## OKAPI:Orbits

We are a spin-off from the Institute of Space Systems of the Technische Universität Braunschweig. We provide easy access to state-of-the-art algorithms for:

• Highly accurate object propagation
• Pass prediction
• Initial orbit determination
• Statistical orbit determination
• Object/object and object/tracklet correlation
• Close approach analysis and risk estimation
• Manoeuvre suggestion
• Catalague build up and maintenance
• SST System simulation

All our routines have been developed under scientific standards and will be made available step-by-step using a simple-to-implement Representational State Transfer (REST) interface. This brings some distinct advantages to our customers:

• Our services are easy to integrate into software written in any programming language.
• You only subscribe for the services you need.
• There is no long training needed: All interfaces and processes are lean and well designed, so you can focus on your work.
• You don’t need expensive hardware to run your requests. Everything runs in the cloud, on safe and secure servers. If that is no option for you, of course the server can run anywhere you like.
• Last-but-not-least: Everything can be fully customized. If you need anything, we make sure you get it.

All this is possible thanks to the system running in the background: our stream processing system (SPS). Compared to conventional database management systems, SPS continuously expect incoming data streams, just as they continuously provide outgoing data, while being optimized to control data flow and processing. The SPS brings the great advantage that processes can be defined dynamically within the system. While in the current step of our development, the processes are already defined within the system, in future releases this will bring the possibility to define any process based on all components included in our software – which exceeds the list of tools named above.

## The Space Debris Problem

Space is crowded. As of date, more than 300,000 objects larger than 1 cm are in orbit around Earth1. The origins of these objects are many fold: Most large objects are satellites(active and used ones), rocket upper stages, and mission related objects such as adapter rings etc. Although they by far include most of the mass, their total count is comparably low: About 4500 satellites are in orbit (less than 2000 of them still active), ca. 2000 are upper stages. Greatest contribution in terms of number of objects are fragments, created during explosions and collisions. Most notable events from the latter kind are a Chinese anti-satellite test from 20072 and the collision between the active Iridium 33 and the old Cosmos 2251 satellites3. These three major contributors are accompanied by several other sources, such as sodium potassium droplets, released from nuclear reactors used on satellites between 1980 and 1988 and wastes created using solid rocket motors in rocket stages and satellites. This distribution of objects from these different sources along their orbital altitude is shown in Figure 1. Looking at this figure, it becomes apparent that, naturally, altitudes which are of most interest for satellite operators are also those that are populated the most.

Every one of these objects can cause a critical mission failure or even the loss of the complete satellite. As a guideline, it is said that from an Energy-to-Mass ratio of 40 J/g4 an impact leads to a complete destruction of a satellite. Due to the high relative velocities on orbit (which in LEO can reach up to 15 km/s), small objects can already lead to these catastrophic collisions. Collisions between satellites and space debris objects happen frequently, although the large share of them have no critical impact on the satellite’s mission. A list of some past collision events is given in the table below. Creating an exhaustive list is nearly impossible, as many on-orbit anomalies cannot be fully re-solved. Even though the latest stated collision had a very low Energy-to-Mass-Ratio (EMR), it led to damaging the solar array of Sentinel-1A in an area with a diameter of 40 cm.

Year    |        Object 1        |        Object 2     |   EMR / J/kg    |     Cat. Frags
--------|------------------------|---------------------|-----------------|------------------
1991    |     Cosmos 1934        |     Debris 13475    |      76,600     |        2
1996    |     Cerise             |     Debris 18208    |     830,000     |        1
2002    |     Jason-1            |     Unknown         |      Unkown     |        2
2005    |     DMSP 5B F5 Thor    |     Debris 26207    |     628,000     |      699
2009    |     Cosmos 2251        |     Iridium 33      |  15,900,000     |     2199
2013    |     Blits              |     Unknown         |     Unknown     |        1
2016    |     Sentinal-1A        |     Unkwon          |          ~6     |        0


Of all objects, currently only about 22,300 are actively tracked by the US Space Command. Most of them of size larger than 10 cm in LEO. On higher orbits the minimal visible diameter decreases. Geostationary Orbit(GEO) to about 1 m. If a potential collision is projected, a warning is sent to satellite operators, including information such as the state uncertainties of the objects included in that conjunction, the collision probability, and calculated miss distances. In 2017, about 4,000,000 of these conjunction warnings were sent to satellite operators, leading to more than 100 collision avoidance manoeuvres per year5.

