Daylight space debris laser ranging

The study addresses whether space debris laser ranging (SDLR) can be performed successfully during full daylight, overcoming the long-standing restriction to twilight conditions. The context is the evolution of satellite laser ranging (SLR) from meter-level accuracy in the 1960s to millimeter-level precision enabled by short-pulse lasers and advanced single-photon detectors. Space debris ranging, which relies on detecting diffusely reflected photons from non-cooperative targets, has been limited by the need to visually acquire targets against the sky and by inaccuracies in two-line element (TLE) orbits. The purpose is to demonstrate daylight visualization and real-time correction of prediction biases to enable SDLR during daytime, thereby greatly extending observation opportunities. This is important for significantly improving orbit predictions to support conjunction assessment, collision avoidance, active debris removal, and attitude determination.

Prior work established SLR to cooperative satellites using picosecond lasers and high-sensitivity detectors such as microchannel plates, SPADs, and superconducting nanowire detectors, achieving millimeter precision and enabling kHz repetition rates for attitude/spin studies. Since the early 2000s, SDLR to non-cooperative debris has used higher-energy, nanosecond lasers to detect diffuse returns with ~1 m single-shot precision. Multi-static experiments, where one station transmits and others receive, can improve orbit predictions by up to an order of magnitude, and data fusion with angular optical measurements further enhances orbit determination. However, operations have been confined to twilight when debris is sunlit and the station is dark. TLE-based predictions can have kilometer-level errors, necessitating optical acquisition for centering targets within the field of view prior to ranging. This work builds on these developments by enabling daylight target visualization and real-time bias estimation/correction.

Daylight SDLR procedure: Tracking begins typically above 15° elevation. A piggyback 20 cm Schmidt-Cassegrain telescope and camera visualize the target against the sky. Real-time image analysis detects the illuminated object and computes offsets relative to the predicted path: along-track (time bias) and across-track errors. The tracking orbit is corrected immediately to center the target in the SLR receive telescope field of view; additional across-track residuals are compensated via pointing offsets. Because range biases from TLE errors cannot be directly inferred from images, the detector’s activation (range gate) is shifted experimentally. The SDLR search iteratively adjusts time bias, centers the object optically, and shifts detector activation to maximize detection probability. Guidelines for daylight visualization: Contrast is governed by telescope diffraction (Airy disk), target angular size, detector pixel field of view, atmospheric seeing, and exposure time. The first Airy minimum θa = 1.22 λ/d highlights that larger apertures reduce the Airy disk, concentrating signal into fewer pixels and improving contrast. The pixel field of view should match the Airy disk; undersampling degrades contrast by integrating excessive skylight. Short exposures (few ms) freeze speckle patterns; longer exposures accumulate sky background. A simple contrast model C = 10^((msky − mstar)/2.5) scaled by the image-plane star area was used with a daylight sky brightness of ~3 mag arcsec^-2 to estimate detection limits (contrast ≈ 0.04 as a practical threshold). Visualization setup: Daylight star and satellite imaging used ZWO ASI120 (4.8×3.6 mm, 3.8 µm pixels, 1280×960) and ASI1600 (17.7×13.4 mm, 3.8 µm, 4656×3520) cameras on an 80 cm Ritchey–Chrétien telescope (f = 4.8 m, θa ≈ 0.17 arcsec, per-pixel field ≈ 0.16 arcsec). For SDLR tracking, a 20 cm Schmidt–Cassegrain telescope (θa ≈ 0.69 arcsec, per-pixel field ≈ 0.39 arcsec) was piggyback-mounted on a 50 cm SLR receive telescope. A 780 nm long-pass filter reduced skylight. Custom real-time software (LabVIEW) automatically detected targets and estimated time bias and across-track errors to update pointing and tracking. SDLR equipment: The transmitter operated at 532 nm, 80 mJ per pulse, 3 ns pulse width, 200 Hz (≈16 W average). The beam was expanded to 7 cm to minimize divergence, yielding ≈2 arcsec half-angle, comparable to local seeing. Returns were collected by a 50 cm receive telescope onto a compensated SPAD (C-SPAD) with 200 µm active diameter, operated in gated mode. A range-gate generator (FPGA) derived predicted arrival times from the laser start pulse to activate the detector. Daylight noise is dominated by sky background following Poisson statistics; thus, detection probability increases when returns arrive soon after gate opening. The receive field of view was limited to ≈100 µrad and spectral filters constrained bandwidth to reduce noise. Targets: The Graz catalog included >200 objects (mainly upper-stage rocket bodies and dead satellites) with radar cross-sections from ~0.3 to 15 m² and altitudes from a few hundred to ~1500 km. Daylight star visibility tests were conducted at sun elevations 10–18° to validate contrast theory and system performance.

