Concept for satellite gravity mission proposed in Japan. Two satellites in a near-polar orbit of altitude fly apart. The global positioning system (GPS) continuously tracks the satellite motion for the SST-HL. Precision accelerometers (ACCs) measure the nongravitational disturbing forces acting on the satellites. A satellite laser ranging (SLR) also determines the satellite orbit using laser-retro reflectors (LRRs). A laser interferometer detects changes in the relative velocity between the twin satellite. A laser and photodiode (PD) are contained in the lead satellite 1. The laser beam emitted from this satellite is directly reflected by the following satellite 2.
(a) Optical layout of SSI for satellite gravity mission and (b) conceptual design of ground-based simulator for SSI. This apparatus simulates the laser Doppler frequency shift, optical path-length difference, and differential light power. : light field, V: voltage, : velocity, : interferometer arm length, : wave number, BS: beam splitter, M: mirror, and PD: photodiode.
Schematic diagram of ground-based simulator. It consists of an interferometer, frequency-stabilized laser, and automatic alignment controller. FI: Faraday isolator, PO: pick-off plate, BS: beam splitter, CM: concave mirror, HWP: half wave plate, QWP: quarter wave plate, AOD: acousto-optic deflector, EOM: electro-optical modulator, FSM: fast-steering mirror, PD: photodiode, QPD: quadrant photodiode, DDS: direct digital synthesizer, and DSG: digital signal generator.
Measured open-loop transfer function of fringe-control loop. The bold line and dots represent the calculated and measured open-loop transfer function of the fringe-control loop.
(a) Range noise spectrum and (b) range-rate noise spectrum of Mach-Zehnder interferometer. The black solid and gray dashed lines represent the noise spectra with the power differential of 1.2 and in the interferometer arms, respectively. The goal sensitivity of the SSI for the Japanese satellite gravity mission is also indicated by the gray line.
Frequency-noise spectra of light source for ground-based simulator. The top and bottom traces indicate the free-running frequency noise and the frequency noise of the stabilized light source. The traces indexed with different pressures represent the frequency noise of the light source referenced to the Fabry-Pérot cavity at each pressure. The required noise levels for the ground-based simulator and Japanese satellite gravity mission are also indicated by the dashed and dotted lines, respectively.
Estimated angular fluctuation spectra of laser beam and rms fluctuations (pitch motion). The black and gray solid lines represent the angular fluctuation spectrum with/without alignment control. The black and gray dotted lines are their rms fluctuations.
Estimated angular fluctuation spectra of laser beam and rms fluctuations (yaw motion). The black and gray solid lines represent the angular fluctuation spectrum with/without alignment control. The black and gray dotted lines are their rms fluctuations.
Range-rate signal wave form for signal-injection simulation. This was computed using a satellite-orbit analysis software. Twin satellites were assumed to be flown in a near-polar orbit at an altitude of with an inclination of and separated from each other by in the along-track direction. The geopotential model EGM 96 was used for the orbit analysis. For simplicity, the effect of nongravitational forces was neglected in the analysis.
Residual noise of range-rate signal retrieved. The original signal injected into the ground-based simulator was subtracted from the feedback signal of the fringe-control loop.
Range-rate signal spectra. The dotted and solid lines indicate the spectrum of the original range-rate signal and that of the feedback signal during the signal-injection simulation, respectively. The spectrum of the analog-to-digital converter (ADC) noise is also shown by a gray line.
Range-rate noise level of the ground-based simulator and the contribution of identified noise sources. The present sensitivity in the measurement band is restricted by the DDS phase noise and Mach-Zehnder noise.
Geoid height errors for different satellite altitudes: , 275, 300, and . The intersatellite range is , the range-rate sensitivity is , the mission duration is 60 days and the sampling interval is .
Geoid height errors for different range-rate sensitivity: , 1, 10, and . The altitude of the satellite orbit is . The other parameters are the same as in Fig. 13.
Article metrics loading...
Full text loading...