program) [4] and the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) program [5], have been used in Mars entry vehicles as they fly through Mars atmospheric environment [6]. The JPL Horton program was developed in the 1960s [4] and then used to predict any possible communications blackout problem [2, 6]. These tools output the velocity of the shock layer and the electron number density at the stagnation point by using the composition, pressure, temperature, and density of the Mars atmosphere [6]. Morabito analyses the communications blackout problem of the Mars Pathfinder at X-band based on the wake-region electron density estimates produced by the JPL Horton program and the LAURA program and compares the predicted results of the two programs [6]. Morabito and Edquist predicted the communications blackout at UHF for different MSL entry cases by using the above two tools [3]. After the MSL successfully landed on Mars, Morabito and Edquist analyzed the UHF communications brownout and blackout experienced by the MSL during Mars atmospheric entry, which coincided with the predicted signal degradation from preflight analyses in literature [3]. Schratz et al. discussed the performance of the MSL telecommunications for UHF during the EDL phase and analyzed the signal strength and the plasma blackout during Mars entry [7].