USE OF EISCAT 3D FOR OBSERVATIONS OF SPACE DEBRIS J. Vierinen(1), J. Markkanen(2), H. Krag(3), J. Siminski(3), and A. Mancas(3) (1)University of Tromsø, Hansine Hansens veg 18, 9019 Tromsø, Norway, Email: [email protected] (2)EISCAT Scientific Association, Sodankyla,¨ Finland (3)ESA/ESOC Space Debris Office, Darmstadt, Germany ABSTRACT large-aperture (HPLA) radar planned in Northern Fenno- Scandinavia. The primary mission for the radar is how- ever ionospheric science, which means that it the driver We investigate the capabilities of the next generation for the radar design was not space object observations. In ionospheric research radar EISCAT 3D (E3D) for ob- this study, we have set out to investigate in greater detail servations of space objects. The radar is multi-static, the utility of E3D for space object observations – both for and is therefore capable of observing instantaneous three- beam-park observations, and for surveillance purposes. dimensional vector velocity and position by observing We will use the current projected design of E3D as basis round-trip delay and Doppler shift between the transmit- of this study. ter and three receiver sites. The radar is to be located in Northern Scandinavia, which provides a high revisit- rate for high inclination objects. To model the perfor- mance of E3D for space object observations, we have in- To model the performance of E3D for space debris ob- cluded radar equation based analysis of object detectabil- servations, we have studied radar equation based de- ity as a function of range and size. To study the perfor- tectability and performed an error analysis for range and mance of the radar for orbital elements determination, we range-rate observables [MLHV02]. Because the radar have used a linearized error covariance analysis for ide- will be multi-static, it is capable of observing instanta- alized Keplerian elements. The analysis includes range neous three-dimensional vector velocities and positions and range-rate errors due to signal-to-noise and iono- by observing delay and Doppler shift between the trans- spheric radio propagation. To estimate the fraction of to- mitter and three or more receiver sites. We have included tal debris that can be observed with E3D, we have used linearized error estimates for Keplerian orbital elements the MASTER model [FGW+09]. E3D uses a relatively based on multi-static observations of targets. We have low VHF frequency (233 MHz), which experiences more intentionally chosen not to use a more complicated orbit radio wave propagation effects than more conventional model, in order to get a simple and intuitive first order higher frequency space surveillance radars. Our model- idea of orbit determination accuracy and repeat-rate of ing shows that ionospheric ray-bending and group delay observations. are severe enough that these effects need to be modeled in order to determine accurate orbital elements. As EIS- CAT 3D is an ionospheric research radar, there will be high quality ionospheric electron density measurements that can be utilized for radio propagation modeling. Our E3D uses a relatively low VHF frequency (233 MHz), simulations indicate that the radar can be used for ob- which experiences more radio wave propagation effects servations of orbital elements of objects down to 5 cm than more conventional higher frequency surveillance in diameter. It is therefore feasible that the radar could radars. Our modeling shows that ionospheric ray-bending provide to be a useful source of accurate information of and group delay are severe enough that these effects need orbital elements of space debris. to be addressed if one were to utilize E3D for accurate orbital elements determination. Without ionospheric cor- Keywords: Radar, Space object tracking. rections, measurements performed using E3D are unus- able for determining useful orbital elements. Because E3D is an ionospheric research radar, it can measure elec- tron density profiles at the same time with every measure- 1. INTRODUCTION ment of a space object, and this can be used to correct for propagation errors. In order to obtain a more realistic model of errors, we have estimated residual ionospheric Study of space debris is listed in the Science Case for propagation errors after ionospheric propagation correc- EISCAT 3D [MAA+15] as one of the application areas tions have been applied, and used these errors when esti- of the planned next generation multi-static high-power mating orbital elements determination performance. Proc. 7th European Conference on Space Debris, Darmstadt, Germany, 18–21 April 2017, published by the ESA Space Debris Office Ed. T. Flohrer & F. Schmitz, (http://spacedebris2017.sdo.esoc.esa.int, June 2017) 2. EISCAT 3D Site Gain Nbeams TX Trec Btx dBi K MHz Skibotn 43 100 