“In those days only the superpowers were involved in this activity, so the knowledge gained up to that time was not available or accessible to the Israeli engineers, who had to invent and develop everything from the ground up,” explains Ilan Porat, Head of the Space Unit at Elbit Systems. “The objective was to provide high-resolution intelligence for military purposes. Our primary client, then and now, is IMOD.”
The space unit of Elbit Systems, headed by Porat, is located at the Science Park in Rehovot. Even the entrance to the building states that this is a national asset. Numerous certificates of merit issued by IMOD for various developments are posted on the walls, as a monument commemorating the engineering and scientific potential of this small country. “We squeeze out of our systems the maximum resolution for the given size and weight,” says Porat. “The launcher is relatively small, because of the fact that owing to our location and geopolitical constraints imposed on Israel, we are forced to launch westward, contrary to the physical advantage of launching eastward – just the opposite of the rest of the world.
“These constraints compel us to work with a compact, lightweight camera. Israel is unique with regard to the weight and size considerations in the development of satellite cameras, and is a global leader in the ratio between the weight of the camera and its relative performance. Israel is also among the world leaders in resolution, along with such countries as the USA and France.
“One thing must be said: the development of cameras for satellites is not a viable business economically,” explains Porat. “Export is practically nonexistent, and financing is provided mostly by IMOD. Additionally, there is a financing mechanism intended to retain national knowledge centers or in other words – retain the professional personnel of this field. This involves funding in ILS that does not come from US foreign aid. These funds enable us to develop the building blocks of the next generation of space cameras for the State of Israel.
“There is nothing unusual about it. Most of the world’s space industries do not generate profits. It is a highly competitive field that demands massive resources as well as political flexibility. The French, for example, develop satellites for their military and market them worldwide. In Israel that is not the case. The Americans operate in the same way. In those countries, the defense ministries are in charge of marketing the satellites, sometimes with the assistance of the top political echelon. In Israel it works differently, and as there is no export activity, this field must rely on government funding.”
The space cameras made by El-Op weigh only a few dozen kilograms only. The first system developed at Elbit had a lens diameter of 30 centimeters which provided a resolution of 2 meters (the minimum size of an identifiable ground object). Subsequently, they moved up to 50 cm diameter, which enabled a resolution of 70 cm. In the third and present generation, lens diameter is 70 cm and resolution is 50 cm. The latest camera weighs less than 100 kg. As the lens diameter increases, the camera becomes larger and heavier.
Another point that must be remembered is that no corrections or repairs may be made in space. If anything goes wrong during the launch or while the camera is operating in space, the satellite will become space waste. Porat explains that the challenge is to develop and adjust such a system on the ground, under gravity conditions. In space there is no gravity, and as a result optical components tend to warp. As there is no place in the world where this may be tested, El-Op invests considerable efforts in proper design and evaluation of the effect of the absence of gravity on the system once it arrives in space.
Additionally, in order to test the camera while simulating the working conditions in space, Elbit designed a special testing facility, the size of a three-story building. Construction was completed about three years ago, and the new facility is used to test the camera before it is installed in the satellite. The testing is based on the simulation of extreme space conditions, including vacuum, temperature, radiation, vibrations and other parameters.
One of the elements of the simulation facility is a collimator. Its function is to inject targets into the camera from a simulated distance of 600 kilometers, which is the distance between the satellite and planet Earth. The collimator is a sort of an inverted telescope – a specially requisitioned Israeli development. Porat explains that in order to test the camera for a resolution of 50 cm, a more accurate measurement array is required (you cannot measure a meter with a meter). In other words, the measurement and testing array for the space camera should possess the stability and accuracy characteristics that are equal to a resolution of 5 cm from a distance of 600 km. In order to overcome the natural vibrations of the planet as well as interference caused by moving trains, vehicles, noise sources, etc. – the array ‘rides’ a 250 ton concrete slab that floats over pneumatic feet and is not connected to the floor. All of the assemblies of the measurement array are not connected to the floor either, so as to isolate them from vibrations and external interference.
Elbit specified the requirements for the simulation facility, including the thermal vacuum compartment, the floating concrete slab and its feet, the collimator and other elements, and these were subsequently manufactured by various manufacturers in Israel and overseas. The construction process took about three years to complete. Along with the actual construction of the building, the entire array had to be integrated as it was delivered by the various manufacturers and tested for flawless operation. “There are fewer than five such installations worldwide,” says Porat. “Space is a difficult environment to operate in. If you face the sun, the camera might heat up and reach extreme temperatures in minutes, which could lead to warping and irreversible damage to the optical components – even to melting.
“On the other hand, the rest of the sky is at a temperature of absolute zero, which will cool the camera components to temperatures that are lower than those where the camera can survive. Consequently, we are required to come up with an accurate thermal design which includes a strict thermal control system while the camera is in space. Another aspect is the radiation in space, which damages the electronics and optical components. The optics should also survive the high acceleration rates that develop during the launch. It all adds up to a fairly complex simulation challenge.”
Developing a camera generation takes about ten years. Duplicating an existing camera takes about three years. This includes the final testing that takes about six months in the simulation facility. The main lens is processed with a deviation measured by atoms. There are no such off-the-shelf products on the market, they explain at Elbit. Additionally, owing to the very high optical sensitivity, mirror quality must be excellent and distortion must be minimal. The acceptable deviation of the mirror position is measured in microns (thousandths of a millimeter).
In order to reduce the camera size, Elbit uses optics that are based on mirrors rather than on lenses, which enables them to reduce the camera length by more than 10 times. In other words, an optical length of 10-15 meters is minimized into less than one meter. Each mirror has a specific magnification factor and a specific shape, so every distortion or deviation will be magnified accordingly.
