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A space station-based orbital debris tracking system
Adv. Space Res. Vol. 13, No. 8, pp. (8)65—(8)68, 1993
0273—1177)93 $24.00
Printed in Great Britain.
A SPACE STATION-BASED ORBITAL DEBRIS TRACKING SYSTEM G. D. Amdt,* P. Fink* and W. B. Wan.en** * NASeVJohnson Space Center, Houston, IX 77058, U.S.A. ** GBWResearch Inc., Atlanta, GA, U.S.A.
ABSTRACT’ Orbital debris with dimensions between 1 and 10 cm is an increasing threat to the Space Station Freedom. A possible solution is to track the debris with a station-based radar and passive JR sensors. Studies atJSC have shown that the approach window of the debris is relatively narrow, ±10°in elevation and ±(30°to 90°)in azimuth. Preliminary JSC studies suggest that angle and range accuracies of 10~radians and 6 m are required for trajectory determination. Design considerations for minimizing the costs of a microwave array antenna are discussed. A proposed ground-based radar/antenna experiment for tracking small particles with high angular rates is described. INTRODUCTION Accumulation of man-made space debris in low Earth orbit (LEO) is an increasing threat to the Space Station Freedom, a large structure in orbit for an extended length of time. This impact threat is focusing auention on measuring the debris environment as well as on the need for collision detection, warning, and avoidance systems. Although some of the orbital debris can be detected and tracked with ground-based radars, there remains a significant number of debris particles below the detection threshold ofthe ground-based systems which are large enough to severely damage the Station. A possible solution to this problem is to provide an on-board radar to detect and track in-coming debris. The purpose of this paper is to describe the performance requirements for an on-board detection system as well as a proposed low-cost ground radar/antenna experiment for tracking small particles with high angular rates. A briefdiscussion of the microwave system studies conducted at the Johnson Space Center (JSC) is also included. ORBITAL DEBRIS CHARACTERISTICS Data from the U. S. Space Command (USSPACECOM) orbital debris catalog and from on-going debris studiesby the Space Science Branch atJSC suggest that the probability of being hit by an approaching meteoroid or manmade debris on a first pass by the Station is smalL Rather the debris will usually encounter the Station atorbital intersections on successive revolutions. These periodic intersections could precess closet, thereby increasing the chance ofa collision, before diverting. By having an on-board tracker with sufficient accuracy, an advanced warning time of a complete revolution is possible. Studies at JSC ofthe 7000 trajectories in the USSPACECOM catalog have determined that the angles of approach of debris passing close to a Space Station ina reference orbit are normally confined to low elevation angles and very specific angles off the velocity vector /1/. This approach window, as shown in Figure 1, is approximately ±10° in elevation and ±(30°to 900) in azimuth off the velocity vector. This relatively narrow angular approach for large (> 10cm) objects simplifies debris detection, making a structured microwave tracking system possible. SPACE STATION REQUIREMENTS Because of its large structure, low thrust to weight ratio, and operational complexity, the Station requires several hours of warning time to potential encounters with debris. It is also important to minimize or eliminate false alarms. The Station’s detection and tracking system must provide measured parameters with sufficient accuracy to
(8)65
(8)66
(3. D. Arndt et aL
predict the debris’s trajectory one orbit in advance to a few hundred meters. Inaccurate measurements could be worse than no measurements. The collision avoidance criteria for the Space Station is not based upon a predicted intersectional zone as the ShuuJe. The Station will use a collision probability criteria based upon the predicted trajectories of the debris and the Station. 30— 0
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Azimuth Fig. 1. Orbital Debris ~pproach Window. Preliminary JSC studies indicate an angle accuracy of l0~radians and a range error of less than 6 meters is needed /3/. This angular resolution would have to be achieved with either microwave or optical sensors. The study did not resolve the requirement for range-rate data because prior experience with Kalman filters indicate range-rate tends to be redundant information and complicates the filter implementation. Additional studies into the range, angle, and range-rate requirements are planned if funding becomes available. The Space Station’s habitation modules are to be designed with waffled shielding to protect against the penetration of particles up to 1 cm in diameter. This sets the lower size limit for a debris tracking system. The debris of maximum interest for an on-board tracking system are objects in the 1 to 10cm diameter range since smaller particles are shielded against (for the habitation modules) and particles larger than 10cm are tracked by USSPACECOM. However, since the optical tracking study conducted by JSC showed 30% more objects with diameters greater than 10 cm than were predicted in the tracking catalog, some debris greater than 10cm must have radar cross-sections below the sensitivity of the ground-based radars. ON-ORBIT DETECTION AND TRACKING To answer the questions of how would an on-orbit detection and tracking system be implemented and what would constitute representative parameters for the microwave portion of the system, let us consider the following: Detection Trajectory information on debris greater than 10 cm that could be tracked by ground radars would be communicated to a risk management system, either on-orbit or on the ground, which would assess the collision probability with the Station. The pointing angles to the incoming debris would be routed to a microwave (lasers are also possible) tracking system on the Station. This tracking system would then measure the orbital parameters of the debris for trajectory calculations one orbit in advance. For particles 1 to 10cm in diameter, the initial detection could be done with passive thermal infrared or visible spectrum sensors. There are many technical issues with the passive sensors, some of which could be answered with a proposed Debris Collision Warning Sensors (DCWS) flight experiment. A combination of active and passive optical sensors for long-range detection of debris is also being studied. In addition to providing initial pointing
A Space-Based Tracking System
(8)67
information to the microwave tracking antenna systems, the optical detector might be used to provide updated angular information (with the 10 radian accuracy requirement) for trajectory calculations. Trackina In order to improve the accuracy of the trajectory prediction, measurements of angle and range should be made with as large a spacing as possible, i.e., measurements with the debris approaching as well as departing from the Station are needed. If the antenna is to continuously track the particle, the angular scan rate at the point of closest contact can be large. For a particle traveling at 12 Km/sec and within a tangential distance of 5 Km from the Station, the scan rate is 2.4 radian/sec. An electronic phased array mounted on a mechanical rotating pedestal would allow tracking in front (closing), behind (departing), and during the region of closest encounter (high scan rates). In order to conserve prime DC power and prolong the operating lifetime of the electronics, the microwave antenna would only be activated when an approaching particle was detected (an average ofonce every 2 weeks). Once activated, it would have to be fully operating within a few seconds. Preliminary performance parameters for an onboard microwave/optical tracking system can be summarized as follows: o Radar cross section o Tracking range o o o o o o
Angular accuracy Maximum angular rate at 5 Km Range accuracy Range rate Vertical scan (elevation) Horizontal scan (azimuth)
-20dB to -40 dB sq. meter 5 Km mm. to 100 Km max. for 1 cm particles, 500 Km for large debris 10 radians 2.4 red/sec for 1 sec ±6m TED if necessary ±100 60° —
The associated microwave characteristics were chosen to be: o Operating frequency range 15 GHz to 300Hz (for the system sizing calculation 300Hz was chosen) o Pulsewidth=.67usecfora6mrangeaccuracy o S/N required = 13 dB for a Swerling 1 model o Antenna gain = 57 dB for a 16m2 antenna with 50% efficiency o System losses = 5 dB excluding antenna losses o Noise figure =3 dB o Transmit peak power= 10Kw o Pulse Repetition Frequency (PRF) = 300 Hz -
Using these parameters and the standard radar equation with nortcoherent integration, the signal processing gain required to track a 1 cm particle at 100 kilometers is 30 dB. One option is to record the tracking data, telemetry it to the ground for processing on a high-speed computer, and then transfer the data to the risk management system for trajectory calculations and collision assessment. Antenna Considerations There are a number oftracking characteristics which may be used in the design of an on-orbit antenna aboard the Station. The antenna costs may be reduced by consideration of the following: o The angle of the approaching debris is almost invariant during the 10 seconds of tracking time as the particle nears the Station; likewise, the departure angle of the debris after it passes the Station is nearly constant. o Only during the several seconds as the debris passes the Station will the tracking angle approach the 2.4 radian/second rate of change. Since this condition is also the shortest range, only a small portion of the antenna need be active. o On an average, the antenna will be used for only 100 seconds of tracking (assuming five passes of 20 seconds each) every 2 to 3 weeks. The amount of stand-by power required to keep the microwave system ready for use within a few seconds will be a major consideration in determining the type of power amplifiers used.
