Factors in SST Design

· Write a comment · Categories: Space Situational Awareness, Space Surveillance · Tags: , , ,

The development of an effective space surveillance network requires the creation of an comprehensive, accurate and trustworthy catalogue of objects in orbit. This forms the basis for the multitude of services such as conjunction prediction and re-entry warning are expected.

By comprehensive, the system must be able to those objects which may pose a threat to operational satellites. This means that they should be able to detect objects with a minimum diameter to cause a detectable or cumulative deterioration in the performance of a satellite. This minimum diameter is defined by both the kintetic energy contained within a piece of debris (related to the mass and velocity) and the probability that a satellite will be operationally damaged by the impact of such a piece of debris.

As with the diameter of an object, the accuracy that will be achieved is limted by numerous factors, these include the limits of the technology used, the number of sensors within the surveillance network and the observation conditions (at both sensor and target) when detecting a specific piece of debris.

When discussing the degree to which a system can be considered trustworthy, three main factors come into play. These are the degree of false readings created by a sensors, the method used to convert the detections into an orbit and the techniques used to relate individual detected passes to objects which already exist in the database. These are all affected by the way in which the sensor system is designed, the weighting or importance given to specific sensors and the actual technology employed when looking at orbiting debris.

In order to develop a system which provides the needed performance, as well as being economically viable, requires a careful monitoring of the interplay between the multiple factors which determine the final design. This is the purpose of the top-down stream of ESA SSA programme and development. Not an easy task – and there are no absolute answers – but in the area of complex system-of-system design, the Agency has a long history of successful and effective solutions.

Space Imaging for SSA

· Write a comment · Categories: Space Situational Awareness, Space Surveillance · Tags: , , , , , , , ,

When satellite imaging is mentioned, we usually think of satellites taking pictures of the weather, floods or downtown New York. This is one of the primary uses of satellites and one which has brought great benefits to many people. Within space surveillance, satellite imaging refers to taking pictures of satellites on orbit. But why would we want to do that?

One reason is when things don’t go as planned. Satellite operators cannot always see what is happening on the satellite directly, but rely on specific sensors to tell them if something has – or hasn’t – worked. This could be a signal to say that the solar panels have been set correctly. If this signal hasn’t been received, it could mean one of two things. Either the solar panel hasn’t been set correctly or the sensor has malfunctioned. But which one of the two is it? Sometimes this specific situation can be verified because there is no power being generated by the solar panel or the satellite’s attitude moves in such a way that indicates the solar panel is not sticking out of the side of the satellite body. In either case, an independant way to verify this would be useful.

Another reason is during re-entry. The way a space object is oriented can effect large changes in the re-entry profile. It can determine if the satellite will break up high in the atmosphere and these small pieces vapourise without touching the Earth’s surface or if the orientation will cause some drag or lift as it comes through the atmopshere and hence change the impact point. Being able to image an object as it comes close to re-entry and begins to be affected by the atmosphere can really help reduce the uncertainty in both these areas.

A final reason why satellite imaging is important is – as can be guessed – military. Having intelligence regarding the capabilities of satellites in orbit is very useful to military commanders. Using satellite imaging could be a good way to do this.

Of course any specific military requirments are out of the scope of the ESA SSA programme. It can be predicted that the resolution required to perform the first two functions of anomaly resolution and re-entry prediction is much less than that required for the third one.

Survillance and Tracking for SSA

· Write a comment · Categories: Space Situational Awareness, Space Surveillance · Tags: , , ,

The ESA OGS (image credit: Emmet Fletcher)

There are sometimes confusing statements made regarding the different sensors used within a Space Situational Awareness (SSA) system. Even if we are discussing radar or optical, there are – in essence – three types of sensor: surveillance, tracking and imaging. Since imaging sensors are a very special type – and do not figure in the development of the ESA SSA system, we will ignore them for this specific discussion. What we will discuss here is the difference between surveillance and tracking sensors – a difference that is often overlooked or ignored or just not understood, but is of prime concern when discussing the development of an effective space surveillance system.