In order to reach high accuracy rates and light weight, Elbit developed some highly specialized machines. One of them machines the rear part of the mirror. This machining method uses an ultrasonic technique which enables, theoretically, the removal of layers only a few molecules thick. Porat explains that this was the life’s work of a German engineer who aspired to be able to etch serial numbers on diamonds. He worked on this technology for 30 years, but then laser technology appeared and the whole project went down the drain. Elbit took his technology and used it as a basis for the development of this tool. You start off with a mirror weighing 120 kg and in the end remain with a mirror weighing only 20 kg. “Why are there no such machines anywhere in the world? Because no one needs them except Israel,” says Porat.
Another device polishes the mirror using a specialized magnetic fluid. In this way, no pressure is exerted on the mirror. This method was developed abroad for the polishing of flexible contact lenses. Elbit took the idea and developed it into a device for polishing nearly hollow mirrors (after the machining stage, the rear part of the mirror looks like a honeycomb). The entire process is computer controlled, thereby making it possible to reach a maximum error of a few atoms. At the end of the process, the mirror is coated with a reflective finish so as to enhance reflection. In order to exploit most of the energy received from the light reflected off the Earth, each one of the camera mirrors must produce maximum reflection. The entire machining, polishing and coating process takes about one year to complete.
After the camera is installed in the satellite and launched into space, the only parameter that may be corrected is focus, using an assembly that remains adjustable even when the camera is in space by a suitable command from the ground. All of the other elements remain fixed. If anything moves on the way to outer space, the camera will not work and the satellite will become space waste.
Can the satellite track moving targets?
“This only happens in Hollywood,” says Porat. “The satellite covers the distance from sunrise to sunset within a few minutes (depending on the altitude of its orbit). If you want to track a target, you need to see it constantly. Consequently, even in an optimal profile, the theoretical maximum interval for tracking a target is a few minutes over the course of an hour and a half, which is the time it takes the satellite to complete a full circle around the Earth. Tracking moving targets is science fiction. It is impossible for a satellite to track a moving target in real time. Even then, the angle, as well as the range, will change significantly during the actual photography cycle. The only way to remain permanently above a given area is using a geostationary orbit, which is above the equator at an altitude of almost 36,000 kilometers (like the communication satellites). From these ranges, it is impossible to provide photographs with usable resolutions unless you have a camera whose lens diameter is dozens of meters.
Admittedly, using nanosatellites is a new and “sexy” activity, but for space photography you need a large lens. Porat explains that there are thoughts of splitting the main mirror into several small satellites, but the problem is maintaining the relative accuracy so that they work in harmony. “We do not know how to do that yet. It would be feasible in the future, but that will take time,” says Porat. “There are also thoughts about deployable optics, namely the optical array will be launched into space in a retracted state, to be deployed in space. Once again, in this case, too, system accuracy is a sensitive issue.
“One of the possible uses of nanosatellites is testing electronics for space use. In the past, we used to purchase space-standard products. Unfortunately, these products are costly, and companies stopped manufacturing such parts as it is not profitable and there is hardly any demand for them. Today, both we and other manufacturers go to military or civilian components and subject them to a series of tests for withstanding space radiation. If you have a cheap nanosatellite whose launching is also cheap, you will be able to use it to conduct such resistance tests in space.”
How much Resolution is Enough?
One of the questions in the context of photo-surveillance satellites concerns the resolution that is sufficient in order to provide good intelligence. Well, the answer depends on the person being asked. Some terrorist organizations collect intelligence using the Google Earth service. Another option is to purchase photographs on the commercial market. Even countries use this option and today it is possible to purchase satellite photographs in black and white, as well as in color, from commercial companies worldwide, with a 50 cm resolution and more recently even with 25 cm resolution.
Assuming there are no size or weight restrictions that apply to the camera, according to statistical calculations, the optimal resolution that may be achieved is around 10-15 cm, owing to the atmospheric transmittance limitation. This is a physical limitation that applies to space photography through the atmosphere, owing to the turbulence phenomenon. Turbulence is the result of changes in air density (winds, jet streams, temperature differentials, humidity, etc.) in the atmosphere which prevent the image reflected off the ground from reaching the aperture of the space camera with sufficient quality. The most advanced country with regard to this aspect is the USA.
“The more you improve the resolution, the more pixels you will require, as otherwise your coverage area will be too small,” explains Porat. “Eventually, there is a trade-off between resolution and coverage area. If you want to scan a whole country and you only have a small coverage area, by the time you scan it again it will no longer be the same country.”
Although resolution is normally the issue being addressed, Porat insists on directing the spotlight at the data processing capability. “A third-generation camera (<50 cm) generates data at a rate of almost 6 gigabytes/second. You must do something with it,” explains Porat. “Admittedly, there are compression and storage mechanisms onboard the satellite, but eventually, you have to get the data to the ground, as otherwise, why would you launch a satellite in the first place? Today, with a compression ratio of 1:6 or 1:7, you lose some of the resolution and sometimes some of the details, but that is acceptable. The question is how the interpreters on the ground can deal with this amount of data.
“The satellite completes a full circle around the Earth every ninety minutes, travelling at a speed of about 7 km per second. Assuming the size of the Dan region is about 15x15 km, photographing it will require two seconds of photography out of ninety minutes. Consider the number of people who would have to sit down to interpret that. Within ninety minutes you should interpret 12 gigabytes of data. If you fail to do it, you will not be able to notice the changes. The entire chain should be ready for such a satellite. Admittedly, the advantage of a satellite of this type is the ability to photograph at near real time, but it all depends on the interpretation capability – that’s where the bottleneck is located.”