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High precision space debris laser ranging with 4.2 W double-pulse picosecond laser at 1 kHz in 532nm
Optik - International Journal for Light and Electron Optics 179 (2019) 691–699
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Optik journal homepage: www.elsevier.com/locate/ijleo
High precision space debris laser ranging with 4.2 W double-pulse picosecond laser at 1 kHz in 532nm
T
⁎
Zhongping Zhang, Haifeng Zhang, MingLiang Long , Huarong Deng, Zhibo Wu, Wendong Meng Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Picosecond laser Laser ranging Space debris Double pulse
Space debris laser ranging with a 4.2 W double-pulse 532 nm picosecond laser at a pulse repetition frequency of 1 kHz was achieved at the Shanghai satellite laser ranging (SLR) station, which receiving telescope was 60 cm. Space debris’ radar cross sections were from 2 m2 to 12 m2, and ranging precision was up to centimeters. A compound regenerative amplifier was used to realize double pulses output with pulse space of 4.1 ns. Double pulses would have the same detection ability as the equal energy single pulse. A 500 μm diameter avalanche photo-diode detector with less than 10 kHz dark noise and a quantum efficiency of 40% at 532 nm was developed to improve the measurement capability. And also a spectral filter with high efficiency and narrow bandwidth were applied. Basing on the Shanghai SLR system, the laser ranging to space debris with picosecond laser at 1 kHz was successfully implemented for the first time among the global SLR stations. The results showed that the ranging precision root-mean-square can be achieved about 5 cm. It will be very useful to monitor and study the space debris.
1. Introduction As the development of space technology in the world, more and more spacecraft are launched into space, meantime, lots of space debris orbiting the Earth are produced by rocket bodies, upper stage engines, decommissioned satellites, and fragmentation. The most prolific example of the threat was the well-known collision between Iridium 33 and Cosmos 2251 in 2009 [1,2]. This catastrophic event injected thousands of damaging debris into an already cluttered region of the Low Earth Orbit (LEO) environment and shown the necessity of maintaining an accurate catalogue of tracking information and removed [2–7]. And 1307 fragments were cataloged through 168 days from the aftermath of this collision. Recently, the passive Russian BLITS nanosatellite was reportedly struck by a piece of space debris: in a near-circular orbit with a perigee of 818 km, BLITS occupies a crowded region of LEO environment [2]. Therefore, improving the predicted orbit accuracy is necessary to avoid unnecessary anti-collision manoeuvres, or even to remove space debris by laser ablation or other ways. The great threats to the active spacecraft have become the major problem for all spaceactive nations in recent decade years [4]. In recent years, space debris is measured and cataloged by many countries. Currently, approximately 17,000 objects, mostly larger than 10 cm in diameter, were tracked by the US Air Force in their unclassified catalogue. In addition, the estimated number of space debris objects down to a size of about 1 cm was in the order of more than 600,000. Many techniques were developed to observe and move the space debris [3,5–12]. However, monitoring and tracking of space debris is an ongoing challenge, and there is some degree of uncertainty on the populations of different sizes and orbits. The most
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Corresponding author. E-mail address: [email protected] (M. Long).
https://doi.org/10.1016/j.ijleo.2018.10.219 Received 30 September 2018; Accepted 31 October 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
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effective debris removal and mitigation methods can do little about this unless the debris objects' orbital characteristics are known more precisely [8]. Among the techniques of observing space debris, laser ranging is one kind of real time measurement with the high precision, which was one or two orders of magnitude higher than microwave radar. Space debris’ orbit prediction (OP) accuracy of 10–20 arc seconds for the next 24–48 h can be achieved from a single laser ranging station’s data [9]. In recent years, nanosecond laser unit was the most widely used light source for space debris laser ranging owing to the readily available high pulse energy. The first experiment of laser ranging to space debris in China was successfully performed at the Shanghai satellite laser ranging (SLR) station in July 2008 with 20 Hz repetition rate and the power of 40 W @532 nm [11]. In 2012, a 532 nm Nd:YAG laser with pulse repetition frequency (PRF) of 1 kHz, a pulse width of 10 ns, and a pulse energy of 25 mJ was used for space debris laser ranging (SDLR) in Graz SLR station. RCS from > 15 m2 down to < 0.3 m2 were obtained from more than 2500 km, but their ranging precision RMS were about 0.7 m [6]. Laser ranging precision RMS of 39 cm was realized at Shanghai SLR station with a 200 Hz, 50 W nanosecond laser system in years of 2014 [12]. And GSFC's ground-based laser ranging facility with 1.2 m telescope was using to provide meter-level or better ranging precision on optically passive 10∼30 cm orbital debris targets, which goal was to improve current predictions up to 85%. Over the next decade, NASA plans to build ten new SLR systems and deploy them globally. Other countries are expected to partner in this field to create a total of more than 20 new systems distributed globally over the next few decades. Designed specifically for cooperative Earth orbiting satellite ranging, these systems could be upgraded to support orbital debris tracking [8]. The size of space debris expressed by radar cross sections (RCS) and an average precision were limited to the laser’s power and pulse width [7,11]. As the simple ranging equation (1)
R = c•t The range precision ΔR was as shown:
(2)
ΔR = c•Δt
So, the shorter the time of Δt was, the better range precision ΔR would be. Picosecond laser has a much narrower pulse width than nanosecond laser, and that makes it more beneficial for high ranging precision. On satellites which carried the laser retro-reflector, the laser ranging has been routinely performed in global SLR stations and the ranging precision RMS was up to sub-centimeter [13,14]. However, space debris don’t have the retro-reflector, laser was diffuse reflected. Up to now, there is no report on the application of laser ranging to space debris with picosecond laser. For picosecond laser, it is not easy to obtain high pulse energy owing the pulse width of picosecond. Generally, regenerative amplification is the best method to per-amplifier picosecond seed laser with low pulse energy. In this way, low energy mode-locked laser (nJ) can be amplified to micro joule (uJ) or mill joule (mJ) level through the single stage regenerative amplifier (RA) and then amplified to tens of mJ or even joules by the other techniques. In order to improve the power, a compound regenerative amplifier was used to obtain double pulses, and fourteen number of space debris was measuring with the double-pulse picosecond laser in this paper. The best ranging precision RMS can be achieved at 46.9 mm for space debris (ID.19574, rock bodies) with RCS of 4.96 m2. It’s also the first time using a double-pulse picosecond laser for tracking space debris. It makes the ranging precision of laser measurements to space debris step into the centimeters level from the meters and decimeters.