Tracking Sensors

It is probably easier to start any discussion of the differences between tracking and surveillance sensors by focussing (no pun intended) on the major characteristics of a tracking sensor.

Tracking sensors usually have a very small field of view – rather like when you use a domestic telescope and you can see a very small area of the sky. Given a fixed detector performance, the smaller the field of view, the more precise the locations of the objects detected within this field of view (when comparing like-for-like). This is fantastic when you want to increase the precision of an object for when you already have some orbital data of, such as a piece of debris for which you have a rough orbit and may collide with an operational spacecraft. You just take this rough orbit and set your tracking sensor to point along this orbit at the position you think the debris should be. When you see the debris, you can then create a more precise orbit – since your detector is looking at a very small region of space and so has a high precision.

The problem is – of course – that since you only see a small area of the sky, if the error on your rough orbit is too high, you might not see the debris at all (it might slip by outside your field of view). It also makes these sensors very inefficient (read: almost useless) for the build-up a catalogue of objects. Since the view is small, it is difficult to trap new objects, unless you are very lucky. Even then, given the small view, you only have a very short reading as the debris passes across the sensor. This results in an initial orbit guess (orbit determination) which can have very high errors. For the development and maintainence of a catalogue, we need a surveillance sensor.

Examples of European tracking sensors: TIRA (Germany), BEM Monge (France), OGS (ESA), CAMRA (UK)

Surveillance Sensors

The TFRM (image credits: Emmet Fletcher)

A surveillance sensor is the workhorse of a surveillance system. It provides the data for both the initial catalogue development (the so-called ‘cold start’) as well as the day-to-day maintenence of the catalogue.

The main difference between the tracking and surveillance sensor is that the surveillance sensor sees a very large area of the sky at the same time. It is also not actively looking for objects, but rather passively (which counterintuitveilly can be active) waiting for debris – any debris – to pass over it. Once it detects something passing over it, the data related to this pass is processed and passed to the catalogue maintainence system.

In this way, the surveillance sensor creates a ‘fence’ which is triggered by any object passing through it. No prior information is needed by the sensor to generate new data regarding a specific debris object and the system therefore does not need to be ‘tasked’ to look out for an object. In reality, the fence can also be generated using an active sensor scanning the sky with a frequency that ensures nothing will be missed. This is the case for radar systems which quickly scan across a path. It doesn’t look in all directions at all times, but still forms an effective fence.

Through the use of surveillance sensors, a catalogue can be built up. The precision of this catalogue will not be very high initially, although the design of the surveillance network should be such that the eventual precision using just the surveillance assets will be enough to give a reliable warning of potential collisions with operational satellites. When the warning is triggered, then comes the turn of the tracking sensors to refine the orbit of this debris and provide the precise information that satellite operators need to plan their maneouvres.

Examples of European surveillance sensors: GRAVES (France), RAF Fylingdales (UK/US), TFRM (Spain)

If you have any comments, clarifications, corrections or suggestions – please comment!

Phobos-Grunt re-entry position uncertainty

· Write a comment · Categories: interviews, media · Tags: , , , ,

Phobos-Grunt re-entry (image: Michael Carroll)

There’s a nice article with a portion of an interview with me on space.com here: http://www.space.com/14312-russia-mars-probe-phobos-grunt-conspiracy-theories.html.

Working together to be able to get the maximum amount of data spread over as much area as possible is critical to ensuring we can locate where re-entry objects could have landed. It’s a long road towards complete harmony, but the IADC is doing a lot of work towards that aim.

It could be quite a while before we know where exactly the probe landed – there may be a lot of data to sift through before a true picture emerges, but at the moment, I still think the probe landed in the empty areas of the Pacific ocean (just where we wanted it to go!). I hope that the prediction stays that way.

Interview with Antena3 regarding Phobos-Grunt

· Write a comment · Categories: interviews, media, video · Tags:

With all the interest regarding the re-entry of Phobos-Grunt, it’s not surpising that we were approached to give our comments on the national news. Here is an interview made with me. The nice thing is that you can see the new Space Surveillance and Tracking Centre (SSTC) that we have developed in ESA / ESAC. It took some time, but I hope you like the results!

