Development of the first Portuguese radar tracking sensor for Space Debris
João Pandeirada, Miguel Bergano, João Neves, Paulo Marques, Domingos Barbosa, Bruno Coelho, Valério Ribeiro
AArticle
Development of the first Portuguese radar trackingsensor for Space Debris
João Pandeirada , Miguel Bergano , João Neves , Paulo Marques , Domingos Barbosa , BrunoCoelho and Valério Ribeiro Instituto de Telecomunicações, Aveiro, Portugal; [email protected] (J.P); [email protected] (M.B);[email protected] (D.B); [email protected] (B.C); [email protected] (V.R) Universidade de Aveiro, Aveiro, Portugal ESTGA - Universidade de Aveiro, Aveiro, Portugal CINAV - Escola Naval, Alfeite, Portugal; fi[email protected] (J.N) Instituto de Telecomunicações / ISEL-IPL, Lisboa, Portugal; [email protected] (P.M)Received: date; Accepted: date; Published: date
Abstract:
Currently, space debris represents a threat for satellites and space-based operations, bothin-orbit and during the launching process. The yearly increase in space debris represents a seriousconcern to major space agencies leading to the development of dedicated space programs to deal withthis issue. Ground-based radars can detect Earth orbiting debris down to a few square centimeters andtherefore constitute a major building block of a space debris monitoring system. New radar sensorsare required in Europe to enhance capabilities and availability of its small radar network capable oftracking and surveying space objects and to respond to the debris increase expected from the NewSpace economy activities. This article presents ATLAS, a new tracking radar system for debris detectionlocated in Portugal. It starts by an extensive technical description of all the system components followedby a study that estimates its future performance. A section dedicated to waveform design is alsopresented, since the system allows the usage of several types of pulse modulation schemes such asLFM and phase coded modulations while enabling the development and testing of more advancedones. By presenting an architecture that is highly modular with fully digital signal processing, ATLASestablishes a platform for fast and easy development, research and innovation. The system follows theuse of Commercial-Off-The-Shelf technologies and Open Systems which is unique among current radarsystems.
Keywords: radar, sst, ssa, space debris, debris, leo, tracking, doppler, eusst, ATLAS
1. Introduction
The use of sensors (which include telescopes, lasers and radars) has been a growing area of interestfor monitoring the space environment around Earth. Due to the growing number of space debris, there isa need to predict and adjust the orbits of the more than 2000 active satellites [1], avoiding collisions withother inactive satellites or debris, to guarantee their long-term operation and investment.These actions are core activities of the Space Surveillance and Tracking (SST) concept or, in a broadersense, are framed within the Space Situational Awareness (SSA) domain, notwithstanding the differentconcepts acknowledged between a USA concept, strongly related to the Defense, or the European concept,strongly related to a dual-use or purely civil use compliant with the European Union (EU) or the EuropeanSpace Agency (ESA) guidelines.The Earth near-space environment has been getting polluted with space debris since the onset ofthe space exploration era in 1957. The envisaged space launching actions to put more satellites in orbitincluding the announced mega constellations or broadband internet satellites, e.g. Starlink Space X and a r X i v : . [ phy s i c s . s p ace - ph ] F e b of 17 One Web, is a growing concern which drives the need to create, maintain and improve accurate sensornetworks to collectively contribute to a safe use of space. The safety and security of space assets is animportant matter of national interests and central to international treaties and regulations, namely thoseagreed to in the framework of the United Nations, at the Committee on the Peaceful Uses of Outer Space[2]. Sensors networks comprise both ground based and space based assets, although only a few nationscan afford to have space based ones. Regarding the ground based sensors, these are usually divided inthree main groups: telescopes, radars and lasers.Radar usage for SST and SSA purposes is increasing due to its capacity for monitoring the spaceenvironment during day and night, and insensitivity both to light pollution and to weather conditions.A tracking radar distinguishes itself not only by recognizing the presence of a target and determining atarget location in range and in angle coordinates, but also because it is able to follow the target and observeit over some time range thus improving the information accuracy about its trajectory [3].The principle of tracking radar is to use the angular error signal, defined by the difference betweenthe target direction and the reference direction given by the axis of the