Fig. 1. The structure of space debris laser ranging system. 692
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2. Experiment design The space debris laser ranging system was established based on the Shanghai SLR system. Fig. 1 shows the structure of space debris laser ranging system, including orbit prediction, control system, laser system, laser transmitting system, telescope tracking mount system, high precise timing system, return detection and receiving system [11]. Space debris laser ranging system was as shown in Fig. 1. Firstly, the space debris orbit prediction parameters were downloaded to the space debris laser ranging control system computer, and the computer processes and converts the space debris orbit prediction parameters, then it sent out control command to servo system, which controlled the telescope mount to track the space debris with accurate feedback track from the encoder. So the space debris would be monitored by the receiving system's monitoring charge coupled device (CCD). At this time, the picosecond laser was ignited and put out command from the computer, through the coude telescope launch system to the target of space debris, and also pulse signals were received at the laser emission with the photodetector. Pulse signals were processed by discriminator, and sent to the event timer. The event timer would be started, and sent the start time t1 to the computer. And also the start time signal would be sent to the range gate generator to produce gate signal. The space debris laser returns signals passed through the receiving system to the APD and the return time t2 was obtained by event timer. In all, the range gate generator, computer and event timer were provided high precision time by the time/frequency reference. The range of space debris R was as shown:
R=
(t2 − t1 − t0)•C 2
(3)
The t0 was the space debris laser ranging system time-delay and c in the speed of light. The range precision ΔR was as shown:
ΔR =
C•(Δt2 − Δt1 − Δt0) 2
(4)
The time t1 was very precision with a few picosecond by the discriminator, so the Δt1 would be the precision of event timer; Δt0 was very little in the space debris laser ranging system. However, the return time t2 was influenced very much for the laser pulse width and the shape and size of the space debris. The return pulse was as shown in Fig. 2, pulse width was the half of pulse intensity, the avalanche photodiode (APD) maybe detect the single photon in time of T0, T1 or T2. Therefore, t2 would be one of the times of T0, T1 or T2. That made the time of t2 uncertain and improved its time Δt2, and so the narrower pulse width was, the smaller of the time of Δt2 would be. Therefore, if the pulse width of pulse laser was narrow, the time of t2 would be more precision. In addition, the laser pulse width returning from the space debris was different because of the debris shape and posture. Different shape and posture of debris maybe produce different pulse width. If space debris was right ahead of the SLR station, the optics path of returning laser would be the same distance to the SLR station. While, space debris had an angle of Φ or δ, the optics path reflected by the space debris would have different distance to the SLR station. Those optical path differences made the pulse width much wider. Pulses from different part of space debris would be composed of a new pulse, and the new pulse has a wider pulse width. When the space debris shape was large, and posture was suitable, the pulse width would be broadening more.
2.1. Double-pulse picosecond laser The picosecond laser system was designed by us like the ref [14], in order to obtain higher power, by changing the buildup time of the regenerative amplifier and quarter-wavelength plate (QWP2), double-pulse in a burst was output. Moreover, pulse space was 4.1 ns as shown in Fig. 3. An average power of 4.2 W at 532 nm was achieved with double-pulse, beam quality of M2 factor was 1.2, stability of power RMS in four hours was less than 2% pulse width was compressed to 30 ps, an angle of divergence was 0.6 mrad.
Fig. 2. The return pulse from space debris in the SDLR. 693
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Fig. 3. Waveform of pulse space of 4.1 ns in a burst.
2.2. Low noise and high efficient detector For laser ranging to satellites which carried with retro-reflector, the C-SPAD (Single Photon Avalanche Diode, Peltier cooled) detector was generally used to detect laser return signal [14]. For such detector, the dark noise is high (> 300 kHz @ 1 kHz repetition rates) because of the time walk compensation and a quantum efficiency was about 20% at 532 nm [15]. Therefore, it is not benefit to detect the laser signal from space debris. A breadboard prototype of an avalanche photodiode detector (APD) with low dark noise and high efficiency has been developed based on a compass laser time transfer (LTT) detector. A 500 μm diameter avalanche diode was used with < 10 kHz dark noise @ 1 kHz repetition rates, and a quantum efficiency of 40% at 532 nm by the Czech Technical University [15]. In addition, a spectral filter with high efficiency and narrow bandwidth was also adopted to reduce the level of background noise. Which main characteristics were as follows: (1) center wavelength: 532.15 nm; (2) bandwidth: 1 nm; and (3) efficiency: > 90% @532.15 nm. And the measuring software was also updating to allow for identification of laser returns out of noise in real time, with larger time bias and range bias values requiring large range gates. For comparison of the performances of APD detector, an experiment of measuring high orbit satellite (HEO) satellites with or without filter was performed. Fig. 4 was shown the measuring results for the glnoass107 satellite. It can be seen from results that the noise of APD with filter was much less than the one without filter. The detector with the performances of low noise and high sensitivity is very suitable for detecting the weak laser signal from space debris.