(8)68
0. D. Arndt et aL
o The cost of a phased array antenna can be greatly reduced by taking advantage of the small angular scan region required for tracking the debris. There is no need to have phase control and power amplification at the individual antenna element level. Rather the antenna should consist of higher gain subarrays whose gains are determined by the 3 dB scanning beamwidth required to track the debris. For example, if the elevation and azimuth scans are limited to 5~,the gain of an individual subarray is approximately 30dB. The number of subarrays required to achieve the 57 dB total antenna gain is 500. Reducing the angular scan to 30 in azimuth and elevation will further reduce the number of subarrays to 187. The lower limit of the number of subarrays may be determined by the achievable peak power per power amplifier. Since the cost of an array is largely determined by the number ofpower amplifiers, low-noise amplifiers, phase shifters, etc., associated with each individually controlled subarray, reducing the array to 200 or less subarrays represents a significant cost savings. GROUND-BASED DEMONSTRATION Background A ground based demonstration will require either the use of an existing radar whose characteristics are well matched to the needs of the demonstration, or a suitable radar that is assembled expressly in support of the demonstration. Since the costs of using an existing radar are expected to be much less than that of assembling a radar for this special application, initial efforts are being concentrated on finding an existing radar with the appropriate characteristics. Concurrently, a scaled breadboard radar is being developed at JSC to investigate various implementational aspects of a possible phased array radar system. Angular Rates for Bullet Target The angular tracking rates required for objects crossing the Space Station orbit will be very high if the object passes close to the station. For the ground antenna test, the bullet will be fired cross-range with a down-range component at a distance, D, of 1 km and a velocity of 1.5 km/sec. The maximum Angular Rate is 1.5 rads/sec. By arranging for the bullets in the ground-based demonstration to pass the radar at the correct range, angular crossing rates similar to those to be seen in orbit should be possible. System Sensitivity In order to estimate the sensitivity required of the radar used to support the ground-based demonstration, estimates are needed of the gain of the test bed antenna as well as the radar cross section of the test target, i.e., a 30 calibre bullet. The target cross section ofa 30 calibre bullet has been measured at -40 to -30 dBsm from 6 tol5 0Hz. For a transmit power of 1 Watt, an antenna gain of 40 dB, and a pulse repetition frequency of 0.1, the estimated average receive power is -127 dBm at 15 0Hz for a 1 km range. The receiver, then, must provide significant post detection procession gain if a reasonable signal-to-noise ratio is to be available to the range and angle tracking portions of the
References 1. W. L. Jackson, Johnson Space Center, Private Communications, (1989). 2. F. Vilas, et.al, ‘Collision Warning and Avoidance Considerations for the Space Shuttle and Space Station Freedom,” AIAA Orbital Debris Conference, Baltimore, MD, April 1990. 3. J. H. Suddath, Johnson Space Center, Private Communications, (1985). 4. I. Paz, J. Kovitz, R. Shaw, J. Carl, G.D. Arndt, “Design of an Orbital Debris Radar Ground Demonstration”. Paper presented at the 1989 IEEE Aerospace Applications Conference, Breckenridge, Colorado. 5. Shaw R., Kovitz J. K., Paz I., Johnson L., Arndt 0. D., “Design of a Planar Array of Parasitic Microstrip Patch Antennas”. 6. Warren B., “A Report on Ku-Band/Ka-Band Technology Study,” May 1992.
0273—1177)93 $24.00
Printed in Great Britain.