antenna, to adjust the antenna’spointing direction. The tracking radar attempts to position the antenna with zero angular error (i.e., tolocate the target along the reference direction) [4] .The first developments of tracking radars date back from World War II [5] but their biggestdevelopments date from the Cold-War [6]. During those times, both the USA and the Soviet Unionbuilt large networks of ground-based tracking facilities that collected data on man-made objects in Earthorbit. They consisted, primarily, of large tracking radars and optical telescopes, often dual-purposedwith other military missions, such as ballistic missile warning and tracking. The main focus of spacesurveillance at that time was on determining the precise orbit of man-made objects around the Earth[7]. Later, they were improved to make them useful for Combat Weapon Systems, such as the ThalesSTIR-Tracking and Illumination radar [8].The tracking of distant objects in the higher layers of the atmosphere, in the lower orbits aroundEarth or even far distant objects throughout the galaxy, continued to develop up to complex phasedarray systems such as the Square Kilometer Array (SKA) radio telescope [9] or the brand new US SpaceFence, costing several million dollars [10], to name a few. Tracking radars are now suitable for Low-EarthOrbit (LEO) tracking or space object catalogue accuracy improvement due to electronic componentsdevelopments and increased digital processing capability.Referring to existing operational tracking radars for in-orbit object detection (and disregarding theones based on phased arrays, which usually also have surveillance capability), these can be found all overthe world and with different capabilities.On the US side, the development of these radars has a long history with the development ofseveral radars, most of them for the Ballistic Missile Defense program, supported by DARPA (DefenseAdvanced Research Projects Agency), which were closely related to the detection of objects in orbit [6].Notwithstanding the huge variety of radars that were developed and used (some of them have beendecommissioned), two of the most interesting and relevant radars used for Space Debris detection, trackingand characterization are the Haystack and the HAX (Haystack Auxiliary Radar) radars. The HAX radarhas a 12.2m antenna, operating at 16.7GHz and is much smaller than Haystack which has a 36.6m antennaoperating at 10GHz [11] , both in a Cassegrain configuration. HAX had twice the bandwidth of Haystack,being capable of producing correspondingly sharper images of near-earth orbit objects. The cost of buildingHAX, which became operational in 1994, was reduced by using a refurbished antenna and by sharingsignal processing and data processing systems with the Haystack Long Range Imaging Radar (LRIR)(means that the two radars could not operate simultaneously). The Haystack radar has been operationalsince 1964, but from 2010 to 2014 it underwent several upgrades becoming the Haystack Ultra-Wideband of 17
Satellite Imaging Radar (HUSIR), able to also operate in the W-band with a bandwidth up to 8GHz, whichallowed the HAX to detect objects down to 3cm and the HUSIR down to 3mm at an altitude up to 1000km[12]. The Space Debris networks around the world includes facilities from other major players, such asChina [13] and Russia and other spacefaring nations such as India [14] [15], Iran, Brazil [16] [17], Japanor North Korea. However, most of those nations projects’ and infrastructures are linked to the militaryand involved in deep secrecy. It’s highly probable that radio-telescopes and other related ground-basedsensors support missions including intelligence collection, counterspace targeting, ballistic missile earlywarning, spaceflight safety, satellite anomaly resolution, and also space debris tracking [18].In the European continent the development of tracking radars for orbit objects detection has evolvedfor some time and nowadays there are around 50 different radio telescopes. Just to mention some whichhave played a relevant role in space debris and satellite detection, it is worth mentioning the GermanTracking and Imaging Radar (TIRA) system, with a 34 meter antenna, located near Bonn. In a monostaticconfiguration it is able to detect objects of diameters down to 2 cm at 1000 km, but in a bistatic configuration,in synergy with the Effelsberg Radio Telescope of the Max Planck Institute for Radio Astronomy, whichhas a 100 meter dish antenna, the system can detect objects down to 1 cm diameter [19] [20]. The TIRAradar has been one of the most useful radars in Europe in the SST context and its accuracy and imagingcapabilities, along its continued upgrades, made it a valuable asset. The Sardinia Radio Telescope (SRT)is an Italian radio telescope facility with recent radar capabilities with a 64 meter antenna located in theSardinia island, operating only since 2012. Compared to equivalently sized radars, it has a fairly goodspeed in azimuth