bits per second with a power budget of few watts. The directivity of ... 20 grams and launched by a homemade launcher .... Other option is to put solar panels but.
A SYNTHETIC APERTURE ANTENNA FOR FEMTO-SATELLITES BASED ON COMMERCIAL-OF-THE-SHELF Enric Fernandez-Murcia, UPC Barcelona Tech, Barcelona, Spain Luis Izquierdo, Nebrija University, Madrid, Spain Joshua Tristancho, UPC Barcelona Tech, Barcelona, Spain
Abstract Femto-satellites are very small satellites weighing less than 100 grams and by concept they are low cost. These kinds of satellites are very often based on the so called Commercial-Of-The-Shelf components. They are suitable for applications with low budget and with short time of development. The satellite remains in a Low Earth Orbit (LEO) of 250 kilometers for few weeks, and afterwards the orbit can be reused. A new strategy is considered that consists in dedicating the whole satellite to a particular mission, the satellite can be reprogrammed in orbit to provide dynamic support for Disaster Management in natural disasters such as earthquakes. A big issue for such tiny satellites is the communications. They should achieve a link budget of hundreds of kilometers with hundreds of thousands bits per second with a power budget of few watts. The directivity of the antenna and the accuracy to point ground stations should be increased. Also the antenna works as the satellite structure. In this paper a synthetic aperture antenna for femto-satellites based on Commercial-Of-The-Shelf is presented. This implementation meets all the previous requirements as a proof of concept. Different configurations of ceramic antennas array were used. Some transceivers and High Gain Amplifiers were tested. A semi-anechoic chamber results are also presented describing the antenna radiation pattern.
satellite has been launched1 but the lower the mass, the lower the launch cost. It is possible to increment the number of satellites in a single launch acting like swarms. In order to have this advantage, all the subsystems of a traditional satellite should be implemented in such a small mass budget. One of the most difficult parts to be implemented is perhaps the communications subsystem. The antenna size depends on the working frequency [3] but transmitting power also increases to the fourth power of the frequency; i.e. 2.4 GHz has a wave length of few centimeters and the power to communicate with ground from a LEO orbit is about few watts and hundreds of kilobits per second of bandwidth. In the following sections, some applications for femto-satellites will be discussed. In this sense, the design, implementation and validation for space of a micro-strip antenna is presented as a Synthetic Aperture Antenna (SAA). This design was done by the Wikisat team as a part of the N-Prize2 contest and it is based on the use of Commercial-Of-The-Shelf components. The N-Prize is a contest consisting in putting a private founded satellite weighing less than 20 grams and launched by a homemade launcher costing less than 1,000 Sterling pounds [4]. The WikiSat is a Satellite-on-a-board that meets the NPrize rules (Figure 1).
In order to change the phase of each ceramic antenna, passive components were used like microstrips.
Introduction The idea of building a complete satellite with a budget of less than 100 grams is not new. Helvajian’s design in [1] and Barnhart in [2] have proposed a complete femto-satellite. Up to date, no such a femto-
Figure 1. WikiSat V4 Femto-satellite W/O Battery
1 2
http://www.planet4589.org/space/log/launchlog.txt http://www.n-prize.com/
Femto-satellite Applications One of the most useful applications of this kind of technology is in Disaster Management (DM), and this has never been truer than in developing countries, where more than 95 per cent of all deaths are caused by natural hazards. For instance, in 2010, a total of 385 natural disasters killed 297,000 people worldwide and 222,570 fatalities occurred by the Haiti heartquake (Jan 12th) [5]. In this sense, femtosatellites have several strengths to bear in mind:
the opportunity to choose the technology in this very important matter: saving human lives in natural disasters. In Figure 2 the disaster management cycle is represented:
(1) Short space missions can be programmed; since they work in a V(ery)LEO, so they have a very limited period of life (few weeks depending on satellite mass), after that they just fell down and disappear in the atmosphere, leaving no space junk and allowing to reuse the orbit for another mission. (2) Due to the cheap of this technology, a femtosatellite can be used for a very specific mission. In fact, a constellation of them can be used to cover the different needs during an emergency [6] like remote sensing and communications. (3) A specific launch can be programmed for a specific disaster, which means that if there is no disaster there is no launch and therefore there is no cost. (4) The programming of the mission is relatively simple; we are developing one software3 (Moon2.0) that basically just needs three groups of data; (a) where do you want to launch the satellite, (b) the bounding box of the data acquisition and (c) the data type that we want to obtain. Satellite technology can be used in the different phases of DM (Response, Recovery, Mitigation and Preparedness) [7] but maybe, where the use of femtosatellites can be more efficient is during the Response phase, where it is very important to have a Common Operational Picture as soon as possible in order to help Decision Makers to manage the situation. The use of femto-satellites can help developing countries to have their own space technology. In fact they have been criticized by developed countries to waste the money in satellite technology, instead of eradicate child malnutrition [8]. We think that the price of this kind of missions is affordable for all countries, even for the poorest. And they should have 3
http://code.google.com/p/moon-20/
Figure 2. The Disaster Management Cycle
Femto-satellite Specifications The aforementioned femto-satellite uses some synergies to achieve such low cost and low mass. The structure of the satellite, which is a board by itself, is used at the same time to hold the ceramic antenna array that was presented by De las Heras in [9] and at the same time works as a passive thermal control subsystem. The femto-satellite design turns around an Inertial Measurement Unit (IMU) designed, implemented and validated in near space by Bardolet in [10]. The IMU proposed by Bardolet is based on two Micro-Electro-Mechanical System (MEMS). One MEMS is in the acceleration range of +/-24 g and 3 axis with 16 bits of data resolution and a bandwidth of 500 Hz. The other MEMS is in the rate of turn range of +/- 2,000 º/s, 3 axial gyros and a built-in temperature sensor. Both components are connected to the same bus 2-Wire (I2C) to the Main Control Unit (MCU) and other sensors. The accuracy of this IMU installed in the femto-satellite was designed well enough to control remotely the lowcost launcher from the satellite. This idea was presented by Tristancho in [11] as the Space Payload Paradigm where the integration of both, the launcher and the satellite in the design cycle of a space mission was proposed. The satellite attitude control
and the launcher vector control was studied by Navarro in [12] having the same inputs but different outputs. In order to reduce 10 times the launcher size, a balloon launching ramp was proposed by Bonet in [13]. When a launch is done at an altitude of 35 kilometers; the entire hard atmosphere is avoided and a large quantity of propellant is saved. An important problem to overcome in a femtosatellite is the low available power that often is based on a coin battery or even on a LiPoly battery of no more than 500 mW. The first type of batteries have the best power to mass ratio in the marked but the LiPoly batteries have the best peak-power to mass ratio and also are rechargeable. Other good performance is the wide thermal range and long number of charge-discharge cycles. The use of solar cells is very limited due to the small femto-satellite size [14]. For this reason, link budget take into account the use of onboard high directive antennas and high accuracy to point the antenna towards a single ground station. Other expected problem is the high noise level in the vicinity of ground stations. The massive use of WiFi and Bluetooth devices that work in the 2.4 GHz frequency requires a better link budget. Under this scenario, many electromagnetic compatibility problems should be studied because the near field [15] and shielding are not feasible because of the high impact in terms of mass.
The Synthetic Aperture Antenna Challenge There are a lot of different array configurations and each one has its advantages and disadvantages. For the femto-satellite mission, the main important features will be size, easy to implement, resilience and flexibility, which is a challenge. A bigger array will achieve more directivity but will make the design harder and increase the mass budget. Also it can be a problem in order to correctly aim both sides of the transceivers. Therefore a maximum number of elements must be defined for the project. For each number of elements different array configurations are possible (See Figure 3). To date, no femto-satellite was sent to orbit4. To achieve this goal, a radio-link should be implemented in such a small mass and power budget increasing the antenna gain by order of 2 or 3.
4
http://www.planet4589.org/space/log/launchlog.txt
Figure 3. Main Configurations.
