- Astrome Technologies – Providing Connectivity Anytime, Anywhere (NewSpace India, March 18 2016)
- India’s NewSpace Entrepreneur Series: Neha Satak interviewed by Susmita Mohanty (NewSpace India, April 20 2016)
- Beaming Internet From Space Through Astrome (Electronics For You, May 10 2016)
- Entrepreneurship Experience in Bangalore (Club SciWri LinkedIn Page, July 28 2016)
- Neha Satak, Astrome Technologies (YourStory YouTube Channel, August 10 2016)
- Startup Astrome Technologies | YourStory (YourStory YouTube Channel, August 10 2016)
- This space entrepreneur from a small town in Rajasthan is taking on the Elon Musks of the world (YourStory, August 11 2016)
- A dream to beam Internet from space (The Hindu, September 4 2016)
- High-speed internet for India’s remotest corners by 2020 (Bangalore Mirror, September 5 2016)
- The race to provide Internet from space to India & other emerging markets (The Economic Times, October 24 2016)
- The Astrome Plan Is to Launch 150 Satellites By 2020 (SatNews, October 25 2016)
- Astrome, Internet Satelital de Banda Ancha de la India para el Mundo (Latam Satelital, November 9 2016)
- Exclusive: Former Nokia executive backs spacetech startup Astrome (VC Circle, December 8 2016)
- ORF KC 2017 | Satellite Internet for Digital India (Observer Research Foundation YouTube Channel, February 16 2017)
- Space India 2.0 (Space India 2.0: Commerce, policy, security, and governance perspectives, February 17, 2017)
- Astrome’s Technology to Deliver Internet From Space (YouTube, May 16 2017)
- India Looks for Its Own Elon Musk to Win the Space Race with China (The Wall Street Journal, June 23 2017)
- Internet from space: Why not? (TEDx Talks, June 29 2017)
- These Ideas Are Worth The Moolah (The New Indian Express, July 10 2017)
- Here, Space Is The Limit (Entrepreneur Magazine)
- This Bengaluru startup is readying satellites to send you high speed internet from space – anywhere in India (Bangalore Mirror, September 2 2017)
- A Bengaluru startup is readying satellites to send you high-speed Internet from space (The Economic Times, September 2 2017)
- Final frontier: Sky is the limit for 2 IISc start-ups (The Asian Age, September 24 2017)
- Final frontier: Sky is the limit for 2 IISc start-ups (Deccan Chronicle, September 26 2017)
- ‘Jargon unnecessary, make science simple’ (Times of India, November 16 2017)
- ವಿಜ್ಞಾನವನ್ನು ಕುತೂಹಲಕರವಾಗಿ ಬೋಧಿಸಿ: ನಾರಾಯಣಮೂರ್ತಿ (Prajavani, November 16 2017)
- Infosys Science Foundation panel: how to communicate science without jargon (YourStory, November 29 2017)
- Science jargon and society – do we need to bridge this gap? (YourStory YouTube Channel, November 28 2017)
- As ISRO launches its 100th satellite, meet the Indian startups aiming for the stars (YourStory, January 13 2018)
- Indian start-up’s moon mission in doubt (Nature, January 19 2018)
- Time for India to Consider a Special Track for Space Startup Incubation (The Wire, January 20 2018)
- Don’t Panic! The Hitchhiker’s Guide to Creating a Space Startup in India (The Wire, February 28 2018)
- Awards for tech ideas (The Telegraph, May 12 2018)
- National Technology Awards presented (Business Line, May 11 2018)
- Connecting Everyone (Outlook Business, June 22 2018)
- IIMBue 2018: India will be able to put human being in orbit in next 6-7 years, says ex-ISRO chief (One India, June 23 2018)
- Launch pads: Is Cisco accelerator’s belief in B2B startups paying off? (VC Circle, August 28, 2018)
- List of top internships: Freshers can apply too, earn up to Rs 11,000 (The New Indian Express, June 13, 2019)
Growing demand for mobile broadband
Mobile broadband usage has surged exponentially in the last few years. Some reports indicate that the global mobile data traffic more than doubled between 2015 and 2016. By 2022, the total mobile traffic is predicted to cross 70 exabytes (1 exa is sixth power of 1000(1018)) ) . The rapid growth in the demand for broadband internet is fuelled by a substantial increase in the number of mobile subscribers using data-hungry applications. Most of these mobile subscribers are located in the densely populated cities of the world. Catering to the rising demands of these urban subscribers has been a challenge for the mobile network operators as the infrastructure has almost hit a ceiling.