and elevation tracking (0.85º/s and 0.5º/s, respectively) and is able to operate from300MHz up to 15 GHz. It is intended to work in a bistatic configuration, using the Flight TerminatorSystem (FTS), about 40 km apart from the first, which configuration is named BIRALET (BIstatic RAdarfor LEo Tracking) [21]. BIRALET has been used in the context of the European SST for LEO detection, butits capabilities, namely the SRT antenna, envisage the possibility to go beyond this orbit.On the French side, one of the countries in Europe that has more investments in the space segment,there have been several developments in radars, but the most relevant and interesting of them are inthe military side. The French space surveillance network comprises a set of three tracking radars namedSATAM, in three different locations (one of it with deployable capability). This configuration is notprimarily dedicated to space surveillance, but is used for space events, detection of risk collision andatmospheric reentries, being its speed (about 40º/s) an incomparable added-value, notwithstanding thelower dimension of the dishes, compared with the previous ones.The European Space Surveillance and Tracking (EUSST) is a support framework setup by the EuropeanCommission to create an autonomous monitoring network supporting the detection and tracking of spacedebris and issue alerts when evasive actions may be necessary. EUSST aims to establish an SST capabilityat the European level with an appropriate level of European autonomy [22],[23]. Portugal is a member ofthe EUSST program and is developing capabilities both in optical and radar sensors.This paper describes the development of the first Portuguese tracking sensor for space debris andis organized as follows. Section 1 explains the necessity of monitoring the space environment andhow nations worldwide have tackled this problem throughout the years. It also provides examples ofalready deployed tracking radars with similar functions to the current one in development. Section 2introduces ATLAS (r A dio T e L escope p A mpilhosa S erra) and provides a detailed technical description ofits components: antenna, transmitter, receiver, clock generation, data acquisition and controllers. Section 3consists of simulation studies that use the technical characteristics of the system in order to estimate someperformance metrics: minimum detectable target size, minimum elevation, number of expected observableobjects and maximum simultaneous trackable targets. Section 4 explains the importance of waveform of 17 design on radar systems and provides some examples of waveforms that can be used by ATLAS. Finally,Section 5 concludes the article and reveals the next steps on the realization of this project.
2. Implemented System
Figure 1.
ATLAS system block diagram
In order to support and consolidate the pooling of national resources to prepare for the establishmentof a SST sensor network, Instituto de Telecomunicações initiated in 2019 a major upgrade of its CassegrainAntenna located in Pampilhosa da Serra, Portugal. Attached to the antenna will be a completeground-based radar system, named ATLAS, operating in the C-band (5.56 GHz).ATLAS is a monostatic pulse radar using solid state power amplifiers (GaN) with a peak outputpower of 5 kW for tracking debris objects in LEO. Regarding the receiver side, the system is fully coherentwith detection and processing fully in the digital domain with a bandwidth of 50 MHz and with thecapacity to detect Doppler velocities up to 10.79 km/s. The architecture, depicted in Figure 1, is highlyinspired in the HAX/Haystack system [24] developed by NASA , which is a very successful ground-basedradar used for debris detection and tracking. The system follows the use of Commercial-Off-The-Shelf(COTS) technologies and Open Systems (OS) which enables a major downstream cost reduction in thedevelopment and maintenance of the system [25]. Table 1 provides a comparison between ATLAS andother tracking radars currently used in SSA activities.
Table 1.
Comparison between ATLAS and other tracking radars
ATLAS TIRA BIRALET HAX HaystackOperating Frequency
Peak power
Waveform type
Pulsed Pulsed Continuous Pulsed Pulsed
Antenna Gain
46 dB 49.7 dB 13 dB (TX), 47 dB (RX) 63.64 dB 67.23 dB
Antenna Beamwidth
Receiver Bandwidth
80 MHz 250 kHz 5 MHz 1 MHz 1 MHz
Topology
Monostatic Monostatic Bistatic Monostatic Monostatic of 17
Table 2.
Antenna specifications
Optical Configuration
Cassegrain
Mount configuration
Alt-Azimuthal
Primary aperture
Primary depth
Secondary aperture
F/D
Surface rms (static)
Azimuth Travel
0º to 360º
Azimuth Travel Rate
Elevation Travel
35º to 90º
Elevation Travel Rate
Maximum Operational Wind (5GHz)
25 km/h
Survival Winds
150 km/h
Reflector Weight
Pedestal Weight
Foundation Size
Concrete Volume
12 ms
Beamwidth @ 5 GHz
44 arcmin
Beamwidth @ 10 GHz
22 arcmin
Pointing Accuracy (wind limited) <1/20 beamwidth
Gain @ 5 GHz ( G )
46 dB
G/T (5 GHz)
39 dB/K
Figure 2.
