Construction Of X/S Band For Nigeriasat-2 Mission Control Centre And Validation Of Data From The Satellite
A fully automated X/S band Mission Control Centre (MCC) for the Nigeriasat-2 (N2) and Nigeriasat X (NX) satellites was constructed in Abuja. The construction of the MCC consisted of the installation of the appropriate size of antenna and computer systems used for the control of the entire satellite mission. The MCC of N2/NX consists of three main segments: the ground station (GS) used for direct communication link with the satellite, the mission control suit (MCS) used for mission control and satellite telemetry and the mission planning system (MPS) primarily used for satellite’s imaging tasks, data pre-processing and archiving. The geo-spatial image data obtained from the MCC of Abuja and Enugu from the N2 and NX satellites were of specified quality, with resolutions of approximately 5 m and 22 m respectively. Comparative analysis of N2 and NX of geo-spatial data over Abuja and Enugu state and data obtained independently from SPOT 5 and Landsat ETM, with similar capabilities, showed considerable similarities in resolution and geo-location accuracy.
Early astronomers studied planetary bodies, and subsequently developed beliefs, projections and even scientific convictions that understanding the solar and planetary bodies will enable humans to truly understand the earth we live in. The quest to understand earth’s phenomena, complexity of its nature, simple and complex interactions of its numerous features among other things, provided great alluring impetus to the development of various ground based equipment and methods for studying and understanding other planetary bodies (Gruntman, 2004). Invariably, advances in scientific knowledge have led to the development of better techniques such as remote sensing for observing the earth from space. Remote sensing technologies and techniques such as aerial photography and observatories (Satellites) in space have helped in providing better information of the earth (Gleason et al. 2004). Remote sensing consists of using non in-situ methods of viewing, measuring and/or observing features of interest that cannot be seen with the unaided eye. Also applied in remote locations not physically accessible by human beings, or too large to explore or visit for study in a timely and cost effective manner. Remote-sensing devices employ laws of physics to “see” energy and spectral signatures of features via the electromagnetic (EM) spectrum or waves (both generated and induced) of various light and energy wavelengths (spectrum).
The portion of the EM spectrum visible to the human eyes is a small sliver ranging from 400nm to 700nm. Along the spectrum are bands of frequencies/wavelengths that are conventionally designated as infra-red (IR), near infrared (NIR), X-ray, Gamma Ray, etc. Each of these wavelength bands has arbitrarily assigned colours to produce false-colour images. Earth Observation (EO) satellites are specifically designed to carry remote sensing devices optimized to observe earth from space. Most EO satellites carry instruments that operate at a relatively low altitude, mostly at 500-600km above sea level. Altitudes below 500km are generally avoided because of the significant air-drag at such lower altitudes. This is because frequent orbit raising maneuvers are necessary to compensate for this air drag by using the on-board thrusters, which causes early depletion or exhaustion of satellite propellant, thus shortening the life span of the satellite (Hendrickson et al. 2010). In order to get a (or close to) global coverage with such a relatively low orbit, EO satellite has to be positioned at the polar orbit region or at least close to polar. As such a rather low but acceptable orbit a satellite will have an orbital period of roughly 100 minutes to cover the earth, rotating around its polar axis with about 25º between successive orbits with the result that the ground track is shifted towards west with these 25º in longitude (Boain, 2004).
3. Source:http://nortonsafe.search.ask.com/picdetails geo=&prt=cr&o=apn10506&ver=&chn=&q=orbital+track+of+a+low+earth+orb
Figure1.1: Orbital/Ground Track of a Low Earth Orbiting Satellite.
Spacecrafts carrying instruments for which an altitude of 36000 km is suitable, the geostationary orbit is sometimes the preferred choice. From such an orbit one gets uninterrupted coverage of more than one third of the Earth. With three geostationary spacecraft positioned over the equator at longitudes separated by 120º, the whole Earth is covered except the extreme Polar Regions. This type of orbit is mainly used for meteorological and communication satellites (Little et al. 2006).