Fig. 4. The comparison of performances APD detector with or without filter. 694
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2.3. Calculation of return single photon According to laser ranging radar link equation and the parameters of measuring system and cross section of targets, the number of laser echo photons can be estimated in theory, which can provide the reference for the actual laser measurement. The formula of laser link equation is as follows [16]:
n0 =
ληq hc
Et Ar ρS cos θ × T 2 × ηt × ηr × α πθt2 R 4
×
(5)
Where, n0 is the average photoelectrons produced from the photo-detector; λ is the wavelength of laser, λ = 532 nm; h is Planck constant, h = 6.6260693 × 10−34J·s; c is light velocity, c = 3 × 108m/s ; ηq is quantum efficiency of the photo-detector at the wavelength of 532 nm; Et is laser energy per pulse; Ar is effective receiving area of telescope; S is RCS of space targets; T is atmosphere one-way transparency; ηt is efficiency of transmitting optics; ηr is efficiency of receiving optics; α,attenuation factor; θt is divergence of laser beam; R is distance from ground station to space target; the ρ is reflection rate of the space debris; cosθ, suppose the targets are spherical, cosθ = 1. In our SLR system and this paper, T = 0.6, ηq = 0.4, Et = 0.0016, Ar = 0.251, ηt = 0.6, ηr = 0.6, α = 0.39, θt = 25urad with 4 and 6 beam expanding telescopes (0.6/4/6 = 0.025 mrad). When the reflection rate of the space debris ρ was 0.2, R = 1000 km, S = 5m2, return photons n0 = 0.01452. In one second, it’s only 14.52 photons at repetition rate of 1 kHz, it’s too little and very hard for the APD to detect, so laser self-focusing should be take into account for the beam diffraction. According to the pulse laser would be self-focusing if its intensity was too strong, the peak power of the pulse laser is:
P1 =
P τf
(6)
In this paper, P = 4.2 W, τ = 30 ps, f = 1000 Hz, considering a double-pulse, the peak power of the pulse laser was:
P1 =
P = 70MW 2τf
(7)
The diameter of the beam D at the transmitter was about 8 cm with 4 and 6 beam expanding telescopes. The peak power density was:
P2 =
P1 D
π ( 2 )2
(8) 2
So the peak power density was 1.39 MW/cm . This intensity would cause a few nonlinear effects in the atmosphere to make the laser beam self-focusing [17]. So the beam would transmit at beam diffraction. An approximate expression for the beam radius that accounts for beam quality and beam diffraction is:
r = M2
2λL πD
(9)
Where M is a factor that describes the beam quality with respect to an ideal Gaussian beam, λ is the laser wavelength, L is the path length from the beam director to the target and D is the diameter of the beam director [17]. In this paper, M2 was equal to 1.2, λ was 0.532 um, L was chosen to 1000 km, D = 0.08 m. So the beam radius r was equal to 4.233 m, and the area of the laser shining was 14.07 m2. The beam diffraction θ was: 2
θ=
r−D L
(10)
Therefore, the θ was 4.15 urad. It’s much smaller than the divergence of laser beam θt = 25 urad. In this, the return photons n0 would be 0.526 in single pulse. Thus, if the laser beam was self-focusing and transmit at beam diffraction, there would be up to 1052 photons at a repetition rate of 1 kHz with double-pulse in one second. This is more helpful to detect laser signal from space debris. 2.4. Probability of detection for double pulse The detection of photoelectrons follows Poisson statistics. The probability of detecting one photoelectron is given by the equation [11,16]:
ξ = 1 − e−n0
(11)
For pulses, assume that the amplitude of the Nth pulse is bNA, and the number of photons corresponding to amplitude A is n0. The total probability of photoelectrons received by N pulses is: N
ξ1 − N (1) =
∑ ξn (1)=ξ1 (1) + ξ ′1 (1) ξ2 (1) + ξ ′1 (1) ξ ′2 (1) ξ3 (1) + +•••+ξ ′1 (1) ξ ′2 (1) ξ ′3 (1) ξ3 (1) 1
ξ1 − N (1) = 1 −
e−b1 n0
+
e−b1 n0 (1
−
e−b2 n0)
+
•••+e−b1 n0 e−b2 n0•••e−bN − 1 n0 (1
So: 695
−
e−bN n0 )
(12) (13)
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Z. Zhang et al.
Table 1 ranging echo rate with single pulse or double pulses.
The first pulse The second pulsed
Double pulses (%) Total (%) Single pulse (%)
1
2
3
4
5
6
7
0.43 0.43 0.86 0.89
0.4 0.45 0.85 0.87
0.37 0.46 0.83 0.79
0.35 0.48 0.83 0.85
0.44 0.48 0.92 0.83
0.47 0.36 0.83 0.86
0.37 0.43 0.8 0.89
Table 2 the results of picosecond laser ranging to space debris. number
Date
ID
Type of space debris
Measured Distance(km)
RMS (mm)
RCS (m2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2017-8-18 2017-8-18 2017-8-18 2017-8-18 2017-8-18 2017-8-18 2017-8-24 2017-8-27 2018-1-9 2018-1-9 2018-1-9 2018-1-9 2018-3-28 2018-3-28
21423 25,979 19275 19574 19,650 27665 14700 22220 38345 38347 20362 38346 20303 20453
Cosmos 2151 Rocket Helios 1B Rocket Okean 1 Rocket Cosmos 1975 Rocket Cosmos 1980 Rocket GPS 51 Dl Rocket Cosmos 1536 Rocket Cosmos 2219 Rocket GCOM W1 ShrdF GCOMW1 Adptr Debris GPS 2-05 r1 GCOM W1 ShrdG GPS 2-04 r1 GPS 2-06 r1
879∼1132.7 948.7∼1020.8 863.4∼1433.7 683.2∼962.0 885.7∼1156.5 853.7∼870 709∼739.7 860.8∼874.2 888.7∼1005.7 750.9∼979.6 649.1∼853.9 810.8∼1010.7 656.5∼896.8 684.1∼766
56.4 594.3 315.4 46.9 2165 486.3 217.1 1592.4 58.3 164.6 61.4 317.5 400.5 1002.8
4.61 12.01 4.69 4.96 12.11 5.55 4.06 5.72 4.43 10.71 9.99 2.51 2 9.86
ξ1 − N (1) = 1 − e−b1 n0 + e−b1 n0 (1 − e−b2 n0) + •••+e−(b1+ b2 + ••• + bN − 1) n0 (1 − e−bN n0 )
(14)
ξ1 − N (1) = 1 − e−(b1+ b2 + ••• + bN − 1+ bN ) n0
(15)
and：
Owing to
n0 =
ληq hc
×
Et Ar ρS cos θ × T 2 × ηt × ηr × α πθt2 R 4
(16)
that：
ξ1 − N (1) = 1 − e
ληq (b1+ b2 + ••• + bN − 1+ bN ) Et Ar ρS cos θ − 162 × × × T 2× ηt × ηr × α hc π πR 4θt2
(17)
This equation shows that the detection success rate of multiple pulses can be equivalent to the single pulse detection probability of the sum of pulse energies. That is, in the case where the sum of the energies of the plurality of pulses is equal to the single large energy pulse, the detection efficiency would be consistent. The above indicates that the detection success rate is greatly affected by the total energy of the pulse. This means the detection success rate is constant regardless of the number or amplitude of single or multiple different pulses under the same energy. For double pulses, the total energy E was sum of the first pulse energy E1 and the second energy E2. In double pulses: (18)
E = E1 + E2 So echo photons of the double pulses are:
n1 =
ληq hc
×
(E1 + E2) Ar ρS cos θ × T 2 × ηt × ηr × α πθt2 R 4
(19)
The probability of detecting one photoelectron of double pulses was:
ξ = 1 − e−n1 = 1 − e
−
ληq hc
×
(E1+ E2) Ar ρS cos θ πθt2 R 4
× T 2× ηt × ηr × α
(20)
Therefore, the probability of detecting one photoelectron from double pulses laser was equal to the single pulse laser, which single pulse energy was the same as the sum of the double pulses. Large energy in double pulses was easier to obtain than the single pulse for picosecond laser. It’s very beneficial to laser ranging in weak signal. 696
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Fig. 5. Laser returns from space debris of ID.19574 (a) and 25,979 (b).