ATLAS Antenna
We obtained a Vertex RSI C band, 9 metre Cassegrain Antenna originally used for SatelliteCommunications in the Azores Terceira Island, Portugal. The antenna was moved to Pampilhosa daSerra, Portugal, a site with a low Radio Frequency Interference (RFI) environment [26]. New foundations,electrical power and lightning protection were installed at this new site. The mounting pedestal underwenta major adaptation in order to comply with a fast, continuous azimuth antenna rotation. Originallyintended for radio astronomy surveys, this facility represents a step forward in our technologicaldevelopment and evolution of high-performance space radar instrumentation systems.The major improvements have given the station space telemetry and tracking capabilities. Theseimprovements will represent one of Portugal’s main support pilot facilities for space monitoring and willprovide support to current and future space missions. The upgrade approach manages to automatically,and quickly, attain the best antenna steering strategy to successfully track the objects.The data output capabilities have been upgraded for wide bandwidth. These capabilities are nowavailable to users on site and remotely via a high throughput connection. Extensive user support will beprovided for these new facilities, including assistance, monitoring and control, antenna interfaces andoutput interfaces. Operation and data reduction is also available for object tracking.The antenna is depicted in Figure 2 and specifications are detailed in Table 2.
Active radars require Power Amplifiers (PAs), which should be small, efficient and low-cost. Weimplemented a C-band PA centered at 5.56GHz with an output peak power of 5 kW (67dBm) from a 30dBmdrive with a Power Added Efficiency (PAE) of 67 %. GaN technology is suitable for these requirementsand can do so with high efficiency. The PA was implemented by readily available COTS technology.Compact Radio Frequency (RF) power sources now exist with the recent release of high-power solid-stateRF amplifiers such as the Wolfspeed/Cree CGHV59350 high-electron-mobility transistors (HEMTs) [27]. of 17
These HEMTs are capable of 500 W of RF power each. Our design follows the traditional approach ofcombining a large number of solid-state RF amplifiers into a high-power all-solid-state RF system [28].The transmitter, depicted on Figure 1 in blue, is responsible for outputting the RF signal. All the relevantoperating parameters are summarized in Table 3. The signal generation starts with a frequency synthesizerat 5.160 GHz which is fed to a frequency translator. Along with the 5.160 GHz signal, the frequencytranslator receives an intermediate frequency of 400 MHz from the reference module and generates thedesired 5.560 GHz signal. This signal is fed into a PIN modulator (PIN diode based) responsible formodulating the 5.560 GHz carrier in amplitude. The PIN modulator is also connected to the controllerboard in order to receive the user-defined pulse shape. The modulated signal goes to a driver, to achieve a30dBm minimum signal to be provided to the PA. We selected the previously mentioned Cree HEMTs at5–6 GHz because these HEMTs provide the highest available RF power as required (67dBm). The targetperformance requirements for the amplifier are listed in Table 3.
Table 3.
Transmitter Operating Parameters
Peak power
Transmitter Frequency ( λ ) Waveform
Arbitrary amplitude modulation
Max. Pulse length ( τ )
10 s
Phase Noise -91.3dBc[Hz] @ 100 kHz
Modulator
Modified D195 [29]
PA Transistors
CGHV59070 [30], CGHV59350 [27]
The antenna is equipped with a corrugated feed horn, followed by a polarizer and by an orthomodetransducer (OMT) to carry the left and right circular polarizations (LHCP for Tx and RHCP for Rx). Awaveguide to coaxial transition performs the connection to the receiver. The receiver has a limiter andswitch right at the input that handles an excess peak power of 20 W for about 10 ms and blocks the signalduring transmission with an attenuation of 30 dB.The receiver, depicted on Figure 1 in purple, includes an analog chain that is responsible for amplifyingthe signals (target echoes) to the required digital entry levels of the acquisition board (yellow). The receiverstarts with an low noise amplifier (LNA), internally developed, using GaAs technology, presenting anoise factor of 0,7 dB at 5.56GHz. Next, there is a local oscillator at 5.160 GHz to convert it down to a400 MHz intermediate frequency (IF). The local oscillator is provided by the same reference signal fromthe transmitter, guaranteeing a coherent conversion. The IF then passes through an IF filter and a digitalvariable gain amplifier (DVGA) which is connected to the controller board.Finally, the I/Q detector converts the signal to base-band (or 400 kHz) and outputs the I and Qcomponents of the echo signal. The conversion to 400 kHz is used for accommodating Doppler shifts,providing velocity measurements up to 10.79 km/s. Table 4 summarizes all relevant parameters of thereceiver chain.