yes 150 5 The EISCAT 3D radar (E3D) is a a multi-static HPLA Bergfors 43 100 no 150 ≤ radar, which is to be located in Northern Scandinavia. Karesuvanto 43 100 no 150 The radar uses phased array antennas for transmit and Jokkmokk 43 100 no 150 receive. The radar will initially have one transmit and Karesuvanto 43 100 no 150 receive site in Skibotn, Norway (69.340◦N, 20.313◦E). There will also be four receive-only sites: Karesuvanto (68.463◦N,22.458◦E), Bergfors (68.148◦N, 19.769◦E), Table 1. The performance parameters for E3D. Andoya Norway (69.251◦N, 16.096◦E) and Jokkmokk (66.60◦ N, 19.81◦ S). The technical description of the planned radar is described in detail in the technical note published by EISCAT [EIS14]. The planned locations are shown on a map in in Figure 1. The plan is to build E3D in two stages. In the first stage, a transmitter and receiver will be located in Skibotn. Re- ceivers will be located in Karesuvanto and Bergfors. In the next stage, two more receivers will be built: one in Andoya and one in Jokkmokk. Currently planned parameters for E3D are listed in the Table 1. In this table, Nbeams stands for the number of si- multaneous beams that can be formed, Trec stands for re- ceiver noise temperature, and Btx is transmit bandwidth. The transmit power Ptx is planned to be 10 MW. The radar will have a center frequency of 233 MHz. Each antenna array will consist of 9919 inverted-v dipole Figure 1. Planned E3D sites. The core site in Skibotn antenna elements similar to the PAVE PAWS design is colored with red. Receive sites are located in Karesu- [BPC+10], grouped under 91 antenna subgroups. The vanto, Bergfors, Andoya, and Jokkmokk are colored with array will be a planar array placed horizontally with the blue. on-axis direction pointing towards zenith. The antenna arrays have a full-width half maximum beam width of approximately 0.9◦. The antennas are steerable to 60◦ beams are then used by the second level beamformer to off zenith, with gain falling off approximately according form up to 100 dual polarization 30 MHz beams. On the to projected geometric area G(α) = G0 cos(α), where α core site, most of these beams will be utilized for inter- is the zenith angle. ferometry. On the receive sites, these beams will be used to form beams that intersect the transmitter beam across The Skibotn transmitter site will have interferometric out- all altitudes of interest. The number of beams is suffi- lier antennas, which are approximately 10 meters in di- cient to simultaneously cover all common volumes with ameter. The longest baseline will be approximately 1.5 the transmit beam. An example of a transmit beam inter- km. The main purpose of the spaced antenna interfer- secting with 100 receive beams is shown in Figure 2. ometer is to perform imaging of small-scale ionospheric structure in the E-region of the ionosphere during auroral energetic electron precipitation. The interferometry will The second level beamformer and the high level signal also have uses for measurements of trajectories of mi- processing will be implemented using a general purpose crometeoroids [KSN+12], and for planetary radar mea- high performance computing cluster. The e-infrastructure surements of the Moon [Tho87]. These interferometric for E3D is still in somewhat formative stages, and it is not antennas may also have utility when estimating the angle yet decided if the second level beamformed data will be of arrival of high signal-to-noise ratio space debris targets transmitted over a fast internet connection to a central su- with high precision. It is also possible to divide the core percomputer for analysis, or if the supercomputer will re- antenna into subarrays for interferometry. Figure 7 shows side at the location of the radar site. The most likely sce- an example of a mono-pulse type of an interferometer, nario is that the vast majority of computing will be per- achieved by dividing the core array into four quadrants. formed on-site and accomplished by having extra com- putational resources for analyzing and reducing the radar The antenna will have an all-digital beamformer, with a data before sending it to a central archive. The latter sce- direct sampling 105 MHz digitizer connected to two po- nario will allow low-latency analysis to be performed, al- larizations on each one of the 9919 antenna elements. A lowing that radar to quickly adapt to changing conditions. level one beamformer will reduce the bandwidth to 30 It is feasible that the radar will be able to allocate beams MHz and form up to 10 dual polarization beams on each in a sub-second timescale to follow transient targets, such one of the 91 element tiles. These first level wide angle as meteors, or space debris. Figure 2. Conceptual diagram of a 1◦ transmit beam in- Figure 3.