Femto-satellite Link Budget First of all, an accurate power link budget for the communication is necessary in order to determine which elements will be required to increase the transceivers radio link (initially designed for less than 100 meters). The maximum link distance is fixed at 500 km, with a minimum of 250 km and it will be unidirectional from the satellite to the base station. It will be assembled on a Satellite-on-a-Board. It is selected the transceiver nRF24LE1 from Nordic Semiconductor. It is a 2.4 GHz System-on-Chip (SoC). It will work at 250 kbps with a 0 dBm power signal (GFSK modulation). The system must be upgradable, ready to be updated with new elements constantly (step-design). It starts as a medium range communication system able to evolve to a long range system. By this reason, some extra components like amplifiers (in both sides of the radio link) are necessary to increase the range. It is mandatory to begin with the computation of how many extra gain (dB) will be necessary for the worst case condition (500 km). Received power equations were obtained from [16]. The Polarization Loss Factor (PLF) is 3 dB because one antenna works with a linear polarization (satellite antenna) and the other one with circular polarization (ground antenna). This is critical because polarization may change due satellite rotation or any other undesired effect. A circular polarization (20 dB of gain) Yagi antenna is selected for reception and a 6 dB antenna (the gain expected for the array) for transmission is considered. Finally, 0.5 dB losses due efficiencies have been considered at each side of the link. The operating frequency (f0) selected is 2.49 GHz, but it may change a bit if necessary (selecting a different operating channel).
(Eq. 1) (Eq. 2) Following a transformation done by Cardama in [16], Eq. 1 can be easily transformed to obtain the extra gain necessary as shown in Eq. 2. As noted, at least 50.3 dB gain is necessary without considering noise effects (to be considered later on and not in the previous considerations) for a 500 km link. Typical amplifiers offer between 10 and 20 dB of gain, for this reason at least 3 amplifiers will be required. Also will be helpful if them are feed at 3.3 V, which is the voltage supplied by common LiPoly batteries. These calculations are only approximations and no fade margin is considered. In Figure 3 were represented the different modules composing the system.
The pointing accuracy is expressed in terms of angle for a given orbit; loss is about 3 dB as stated in the link budget, for a narrow beam of 15 degrees. The angle β (Figure 4) defined in equation 3 as the semicoverage time, depends on the minimum elevation angle φ, the orbit altitude h and the Earth radius R. For a given orbit of 250 kilometers and an elevation of φ = 10 degrees, the coverage angle computed by Gonzalez in [17] is β = 8.6 degrees which corresponds to a visibility time of 256 seconds. (Eq. 3)
For transmission, a power amplifier (PA) with an adequate gain-consumption trade-off, easy to implement (not too many components) and the best OIP35 and P1dB6 possible are the main specifications considered. After a research it has been found a product from SiGe7, it is the PA2423L. For reception, other parameters are more critical (principally its noise figure and gain), therefore is necessary to find out some low noise amplifiers (LNA in advance). Two options have been found and both will we cascade connected with a filter between them to reduce noise. At Table 1 the main features of each amplifier are shown. The LNA1 selected (because its low noise figure and as close to the antenna as possible) is from Analog Devices 8 and it is the ADL5521 (it is important to place just before the reception antenna an amplifier with the lowest possible noise figure). The LNA2 selected (because the higher gain) is from Maxim IC9 and it is the MAX2644.
Figure 4. Coverage Time Finally, a schematic of complete system is represented in Figure 5 which the achieved power at every step. More than 6 dB fade margin obtained due the excess of gain from the amplifiers (it will help to a correct link connection if any extra loss appears in the system). But this is only a starting point for the project, it may be improved (adding new filters or selecting a different antenna for the ground station). Furthermore, an important constraint is the battery maximum power supply for the satellite (PC). An initial calculation is done using power consumption from Table 1 and transceiver datasheet which means 3 hours of continuous transmitting mode (equation 4): (Eq.4)
5
Output interception point order 3 Input power 1 dB compression point 7 www.sige.com 8 www.analog.com 9 www.maxim-ic.com 6
For this reason, only when a ground station is available, transmission will be allowed in order to save battery. Other option is to put solar panels but for this kind of missions with only few weeks, it is not so efficient in terms of mass.