This only includes about 70% of all objects larger than 10 cm. To reduce the knowledge gap, the US military is in the process bringing the space fence online. It is a space surveillance system built for the US Airforce designed to detect and track objects on Earth’s orbits. It is located on Kwajalein Atoll, in the Marshal Islands. With its capabilities, the aim is to track all objects larger than “softball size”, whereas the current catalogue is stated to be complete in LEO for objects larger than “basketball size”. It is assumed that this lower detection threshold will lead to an increase in the number of objects to ~160,000. Increasing the number of objects in the catalogue will naturally also increase the number of close approaches that can be observed. In a first assessment the conjunction warnings will increase by a factor of 300%. These numbers are valid, if the Low Earth Orbit environment stays similar to how it is today: Currently, Spaceflight is prospering. The number of launched satellites per year has increased steadily from about 40 in 2005, which was the lowest level since the very beginning of spaceflight, to more than 400 in 2017 6. A large share of the new launches comes from an increasingly strong commercial spaceflight, delivering products such as telecommunications, Earth observation and weather data. While these increasing activities help solving a lot of issues on Earth, they also pose new challenges for the Low Earth Environment. If only a fraction of the published plans become realized, altitudes below 500 km will be populated with thousands of new, mostly CubeSat sized small satellite constellations constantly monitoring Earth, more or less classical satellite missions in altitudes between 500 km and 800 km. In higher altitudes, huge communication constellations will be built, consisting of up to several thousand of satellites with masses between 100 kg and 500 kg, constantly cycling up from their launch altitudes in 500 km, and spiralling down for their safe disposal in the atmosphere6,7. To keep spaceflight working, current means of safe satellite operations and collision avoidance won’t be feasible anymore. Therefore, OKAPI:Orbits provides software solutions, to easily integrate and automate all aspects of safe operations into mission control systems, starting from pass prediction, orbit determination, orbit and co-variance propagation, up to collision avoidance and manoeuvre optimization.

## Our Heritage

The core of our software has been developed in the past decade, mostly during two ESA and DLR funded grants8,9. In this time a complete SST (Space, Surveillance and Tracking) software, called Apollon, has been created, which can perform all relevant tasks from initial and statistical orbit determination of arbitrary measurements (range, range rate, angular, GPS, etc.) over object and uncertainty propagation, catalogue build-up and maintenance up to conjunction and re-entry services.

1. S. K. Flegel. Maintenance of the ESA MASTER Model. Final Report to ESA/ESOC contract 21705/08/D/HK, , Institute of Aerospace Systems, TU Braunschweig, 2011.

2. N. L. Johnson, E. Stansbery, J.-C. Liou, M. Horstman, C. Stokely, and D. Whitlock. The characteristics and consequences of the breakup-up of the FengYun-1C spacecraft, Paper ID IAC -07- A6.3.01. In Proceedings of the 58th International Astronautical Congress (IAC). 2007.

3. J.-C. Liou. Satellite Collision Leaves Significant Debris Clouds. Orbital Debris Quarterly News, April 2009.

4. D. S. McKnight. Collision and breakup models:pedigree regime, and validation/verification. Briefing presented to the National Research Council Committee on Space Debris Workshop, Irvine, CA, USA, 1993.

5. Guadalupe V. and Mollett R. 18 SPCS: Who We Are, What We Do, and Where We’re Going. CNES CA Workshop, 2017.

6. B. Bastida Virgili, J. Dolado, H. G. Lewis, J. Radtke, H. Krag, B. Revelin, C. Cazaux, C. Colombo, R. Crowther, and M. Metz. Risk to space sustainability from large constellations of satellites. Acta Astronautica, 126:154 – 162, 2016.

7. J. Radtke, E. Stoll, H. G. Lewis, J. Beck, and B. Bastida Virgili. To launch or not to launch -responsibilities of small satellites for a sustainable space environment. In Proceedings of the 68th International Astronautical Congress, Adelaide, Australia. 2017.

8. Contract No. 4000103850/11/D/JR (ESA/ESOC), 01.08.2011-31.08.2015. Network Partnering Initiative: Definition of Orbit State, Orbit Ephemerides and Orbit Covariance Formats. Performed at Institute of Space Systems, TU Braunschweig.

9. Contract No. 50LZ1404 (DLR), 01.01.2014-31.05.2019. DLR-RSS – Entwicklung eines Radar System Simulators (Development of a Radar System Simulator). Institute of Space Systems, TU-Braunschweig.