1.2 Mission Control Centre (MCC)
A Mission Control Centre (MCC) is a terrestrial terminal station designed for extra planetary telecommunication with spacecraft, or reception of radio waves from an astronomical radio source (Hermansen, 2006). MCC’s are located either on the surface of the earth, within the atmosphere or in space. MCC communicate with spacecraft by transmitting and receiving radio waves in the super high frequency or extremely high frequency bands (e.g., microwaves). The MCC is able to command and give specific task to spacecrafts via this communication channel and is also able to monitor the workings on board such a spacecraft. Furthermore, the MCC is capable of processing data received from spacecraft to ascertain the performance of such spacecraft (Larson and Wertz, 1992). 4 When a satellite is within a MCC line of sight, the station is said to have a view of the satellite. It is possible for a satellite to communicate with more than one MCC at a time. A pair of MCC are said to have a satellite in mutual view when the stations share simultaneous, unobstructed, line-of-sight contact with the satellite.
1.3 Communicating Satellite
The telemetry, tracking and command system usually monitors the working of the satellite system in orbit. These systems help to direct and locate signals from the satellite and MCC or other ground control segment. The transmission and carrying of signal between satellite and MCC can be explained best considering a command and track module (Keesee, 2003). The command system is typically present on the receivers end, like on earth based stations. This system works by decoding, receiving and sometimes sending information, while the track system on the other hand, guides and monitors the working of the satellite and its related movements.
1.4 MCC Transmitting Chain
Information to be transmitted by the MCC is delivered via coaxial cable, fibre, terrestrial microwave, or satellite. The devices in the transmitter chain typically consist of the multiplexer, the modulator, the up-convertor, a high power amplifier (HPA), and the antenna (Love, 2009). The multiplexer combines the individual channels onto a single data stream. The information can be encrypted and encoded with a forward error correction code. The modulator modulates the baseband signal containing the desired information onto an intermediate frequency (IF) carrier (Hausman, 2009). The up-converter changes the 5 carrier to the radio frequency (RF) signals used to transmit the signal. The HPA amplifies the modulated RF signals from the output of the up-convertors to the required power at the input terminals of the antenna. Finally, the antenna transmits the amplified RF signal to the satellite. A common form of modulation used in digital satellite communication is M-ary phase shift keying. In this technique, the carrier can assume one of M phase states, each of which represents a symbol. In binary phase shift keying (BPSK), there are two phase states, 0° and 180°, representing a binary one or zero (Schneider, 1968). In quaternary phase shift keying (QPSK), there are four phase states that represent the four symbols 11, 01, 00, and 10. A QPSK modulator is equivalent to two BPSK modulators out of phase by 90° (Spilker, 1996). It has been shown that both BPSK and QPSK modulation require the same power per bit for the same bit error rate (BER), but QPSK modulation requires only half the bandwidth. Moreover, all other forms of digital modulation require more power (Barry and Nel, 2002). Thus QPSK is by far the most prevalent form of modulation used in satellite communication and is the present day industry standard.
1.5 MCC Receiving Chain
The devices in the MCC receiver chain reverse the process of the transmitting chain. The MCC antenna receives the modulated RF signals from the satellite (Pritchard, 1986). The power level at the output terminals of the antenna is about a Pico watt. This extremely low power level is comparable to the sound level from a barely audible mosquito. A low noise amplifier (LNA) amplifies the received RF signals. 6 The downconverter changes the received RF signals to IF signals for the demodulators. The information is extracted from the received IF signal by the demodulator and is decoded and decrypted (Huang, 2005). The LNA is mounted on the antenna itself to minimize waveguide loss. This is the first active component and its performance is the primary factor in determining the capability of the receiver. The LNA must have a high gain and contribute very little noise. It is essential to place the LNA, with a high gain and low noise temperature, at the head of the receiver chain (David, 1994). Instead of an LNA, a low noise block downconverter (LNB) may be used. An LNA only amplifies the signal, while an LNB amplifies the signal and also downconverts the frequency to minimize losses.