2.5. Other setup of the SLR system At Shanghai SLR system, the aperture of the receiving telescope and transmitter was 60 cm and 21 cm respectively. The mount of the telescope was Alt-Azimuth type, directly driven with motors. In order to satisfy the requirement for tracking space debris, the tracking error of was less than 1′′ [11], and the pointing accuracy after star calibration was about 3′′. The event timer Model A033-ET from Riga University in Latvia was used for measuring time intervals, which precision was 10 ps. The orbit predictions were tracked by the US Air Force in their unclassified catalogue and two-line element (TLE) data sets for these 697
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Fig. 6. Laser returns from space debris of ID.38346.
objects are available through www.Space-Track.-Org. The TLE produced with a range error of hundreds of meters and kilometers by North American Aerospace Defense Command (NORAD). 3. Results and discussion In order to prove the probability of detection for double pulses, laser which can put a single pulse or double pulses in same power was used to laser ranging with the target replace of the space debris on the ground. Laser ranging echo rate was shown below:
β=
N (t ) f ×t
(21)
Which N(t) means the number of laser ranging echo in the time of t. f was mean the PRF of the laser. Ranging echo rate with single pulse or double pulses in same power was shown in the Table 1.The total of double pulses ranging echo rate was almost equal to the single pulse’s, it’s very in accord with the analysis. 3.1. Results of picosecond Laser ranging to space debris With the above configure of the SLR system, much space debris was measured, such as the space debris cataloged by the NORAD of ID.21423, 19574, 25979, 19650 and so on, as shown in Table 2. And also the type of targets, measured distance, ranging precision RMS and RCS from the space debris was also shown in Table 2. Five Cosmos space debris was measured in Table 2, the measured distance was about 1000 km, the smaller RCS of space debris was 2m2 with the ranging precision RMS of 400.5 mm, and the best ranging precision RMS was 46.9 mm with the RCS of 4.96 m2. The range residuals (difference of the observed and predicted) from the space debris of ID.19574 with RCS of 4.96 m2 were shown in Fig. 5(a), the central points were the laser returns, and the other points were noise from the detector and the background. The laser returns were double lines with double lines RMS of 344.1 mm and single lines RMS of 46.9 mm. Moreover, the laser returns of ID.21423 were double lines. In Fig. 5(b), the laser returns from the space debris of ID.25,979 with RCS of 12.01 m2 were just single line with RMS of 594.3 mm. For the larger RCS targets, like ID.225979, 19,650, which RCS were 12.01 m2 and 12.11 m2 respectively, laser returns were single lines. The total ranging precision was 300∼400 mm in RCS of 4∼6 m2 and it’s in accord with the two pulses in a burst with the pulse space of 4.1 ns. When choosing one line of data, the RMS was about 40∼60 mm. For the one line of data, the RMS was limited by the resolution of APD. These results for space debris with double pulses maybe thank to the first pulse and the second pulse in bursts was less returned back to APD at the same time, and then the two pulses were made coherent. And also the different shape and posture of space debris made the difference ranging precision even the RCS of space debris was nearly the same. The different RMS for the nearly same RCS (ID: 25979 and 19650) was in accord with the different shape and posture of space debris. In addition, with the high precision, some more detailed information of space debris could be provided. Like as shown as Fig. 6, from the laser echoes of the space debris ID: 28346, it’s very obvious that this space debris was spinning. These results from picosecond laser system are the first ones among global SLR stations and the feasibility of laser tracking debris 698
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with low power. The laser beam self-focusing made space debris measured come true with the low power of picosecond laser. That picosecond laser will be validated to change the common view of high power and nanosecond laser system for tracking debris. Double pulses would be a good way to achieve large energy picosecond laser for the SDLR. 4. Conclusions We developed the 4.2 W, 30 ps at 532 nm picosecond laser and low noise detector and successfully measured space debris for the first time among global SLR stations based on Shanghai SLR system. For picosecond laser system, the compound regenerative cavity was a new way to obtain double-pulse in a burst. From the measuring results, the best ranging precision RMS can be achieved at 46.9 mm for space debris (ID.19574, rock bodies) with RCS of 4.96 m2. It’s also the first time using a double-pulse picosecond laser for tracking space debris. It makes the ranging precision of laser measurements to space debris step into the centimeters level from the meters and decimeters. It’s shown that picosecond laser have more advantage than nanosecond laser. Using more precision and better detector with higher power picosecond laser system, more and more space debris would be measured, and also for the better RMS. It is very helpful to research for space debris. As a next step, we are going to further update the picosecond laser system to increase the output power to measure smaller RCS of space debris with the ranging precision RMS of centimeters level. Acknowledgements Thanks the US Space Track Organization (www.space-track.org) for making available the Two-Line Elements of cataloged objects. And this work was supported by the National Natural Science Foundation (NSF) of China (U1631240 and 11503068); CAS Key Technology Talent Program; Project funded by China Postdoctoral Science Foundation (BX201700270 and 2017M621562). 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High precision space debris laser ranging with 4.2 W double-pulse picosecond laser at 1 kHz in 532nm
T
⁎
Zhongping Zhang, Haifeng Zhang, MingLiang Long , Huarong Deng, Zhibo Wu, Wendong Meng Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Picosecond laser Laser ranging Space debris Double pulse
Space debris laser ranging with a 4.2 W double-pulse 532 nm picosecond laser at a pulse repetition frequency of 1 kHz was achieved at the Shanghai satellite laser ranging (SLR) station, which receiving telescope was 60 cm. Space debris’ radar cross sections were from 2 m2 to 12 m2, and ranging precision was up to centimeters. A compound regenerative amplifier was used to realize double pulses output with pulse space of 4.1 ns. Double pulses would have the same detection ability as the equal energy single pulse. A 500 μm diameter avalanche photo-diode detector with less than 10 kHz dark noise and a quantum efficiency of 40% at 532 nm was developed to improve the measurement capability. And also a spectral filter with high efficiency and narrow bandwidth were applied. Basing on the Shanghai SLR system, the laser ranging to space debris with picosecond laser at 1 kHz was successfully implemented for the first time among the global SLR stations. The results showed that the ranging precision root-mean-square can be achieved about 5 cm. It will be very useful to monitor and study the space debris.