Más vídeos en Antena3

 

 

Phobos-Grunt: Interesting ground-based imagery

· Write a comment · Categories: Space Situational Awareness, Space Surveillance · Tags: , , , , ,

imageThis is an image taken by Ralf Vandebergh using commercial telescopes as the basis for the observations. This show the power of ground-based telescopes to take images of low-Earth orbit objects. Needless to say, the cost of Ralf’s system is probably an order of magnitute (or less) than the cost of a comparable radar-based system.

Of course, weather conditions and the illumination conditions from the Sun play a great deal in how good the image will be on any given observation; something that is not generally a problem for radar systems or space-based space surveillance. But I think the work that Ralf shows here is very interesting and could teach a few lessons to more formal systems.

Ralf’s home page is here: http://ralfvandebergh.startje.be/vieuw.php?qid=328303 and some articles he has written for Space Safety Magazine can be found here: http://ralfvandebergh.startje.be/vieuw.php?qid=328303

3D documentary on Space Debris (redux)

· Write a comment · Categories: Space Situational Awareness, Space Surveillance · Tags:

This looks pretty interesting. As the synopsis reads:

50 years after launching our dreams into space, we’re left with a troubling legacy: a growing ring of orbiting debris that threatens the safety of earth’s orbits. SPACE JUNK is a visually explosive journey of discovery that weighs the solutions aimed at restoring our planet’s orbits.

Experience mind-boggling collisions, both natural and man-made. Soar for the stunning depths of Meteor Crater to an unprecedented view of our increasingly crowded orbits – 22,000 miles above earth. Join us as foremost expert Don Kessler, the “Father of Space Junk,” guides us through the challenges we face in protecting them, forging a new age of space discovery

More information here.

 

Amenazas del espacio exterior

· Write a comment · Categories: interviews, media

El planeta Melancolía ha aparecido desde más allá de las fronteras del Sistema Solar atraído por la gravedad del Sol. Al modo de los cometas, ha girado alrededor del astro rey y se dirige hacia la Tierra. Los científicos aseguran que se tratará de un mero sobrevuelo y que pasará de largo, pero, seguramente, Melancolía colisionará con la Tierra, ante la impotencia y estupefacción de la raza humana, que será destruida en la brutal colisión.

No teman: esta historia es el argumento de la última y alucinada película del polémico director danés Lars Von Trier, titulada con el nombre de ese planeta intruso, Melancolía. La hipótesis que plantea Von Trier es imposible en el mundo real: ningún planeta va a aparecer de allende los límites del Sistema Solar ni salirse de su órbita para colisionar con la Tierra. Pero sí hay otros peligros en el espacio. Afortunadamente, también hay científicos vigilantes que escrutan el espacio exterior para preverlos.

“Las amenazas son tres: la basura o chatarra espacial, los asteroides y el clima espacial”, explica Emmet Fletcher, responsable del programa de reconocimiento y seguimiento espacial de la Agencia Espacial Europea (ESA). Desde ESAC, el centro de Astronomía Espacial de la ESA en Villafranca del Castillo (Madrid), vigila para que estas amenazas no se concreten.

La basura o chatarra espacial se constituye por residuos tecnológicos de satélites que orbitan alrededor del planeta, hasta 6.000 toneladas, 20.000 fragmentos, de material inútil que se ha acumulado, girando en su danza cósmica, desde el lanzamiento del primer satélite artificial en 1957 (el célebre Sputnik soviético). Ponen en peligro a los satélites operativos que pueden ser dañados incluso por fragmentos de unos pocos centímetros que viajen a mucha velocidad y también a nuestras cabezas si caen a la Tierra y son lo suficientemente grandes para no destruirse en su entrada en la atmósfera. El programa Space Situational Awarenes (SSA) de la ESA se preocupa por tener a mucha de esta chatarra bien controlada. Los científicos e ingenieros sacan de órbitas útiles viejos satélites para aprovechar de nuevo esa órbita o redirigen satélites que vaya a colisionar con la chatarra. También se preocupan por dónde van a caer los satélites viejos y qué parte de esos satélites llegará a la superficie terrestre. El pasado mes de septiembre, precisamente, cayó el satélite incontrolado UARS de la NASA, sobre el Pacífico Norte, cerca de la costa occidental de EE UU.