How are mobile signals transmitted
The most common way to transmit mobile signals is by using what are called ‘base stations’. There is a good chance that you have seen these base stations – they are the big tall towers erected in and around cities. These towers are powerful enough to transmit signals to large areas called ‘macro cells’. However, with more urban subscribers demanding large amounts of data in each cell, base stations have reached their full capacity. Deploying additional infrastructure in the crowded cities has also been a challenge due to logistical constraints.
Small is beautiful
One way to address this is to shrink both the cell and the node that serves it. We can have a number of small nodes each serving a relatively smaller geographical area called a ‘small cell’. Such nodes are so small in size that they can be attached to a light pole, and if required, deployed indoors too. Interestingly, in urban areas, it turns out that a number of small cells deliver better reception and capacity than a single macro cell. But the real advantage of small cells lies in the ease of deployment – when compared to macro cells, it’s much easier to set and scale up small cell infrastructure.
There are other options too. Instead of deploying small cells, one can shrink the macro cells to a certain extent, depending on cell site availability and acquisitions. One more option is to use a mix of macro and small cells – have small cells in highly crowded areas like stadia, and retain macro cells elsewhere.
Supporting the new network
The mobile infrastructure in cities require macro and small cells to work in tandem. Features like Coordinated Multi Point (CoMP), Inter Cell Interference Coordination (ICIC), and Dual Connectivity ensure seamless coordination among the cells. Since these features demand cells to be ‘in touch’ with each other, they require inter-cell backhaul connectivity. Though it is possible, at least in principle, to connect cell-sites by optical fiber and copper cables, the realities of a busy city force the operators to look elsewhere
lThe wireless backhaul
To overcome the limitations of physical connectivity in backhaul, the mobile network operators are looking at wireless options. Certain bands in high frequency millimeter wave spectrum are considered to be very good for high capacity backhaul needs. Moreover, in many countries these bands are unlicensed and light licensed. The availability of large bandwidths in these bands – 7 GHz to 10 GHz – make them ideal candidates for providing multi gigabit capacity wirelessly. Of course, millimeter wave based backhaul products might well enable the network operators to cash in on the exciting opportunities right in front of them.
Posted by: Sanjeev Shankar,Madhukara Putty
LTE is by far the most popular wireless technology. It is deployed in more than 150 countries across the globe. Consumer base has increased manifold as it (LTE) was the technology to truly provide mobile broadband on the go.The strong support LTE has received from Service providers and OEMs is due to the unique advantages it offers from its predecessors:flat-IP architecture, robust Radio,higher spectral efficiency and lower latency.This enabled the reversal of voice to data usage; with data occupying 70% of the total traffic.
The evolving mobile devices, applications,services and content is driving LTE technology to improve in capacity and coverage.LTE-A(LTE-Advanced) has improved the capacity with features like Carrier Aggregation (CA),better spectral efficiency and Multiple Input and Multiple Output (MIMO).LTE-A can deliver cell capacity beyond 500 Mbps. However the capacity improvement is limited to spectrum availability in licensed space.To a large extent the increase in capacity is matched by increase in number of users.Hence the quest for using LTE in unlicensed spectrum.
LTE-U (LTE unlicensed spectrum) extends the benefits of LTE and LTE-A to unlicensed spectrum,enabling mobile operators to offload data traffic onto unlicensed frequencies more efficiently and effectively.
Even with the enhanced capacity of LTE-U,LTE is inadequate to serve the consumers at different coverage areas – e.g. inside buildings,various hot-spots, at crowded events etc. To counter it,MNOs are deploying multiple layers of coverage like outdoor macro cells,indoor small cells (femto access) in buildings and outdoor small cells at specific congested or low coverage areas.LTE has come up with features like Enhanced Inter-cell Interference Coordination (ICIC) which allows macro and small cells to work in tandem and derive benefits from each other.