1.6 MCC Antenna
MCC’s are characterized by the antenna size, the type of service, the frequency band, the Equivalent Isotopic Radiated Power (EIRP), and the Tranmission Gain (G/T). Transmit antennas must conform to international and domestic telecommunication regulations (Hermansen, 2006). EM energy propagates in the form of waves. The spreading of the energy as it leaves the antenna is described by the theory of diffraction. The larger the antenna reflector is in comparison with the wavelength, the less spreading (Warren & Thiele, 2012). The physics of radio waves is identical to the physics of visible light and thus the spreading of radio frequency waves from an antenna reflector is analogous to the transmission of light through an aperture. 7 A reflector antenna is often referred to as an aperture antenna (Harvey, 2010). Monochromatic light, such as from a laser, will produce a series of concentric Airy rings when passed through a small circular hole and projected on a screen. The central bright spot is like the main lobe of an antenna pattern. The surrounding dark and bright rings are analogous to the nulls and side lobes of the antenna pattern. The configuration of the antenna is called a direct feed if the feed horn or LNA is located at the prime focus (Haridas, 2007). Large antennas usually have a sub reflector, of either the convex hyperbolic Cassegrain type or the concave ellipsoidal Gregorian type. The sub-reflector permits the LNA to look into cold space and away from the warm ground, so as to significantly reduce the antenna noise temperature (Williams, 1983). In an offset antenna, the feed is located to one side. The advantage of the offset design is that it eliminates blockage effects from sub-reflectors. Many antennas have tracking capability that permits them to follow a satellite in orbit. The tracking mechanism may be programmed with an ephemeris, that determines the look angle as a function of time of day, or it may have an automatic servo loop with a memory that maximizes the received power. The gain of the antenna is the measure of its ability to concentrate the radio frequency electromagnetic energy in a specified direction which is determined by the size of the physical aperture, the frequency of the radiation, and the efficiency (Struzak, 2007). The antenna gain is proportional to the square of the antenna diameter and the square of the frequency. Factors that affect the efficiency include the geometrical shape of the aperture, the method of illumination, the amount of spill over of energy past the edge of the antenna, surface roughness, blockage, and phase coherence. Another 8 fundamental parameter is the half power beam width. This is the angle between the half power points of the main lobe of the antenna pattern (Love, 2009). The half power beam width varies in inverse proportion to the frequency and the antenna diameter. Two key parameters are the EIRP and the antenna figure of merit. The EIRP is associated with a transmit antenna and is the product of the power (P) to the input terminals of the antenna and the G/T. The figure of merit is associated with a receive antenna. It is also the ratio of the antenna receive gain (Gr) and the system temperature (T), which is a measure of the noise power, accepted by the antenna and must be as low as possible (Fung, 2011).
1.7 Earth Observation Satellites
EO satellites started serving mankind some 50 years ago and were used to produce information on such environmental issues as weather patterns, floods, droughts, winds and tides (Zhou and Kafatos, 2002). The information was and continues to be useful to managers and planners in agriculture and water resources, land, sea, and air transportation, public utilities, environment and disaster/relief operations, among other uses. Over the years, there have been a series of such satellites, notably the American Landsat series, the French SPOT series and the newer NASA Earth Observing System (EOS), but the current king of Earth observation satellites is the European Space Agency’s (and world’s largest) and most capable Earth Observation satellite, called Envisat Environmental Satellite which weighs approximately 8,000 kg. The Envisat satellite features a large collection of disparate payloads sitting on a single platform (Dubock et al. 2001). Some of these and their services include: Ten 9 instruments designed to collect data in four aspects of the Earth’s environment namely, atmosphere, land, oceans and ice, these include; Laser ret reflector for accurate measurement of surface elevation, Advanced Synthetic Aperture Radar (ASAR) for producing images under all natural lighting conditions including night over the oceans and land regions (Bartsch et al. 2012), GOMOS to provide global ozone monitoring by occultation of stars, Michelson interferometer for passive atmospheric sounding (MIPAS), an infrared spectrometer designed to measure concentrations of gases, Microwave radiometer (MWR) to measure total atmospheric water vapour and liquid content of clouds directly below the satellite which provide correction for the radar altimeter payload, Instruments to measure sea surface temperature/chlorophyll concentration and Instrumentation to monitor discharge of pollutantss in waters/ Predicting where fishing fleet should cast their nets/ ocean state forecasting for routing of ship.