A SPACE STATION-BASED ORBITAL DEBRIS TRACKING SYSTEM G. D. Amdt,* P. Fink* and W. B. Wan.en** * NASeVJohnson Space Center, Houston, IX 77058, U.S.A. ** GBWResearch Inc., Atlanta, GA, U.S.A.
ABSTRACT’ Orbital debris with dimensions between 1 and 10 cm is an increasing threat to the Space Station Freedom. A possible solution is to track the debris with a station-based radar and passive JR sensors. Studies atJSC have shown that the approach window of the debris is relatively narrow, ±10°in elevation and ±(30°to 90°)in azimuth. Preliminary JSC studies suggest that angle and range accuracies of 10~radians and 6 m are required for trajectory determination. Design considerations for minimizing the costs of a microwave array antenna are discussed. A proposed ground-based radar/antenna experiment for tracking small particles with high angular rates is described. INTRODUCTION Accumulation of man-made space debris in low Earth orbit (LEO) is an increasing threat to the Space Station Freedom, a large structure in orbit for an extended length of time. This impact threat is focusing auention on measuring the debris environment as well as on the need for collision detection, warning, and avoidance systems. Although some of the orbital debris can be detected and tracked with ground-based radars, there remains a significant number of debris particles below the detection threshold ofthe ground-based systems which are large enough to severely damage the Station. A possible solution to this problem is to provide an on-board radar to detect and track in-coming debris. The purpose of this paper is to describe the performance requirements for an on-board detection system as well as a proposed low-cost ground radar/antenna experiment for tracking small particles with high angular rates. A briefdiscussion of the microwave system studies conducted at the Johnson Space Center (JSC) is also included. ORBITAL DEBRIS CHARACTERISTICS Data from the U. S. Space Command (USSPACECOM) orbital debris catalog and from on-going debris studiesby the Space Science Branch atJSC suggest that the probability of being hit by an approaching meteoroid or manmade debris on a first pass by the Station is smalL Rather the debris will usually encounter the Station atorbital intersections on successive revolutions. These periodic intersections could precess closet, thereby increasing the chance ofa collision, before diverting. By having an on-board tracker with sufficient accuracy, an advanced warning time of a complete revolution is possible. Studies at JSC ofthe 7000 trajectories in the USSPACECOM catalog have determined that the angles of approach of debris passing close to a Space Station ina reference orbit are normally confined to low elevation angles and very specific angles off the velocity vector /1/. This approach window, as shown in Figure 1, is approximately ±10° in elevation and ±(30°to 900) in azimuth off the velocity vector. This relatively narrow angular approach for large (> 10cm) objects simplifies debris detection, making a structured microwave tracking system possible. SPACE STATION REQUIREMENTS Because of its large structure, low thrust to weight ratio, and operational complexity, the Station requires several hours of warning time to potential encounters with debris. It is also important to minimize or eliminate false alarms. The Station’s detection and tracking system must provide measured parameters with sufficient accuracy to
(8)65
(8)66
(3. D. Arndt et aL
predict the debris’s trajectory one orbit in advance to a few hundred meters. Inaccurate measurements could be worse than no measurements. The collision avoidance criteria for the Space Station is not based upon a predicted intersectional zone as the ShuuJe. The Station will use a collision probability criteria based upon the predicted trajectories of the debris and the Station. 30— 0
E 1 e V
a t i 0
10—0
I
IC—— 0
0
a Q
-- —
...~ )~, 0 0~0~b~U
—
.cq
—to
--—--fl-—n———-.-
—10
——~
-30
-.~—•~i —*40
-I~
aea
—
—
-~
-re-
~‘
0
-°—
—
0
— —60
—20
20
6*
*00
*40
Azimuth Fig. 1. Orbital Debris ~pproach Window. Preliminary JSC studies indicate an angle accuracy of l0~radians and a range error of less than 6 meters is needed /3/. This angular resolution would have to be achieved with either microwave or optical sensors. The study did not resolve the requirement for range-rate data because prior experience with Kalman filters indicate range-rate tends to be redundant information and complicates the filter implementation. Additional studies into the range, angle, and range-rate requirements are planned if funding becomes available. The Space Station’s habitation modules are to be designed with waffled shielding to protect against the penetration of particles up to 1 cm in diameter. This sets the lower size limit for a debris tracking system. The debris of maximum interest for an on-board tracking system are objects in the 1 to 10cm diameter range since smaller particles are shielded against (for the habitation modules) and particles