1. Introduction As the development of space technology in the world, more and more spacecraft are launched into space, meantime, lots of space debris orbiting the Earth are produced by rocket bodies, upper stage engines, decommissioned satellites, and fragmentation. The most prolific example of the threat was the well-known collision between Iridium 33 and Cosmos 2251 in 2009 [1,2]. This catastrophic event injected thousands of damaging debris into an already cluttered region of the Low Earth Orbit (LEO) environment and shown the necessity of maintaining an accurate catalogue of tracking information and removed [2–7]. And 1307 fragments were cataloged through 168 days from the aftermath of this collision. Recently, the passive Russian BLITS nanosatellite was reportedly struck by a piece of space debris: in a near-circular orbit with a perigee of 818 km, BLITS occupies a crowded region of LEO environment [2]. Therefore, improving the predicted orbit accuracy is necessary to avoid unnecessary anti-collision manoeuvres, or even to remove space debris by laser ablation or other ways. The great threats to the active spacecraft have become the major problem for all spaceactive nations in recent decade years [4]. In recent years, space debris is measured and cataloged by many countries. Currently, approximately 17,000 objects, mostly larger than 10 cm in diameter, were tracked by the US Air Force in their unclassified catalogue. In addition, the estimated number of space debris objects down to a size of about 1 cm was in the order of more than 600,000. Many techniques were developed to observe and move the space debris [3,5–12]. However, monitoring and tracking of space debris is an ongoing challenge, and there is some degree of uncertainty on the populations of different sizes and orbits. The most
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Corresponding author. E-mail address: [email protected] (M. Long).
https://doi.org/10.1016/j.ijleo.2018.10.219 Received 30 September 2018; Accepted 31 October 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
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effective debris removal and mitigation methods can do little about this unless the debris objects' orbital characteristics are known more precisely [8]. Among the techniques of observing space debris, laser ranging is one kind of real time measurement with the high precision, which was one or two orders of magnitude higher than microwave radar. Space debris’ orbit prediction (OP) accuracy of 10–20 arc seconds for the next 24–48 h can be achieved from a single laser ranging station’s data [9]. In recent years, nanosecond laser unit was the most widely used light source for space debris laser ranging owing to the readily available high pulse energy. The first experiment of laser ranging to space debris in China was successfully performed at the Shanghai satellite laser ranging (SLR) station in July 2008 with 20 Hz repetition rate and the power of 40 W @532 nm [11]. In 2012, a 532 nm Nd:YAG laser with pulse repetition frequency (PRF) of 1 kHz, a pulse width of 10 ns, and a pulse energy of 25 mJ was used for space debris laser ranging (SDLR) in Graz SLR station. RCS from > 15 m2 down to < 0.3 m2 were obtained from more than 2500 km, but their ranging precision RMS were about 0.7 m [6]. Laser ranging precision RMS of 39 cm was realized at Shanghai SLR station with a 200 Hz, 50 W nanosecond laser system in years of 2014 [12]. And GSFC's ground-based laser ranging facility with 1.2 m telescope was using to provide meter-level or better ranging precision on optically passive 10∼30 cm orbital debris targets, which goal was to improve current predictions up to 85%. Over the next decade, NASA plans to build ten new SLR systems and deploy them globally. Other countries are expected to partner in this field to create a total of more than 20 new systems distributed globally over the next few decades. Designed specifically for cooperative Earth orbiting satellite ranging, these systems could be upgraded to support orbital debris tracking [8]. The size of space debris expressed by radar cross sections (RCS) and an average precision were limited to the laser’s power and pulse width [7,11]. As the simple ranging equation (1)
R = c•t The range precision ΔR was as shown:
(2)
ΔR = c•Δt
So, the shorter the time of Δt was, the better range precision ΔR would be. Picosecond laser has a much narrower pulse width than nanosecond laser, and that makes it more beneficial for high ranging precision. On satellites which carried the laser retro-reflector, the laser ranging has been routinely performed in global SLR stations and the ranging precision RMS was up to sub-centimeter [13,14]. However, space debris don’t have the retro-reflector, laser was diffuse reflected. Up to now, there is no report on the application of laser ranging to space debris with picosecond laser. For picosecond laser, it is not easy to obtain high pulse energy owing the pulse width of picosecond. Generally, regenerative amplification is the best method to per-amplifier picosecond seed laser with low pulse energy. In this way, low energy mode-locked laser (nJ) can be amplified to micro joule (uJ) or mill joule (mJ) level through the single stage regenerative amplifier (RA) and then amplified to tens of mJ or even joules by the other techniques. In order to improve the power, a compound regenerative amplifier was used to obtain double pulses, and fourteen number of space debris was measuring with the double-pulse picosecond laser in this paper. The best ranging precision RMS can be achieved at 46.9 mm for space debris (ID.19574, rock bodies) with RCS of 4.96 m2. It’s also the first time using a double-pulse picosecond laser for tracking space debris. It makes the ranging precision of laser measurements to space debris step into the centimeters level from the meters and decimeters.