Table 4.
Receiver Operating Parameters IF
400 MHz
LNA Noise figure (<15ºC) ( N f ) Receiver Temperature ( T ) IF Filter BW ( B )
80 MHz @ -3dB
IQ Detector output BW
50 MHz of 17
The reference module, depicted on Figure 1 in orange, generates reference signals at 100, 400 and400.4 MHz. The main reference oscillator is a 5 MHz oven-controlled crystal oscillator (OCXO) from MTI[31]. The 5 MHz reference locks a pair of 100 MHz XPLO’s that are then multiplied by 4 to generate the 400and 400.4 MHz. This module is connected to the controller board in order to switch between IQ detectionat base band or with 400 KHz offset.
The controller, depicted on Figure 1 in green, is responsible for the digital control of the whole system.It is composed of a controller board, a pulse generator and an arbitrary waveform generator (AWG).The board establishes a telnet connection with the host PC via Ethernet (ETH) where the user canprogram the radar using a command line interface (CLI). The CLI can be used to define a great range ofradar parameters such as the pulse length, shape, time between pulses, number of pulses and a lot ofother internal parameters such as gains and delays. It is also possible to display several system statusparameters such as pulse configurations and temperatures. Table 5 shows all the variables that can bemonitored and configured.The pulse generator is responsible for timing all the components of the system. It enables/disablesthe transmitting and receiving chains and triggers the arbitrary wave generator as well as the acquisitionboard. This enables the system to be fully coherent and synchronized from signal generation to acquisition.Finally, the AWG enables the design of arbitrary amplitude modulated waveforms in the digitaldomain that can be uploaded to the system.
Table 5.
Controller board parameters
Waveform resolution
10 ns
Pulse repetition frequency ( f p )
10 MHz (max)
Duty Cycle
10% (max)
Number of pulses for integration
Variable
Temperature Monitoring
Tx, Rx, LNC, Driver, PA, Coupler
System Monitoring
Pulse shape file name, Tx/Rx gain,No. Pulses, PRF, Pulse length ...
The data acquisition, depicted on Figure 1 in yellow, contains the acquisition board (AQS) and asingle board computer (SBC). The acquisition board is triggered by the controller board and is responsiblefor converting the I/Q signal components to the digital domain, by employing two 16 bit ADC’s at amaximum sample rate of 100 MS/s. The computer (MIO 2360 N-32A1E by Advantech [32]) receives theI/Q signal in the digital domain and performs the necessary digital signal processing. Communicationbetween the AQS and the SBC is done via full speed USB 3.0 (100 MS/s) and a Fast/Slow mode buffer isselectable, where a compromise between sample resolution and sample rate can be chosen.
3. System expected capabilities
Given the technical specifications of the current system, it is possible to make estimates on how it willperform. These estimates are not only useful for predicting the systems applicability but also to identify itslimitations and guide to future upgrades. of 17
The radar range equation represents the physical dependencies of the transmitted power and is usedto obtain the power in the receiving antenna [33]. Using the radar equation, we can obtain the dependenceof the signal to noise ratio (SNR) with the radar specifications:SNR = P av G λ σ ne ( n ) F π τ f p R N F kTBL s (1)where:• P av is the average transmitted power ( W )• G is the antenna gain• λ is the operating wavelength ( m )• σ is the target radar cross section (RCS) ( m )• n is the number of integrated pulses• e ( n ) is the integration efficiency• F accounts for all the propagation effects• τ is the pulse width ( s )• f p is the pulse repetition frequency ( Hz )• R is the distance to the object ( m )• N F is the noise factor of the receiver• k is the Boltzmann constant ( JK )• T is the receiver temperature ( K )• B is the bandwidth of the receiver ( Hz )• L s accounts for all system lossesSome of the radar specifications are fixed and have already been shown during this article. Regardingpropagation losses it is known that for frequencies below 10 GHz losses due to atmospheric absorptionmay be neglected [34], for that reason we neglected for now the F factor. In future work we will include adetailed modelling of the atmospheric propagation effects including tropospheric absorption. Accountingfor all the losses on the waveguide transition from the antenna to the polarizer/orthomode transducer(OMT)/switch and the coaxial transition from the transmitter/receiver to the switch, we obtained L s = e ( n ) = τ = f p =