1.7.1 The Nigeriasat-2 (N2) Project
The launch of Nigeria’s first EO satellite, Nigeriasat-1 (N1) on 27th September, 2003, heralded Nigeria’s entry into the elite Committee of satellite owning nations. The epoch making event drew ovation as well as bewilderment from many people. To the sceptics, it was seen as one of those foolhardy events resulting from misplaced priorities and one that was going to be the first and the last of its type. After three years of near-flawless operation in orbit, and delivery of over 3000 images that have been utilized locally and internationally, the National Space Research and Development Agency (NASRDA) for the monumental success of N1 was directed by the Federal Government of Nigeria to procure two other satellites, a communications 10 satellite, Nigcomsat-1 and a second EO satellite, Nigeriasat-2 (N2). The latter is expected to replace N1. Current worldwide trend is in favour of the building and launching of small satellites with just one or a few payloads working alone or in constellation. This ensures a faster and cheaper space mission especially where individual satellite of the constellation is owned by collaborating countries/ partners. This is the path Nigeria has chosen in its N1 and N2 projects. Apart from the need to sustain the image continuity of N1, the N2 is designed to have more powerful and flexible capabilities to support the National Geospatial Data Infrastructure (NGDI) project (Chizea and Ejimaya, 2006). Besides, having operated the N1 for over eight years, performance of the mission has been appraised, limitations identified and a variety of experience has been gained during the period. These experiences have been taken into consideration in specifying the design requirements of N2. The main component of the Nigeriasat-2 project is the N2 satellite, the MCC which is located in Abuja and the flight standard training model, Nigeriasat-X (NX), built by Nigerian scientists and engineers using the facilities of Surrey Satellite Technology Limited (SSTL) , United Kingdom.
18.104.22.168 Nigeriasat-2 (N2) Satellite Main Features
The N2 satellites system was implemented as a stand-alone system that will, at the same time, have the capability to interface with existing NASRDA data collection, dissemination and analysis infrastructure (Chizea, 2009). Table 1.1 below shows clearly the capabilities of N2 in such areas as resolution, computing power, on-board data 11 storage, images captured per day, minimum orbit life and geo-location accuracy among others. Table 1.1: Some Design Features of N2 Spacecraft N2 Wet/launch mass 300 kg Orbit Low earth orbit, sun synchronous, near polar Altitude 700 km Propellant Xenon Minimum orbit life 7 years Payload Camera-based; 2.5 m Ground Sampling Distance (GSD) panchromatic; 5.0 m GSD in 4 spectral bands and 32 GSD also in 4 spectral bands On-board-data storage 32 GBytes hard drive data storage Swath width 20 x 20 km (2.5 and 5.0 GSD) and 300×300 km (32 GSD) Imaging mode Strip, aerial, stereo and Near real time imaging modes Image capture One hundred and fifty 20 x 20 12 a day Geo-location accuracy 35-45 m Data transmission link X-band and S-band On Board Computer (OBC) 3 No. 386 1 No. 760 Size of ground station antenna 7.3 m diameter parabolic antenna. Attitude Determination and Control (ADC) 3-axis stabilized using star camera The flexibility afforded the user by the N2 satellite’s Electro-Optical payload (E-OP) can be seen in the wavelength ranges of the four multispectral bands that cover the spectrum from 450nm to 900 nm. The intent here is to ensure that the N2 is compatible with other EO missions with Electro-Optical payloads. Table 1.2 shows the range of frequencies and the wavelength range of the panchromatic band is 450 nm ± 20 nm to 900 nm ± 20 nm. The 32 m resolution imagery ensures compatibility with the N1 imagery while the high resolution imagery of 2.5 m and 5 m GSD, with improved geo-location accuracy supports several new high/very high resolution applications. 13 Table 1.2: Multispectral Bands of the N2 for High and Low Resolution Band Wavelength Range Colour 1 450 nm ± 10 nm to 520 nm ± 10 nm Blue 2 520 nm ± 10 nm to 600 nm ± 10 nm Green 3 630 nm ± 10 nm to 690 nm ± 10 nm Red 4 760 nm ± 10 nm to 900 nm ± 10 nm Infra-red Figure 1.2: Configuration of the Nigeriasat-2 spacecraft.
22.214.171.124 Nigeriasat-2 (N2) Payload.
The N2 carries optical payload that can produce very high resolution images (HRI) of 2.5m in the panchromatic band, 5m in 4 spectral bands of red, blue, green and 14 near infrared and medium resolution images (MRI) of 32m in 4 spectral bands of red, blue, green and near infrared. The imager has a geo-location accuracy of 35-45 m.