Beyond LTE : 5G
The evolution in LTE clearly indicates a trend towards supporting enhanced mobile broadband, massive IoT and mission critical services.LTE-A is the building block or a strong foundation on which the next generation wireless is being built.
5G wireless system is designed to offer multi gigabit capacity and super low latency.It has the potential to enable the augmented/virtual reality (AR/VR) in different sectors like education, health, automobiles, manufacturing etc. 5G trials in millimeter wave frequencies like 28 GHz, 39 GHz has shown positive results in delivering multi-gigabit capacity in countries like South Korea, US and …. .
Posted by: Sanjeev Shankar,Madhukara Putty
We live in an era of data deluge. We upload pictures of a birthday party to Facebook, Twitter, and Instagram simultaneously. If bandwidth permits, we also like to livestream the party to those who couldn’t attend. Mobile apps belonging to taxi aggregators like Ola and Uber gather data corresponding to thousands of rides everyday. When the astronauts go for a ‘space walk’, NASA wants to share the excitement by live streaming the event to those living on earth.
Studies suggest that our hunger for data is set to increase. This provides a great business opportunity and a great challenge for the telecom service providers. They want to cash in on this ever expanding realm of business, but are limited by technology options. Interestingly, the challenge lies not in transferring data over continents, but over the last few hundred miles.
Optical fibers: Still the king, but not at the doorstep
Despite phenomenal growth in mobile technologies, the good old optical fiber remains the best mode for high speed, high bandwidth data transmission. Whether we are connecting continents or connecting cities, they win hands down over other technologies. But, these rugged cables have a drawback: it is not economically viable to lay them over short distances from a home or village to the nearest fiber access point, say a few hundred metres to a couple of kilometers distance. It is not only, economically less viable to lay but also very costly to maintain these cables for last mile connectivity.
Copper: Data dies far too quickly
A time-tested way to overcome this limitation is to fall back to copper cables instead of optical fibers as one approaches the destination. This works pretty well as copper based technologies have advanced to support bandwidths up to 500 Mbps and beyond. However, copper cables work well only for short distances, say about half a kilometre. Other issues like signal degradation, and interference by electric and magnetic fields make copper unsuitable for long distance, high bandwidth data transmission.
Fixed wireless: Easy to set up, hard to rely on
One way to get over the problems of copper is to get rid of copper itself and use radio signals to provide internet. Called ‘fixed wireless’, this mode is preferred in places where laying cables, be it copper or optical fiber, is cumbersome and expensive. It provides decent internet even though the speeds are quite less than what is provided by optical fiber. However, the fact that fixed wireless operates in the unlicensed spectrum – a band of frequencies that can be used freely by anyone – makes it vulnerable to signal interference. Moreover, the quality and the speed of the internet comes down as the number of users served by a fixed wireless tower increase.
Riding the millimeter wave
Building networks that can support the rising demand for data transmission requires looking beyond copper cables and fixed wireless. A promising way forward is to transmit data in the form of high frequency electromagnetic waves (millimeter waves). Telecom service providers can operate in some of these bands upon paying a small but fixed licence fee and provide fiber-like internet. In many countries, a few bands in the millimeter range remain free for operation.
Unlike traditional data transmission techniques, millimeter waves carry data in highly focussed beams. They are free from interference and can provide quality internet to a large number of users. Once fully deployed, they will usher a new era of exciting technologies like connected cars, virtual reality classrooms, and the next generation of the internet of things.
Posted by:Sanjeev Shankar,Madhukara Putty
If you are a first-timer in satellite design and looking for information about how to choose and start design of your satellite structure, then you are at the right place!
A satellite structural platform is a subsystem of the satellite which provides a mechanical base to hold its other subsystems. A satellite platform is expected to (a) withstand structural load, stresses and vibration experienced during launch, (b) maintain structural integrity and stability while in orbit, as well as, (c) protect the satellite from the damage due to the harsh spacecraft environment.