Fig. 1. The structure of space debris laser ranging system. 692
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2. Experiment design The space debris laser ranging system was established based on the Shanghai SLR system. Fig. 1 shows the structure of space debris laser ranging system, including orbit prediction, control system, laser system, laser transmitting system, telescope tracking mount system, high precise timing system, return detection and receiving system [11]. Space debris laser ranging system was as shown in Fig. 1. Firstly, the space debris orbit prediction parameters were downloaded to the space debris laser ranging control system computer, and the computer processes and converts the space debris orbit prediction parameters, then it sent out control command to servo system, which controlled the telescope mount to track the space debris with accurate feedback track from the encoder. So the space debris would be monitored by the receiving system's monitoring charge coupled device (CCD). At this time, the picosecond laser was ignited and put out command from the computer, through the coude telescope launch system to the target of space debris, and also pulse signals were received at the laser emission with the photodetector. Pulse signals were processed by discriminator, and sent to the event timer. The event timer would be started, and sent the start time t1 to the computer. And also the start time signal would be sent to the range gate generator to produce gate signal. The space debris laser returns signals passed through the receiving system to the APD and the return time t2 was obtained by event timer. In all, the range gate generator, computer and event timer were provided high precision time by the time/frequency reference. The range of space debris R was as shown:
R=
(t2 − t1 − t0)•C 2
(3)
The t0 was the space debris laser ranging system time-delay and c in the speed of light. The range precision ΔR was as shown:
ΔR =
C•(Δt2 − Δt1 − Δt0) 2
(4)
The time t1 was very precision with a few picosecond by the discriminator, so the Δt1 would be the precision of event timer; Δt0 was very little in the space debris laser ranging system. However, the return time t2 was influenced very much for the laser pulse width and the shape and size of the space debris. The return pulse was as shown in Fig. 2, pulse width was the half of pulse intensity, the avalanche photodiode (APD) maybe detect the single photon in time of T0, T1 or T2. Therefore, t2 would be one of the times of T0, T1 or T2. That made the time of t2 uncertain and improved its time Δt2, and so the narrower pulse width was, the smaller of the time of Δt2 would be. Therefore, if the pulse width of pulse laser was narrow, the time of t2 would be more precision. In addition, the laser pulse width returning from the space debris was different because of the debris shape and posture. Different shape and posture of debris maybe produce different pulse width. If space debris was right ahead of the SLR station, the optics path of returning laser would be the same distance to the SLR station. While, space debris had an angle of Φ or δ, the optics path reflected by the space debris would have different distance to the SLR station. Those optical path differences made the pulse width much wider. Pulses from different part of space debris would be composed of a new pulse, and the new pulse has a wider pulse width. When the space debris shape was large, and posture was suitable, the pulse width would be broadening more.
2.1. Double-pulse picosecond laser The picosecond laser system was designed by us like the ref [14], in order to obtain higher power, by changing the buildup time of the regenerative amplifier and quarter-wavelength plate (QWP2), double-pulse in a burst was output. Moreover, pulse space was 4.1 ns as shown in Fig. 3. An average power of 4.2 W at 532 nm was achieved with double-pulse, beam quality of M2 factor was 1.2, stability of power RMS in four hours was less than 2% pulse width was compressed to 30 ps, an angle of divergence was 0.6 mrad.
Fig. 2. The return pulse from space debris in the SDLR. 693
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Fig. 3. Waveform of pulse space of 4.1 ns in a burst.
2.2. Low noise and high efficient detector For laser ranging to satellites which carried with retro-reflector, the C-SPAD (Single Photon Avalanche Diode, Peltier cooled) detector was generally used to detect laser return signal [14]. For such detector, the dark noise is high (> 300 kHz @ 1 kHz repetition rates) because of the time walk compensation and a quantum efficiency was about 20% at 532 nm [15]. Therefore, it is not benefit to detect the laser signal from space debris. A breadboard prototype of an avalanche photodiode detector (APD) with low dark noise and high efficiency has been developed based on a compass laser time transfer (LTT) detector. A 500 μm diameter avalanche diode was used with < 10 kHz dark noise @ 1 kHz repetition rates, and a quantum efficiency of 40% at 532 nm by the Czech Technical University [15]. In addition, a spectral filter with high efficiency and narrow bandwidth was also adopted to reduce the level of background noise. Which main characteristics were as follows: (1) center wavelength: 532.15 nm; (2) bandwidth: 1 nm; and (3) efficiency: > 90% @532.15 nm. And the measuring software was also updating to allow for identification of laser returns out of noise in real time, with larger time bias and range bias values requiring large range gates. For comparison of the performances of APD detector, an experiment of measuring high orbit satellite (HEO) satellites with or without filter was performed. Fig. 4 was shown the measuring results for the glnoass107 satellite. It can be seen from results that the noise of APD with filter was much less than the one without filter. The detector with the performances of low noise and high sensitivity is very suitable for detecting the weak laser signal from space debris.