High and Medium Imaging Payload of the N2
Cross section of the N2 HRI The very high resolution imager design is based on the Cassegrain system. Light from an observed target is reflected from the primary mirror to a secondary mirror, which then reflects the light back down a concentrating path created by the internal 15 baffles, through a corrector lens to long Linear CCD arrays located at the focal point of the mirror system (Xiao et al. 2010).
Cassegrain System Though the designs of the imager are implemented using low expansion materials, in order to compensate for moisture release from the carbon fibre composite in orbit, which cannot be accurately predicted, a focus control mechanism is implemented by moving one of the corrector lenses located near the center of the primary mirror. The N2 imager design uses an In-field color channel separation, where different colors are generated at different times on the ground. This separation technique coupled with the long linear CCD arrays help to avoid complex beam-splitting focal plane assemblies. As the linear arrays of pixels suggest a “pushbroom” imaging technique is used, where the swadth width is a product of the number of pixels on the CCD array and the ground sampling distance of the imager (Curiel et al. 2012).
The N2 uses 5 linear CCDS with 8,000 pixels to attain a swath width of 20 km for its 700Km altitude. The panchromatic channel has the full resolution required and the color channels which have two times the GSD, are read out slower by the detector so that it always increases the along track GSD captured by the detector by the same amount as the Pan channel. This produces more light in the color pixel to compensate for the loss of light due to the color filter and allows for a full resolution color picture after image processing. Signals read out from the CCD are stored on an on-board high speed data recorder. There are 3 high speed data recorders of a total capacity of 32GB for data storage on board the N2. The medium resolution imager on the N2 has a configuration of 1 bank of imagers in 4 spectral bands .In this image design; each channel has its own lens and detector with different filters. The spectral filters are located in front of the camera lens. The N2 is a very agile spacecraft capable of imaging in several modes which is made possible by the inclusion of an Advance Altitude and Orbit Control System (AOCS). The AOCS consist of two different sets of reaction wheels with high torque, high precision star tracker, sun sensors, GPS and Magnetometers (Chizea et al. 2011). The imaging modes of the N2 include; the stereo imaging mode (which is used to produce data with height differentials for creating Digital Elevation Models), aerial imaging mode (to produce wider swath width of up to 3 times the original capability of 20km by 20km), Near Real Time imaging modes (used to produce real time images within a radius of 2000km from the MCC) and the strip imaging modes (used to produce long strips of images during a pass).
The imaging modes of the N2 are more effective using the On and Off pointing capability of the satellite which is also attributed to the inclusion of advance AOCS which allows for row, yaw and pitch movements within a specific limit of time and angle. This capability is utilized for imaging over an area without an overhead pass thus making possible a wider area of coverage during each pass.
1.7.2 Nigeriasat X (NX)
Project The Nigeriasat-X (NX) is the flight standard training model of the N2 built using the facilities of SSTL by Nigerian scientists and engineers that underwent the satellite Know How Technology Training (KHTT) at SSTL (Jason et al. 2010). The Nigeriasat-X is based on the enhanced micro-satellite 100 platform of SSTL which was used for the N1. NX weighted approximately 87kg and was launched alongside the N2 into a 700km polar sun synchronous orbit. The satellite has a design lifespan of 5 years and carries an optical imager that can acquire images of 22 m resolution in three spectral bands (Red, Green, and Near Infrared) with a swath width of 600 x 600km with a revisit time of 3-4 days. The NX uses the 3-axis momentum-biased attitude control provided by the Attitude and Orbit Determination and Control Subsystem (AODCS). The dual-axis sun sensors on the four sides of the spacecraft and the dual redundant vector magnetometers provide attitude knowledge for measuring the sun angle and Earth’s magnetic field. Three micro-satellite reaction wheels and three magnetorquer rods are used to control the attitude and attitude rates to maintain nadir attitude pointing. 18 In addition, the AODCS features an experimental star tracker head and electronics that is integrated into the -Y panel. The start tracker on the satellite was a sole innovation of the Nigerian KHTT team. An onboard GPS receiver is used to determine the orbit and provide accurate timing information for the NX. The satellite has an onboard propulsion system that uses butane fuel, for small orbit change maneuvers/ station keeping. Power generation in NX is provided by three body-mounted GaAs solar panels on the -X, +X and +Y axes of the satellite. This power is managed by a Battery Charge Regulator (BCR), Power Conditioning Module (PCM) and Power Distribution Module (PDM). A raw 28 V and a regulated 5 V power bus are provided to all modules on the satellite. A Li-ion battery with a capacity of 15 Ah provides power storage for eclipse durations and periods of peak power demand. The Electric Power Subsystem (EPS) delivers over 30 W Orbit-Average Power (OAP) to the platform and payloads, with ~ 12 W for the platform, and ~ 16-20 W for payload operations.