Design drivers for Selecting a Structural Platform
The design of the structure involves determining the desired shape and size of the structure so that it is big enough to house all the components of the satellite. After this, material selection is done and the loads acting on the structure are calculated. The satellite is iteratively redesigned until all components fit and the structural loads are manageable. The following parameters usually drives the design:
- Size and weight of the satellite subsystems
- Availability of materials with high strength-to-weight ratio and thermal resistivity
- Ease of manufacturing
- A ready availability of other raw materials
- Radiation protection coating
- Cost considerations
- Launch adapter integration
- Expected space environment
- Ease of assembly, reusability and extensibility
These aspects would act as objectives and constraints for selecting the optimal design of the structure.
Aluminium and Aluminium alloys are most widely used in space applications because of their high strength-to-weight ratios. Recent advances in the manufacturing industry has introduced the use of composite materials integrated with the Aluminium metal. A list of desirable material properties with recommended materials to be used is given below.
|High strength||Stainless steel, Al alloy, Composites|
|Light weight||Composites, Al|
|Easily available||Al, steel|
|Protect against radiation||Lead|
|High thermal resistivity||Ceramic, Nickel and cobalt alloys|
Estimation of loads on the satellite
The load acting on the satellite while in orbit is negligible, whereas during launch, the load due to the launch vehicle vibration and dynamic load due to gravity is enormous. Typically, the launch vehicle provider furnishes details about the expected launch loads. The satellite platform is to be designed to withstand these launch load by a suitable margin. The launch service provider also defines an adapter for fitting the satellite onto the launch vehicle, with suitable auto-eject mechanism. For instance, IBL230/298 adapters are used for PSLV of ISRO. Finite Element Models (FEM) analysis of the whole structure including the launch vehicle, is usually carried out to verify that the satellite-adapter system can withstand launch loads. See here for more details.
Platform Shape Selection
Common shapes used for micro and nano satellites are;
- Hexagonal prism
- Octagonal prism
The selection of the shapes depends on subsystems used for the mission. Also, the surface area of particular shapes plays a major role in heat transfer and radiation when the satellite is in orbit.
Pros and cons of various satellite platform are listed in the table below.
|Cube||Six plates of metal joined together to form a cube therefore easy manufacturing||i) Lesser surface area, implantation surface
ii) mounted solar panel may not be sufficient for the mission demanding high power. Deployment of solar panel involves, extra weight and higher risk of failure due to deployment.
|Less material used which corresponds to lesser weight and lower cost||iii) Problems in arranging subsystems in the sharp acute corners.||CanX5|
|Larger surface area for the same volume compared to other shapes would help in heat dissipation and larger area for solar panel|
|Less number of joints compared to other shapes|
|Easy integration of constellation solar arrays.|
|Sphere||Larger volume for lesser material||Difficult to manufacture||SpinSat|
|Cannot utilise full space available due to the curved surface|
|Difficult to integrate with launch vehicle|
|Hexagonal prism||Little larger volume compared to cube for the same quantity of material||Difficult to build a perfect structure due to the angular constraints||OCO-2|
|Sharp corner problems are lesser as compared to the cube shape||Too many joints which need many bolts and nuts, this slightly increases the weight||NanoSat-1|
|May affect panel folding|
|Octagonal||Similar to hexagonal prism but it has slightly lower volume than that of cube for the same quantity of material||Increased difficulty in building||Spartnik|
|Increased number of joints and therefore increased risk of failure due to the joints and increased weight|
In general, the space provided in the launch vehicle for a satellite is cuboidal. It is necessary that the designed satellite structure fits within this space. So, naturally cube platform is preferred over other shapes to utilize fully the space provided in the launch vehicle for the satellite.
From our literature survey, we found that about 80% of the satellite platforms are cube shaped. The graph below shows the result of the survey. One rich source of information about satellite missions is the eoPortal, which we have used for this survey.
The reason that most of the micro and nano satellites are cube is because of its various advantages over other shapes. It is wise to choose cubic platform unless the mission has specific requirements like high surface area for mounted solar panels where other shapes may be more effective. A compact structure with lower length-to-width ratio is preferred not only to minimize the size of the satellite but also to avoid vibration. Being abundant, light-weight and inexpensive, Aluminium and Aluminium alloys are preferred materials for micro or nano satellite structures.
