Since communication subsystems are of key importance for all Space missions, the GAMALINK Project will develop a CubeSat compatible communications platform, which will provide a dedicated solution for these small satellite missions and validate several new technologies in space. In fact, GAMALINK will go even further and will use the same communications platform to implement other functionalities like GPS position decoding, attitude determination and ranging techniques based on RF signals. Ultimately, GAMALINK could merge COMM and AOCS sensors in just one device, which would greatly reduce the mass, dimension and even power budgets, an extremely interesting feature for such small platforms. GAMALINK will combine expertise on satellite navigation, ad hoc networking, attitude determination, antenna design and beam forming into a compact common technological platform, suitable for LEO CubeSat and small satellite missions. The hardware communications platform will be based on Software-Defined Radio (SDR), an innovative terrestrial concept that enables the development of various waveforms using a common hardware platform. Its characteristics can result in tremendous mass and volume savings, while increasing flexibility to a point where a radio system could be completely modified by just sending a command from ground. Moreover, it allows operating different subsystems simultaneously in the same hardware such as radio communications, GNSS reception for navigation and distance and orientation measurement. On top of this radio platform, a set of different techniques will be implemented. The first is mobile ad hoc networking, an enabler for creating Inter-Satellite Links (ISLs). Their self-discovery, self-organization and autoconfiguration capabilities guarantee the autonomy, required for future space missions using flexible distributed architectures, like formation flying or planetary surface exploration. Ultimately, a mobile ad hoc space network can enable lower communication costs and latencies between satellites, space vehicles and astronauts or satellites and ground stations. Other techniques to be studied and implemented on the SDR platform are attitude determination of one station relative to another,through the measurement of carrier phase delays between signals transmitted from multiple antennas, GPS waveform reading and signal decoding and ranging between different satellites, based solely on the transmission of communication signals. GAMALINK will also focus on innovative antenna and RF frontend design and beam forming techniques, trying to develop hardware solutions that can cope with the flexibility introduced by SDR. Development of two functional prototypes is planned. The first will be mainly to test the hardware and preliminary algorithm developments, assuring that the SDR platform is properly working and adequately designed. The project will then end with a fully-functional integrated prototype of GAMALINK, featuring all the different protocols developed within the project. The table below summarizes the innovations proposed by the GAMALINK Project.
SDR Preliminary versions of Software Radios have already been tested in orbit but some of its features, like the reprogramming ability, haven’t yet been tested and validated in space. GAMALINK will develop its hardware platform based on extensive research for terrestrial civil and military applications and has the chance of creating a major impact by helping in fully validating this technology in space and proving the wide range of promising applications. Having a satellite communication system based on SDR holds many direct advantages. In case of a communications failure for instance, the chance of being able to solve the problem by uploading new software is high, especially since there will be less hardware involved. This has a major impact on the way current satellites are designed and operated. A failure in communications can compromise an entire million-dollar mission, which SDR can avoid up to some extent. Therefore, the use of this technology in space has a relevant economic impact in the entire space sector. Another big advantage is flexibility, as already mentioned, which has an impact in several aspects. For instance, some missions require a high speed link for payload data transmission and a low-bandwidth link to be used for transmitting housekeeping data and receiving commands from Ground. With SDR, it is possible to have one device transmitting in different frequencies, i.e. the same hardware platform can transmit in low frequencies and reconfigure itself to transmit in high-speed, consuming more power, when needed. This has a huge impact on mass and volume, which is particularly critical for small satellites (and even more in CubeSats). As an underlying platform, SDR can run several algorithms on top of it, bringing RF communications to another level.
Ad-hoc Networking On one side, it will allow to test modifications and evolutions of relevant terrestrial protocols in space, like WiFi and the Internet Protocol (IP) (Cisco has already done some work with IP routers in space for instance). When these protocols are validated in space, they will have a huge impact on taking Internet into orbit and on using terrestrial wireless protocols in space. On the other side, ad hoc networking is a key enabler to missions with more than one satellite, where collaboration between platforms is essential. Validating ad hoc networking in space will have a large impact on the feasibility of new advanced missions, like formation flying and on-orbit servicing, which conceptually can be performed using small satellites.
Antenna Design and Beam Forming A large number of antennas will be necessary and their placement will need to be strategically chosen to enable and optimize the communication processes. Beam forming, on the other hand, is a technique that can greatly reduce the power consumption when communicating. Its beam steering capabilities allows it to avoid power dispersion, focusing the beam towards the target. It also filters noise coming from undesired directions. Beam forming will depend on the use of smart antennas which will increase antenna design complexity, but can represent tremendous power savings when communicating. This has a big impact in small satellite missions where the power budget is extremely limited, allowing for longer range communications for the same power consumption.
Attitude Determination Attitude determination in GAMALINK is based on the differential carrier phase measurements of the signals transmitted from different antennas of one satellite and received by different antennas of another satellite. These phase measurements translate into angular measurements along two independent vectors orthogonal to the line of sight between the satellites. These angular measurements give both the location in the sky and the attitude of the transmitting satellite relative to the receiving one. Exchange of roles in a set of satellites allows all the satellites to gather such information about the others. Attitude information allows for more efficient communications between satellites through beam forming and to transform range measurements into vector arms.
Ranging Ranging is the term used for distance measuring of moving objects [see Introduction to RADAR Systems. Merrill I. Skolnik, Third edition, Mcgraw-Hill, April 2003]. Unlike passive RADAR systems where a transmitted ranging signal is passively reflected by the measured target, cooperative systems are based on an active target or what is called the transponder. The use of active transponder reduces the effect of clutter and allows higher ranging accuracy and power [see Cooperative multi-user detection and ranging based on pseudo-random codes. C. Morhart and E. M. Biebl. Advances in Radio Science 2009. pages 55-59.]. Two basic methods may be applied for ranging using electromagnetic waves:1- Received Signal Strength (RSS); 2- Time of Flight [see Wireless Positioning Technologies and applications. Alan Bensky. 2008].
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