Los asteroides también pueden alcanzar la Tierra. En realidad la caída de meteoritos es muy frecuente, pero su tamaño es tan reducido que no supone ninguna amenaza. “Buena parte del polvo que se deposita sobre la carrocería de un coche cuando está mucho tiempo aparcado proviene de los meteoritos”, explica Fletcher. “El riesgo de colisión con un asteroide peligroso es pequeño, aunque sus consecuencias serían muy grandes”. Para evitarlo los astrónomos someten al cielo a un constante escrutinio. “Si detectásemos un asteroide peligroso tendríamos todavía décadas para prepararnos, aunque lo óptimo sería desviarlo cuando este más lejos: así serviría una mínima desviación para que no chocase con la Tierra”, explica Fletcher. “Destruirlo con explosivos sería la peor opción: es mucho mejor tener un asteroide grande controlado que incontables fragmentos pequeños incontrolados”. También hay un asteroide que nos ha visitado recientemente, el 2005 YU55, que pasó a 324.600 kilómetros causando gran expectación y también un poco de temor. El asteroide Apofis se acercará mucho a nuestro planeta el 13 de abril de 2029. En 2004 se calculó que Apofis, de 270 metros de diámetro, tenía una probabilidad de 1 sobre 37 de colisionar con la Tierra con consecuencias terribles. “Pero ya estamos seguros que no es ningún peligro. Lo dice la Física y no cabe discusión”, asegura Fletcher.

El clima espacial son los fenómenos causados en el medio interplanetario por la actividad del Sol. Las fulguraciones solares emiten radiaciones muy energéticas, como rayos X, o partículas de más baja energía, como protones, que serían fatales para la Tierra de no ser por el campo magnético terrestre, la magnetosfera, que las frenan y las desvían a los polos, donde forman las vistosas auroras boreales y australes. Cuando la llegada de este viento solar es muy intenso pueden variar nuestro campo magnético y afectar a nuestras redes eléctricas “que son, al fin y al cabo, como cables muy largos de electricidad” o a los oleoductos, según explica Fletcher. “Cuando tienes un cable en un campo magnético variable se inducen corrientes que pueden afectarnos”, a veces revientan aparatos o provocan apagones (como el gran apagón en Quebec, en 1989, que afectó a seis millones de personas durante 12 horas). “También pueden arruinar satélites, como los del sistema GPS, y, cuando estos fenómenos se producen, es mejor decir a nuestros astronautas, si están operando fuera de la nave reparando algo, que se metan dentro, pues pueden ser peligrosos”, explica el astrónomo. Los científicos observan cuidadosamente la actividad del Sol (que alcanzará un máximo a finales de 2012) y, por tanto, el clima espacial, para evitar estos problemas.

Una red mundial de científicos de instituciones como la NASA o la ESA vigilan constantemente el firmamento. El espacio tiene sus peligros pero, por el momento, podemos dormir tranquilos.

Originally published by El Pais 26/12/2011. Link

Security and space surveillance

· Write a comment · Categories: Events · Tags:

The issue of security comes up often when talking about SSA. For some background, it’s worth having a look at what the European Union Satellite Centre (EUSC) are doing as part of the SpA project.

As it says in their project goals:

The SPA project is contributing to the technical definition of European SSA Governance and Data Policy testing possible models in the EUSC secure premises

The EUSC are investigating three representative services for SSA:

  1. Satellite Overflight
  2. Satellite Conjunctions
  3. Satellite Re-entries
The hope is that at the end of the study, they will be able to provide solid recommendations for the governance and data policy for a future European SSA system.