NX in x,y and z direction Two Intel 386 On-board Computers (OBCs) are used for onboard data handling on the NX. The OBCs provide communications between subsystems, monitor the temperature and current consumptions, maintain log files and execute the imaging schedule. The data handling subsystem uses a Controller Area Network (CAN) bus for 20 onboard data exchange. The onboard data storage is the Solid State Data Recorders (SSDR) with a capacity of 4GB. The NX Uplink tele-command communications is supported in the S-band frequency at 9.6 kbit/s uplink rate. While the receiver subsystems comprise of two receiver electronics modules and four receiver patch antennas, providing an omnidirectional antenna pattern to maintain the RF link at all times when in view of the MCC. Downlink telemetry communications on the NX is supported in the S-band frequency at 38.4 kbit/s downlink rate. The transmitter downlink modules are comprised of one dedicated low-rate transmitter with two monopole antennas. A redundant low-rate downlink is provided by the 8 Mbit/s payload transmitters that can operate, if required, in a low-rate mode of operations. The payload data is downlinked via either the high rate S-band transmitter operating at 8 Mbit/s, or the high rate X-band transmitter operating at 20 Mbit/s.
1.7.3 N2/NX Orbit
The N2/NX orbit was designed to meet several mission requirements which include very high imaging access over Nigeria at specified resolutions. This means an orbit suitable for passive imaging of the earth, with best imaging times over Nigeria, a short re-visit time and also provides for high data download capabilities. A 700km, near-polar sun synchronous circular orbit is chosen for the N2/NX Mission, with a Local Time of Descending Node (LTDN) of 10:30am. Using a sunsynchronous orbit ensures that successive orbital passes over a particular location on the earth occurs at the same local time of the day (Bock, 2002). This is particularly 21 important in environmental monitoring as changes over time can be better monitored without accounting for changes in earth illumination by the sun and local time (Watson, 2012). At a near polar orbit, the N2/NX experiences a torque, due to the Earth’s equatorial bulge, that causes precession. To maintain a sun synchronous orbit, the rate of precession of the satellite is matched to the rate of precession of the earth (Strizzi, 1993). With a near polar orbit, global access is achieved as the earth rotates underneath. In order to keep the same local time for a given satellite pass, it is desirable to keep the orbit as short (low) as possible (Bock et al. 2001). However, a too low orbit would mean rapid orbital decay due to atmospheric drag (Vallado and Finkleman, 2007). The 10:30am LTDN offers excellent imaging conditions with cloud mitigated imaging conditions.
N2 Pass over the Abuja MCC 1.8 Purpose of study Nigeria constructed its first MCC in 2003 for the in orbit control of her first earth observation satellite, N1. The N1 MCC experienced numerous local problems 22 such as erratic power supply, frequency interference and harsh weather conditions. These issues immensely affected the overall performance of the MCC and the entire Nigeriasat-1 mission. In preparation for the launch of the N2/NX, we had to construct a more advanced S/X band MCC, taking into considerations some of the problems faced by the N1 MCC, in order to achieve optimal performance of the N2/NX MCC. Hence the purpose of this study is:
i. To construct a suitable X/S band MCC for the N2/NX satellite that is capable of carrying out multifunction, such as in orbit calculation, satellite tracking and telemetry processing, satellite data pre-processing to the level of geo-correction and data archiving and cataloguing.
ii. Download and analyse of geo-spatial data from the MCC of Abuja and Enugu