larger than 10cm are tracked by USSPACECOM. However, since the optical tracking study conducted by JSC showed 30% more objects with diameters greater than 10 cm than were predicted in the tracking catalog, some debris greater than 10cm must have radar cross-sections below the sensitivity of the ground-based radars. ON-ORBIT DETECTION AND TRACKING To answer the questions of how would an on-orbit detection and tracking system be implemented and what would constitute representative parameters for the microwave portion of the system, let us consider the following: Detection Trajectory information on debris greater than 10 cm that could be tracked by ground radars would be communicated to a risk management system, either on-orbit or on the ground, which would assess the collision probability with the Station. The pointing angles to the incoming debris would be routed to a microwave (lasers are also possible) tracking system on the Station. This tracking system would then measure the orbital parameters of the debris for trajectory calculations one orbit in advance. For particles 1 to 10cm in diameter, the initial detection could be done with passive thermal infrared or visible spectrum sensors. There are many technical issues with the passive sensors, some of which could be answered with a proposed Debris Collision Warning Sensors (DCWS) flight experiment. A combination of active and passive optical sensors for long-range detection of debris is also being studied. In addition to providing initial pointing
A Space-Based Tracking System
(8)67
information to the microwave tracking antenna systems, the optical detector might be used to provide updated angular information (with the 10 radian accuracy requirement) for trajectory calculations. Trackina In order to improve the accuracy of the trajectory prediction, measurements of angle and range should be made with as large a spacing as possible, i.e., measurements with the debris approaching as well as departing from the Station are needed. If the antenna is to continuously track the particle, the angular scan rate at the point of closest contact can be large. For a particle traveling at 12 Km/sec and within a tangential distance of 5 Km from the Station, the scan rate is 2.4 radian/sec. An electronic phased array mounted on a mechanical rotating pedestal would allow tracking in front (closing), behind (departing), and during the region of closest encounter (high scan rates). In order to conserve prime DC power and prolong the operating lifetime of the electronics, the microwave antenna would only be activated when an approaching particle was detected (an average ofonce every 2 weeks). Once activated, it would have to be fully operating within a few seconds. Preliminary performance parameters for an onboard microwave/optical tracking system can be summarized as follows: o Radar cross section o Tracking range o o o o o o
Angular accuracy Maximum angular rate at 5 Km Range accuracy Range rate Vertical scan (elevation) Horizontal scan (azimuth)
-20dB to -40 dB sq. meter 5 Km mm. to 100 Km max. for 1 cm particles, 500 Km for large debris 10 radians 2.4 red/sec for 1 sec ±6m TED if necessary ±100 60° —
The associated microwave characteristics were chosen to be: o Operating frequency range 15 GHz to 300Hz (for the system sizing calculation 300Hz was chosen) o Pulsewidth=.67usecfora6mrangeaccuracy o S/N required = 13 dB for a Swerling 1 model o Antenna gain = 57 dB for a 16m2 antenna with 50% efficiency o System losses = 5 dB excluding antenna losses o Noise figure =3 dB o Transmit peak power= 10Kw o Pulse Repetition Frequency (PRF) = 300 Hz -
Using these parameters and the standard radar equation with nortcoherent integration, the signal processing gain required to track a 1 cm particle at 100 kilometers is 30 dB. One option is to record the tracking data, telemetry it to the ground for processing on a high-speed computer, and then transfer the data to the risk management system for trajectory calculations and collision assessment. Antenna Considerations There are a number oftracking characteristics which may be used in the design of an on-orbit antenna aboard the Station. The antenna costs may be reduced by consideration of the following: o The angle of the approaching debris is almost invariant during the 10 seconds of tracking time as the particle nears the Station; likewise, the departure angle of the debris after it passes the Station is nearly constant. o Only during the several seconds as the debris passes the Station will the tracking angle approach the 2.4 radian/second rate of change. Since this condition is also the shortest range, only a small portion of the antenna need be active. o On an average, the antenna will be used for only 100 seconds of tracking (assuming five passes of 20 seconds each) every 2 to 3 weeks. The amount of stand-by power required to keep the microwave system ready for use within a few seconds will be a major consideration in determining the type of power amplifiers used.