Un asteroide como un superpetrolero se aproxima a la Tierra

· Write a comment · Categories: interviews · Tags:

09/11/2011 Angela Gómez – Diario Expansión

Su nombre es 2005 YU55 y ayer pasó tan cerca de la Tierra (a unos 320.000 kilómetros) que se podía ver con un telescopio.

Este asteroide ha traido a la memoria colectiva las lluvias de rocas espaciales tan habituales en películas como ‘Superman’ o ’2001 una odisea en el espacio’. Algunos han llegado a vaticinar que su impacto sobre la superficie terrestre podría ser catastrófico. Sin embargo, los científicos no dejan margen para la imaginación: “Este asteroide va a pasar muy cerca de la Tierra pero no va a impactar. Por lo tanto, no es peligroso”, afirma con rotundidad la investigadora Julia León, del Instituto Astrofísico de Andalucía, del CSIC.
Con todo, “es un evento raro, que no suele ocurrir y, según nuestras previsiones, para que otro objeto similar se acerque tanto a nuestro planeta tendremos que esperar hasta 2028 o 2030″, puntualiza la astrofísica.

Objetivamente, sí hay motivos para una cierta alarma. El primero es que se trata de un asteroide grande, de 400 metros de diámetro (más grande que el portaaviones español Juan Carlos I o el Titanic y de la misma eslora que el superpetrolero Esso Pacific), y otro es la proximidad a la Tierra. Como explica León, “un asteroide se considera potencialmente peligroso cuando su diámetro es mayor de 150 metros y se acerca a nuestro planeta a menos de 20 veces la distancia de la Tierra a la Luna”. Y con estos requisitos, hay más de un millar de asteroides catalogados.

Los científicos de la NASA y de la Agencia Espacial Europea (ESA) han hecho un seguimiento minucioso de 2005 YU55. Desde que se descubrió, en 2005, los astrónomos han estudiado su órbita para determinar las posibilidades de que impacte en la Tierra, y la conclusión final es tranquilizadora.

Pero, ¿cómo saber que la información que nos llega es la correcta? Emmet Fletcher, Responsable del programa de Reconocimiento y Seguimiento espacial de la ESA explica que “hay que validar los datos que llegan de grupos independientes. En este caso, lo han hecho la Universidad de Pisa y un centro de Estados Unidos. Después de cotejar sus cálculos comprobaron que coincidían”.

Tal vez, para algunos sea una decepción saber que nuestro planeta no está, de momento en peligro, pero “la cercanía de este asteroide tiene un enorme interés científico, ya que nos permitirá estudiarlo desde tan cerca que podremos ver bien su superficie, estudiar su composición y otros datos que pueden arrojar más luz sobre el origen del universo”, apunta León. “El rádar Goldstone lo podrá observar con una resolución de cuatro metros”.

El astrónomo de la ESA añade que habrá que esperar cien años para que las alarmas vuelvan a saltar. “Son situaciones excepcionales, pero la caída de meteoritos a la Tierra es muy frecuente. Disponemos de cientos de fragmentos que hemos recogido de distintos lugares y que nos ayudan a conocer la composición del universo”.

Métodos de defensa

Aunque por ahora no hay que temer un gran impacto, los científicos trabajan desde hace años en el desarrollo de métodos defensivos para evitar una colisión potencialmente catastrófica. “Lo más sencillo y barato parece ser bombardear el cuerpo peligroso”, indica León, pero “la solución puede ser peor que la amenaza, porque hay que saber como controlar los fragmentos en los que se rompe ese cuerpo”.

Fletcher añade que otra opción sería enviar cohetes capaces de alterar la trayectoria del cuerpo amenazante. “Las soluciones son muy diferentes, pero se necesitan varios años para preparar una estrategia defensiva eficaz”.

Original article: http://www.expansion.com/2011/11/08/entorno/1320775668.html?a=e3d2fb3847e30689fbbdfc05166ceb21&t=1321874901

« Older articles