Study carried out by Indra Muthuvijayan, Intern at Astrome Technologies.
Satellites orbiting within few hundred kilometers from the Earth’s surface are multiplying every day due to the rise in demand for communication, navigation and monitoring technologies. Typical applications of these so called Low – Earth Orbit(LEO) satellites include, air , sea and road traffic monitoring, remote sensing, communication services, atmospheric research and weather forecasting. In space, these satellites are exposed to a harsh space environment that varies widely in temperature as the satellite orbits around the Earth. These extreme temperatures pose a major threat to the electronics housed inside the satellite. Generally, the electronic boards are designed to operate optimally within a certain range of temperatures defined by the manufacturer. In addition to the thermal loads from the environment, the electronic components themselves generate heat which has to be managed. Hence, it is critical that the satellites maintain operational temperatures to avoid any subsystem failures.
The responsibility is on the Thermal System Design Engineer to solve these challenges with an efficient and affordable system. This article will present important factors that govern the design of the Low Earth Orbiting satellite from a thermal point of view.
Orbits and their Environments
Before we get into the details of the thermal system design, it is wise to find out about the different types of orbits and the nature of space environment experienced by a satellite in each of these orbits. Most of thermal design choices are heavily dependent on the type of orbit chosen for a particular satellite. Typical LEOs are: Sun – Synchronous Orbits (SSO), Polar Orbits, Inclined Orbits or Elliptical Orbits. Sun – Synchronous Orbits are of two types: Dawn – Dusk SSO and Noon – Midnight SSO. The two orbits are completely different from the thermal perspective as will be explained shortly.
Space thermal environment experienced by a LEO satellite is, for all practical purposes, defined by three parameters: Solar Flux (S), Albedo and Earth’s Infrared radiation. The latter two parameters are a function of altitude of the orbit while the first one is that of distance from sun. A typical orbit of a satellite around earth can be divided into two phases – (1) Sun-lit phase, and (2) Eclipse phase. During the sun – lit phase of the orbit, the satellite heats up from all of these three effects. As a result, the temperature of the satellite goes to a maximum. When in the eclipse phase, the Solar Flux and Albedo effects are not encountered and the satellite is exposed to temperatures as low as the Earth’s Average Infrared temperature of -18 ˚C. Dawn – Dusk SSO doesn’t have an eclipse phase and hence it experiences a high temperature environment for the whole time but Noon – Midnight SSO and all other orbits experiences both hot and cold environments as they have both sun – lit and eclipse phases. The duration of these phases though, depends on the type of orbit. Thus, based on altitude and duration of the two phases, satellite in each type of orbit will experience a different hot and cold environment. However, the typical range of temperatures was found to be from -170 ˚C to 123 ˚C for LEO satellites while -250 ˚C to 300 ˚C could be experienced in other orbits. For better understanding, an example providing different thermal loads and the temperature of the satellite orbiting at an altitude of 1280 km is presented below.
Thermal System Design Considerations
Given the extremely cold and hot ambient temperatures that a satellite is exposed to, it is impossible to design a satellite and sustain its operation without a thermal control system (TCS). Once a specific orbit is selected for the mission, a thorough understanding of the space environment for that orbit coupled with the mission requirements, provide the ideal thermal system to be implemented. Representative operational temperature limits of the typical electronic components used in a satellite is given here.
Thermal Control System (TCS)
Having learnt the “why” and “how” a thermal control system is selected, we can further explore the different thermal system design components available. In general, there are three categories of thermal control systems used in satellites: 1) Passive Thermal Control System, 2) Active Thermal Control System, and 3) Partially – Active Thermal Control System. They differ in the way they function and maintain the temperature of a section or the whole satellite. Passive TCS requires no mechanical moving parts or moving fluids and no power consumption. It is simple to design, implement and test. It has low mass and cost and is highly reliable. However, it has limited temperature control capability. Active TCS requires mechanical moving parts or moving fluids or electrical power. It has complex design and generates constraints on spacecraft design and test configurations. It has a high mass and cost and it is less reliable than Passive TCS. Partially – Active TCS is a hybrid system that uses both Passive TCS and Active TCS components. It uses less power and is of lower cost than the Active TCS. It is comparatively low in mass and offers better reliability than active TCS. It also provides better temperature control as compared to the Passive TCS. The figure on the right highlights all the regularly used passive and active thermal system design components.