(8)68
0. D. Arndt et aL
o The cost of a phased array antenna can be greatly reduced by taking advantage of the small angular scan region required for tracking the debris. There is no need to have phase control and power amplification at the individual antenna element level. Rather the antenna should consist of higher gain subarrays whose gains are determined by the 3 dB scanning beamwidth required to track the debris. For example, if the elevation and azimuth scans are limited to 5~,the gain of an individual subarray is approximately 30dB. The number of subarrays required to achieve the 57 dB total antenna gain is 500. Reducing the angular scan to 30 in azimuth and elevation will further reduce the number of subarrays to 187. The lower limit of the number of subarrays may be determined by the achievable peak power per power amplifier. Since the cost of an array is largely determined by the number ofpower amplifiers, low-noise amplifiers, phase shifters, etc., associated with each individually controlled subarray, reducing the array to 200 or less subarrays represents a significant cost savings. GROUND-BASED DEMONSTRATION Background A ground based demonstration will require either the use of an existing radar whose characteristics are well matched to the needs of the demonstration, or a suitable radar that is assembled expressly in support of the demonstration. Since the costs of using an existing radar are expected to be much less than that of assembling a radar for this special application, initial efforts are being concentrated on finding an existing radar with the appropriate characteristics. Concurrently, a scaled breadboard radar is being developed at JSC to investigate various implementational aspects of a possible phased array radar system. Angular Rates for Bullet Target The angular tracking rates required for objects crossing the Space Station orbit will be very high if the object passes close to the station. For the ground antenna test, the bullet will be fired cross-range with a down-range component at a distance, D, of 1 km and a velocity of 1.5 km/sec. The maximum Angular Rate is 1.5 rads/sec. By arranging for the bullets in the ground-based demonstration to pass the radar at the correct range, angular crossing rates similar to those to be seen in orbit should be possible. System Sensitivity In order to estimate the sensitivity required of the radar used to support the ground-based demonstration, estimates are needed of the gain of the test bed antenna as well as the radar cross section of the test target, i.e., a 30 calibre bullet. The target cross section ofa 30 calibre bullet has been measured at -40 to -30 dBsm from 6 tol5 0Hz. For a transmit power of 1 Watt, an antenna gain of 40 dB, and a pulse repetition frequency of 0.1, the estimated average receive power is -127 dBm at 15 0Hz for a 1 km range. The receiver, then, must provide significant post detection procession gain if a reasonable signal-to-noise ratio is to be available to the range and angle tracking portions of the
References 1. W. L. Jackson, Johnson Space Center, Private Communications, (1989). 2. F. Vilas, et.al, ‘Collision Warning and Avoidance Considerations for the Space Shuttle and Space Station Freedom,” AIAA Orbital Debris Conference, Baltimore, MD, April 1990. 3. J. H. Suddath, Johnson Space Center, Private Communications, (1985). 4. I. Paz, J. Kovitz, R. Shaw, J. Carl, G.D. Arndt, “Design of an Orbital Debris Radar Ground Demonstration”. Paper presented at the 1989 IEEE Aerospace Applications Conference, Breckenridge, Colorado. 5. Shaw R., Kovitz J. K., Paz I., Johnson L., Arndt 0. D., “Design of a Planar Array of Parasitic Microstrip Patch Antennas”. 6. Warren B., “A Report on Ku-Band/Ka-Band Technology Study,” May 1992.