30 Hz, obtaining thefollowing performance results:• Maximum Unambiguous Range: 5000 km• Maximum Unambiguous Velocity: 0.4 m/s• SNR for a 1 m RCS at 10 km: 39.55 dB• SNR for a 10 cm RCS at 10 km: 9.55 dBWe can conclude that we can measure targets up to 1000 km in distance with a minimum RCS of 10cm . Since debris objects in LEO have velocities of around 7.8 km/s, obtaining a maximum unambiguousvelocity of 0.4 m/s results in inability to measure the Doppler shift. This is due to the fact that we need a of 17 low pulse repetition frequency (PRF) for maintaining range unambiguity. It is still possible to measurevelocities without Doppler shift by taking several range measurements and inferring velocity from themeasurements, which in fact is what is done in an SST system, by processing several individual tracks ofthe same object.In order to take advantage of the long idle times between the transmitted and received pulses in alow PRF system, we can use a process called pulse interleaving. Pulse Interleaving is a technique thatinserts carefully crafted pulses in the idle time of other pulses. One example is the emission of differentwaveforms with low cross correlation between them, such as Orthogonal Frequency Division Multiplexing(OFDM) signals, in order to suppress range ambiguity [35]. The location of the radar and its surroundings need to be taken into account for object tracking.ATLAS is located at (40.18ºN,7.87ºW) and is surrounded by mountains, which makes the horizon altitudeprofile vary in some directions. In these directions, this decreases the arc length of a debris passage therebyreducing its potential tracking time. We thus defined that the minimum elevation for initial and final targetacquisition as 30º.
ATLAS was designed to track space objects in LEO orbits below 1000 km of altitude, however, severalobjects in this range might not be observable due to several constraints:• Objects with an RCS below the minimum detectable threshold;• Orbits with an elevation range below the minimum elevation required for detection and dataacquisition;• Orbital speeds exceeding the maximum antenna tracking speed;In order to make an estimate on the number of objects trackable by ATLAS, we retrieved the latesttwo-line element (TLE) files (which contain the orbital elements) from all the objects categorized as debriswith an apogee less than 1000km from the Space-Track public catalogue [36]. With the orbital elements,we predicted the orbit of each object for the next 7 days (counting from the epoch of the TLE file) with anSGP4 based algorithm [37]. Then, we intercepted the predicted orbits with the visible portion of the skyfor ATLAS, which is obtained by the information provided in Section 3.2. Objects that do not intercept thevisible sky are discarded.Since the catalogue does not provide detailed information about the objects RCS, it is not possibleto filter out by RCS. All the objects were considered regardless of RCS and the filtering was done byminimum elevation and maximum velocity. Figure 3 depicts the number of expected observable objectsduring a complete day.
Since the antenna has a narrow beamwidth (see Table 2), the amount of sky visible at a specific momentin time is limited and provides an idea on the number of objects illuminated by the radar simultaneously.Figure 4 shows the maximum number of simultaneous trackable targets over the course of the day for abeampark configuration. Figure 3 and 4 share the same shape of the distribution because, if we assumethat the number of objects at each bin in Figure 3 is uniformly distributed over the visible sky, the numberof simultaneous objects is a fraction of that number, thus the shape is retained.
Figure 3.
Number of expected observable debris objects during a 24 hour timespan (regardless of RCS).
4. Waveform design
Designing a proper waveform for the radar pulse is of utmost importance. By manipulating thetransmitted pulses in terms of amplitude, frequency and phase it is possible to create waveforms thatmaximize the range and velocity resolution of the echo signal. In order to evaluate the characteristics of awaveform one can use the ambiguity function (AF). The AF represents the time response of the matchedfilter when the signal received is affected by a delay d and a Doppler shift v relative to the values expectedby the filter and is given by: | X ( d , v ) | = (cid:12)(cid:12)(cid:12)(cid:12) (cid:90) ∞ − ∞ u ( t ) u ∗ ( t + d ) e j π vt dt (cid:12)(cid:12)(cid:12)(cid:12) (2)where u is the complex envelope of the signal [38]. By analyzing the zero delay and zero Dopplercuts on the AF it is possible to define the range and velocity resolution. Since ATLAS will work with alow pulse repetition frequency when detecting debris, waveform design focuses on maximizing rangeresolution instead of velocity.Matched filtering is the most basic form of signal processing commonly used in radar. It consists inthe convolution of the received signal with a filter that is matched with the emitted signal. Matching willresult in the maximum attainable SNR at the output of the filter when the signal to which it was matched,after addition of white noise, is passed through it [39].As defined in Section 3.1, ATLAS will use pulses of 3.3 ms with a pulse repetition frequency of 30 Hzin order to maintain the desired SNR. The waveform used can be arbitrary, within the hardware limitsdescribed previously and as long as those power requirements are met. Figure 5 shows a rectangular pulse of 3.3 ms repeated at 30 Hz and the zero Doppler cut. Using arectangular pulse results in a range resolution of 500 km which is unacceptable when detecting objects at amaximum of 1000 km.