Orbits and suggested TCS
Any one or a combination of the thermal system components mentioned earlier can be used to establish the required thermal equilibrium in the satellite. Whether to choose passive or active or both depends on the type of selected orbit. Satellites in Dawn – Dusk Sun – Synchronous orbit will not require heater power to increase the temperature of the spacecraft as it is sun – lit throughout the mission. But, to avoid the temperature from rising above the maximum allowable operating temperature, a cooling down mechanism is required. Heat can be distributed along the structure of the satellite by suitable construction material (Eg. Aluminum) or through heat pipes or fillers. Multi – Layer Insulation (MLI) blankets and paint on the surface with suitable coating material are also used. These techniques come under the Passive thermal control system.
Satellites in Noon – Midnight Sun – Synchronous, Polar, Inclined and Elliptical orbits will require heater power to increase the temperature of the spacecraft during cold eclipse phases. During the rest of the orbit when the satellite is sun – lit, to avoid temperatures from rising above the allowable operating temperature limit, similar thermal control methods as used in the above case can be used. Since, this thermal control system uses both passive components and active electric heater system, the system is Partially-active.
Satellites in any orbit will require a surface coating with a specific surface property (Absorptivity and Emissivity controls the heat load input and output) as required to manage the thermal loads in that orbit. Secondary surface mirrors (SSM) or Optical solar reflectors (OSR) sometimes replace surface coating but add an extra expense to the cost of the satellite. Thermal radiators are used in satellites to manage internal heat generated by electronics. Thermal doublers are usually used in the radiators of large satellites like RADARSAT-2 which generate enormous amount of heat. Louvers are usually not used unless there is a stringent condition to maintain the temperature of the spacecraft as a function of time as in the ROSETTA mission.
Conclusion: Options are too many. A precise thermal control can be achieved using expensive components which in turn affects the satellite cost and mass budget, while reasonable temperature control can be achieved using partially active or passive components that are low cost and more reliable. It is upon the thermal system design engineer to choose the optimal design. He or she will have to explore all the available options to find the most efficient and affordable thermal control system such that the temperature limit constraints are met with good tolerance and the costs are kept within the budget.
Study carried out by Miracle Israel, Intern at Astrome Technologies.
There is no defined common process for constellation design since it is a process that varies significantly with the mission objectives. During constellation design, the preferred solutions are those that satisfy the mission requirements while minimizing the overall cost of realizing the mission. As a consequence, it is desired that a minimum number of satellites be employed to accomplish the mission objectives. Another important cost factor is the number of orbital planes utilized. Placing satellites on significantly differing orbital planes require multiple launches that increases launch cost and complicates the launch sequence. The primary input for constellation design is the geographical areas that need to be covered and how frequently they needs to be covered. Quite often a mission requires global coverage and sometimes continuous global coverage.
This article describes key parameters that a constellation designer needs to consider and their trade-offs. Also, we will describe the steps involved in designing a frequently used constellation geometry: The Walker Delta Constellation.
How it began
The Space Age began with the launch of Sputnik-1, world’s first Artificial Satellite, on October 1957. The commencement of Space Age created a rapid demand for space-based applications, mainly in areas of navigation, communication and observation. The idea of satellites functioning in a coordinated manner came up in 1958. The synchronized behaviour by a group of satellites, known as a satellite constellation, provides significant improvement in temporal and spatial coverage. The importance of satellite constellations cannot be overstated.