• Demonstrated the first successful daylight space debris laser ranging passes: four daylight passes to SL rocket bodies (Zenit/Tsyklon/Vostok families, launches 1971–1995).
• Maximum sun elevation during successful ranging was 39° (2019/03/22, 10:31 local time). The longest continuous measurement lasted ~100 s.
• Real-time and iterative adjustment of time bias (examples: +9 ms, −76 ms, +140 ms, −150 ms during observations) and detector gate shifts enabled sustained detection despite prediction errors. Correct biasing produced linear observed-minus-calculated residuals.
• Post-processed close-ups correcting time and range biases (e.g., tb = +200 ms with rb = 415 m; tb = −158 ms with rb = 133 m) aligned measurements with predictions, revealing fine structure.
• Detected returns from the front and back of rocket bodies, showing range separations up to ~8 m, providing a lower-bound size estimate of the targets.
• Daylight visualization capability validated: stars from magnitude 0.15 to 8.25 were imaged and automatically detected at sun elevations of 10–18°, matching contrast predictions. Over 40 different upper-stage rocket bodies were visually acquired in daylight using the wide-field camera.
• Operational impact: Including daylight alongside twilight increases potential SDLR observation time at Graz (47.1°N) up to ~22 h per day and at San Fernando (36.5°N) to ~18 h, vastly expanding measurement opportunities.

The work demonstrates that optical visualization and real-time bias correction enable SDLR during daylight, directly addressing the constraint that previously limited debris ranging to twilight. By estimating along-track time bias and across-track pointing errors from real-time images and iteratively adjusting the detector gate, the system compensates for large TLE prediction errors that otherwise preclude ranging. The observed daylight returns and ability to recover fine range structures validate the approach and show that meaningful geometrical information (e.g., target length scales) can be obtained. The substantial increase in potential observation time extends coverage for selected targets, which is critical for rapidly improving orbits needed for conjunction screening, avoidance maneuver planning, and enabling future active removal and laser nudging concepts. Lower-latitude stations still gain significant daylight capacity, suggesting a globally distributed SDLR network could respond promptly to high-priority events and deliver order-of-magnitude improvements in orbit predictions when combined with multi-static and data-fusion approaches.

This study presents the first daylight space debris laser ranging measurements, achieved by combining high-contrast daylight visualization, real-time estimation of along-/across-track biases, and iterative detector gate control. The team validated performance with daylight detection of stars to magnitude ~8 and visual acquisition of >40 debris objects, and demonstrated successful daylight ranging on four rocket bodies at sun elevations up to 39°, including recovery of target-length features (~8 m) from range separations. By enabling daytime operations, potential observation time expands to ~18–22 h per day depending on latitude, opening the door to continuous debris monitoring and rapid orbit refinement. Future work should: (i) deploy and coordinate a global SDLR network for near-real-time orbit improvement; (ii) further automate bias estimation and gate optimization to reduce operator intervention; (iii) integrate SDLR with angular optical and radar data for enhanced orbit determination; and (iv) refine hardware (filters, gating, detectors) for improved signal-to-noise under bright-sky conditions.

• Range bias cannot be inferred from images; only iterative shifting of detector activation (gate timing) can compensate, and incorrect shifts can quickly lose the target in noise.
• Daylight operations are limited by sky background (Poisson) noise rather than detector dark counts, demanding tight gating and narrow fields of view; performance is sensitive to timing alignment so that returns arrive shortly after gate opening.
• Residual biases can arise from imperfect alignment between piggyback and receive telescope optical axes, affecting time-bias estimation.
• TLE prediction errors can reach kilometer scale, requiring large fields of view for acquisition and rapid control loops; objects may fall outside small sensors.
• Results are demonstrated on a limited number of daylight passes (four ranging examples); broader validation across object types, geometries, and conditions remains to be shown.