Figure 4.
Maximum simultaneous number of trackable targets during a 24 hour timespan.
ATLAS enables the design of arbitrary amplitude modulated waveforms in the digital domain with aresolution of 10 ns. This enables the design of waveforms with high autocorrelation properties that can beused to increase resolution. This signal processing technique is usually called pulse compression [40].
The Barker code is a famous binary code used in pulse compression and consists in binary sequencesthat guarantee that the peak-to-peak sidelobe ratio of the autocorrelation is M where M is the size of theBarker code. Another important feature of the Barker code is that the range resolution increases by M inrelation to the rectangular pulse. We can further increase this resolution by using nested Barker codes [41].Figure 6 shows a Nested Barker code composed of two 13 element codes and the correspondingzero Doppler cut. For visualization purposes, only a small fraction of the code inside of the 3.3 ms pulseis shown. As we can see, the pulse is compressed by a factor of 169, resulting in a range resolution ofapproximately 2.96 km which is a massive improvement from the rectangular pulse.Longer binary codes with good auto correlation properties are available which enable even highercompression factors [42]. Linear frequency modulation (LFM), also known as chirp, is probably the most popular waveformused for pulse compression. It consists in sweeping the carrier wave by the frequency band B during thepulse duration T . The complex envelope of a linear chirp is given by [43]: u ( t ) = (cid:112) ( T ) rect ( tT ) e j π kt (3)where k = ± BT for a linear chirp and the signal indicates an increasing/decreasing sweep. Thetime-bandwidth product of the signal B × T is the compression factor in relation to the rectangular pulse,that is, by increasing B the range resolution improves. Since ATLAS currently only supports amplitude Figure 5.
Rectangular pulse (left) and zero cut of AF (right). modulation of the carrier, it is not possible to generate traditional chirp pulses because it requires phasemodulation. In order to overcome this limitation, we developed an AM Chirp.The AM Chirp consists in designing a chirp signal at a much lower frequency and using it formodulating the carrier in amplitude. The amplitude modulation is given by: y ( t ) = A ( t ) cos ( π f c t ) (4)where A ( t ) is the modulating signal and f c is the carrier frequency. In the case of the AM chirp, A ( t ) isgiven by: A ( t ) = α + β cos [ π ( ct + f ) t ] , c = f − f T (5)where α and β define the modulation parameters, f and f are the starting and final frequencies and T is the pulse duration.Figure 7 illustrates a simple case of AM chirp. The left side depicts a 1 kHz carrier modulated inamplitude by a chirp signal with α = β = f = f =
50 Hz. After carrier demodulation,filtering and signal reconstruction in the digital domain, the chirp waveform is successfully recovered asdepicted on the right side of Figure 7.As said in Section 2, ATLAS has a waveform resolution of 10 ns and a receiver bandwidth of 80 MHz,which enables generation of AM chirps with high compression factors. As an example, Figure 8 shows alinear chirp with a compression factor of 1000 inside of the 3.3 ms rectangular pulse. For visualizationpurposes, only a small fraction of the waveform inside of the pulse is shown.A compression factor of 1000 was used to maintain a reasonable simulation time while showingthe benefits of using this waveform for pulse compression. As can be seen on the right side of Figure 8,the range resolution now is approximately 500 m which corresponds to an error of 0.05% at 1000 km ofaltitude.This waveform requires a bandwidth of 300 kHz which is only a small fraction of the capabilitiesof the system to generate and receive it. This indicates it is possible to synthesize waveforms with evenhigher compression factors.
Figure 6.
Part of the Nested Barker Code (left) and zero cut of AF (right).