The first constellation called the TRANSIT was developed by the US Navy in 1960 to provide navigational assistance to their ballistic missiles. Since then, the developments in satellite constellations was propelled by its wide applicability. Satellite constellation design is often misrepresented as a mere act of replicating multiple copies of a single satellite in modified orbits. The satellite constellation design process is somewhat akin to developing a multicellular organism with each cell representing a satellite. The infinite number of choices for the six Keplerian orbital parameters make the constellation design extraordinarily difficult. Various constellation geometries were proposed to reduce this complexity. The most notable constellation geometry is the Walker-Delta constellation proposed by John Walker in 1970. Walker’s geometry made the orbital parameters dependant on one another in a particular way, thereby reducing the complexity. Walker-Delta technique provides the most symmetric geometry among all the constellation design techniques. Thus, it is most suitable for global coverage for several applications related to earth-observation. Off late another technique, called the `flower constellation’ technique developed at Texas A&M University, is becoming popular which can address cases where only local coverage is desirable. In fact, flower constellations can provide “repeating ground track” orbits which are not restricted to an inertial plane, thus widening the varieties of satellite constellation. However, detailed discussions about flower constellations will be deferred for a later article.
Designing a Satellite Constellation
There are no definite rules for designing a satellite constellation. The parameters defining a satellite constellation are ‘mission dependant’. Generally all the satellites in the constellation have similar altitude profiles, eccentricity and inclination so that perturbations affect the satellites in the same way and the geometry can be preserved without much station-keeping. The principal factors to be defined while designing a Satellite Constellation are listed below.
Table: Parameters to be considered during Constellation Design
|Number of Satellites||Affects the coverage and the principal cost|
|Number of Orbital Planes||Varies based on coverage needs. Highly advantageous to have minimum number of orbital planes as transfer between the orbits increases the launch, and transfer costs.|
|Minimum Elevation Angle||Must be consistent with all satellites. Determines the coverage of single satellite.|
|Altitude||Increasing the Altitude increases the coverage and the launch, transfer cost.
Decreases the number of Satellites. For communication applications, increase/decrease in altitude can correspondingly change latency.
|Inclination||Determines the latitude distribution of coverage and selected based on coverage needs.|
|Plane Spacing||Uniform plane spacing results in continuous ground coverage.|
|Eccentricity||Circular orbits are popular, because then the satellite is at a constant altitude requiring a constant strength signal to communicate. For some cases, elliptic orbits are chosen where we need satellites to stay over a particular region for longer duration. Tundra and Molniya orbits are two such examples.|
Apart from the six orbital parameters and the ones mentioned above, one important design consideration is collision avoidance. Apart from loss of mission, collision between satellites in the constellation or between other existing satellites will result in space debris which might have a devastating effect on the other satellites, like it was depicted in a recent movie `Gravity’. The most unfortunate example is the collision between Iridium 33 and Kosmos 2251 on February 2009. This resulted in millions of small debris, most of which still orbit the Earth. To prevent unnecessary space debris, we also require a well defined ‘end of life strategy’. Typically, at end of life, satellites are either de-orbited or transferred to graveyard orbits (suitable for satellites in geostationary orbits).
Figure above shows ground trace for a typical communication satellite. The ground trace (shaded area) is circular with radius and is subtended by a cone with half angle . The continuous coverage often called the street of coverage is represented by considering a chordal range of on both sides of the ground trace (assumed circular), as shown in figure below. The adjacent orbits should be decided such that the bulges of one orbital plane fills the dips of the other orbital plane. Hence to guarantee continuous coverage the maximum distance between adjacent orbit planes can be selected as
As one can guess, coverage increases with the increase in number of satellites or with the increase in altitude. However this also increases the principal and launch costs. Hence there exists a trade-off between coverage and mission cost.
Let us consider a satellite constellation at altitude 1000 km with an inclination of 45° and in circular orbits. Let it have 2 orbital planes with 4 satellites in each plane. The graph below shows the variation in coverage with the variation in number of satellites. Here the ground terminal elevation mask is set at 10°, which means that the ground terminal can see the full sky except 10° from the horizon.
The variation in coverage vs altitude, with the total number of satellites fixed at 8 is shown below. The values of coverage is estimated using the software SaVi.
We can clearly see that the coverage increases significantly with the increase in Altitude. The downside of using satellites at higher altitude apart from the launch cost is the increase in power needs for data transmission and longer signal propagation periods (higher latency).