Fig. 4. The comparison of performances APD detector with or without filter. 694
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2.3. Calculation of return single photon According to laser ranging radar link equation and the parameters of measuring system and cross section of targets, the number of laser echo photons can be estimated in theory, which can provide the reference for the actual laser measurement. The formula of laser link equation is as follows [16]:
n0 =
ληq hc
Et Ar ρS cos θ × T 2 × ηt × ηr × α πθt2 R 4
×
(5)
Where, n0 is the average photoelectrons produced from the photo-detector; λ is the wavelength of laser, λ = 532 nm; h is Planck constant, h = 6.6260693 × 10−34J·s; c is light velocity, c = 3 × 108m/s ; ηq is quantum efficiency of the photo-detector at the wavelength of 532 nm; Et is laser energy per pulse; Ar is effective receiving area of telescope; S is RCS of space targets; T is atmosphere one-way transparency; ηt is efficiency of transmitting optics; ηr is efficiency of receiving optics; α,attenuation factor; θt is divergence of laser beam; R is distance from ground station to space target; the ρ is reflection rate of the space debris; cosθ, suppose the targets are spherical, cosθ = 1. In our SLR system and this paper, T = 0.6, ηq = 0.4, Et = 0.0016, Ar = 0.251, ηt = 0.6, ηr = 0.6, α = 0.39, θt = 25urad with 4 and 6 beam expanding telescopes (0.6/4/6 = 0.025 mrad). When the reflection rate of the space debris ρ was 0.2, R = 1000 km, S = 5m2, return photons n0 = 0.01452. In one second, it’s only 14.52 photons at repetition rate of 1 kHz, it’s too little and very hard for the APD to detect, so laser self-focusing should be take into account for the beam diffraction. According to the pulse laser would be self-focusing if its intensity was too strong, the peak power of the pulse laser is:
P1 =
P τf
(6)
In this paper, P = 4.2 W, τ = 30 ps, f = 1000 Hz, considering a double-pulse, the peak power of the pulse laser was:
P1 =
P = 70MW 2τf
(7)
The diameter of the beam D at the transmitter was about 8 cm with 4 and 6 beam expanding telescopes. The peak power density was:
P2 =
P1 D
π ( 2 )2
(8) 2
So the peak power density was 1.39 MW/cm . This intensity would cause a few nonlinear effects in the atmosphere to make the laser beam self-focusing [17]. So the beam would transmit at beam diffraction. An approximate expression for the beam radius that accounts for beam quality and beam diffraction is:
r = M2
2λL πD
(9)
Where M is a factor that describes the beam quality with respect to an ideal Gaussian beam, λ is the laser wavelength, L is the path length from the beam director to the target and D is the diameter of the beam director [17]. In this paper, M2 was equal to 1.2, λ was 0.532 um, L was chosen to 1000 km, D = 0.08 m. So the beam radius r was equal to 4.233 m, and the area of the laser shining was 14.07 m2. The beam diffraction θ was: 2
θ=
r−D L
(10)
Therefore, the θ was 4.15 urad. It’s much smaller than the divergence of laser beam θt = 25 urad. In this, the return photons n0 would be 0.526 in single pulse. Thus, if the laser beam was self-focusing and transmit at beam diffraction, there would be up to 1052 photons at a repetition rate of 1 kHz with double-pulse in one second. This is more helpful to detect laser signal from space debris. 2.4. Probability of detection for double pulse The detection of photoelectrons follows Poisson statistics. The probability of detecting one photoelectron is given by the equation [11,16]:
ξ = 1 − e−n0
(11)
For pulses, assume that the amplitude of the Nth pulse is bNA, and the number of photons corresponding to amplitude A is n0. The total probability of photoelectrons received by N pulses is: N
ξ1 − N (1) =
∑ ξn (1)=ξ1 (1) + ξ ′1 (1) ξ2 (1) + ξ ′1 (1) ξ ′2 (1) ξ3 (1) + +•••+ξ ′1 (1) ξ ′2 (1) ξ ′3 (1) ξ3 (1) 1
ξ1 − N (1) = 1 −
e−b1 n0
+
e−b1 n0 (1
−
e−b2 n0)
+
•••+e−b1 n0 e−b2 n0•••e−bN − 1 n0 (1
So: 695
−
e−bN n0 )
(12) (13)
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Table 1 ranging echo rate with single pulse or double pulses.
The first pulse The second pulsed
Double pulses (%) Total (%) Single pulse (%)
1
2
3
4
5
6
7
0.43 0.43 0.86 0.89
0.4 0.45 0.85 0.87
0.37 0.46 0.83 0.79
0.35 0.48 0.83 0.85
0.44 0.48 0.92 0.83
0.47 0.36 0.83 0.86
0.37 0.43 0.8 0.89
Table 2 the results of picosecond laser ranging to space debris. number
Date
ID
Type of space debris
Measured Distance(km)
RMS (mm)
RCS (m2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2017-8-18 2017-8-18 2017-8-18 2017-8-18 2017-8-18 2017-8-18 2017-8-24 2017-8-27 2018-1-9 2018-1-9 2018-1-9 2018-1-9 2018-3-28 2018-3-28
21423 25,979 19275 19574 19,650 27665 14700 22220 38345 38347 20362 38346 20303 20453
Cosmos 2151 Rocket Helios 1B Rocket Okean 1 Rocket Cosmos 1975 Rocket Cosmos 1980 Rocket GPS 51 Dl Rocket Cosmos 1536 Rocket Cosmos 2219 Rocket GCOM W1 ShrdF GCOMW1 Adptr Debris GPS 2-05 r1 GCOM W1 ShrdG GPS 2-04 r1 GPS 2-06 r1
879∼1132.7 948.7∼1020.8 863.4∼1433.7 683.2∼962.0 885.7∼1156.5 853.7∼870 709∼739.7 860.8∼874.2 888.7∼1005.7 750.9∼979.6 649.1∼853.9 810.8∼1010.7 656.5∼896.8 684.1∼766
56.4 594.3 315.4 46.9 2165 486.3 217.1 1592.4 58.3 164.6 61.4 317.5 400.5 1002.8
4.61 12.01 4.69 4.96 12.11 5.55 4.06 5.72 4.43 10.71 9.99 2.51 2 9.86
ξ1 − N (1) = 1 − e−b1 n0 + e−b1 n0 (1 − e−b2 n0) + •••+e−(b1+ b2 + ••• + bN − 1) n0 (1 − e−bN n0 )
(14)
ξ1 − N (1) = 1 − e−(b1+ b2 + ••• + bN − 1+ bN ) n0
(15)
and：
Owing to
n0 =
ληq hc
×
Et Ar ρS cos θ × T 2 × ηt × ηr × α πθt2 R 4
(16)
that：
ξ1 − N (1) = 1 − e
ληq (b1+ b2 + ••• + bN − 1+ bN ) Et Ar ρS cos θ − 162 × × × T 2× ηt × ηr × α hc π πR 4θt2
(17)
This equation shows that the detection success rate of multiple pulses can be equivalent to the single pulse detection probability of the sum of pulse energies. That is, in the case where the sum of the energies of the plurality of pulses is equal to the single large energy pulse, the detection efficiency would be consistent. The above indicates that the detection success rate is greatly affected by the total energy of the pulse. This means the detection success rate is constant regardless of the number or amplitude of single or multiple different pulses under the same energy. For double pulses, the total energy E was sum of the first pulse energy E1 and the second energy E2. In double pulses: (18)
E = E1 + E2 So echo photons of the double pulses are:
n1 =
ληq hc
×
(E1 + E2) Ar ρS cos θ × T 2 × ηt × ηr × α πθt2 R 4
(19)
The probability of detecting one photoelectron of double pulses was:
ξ = 1 − e−n1 = 1 − e
−
ληq hc
×
(E1+ E2) Ar ρS cos θ πθt2 R 4
× T 2× ηt × ηr × α
(20)
Therefore, the probability of detecting one photoelectron from double pulses laser was equal to the single pulse laser, which single pulse energy was the same as the sum of the double pulses. Large energy in double pulses was easier to obtain than the single pulse for picosecond laser. It’s very beneficial to laser ranging in weak signal. 696
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Fig. 5. Laser returns from space debris of ID.19574 (a) and 25,979 (b).