In the particular scenario where certain selected man-made targets are of interest, the transmittedwaveform can be further optimized [44]. The basic idea is to change the signal used as reference fordetection in such a way that it is adapted to the predominant scatterers of the targets, resulting in a bettermatched filtering which ultimately leads to better SNR and pulse compression.ATLAS also permits the testing of waveforms typically used on Noise Radar Technology [45], beingable to transmit a virtually infinite set of waveforms produced via a set of random realizations, with thereal-time configuration of the correspondent matched filter. This enables the use of the radar in criticalenvironments where interception and jamming robustness is a requisite. Additionally, mutual interferencebetween two radars that occupy the same transmit spectral band can be made negligible [45] [46].It is also possible to implement pulse-to-pulse diversity in ATLAS by loading a different waveformduring the idle time of the previous pulse. Trains of diverse pulses can lead to reduction of the rangesidelobes of the autocorrelation function. The Golay complementary sequences, for example, can beused to phase-modulate the carrier wave leading to trains of complementary pulses. The sum of theautocorrelation of complementary pulses is zero except for the zero shift, leading to an impulse likeresponse [47].Since the system can be used in passive bistatic mode, it will also be used to test techniques forimproved detection by illumination matching on receive such as the one published in [48].
5. Conclusions and Future Work
In this work we started by doing a review on the current tracking radar systems deployed aroundthe world as well as the networks responsible for SSA and SST tasks (Section 1). Next, we explainedthe necessity of developing a tracking radar in Portugal and presented the current one in development,ATLAS. An extensive technical description of the system was provided (Section 2) and simulation studieson its expected performance were developed and discussed (Section 3). Since waveform design plays a bigrole in radar systems, we gave a description of some interesting waveforms that can be used by the systemin order to increase the tracking quality (Section 4).
Figure 7.
AM Chirp (left) and reconstructed chirp (right).
Figure 8.
Part of the chirp waveform (left) and zero cut of AF (right).
The system is expected to track targets up to 1000 km in distance with a minimum RCS of 10 cm ,with an average of 500-700 objects passing hourly in the visible sky. With proper waveform design andprocessing, it is possible to achieve measurements at 1000 km of range with less than 0.05% error.The ATLAS pilot radar was developed with cutting edge hardware technology and uses a highlymodular architecture with processing fully in the digital domain. This enables a cost reduction in thedevelopment and maintenance of the system and provides a platform for research and innovation in theradar field. We also indicated several emerging fields that can be tested and developed using it, such asmatched illumination, noise radar and cognitive radar (Section 4.2).Since the system is at an early stage, several tests and upgrades are envisioned:• Calibration campaigns for the antenna motorized tracking system;• Operational testing in real scenarios;• Incorporation of the system into an operational SST network;• Addition of a waveform generator with In-Phase and Quadrature Modulation; Author Contributions: conceptualization, João Pandeirada, Miguel Bergano, Domingos Barbosa and Paulo Marques;investigation and writing, João Pandeirada, Miguel Bergano, João Neves and Paulo Marques; simulations, results andvisualization, João Pandeirada, Miguel Bergano, Bruno Coelho and Valério Ribeiro; project administration, MiguelBergano and Domingos Barbosa; funding acquisition, Domingos Barbosa and Valério Ribeiro.
Funding:
The team acknowledges financial support from the Aga Khan Development Network and the Fundaçãopara a Ciência e a Technologia, Portugal, for the Science and Technology Cooperation DOPPLER - DevelOpment ofPaloP knowLEdge in Radioastronomy, project number 333197717 . V.A.R.M.R. acknowledges financial support fromthe Fundacão para a Ciência e Tecnologia (FCT) in the form of an exploratory project of reference IF/00498/2015 andPHOBOS, POCI-01-0145- FEDER-029932, funded by Programa Operacional Competitividade e Internacionalizacão(COMPETE 2020) and FCT, Portugal. The team acknowledges financial support from Enabling Green E-sciencefor the Square Kilometre Array Research Infrastructure (ENGAGE-SKA), POCI-01-0145-FEDER-022217, funded byCOMPETE 2020 and FCT, Portugal and from the European Commission H2020 Programme under the grant agreement2-3SST2018-20.
Acknowledgments:
The team acknowledges the European Commission Horizon 2020 Programme under the SpaceSurveillance and Tracking (SST) grant agreement 2-3SST2018-20.
Conflicts of Interest:
The authors declare no conflict of interest.
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