Coverage by a LEO Constellation
In contrast to Geostationary satellites , many LEO satellites are needed to provide continuous coverage over an area. However satellites in Low Earth Orbits enjoy the benefits of shorter distance to the Earth’s surface. The key advantages of using LEO constellation are listed below.
- Shorter signal propagation periods (low latency). The minimum theoretical latency for LEO satellite is 1-4 milliseconds whereas the latency for GEO satellite is 125 milliseconds.
- Lower power needed for data transmission and instrumentation
- Better resolution for imaging applications, and also for other earth-observation applications.
The high velocities of LEO satellites relative to the surface imply short contact periods to ground stations and short observation periods of specific surface areas by a single satellite. Hence several satellites in appropriate complementary orbits are necessary to provide continuous coverage.
A frequently used design technique is the Walker-Delta pattern constellation for a global coverage of the Earth’s surface by a minimum number of satellites in circular orbits. The Walker constellation is denoted by a notation
- i : inclination
- t : total number of satellites
- p : number of equally spaced orbit planes
- f : relative phase difference between satellites in adjacent planes
A Walker-Delta pattern contains of total of ‘t’ satellites in ‘p’ orbital planes with satellites in each orbital plane. All orbital planes are assumed to be in same inclination ‘i’ with reference to the equator. The phase difference between satellites in adjacent plane is defined as the angle in the direction of motion from the ascending node to the nearest satellite at a time when a satellite in the next most westerly plane is at its ascending node. This is illustrated in figure below. In order for all of the orbit planes to have the same phase difference with each other, the phase difference between adjacent satellites must be a multiple ‘f’ of , where ‘f’ can be an integer between 0 to p – 1.
Designing a Walker-Delta Constellation
After defining the number of satellites, number of orbital planes, semi major axis and inclination, specific to the mission, the true anomaly and the right ascension of ascending node can be calculated using the spacing rule defined by John Walker. The eccentricity and argument of perigee can be ignored as most Walker constellation orbits are circular. The steps involved in designing a Walker constellation are simplified and listed below.
- Calculate the number of satellites needed to satisfy the mission requirements , ‘t’.
- Select the number of orbital planes that provide maximum coverage and at the same time obeys the specified cost constraint, ‘p’.
- The ascending node of the ‘p’ orbital planes should be equally distributed around the equator at intervals of .
- Define the number of satellites per plane, .
- Within each orbit plane, ‘s’ satellites should be equally distributed at interval .
- The spacing between the satellites in adjacent planes should be ‘f’ multiplied by spacing between the satellites in a orbit plane [(i.e) .] divided by the number of orbital planes.
- Spacing (angular) between satellites in adjacent planes = .
Walker-Delta constellation design is a milestone in constellation design process but it should be noted that it is one among the various options available and does not necessarily provide the best characteristics for a given mission. For further reading about Walker-Delta constellation, see here and here.
A famous example of Walker-Delta Constellation is the Galileo constellation. The satellites are placed as a 56°: 27/3/1 constellation, having 27 satellites in orbit, placed in 3 orbit planes separated by 120°. The altitude of the constellation is 23,222 km. The orbital planes are at an inclination i = 56° and hosts 9 satellites at an angular distance of 40° in a plane. The phase shift between adjacent orbits is . The Galileo constellation has been optimised and its orbital parameters are chosen in such a way that it provides continuous global coverage.
The Ground traces (green and pink in color) of two Galileo satellites (marked as two dots) are highlighted in the figure below. The figure also shows the orbit of the two satellites along with their directions.
Conclusion (A New Beginning)
Satellite constellations provides effective solutions for the skyrocketing demands in numerous fields. A breakdown of various parameters influencing a satellite constellation has been presented in this article. The choice of appropriate values to these parameter are limited to the mission needs and to the constellation designer. As a consequence of complexity in the design process, even after 50 years, the constellation design process is still considered to be in it’s infant stage. The time proven simplified constellation geometries and design processes have reached a plateau and no longer satisfy the modern mission needs. This has created a need for new benchmarking processes. Numerous researches are being carried out in this field and one could expect cosmic advancements in the near future.
Study carried out by Raja P, Intern at Astrome Technologies.