Figure 5. Femto-satellite Communication Diagram Block And Link Budget
Design of Communications Subsystem Now, LNA and PA must be tested alone. For each one, a PCB has been manufactured following the proposed circuits from the manufacturer’s datasheet with some simplifications. As it is a low cost project, two options have been considered: selfmanufacturing and professional PCB. Both options should work correctly. By the one hand, the selected PCB for selfmanufacturing is R04003 from Rogers Corporation with 1.5, 0.8 and 0.5 mm substrate height (dielectric constant of 3.38). By the other hand, the selected PCB for external manufacturing10 is a typical FR-4 with 0.8 mm substrate height and a dielectric constant of 4.2 approximately. Basically, both have good properties for not too high frequencies (not too much loss), but first option (R4003) reduces lines width (making designs smaller). They are two layers PCB, which reduce cost and manufacturing time. By the other hand, it makes difficult some pads connection or lines design and some losses or deficiencies could appear. However, a good operation is expected. All the elements will be accurately adapted to a 50 ohms input and output impedances.
PA2423L. It works at 3.3 V and it has 2 control pins (Gain control and Low consumption mode). Both operates from -0.3 volts to VCC (supply voltage). Size 21x15 mm (Figure 6).
Figure 6. PA2324L Eagle Design (21x15 mm) ADL5521. It can work at 3 and 5 volts (designed for 3 volts operation). Size is 20x17 mm (Figure 7).
Finally, all the circuits have SMA connections and a ground plane (bottom). Moreover, a lot of drills connect upper ground to bottom ground only on the professional boards (this improves boards design). All circuits are designed with Eagle software. Figure 7. ADL5521 Eagle Design (20x17 mm) 10
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MAX2644. It can work at 3 and 5 volts (designed for 3 volts operation). Size is 13x13 mm (Figure 8).
Figure 9. CC2591 Eagle Design (23x18 mm) Figure 8. MAX2644 Eagle Design (13x13 mm) CC2591. It will work only in reception mode (unidirectional communication) at 3.3 V. It can be a good solution for a future bidirectional communication design. This system is a complete RF front-end, ideal for a bidirectional communication (may be adequate for a future design). It will work in a single direction (as a transmitter). It is not going to be implemented in this project (it is a proposal for a future project as a complete RF front-end system). Size is 23x18 mm (Figure 9).
BPF. A band pass filter is required to reduce harmonics. It will have a bandwidth between 2 and 20 MHz, no more of 3 dB attenuation in pass band and the best attenuation as possible for the not desired band. It is a passive filter. It is not going to be implemented in this project (it is a proposal for a future project). Finally, an initial design of the array configuration for the transmitting antenna with SuperNEC is represented below (Figure 10) where a) is the Femto-satellite antenna array distribution, b) is the radiation pattern in elevation and c) is a 3D radiation pattern. Its design follows previous work done by Fernandez and Cuadrado in [18].
Figure 10. Femto-satellite Antenna Design. a) Antenna Array Distribution, b) Radiation Pattern in Elevation and c) 3D Radiation Pattern using ceramic antennas
Theoretical Results The first step is the antenna design (it has required a long time design process). The selected antennas are ceramic antennas AT952011 with a 1.5-2 dB max. gain and a radiation diagram similar to a dipole diagram in 2.4 to 2.5 GHz band. As 5 to 10 dB of gain are required a 4 elements array has been considered. It has been simulated with SuperNEC as a dipole array due its radiation diagram similarity. Simulations expected a maximum gain of 6.4 dB (Figure 10) and a -3 dB beam width of 85 degrees in the perpendicular axis to the board. Simulations determined an adequate elements spacing between 45 and 55 millimeters (small variations on elements spacing can increase or decrease gain some decimals of dB). It results in a broadside 12 array. Then, the design process continued with ADS13 software for the micro-strip interconnection of the array elements (all the ceramic antennas must be place in a single-axis array, with the same phase feeding). The antenna input impedance desired is 50 ohms. Basically, it is designed for a small VSWR (