2.5. Other setup of the SLR system At Shanghai SLR system, the aperture of the receiving telescope and transmitter was 60 cm and 21 cm respectively. The mount of the telescope was Alt-Azimuth type, directly driven with motors. In order to satisfy the requirement for tracking space debris, the tracking error of was less than 1′′ [11], and the pointing accuracy after star calibration was about 3′′. The event timer Model A033-ET from Riga University in Latvia was used for measuring time intervals, which precision was 10 ps. The orbit predictions were tracked by the US Air Force in their unclassified catalogue and two-line element (TLE) data sets for these 697
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Fig. 6. Laser returns from space debris of ID.38346.
objects are available through www.Space-Track.-Org. The TLE produced with a range error of hundreds of meters and kilometers by North American Aerospace Defense Command (NORAD). 3. Results and discussion In order to prove the probability of detection for double pulses, laser which can put a single pulse or double pulses in same power was used to laser ranging with the target replace of the space debris on the ground. Laser ranging echo rate was shown below:
β=
N (t ) f ×t
(21)
Which N(t) means the number of laser ranging echo in the time of t. f was mean the PRF of the laser. Ranging echo rate with single pulse or double pulses in same power was shown in the Table 1.The total of double pulses ranging echo rate was almost equal to the single pulse’s, it’s very in accord with the analysis. 3.1. Results of picosecond Laser ranging to space debris With the above configure of the SLR system, much space debris was measured, such as the space debris cataloged by the NORAD of ID.21423, 19574, 25979, 19650 and so on, as shown in Table 2. And also the type of targets, measured distance, ranging precision RMS and RCS from the space debris was also shown in Table 2. Five Cosmos space debris was measured in Table 2, the measured distance was about 1000 km, the smaller RCS of space debris was 2m2 with the ranging precision RMS of 400.5 mm, and the best ranging precision RMS was 46.9 mm with the RCS of 4.96 m2. The range residuals (difference of the observed and predicted) from the space debris of ID.19574 with RCS of 4.96 m2 were shown in Fig. 5(a), the central points were the laser returns, and the other points were noise from the detector and the background. The laser returns were double lines with double lines RMS of 344.1 mm and single lines RMS of 46.9 mm. Moreover, the laser returns of ID.21423 were double lines. In Fig. 5(b), the laser returns from the space debris of ID.25,979 with RCS of 12.01 m2 were just single line with RMS of 594.3 mm. For the larger RCS targets, like ID.225979, 19,650, which RCS were 12.01 m2 and 12.11 m2 respectively, laser returns were single lines. The total ranging precision was 300∼400 mm in RCS of 4∼6 m2 and it’s in accord with the two pulses in a burst with the pulse space of 4.1 ns. When choosing one line of data, the RMS was about 40∼60 mm. For the one line of data, the RMS was limited by the resolution of APD. These results for space debris with double pulses maybe thank to the first pulse and the second pulse in bursts was less returned back to APD at the same time, and then the two pulses were made coherent. And also the different shape and posture of space debris made the difference ranging precision even the RCS of space debris was nearly the same. The different RMS for the nearly same RCS (ID: 25979 and 19650) was in accord with the different shape and posture of space debris. In addition, with the high precision, some more detailed information of space debris could be provided. Like as shown as Fig. 6, from the laser echoes of the space debris ID: 28346, it’s very obvious that this space debris was spinning. These results from picosecond laser system are the first ones among global SLR stations and the feasibility of laser tracking debris 698
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with low power. The laser beam self-focusing made space debris measured come true with the low power of picosecond laser. That picosecond laser will be validated to change the common view of high power and nanosecond laser system for tracking debris. Double pulses would be a good way to achieve large energy picosecond laser for the SDLR. 4. Conclusions We developed the 4.2 W, 30 ps at 532 nm picosecond laser and low noise detector and successfully measured space debris for the first time among global SLR stations based on Shanghai SLR system. For picosecond laser system, the compound regenerative cavity was a new way to obtain double-pulse in a burst. From the measuring results, the best ranging precision RMS can be achieved at 46.9 mm for space debris (ID.19574, rock bodies) with RCS of 4.96 m2. It’s also the first time using a double-pulse picosecond laser for tracking space debris. It makes the ranging precision of laser measurements to space debris step into the centimeters level from the meters and decimeters. It’s shown that picosecond laser have more advantage than nanosecond laser. Using more precision and better detector with higher power picosecond laser system, more and more space debris would be measured, and also for the better RMS. It is very helpful to research for space debris. As a next step, we are going to further update the picosecond laser system to increase the output power to measure smaller RCS of space debris with the ranging precision RMS of centimeters level. Acknowledgements Thanks the US Space Track Organization (www.space-track.org) for making available the Two-Line Elements of cataloged objects. And this work was supported by the National Natural Science Foundation (NSF) of China (U1631240 and 11503068); CAS Key Technology Talent Program; Project funded by China Postdoctoral Science Foundation (BX201700270 and 2017M621562). 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