Optical interference technology for imaging GEO satellites

How to be able to distinguish geostationary satellites is causing widespread interest in the field of space situational awareness. However, even the largest single ground-based telescope can only distinguish large geostationary satellites. To distinguish the details, such as: Antenna or instrument that extends beyond the main body of the satellite, other techniques such as optical interference are required.

The Navy Precision Optical Interferometer (NPOI) was the first such interferometer to successfully detect a geostationary satellite from the ground. We observed DirecTV-9S during the March 2008 glint season, the most common visible twilight point of satellite reflection back to sunlight, and in 2009 we followed. We find that the NPOI has only the shortest baseline of 16 m, which was too long for the observation of the geostationary orbit satellites, which are on the order of a few meters.

The observation wavelength band of NPOI is λ = 556 ~ 845nm with a resolution of 35-50 nanoradians (θres∽λ / B), which is equivalent to the scale of 1.3-2m of geosynchronous height. The structure of this scale produces fringe contrast - the ratio of maximum to minimum reflectance - V∽0.2, and only 20% larger structures do not have fringe contrast. When the size of the structure is large, the fringe contrast will be restored again, only weakly.

The lack of a shorter baseline necessarily reveals two drawbacks. First, making it difficult to detect and track streaks in real time and to adjust the internal light path of the interferometer. This is because atmospheric turbulence forces us to detect streaks and readjust the light path every 20 ms. Second, the long baseline data can not describe the large-scale structural features of the target. Better mathematically indicates that larger baseline satellite bodies require short baseline data or smaller interferometric uv gaps, where u and v are toward the target from east to west, toward the baseline component from north to south Sampling spatial frequencies BEW / λ and BNS / λ.

The fringe power is a function of the frequency fringes scanned for each modulation period and is the data DirecTV-7S data obtained on March 6, 2015 through the spectrometer. Each frame corresponds to a spectral channel, the wavelength from 845nm (Channel 1) -604nm (Channel 2). The spectrometer was observed only through the W4 AC baseline with a stripe scan frequency of k = 1. The black curve represents the power spectrum obtained on the on-fringe scan. The red curve is the deviation of the power spectrum from the off-fringe scan, which is the flux ratio based on coherent and incoherent scans. The green curve shows the corrected power spectrum.

Since 2009 NPOI has enabled several new array stations to enable shorter baselines. In particular, there is now a baseline of 8.8m between W4-ACs with a resolution of 2.3-3.5m for synchronous orbit height and a 9.8m baseline between AC-E3s. These two short baselines share the array cells AC, making it possible to combine them to form a third baseline, W4-E3 of 18.6m. The ultimate goal is to detect streaks with longer baselines for finer detail, but with poor signal-to-noise ratios. If two shorter baselines are used to detect and track fringes, the third baseline can be correctly co-phased and real-time data without detectable fringes can be acquired, a concept originally proposed by Roddier. The closing phase can also be achieved using three or more array elements, which is the sum of the stripe phases around the base of the triangle. Since atmospheric disturbances of the baseline phase cancel out in the sum, these closed phases represent some phase information of the target image. What makes it special is that they can be used to better determine the relative position of satellite components.

We obtained the DirecTV-7S stripes on the W4 AC baseline in March 2015. However, because the combiner software was searching for the second baseline, the data was not recorded. The next night, fringe detection was interrupted due to poor weather conditions, but we were able to detect and record fringes during a 27 second period during which we tracked the fringes through the W4 AC baseline for approximately 4 seconds, tracking streaks through the AC E3 baseline To 4s. The complications of our observations as a result of the last days of the "blinking" season meant that the shorter the "blinking", the less brilliant the goal of the best moment of the season was.

Figure 2 is a streak power spectrum data, 12 spectral channels, acquired on March 6, 2015 by a spectrometer 2 at W4 AC baseline. The observation uses two fringe scanning frequencies for each modulation period. Deviation extraction fringe power spectrum (green curve) Description 845nm channel (channel 1) signal is strong, as the wavelength becomes shorter streak power.

This article discusses an update of early research work, and papers on early work have been published in many places. The next step for NPOI is to try out three 1m telescopes to increase the sensitivity of the instrument so that it can perform observations in non-"blinking" seasons. At present, our work is still in the initial stage of design updating system.

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