The Challenges of Satellite Communications Systems
Satellite systems hold forth the promise of true “anywhere, anytime” access to communications, even in the most rural and remote areas of the globe. These systems provide communications coverage over very wide areas, including over the ocean. In general, they fall into three broad classes: geosynchronous (GEO), “big” low earth orbit (LEO), and “little” LEOs.
Geosynchronous systems, in which the satellites maintain a high orbit that keeps them over a fixed spot on the Earth's equator, have several advantages in terms of long satellite life and wide area coverage by a small number of satellites. They have the disadvantages of round trip latencies that exceed a half a second, poor coverage and inadequate elevation angles (to avoid building radio shadows in urban areas) at the high latitudes. These weaknesses are addressed by the low earth orbit systems, which follow elliptical orbits, allowing them to provide reduced delays and better coverage and elevation angles when close to their orbital perigee. However, LEOs require substantially greater numbers of satellites to provide adequate coverage, and these will need more frequent replacement.
Geosynchronous systems include Inmarsat and OmniTRACS. The former is geared mainly for analog voice transmission. Newer generations of Inmarsats are incorporating digital techniques for use with smaller, less expensive terminals (i.e., the size of briefcase). The Inmarsat system uses allocations in the 6 Ghz band for the ground station to the satellite, 1.5 Ghz for the satellite to terminal downlink, 1.6 Ghz for the terminal to satellite uplink, and 4 Ghz for the satellite to ground station.
Qualcomm's OmniTRACS provides two-way communications as well as location positioning. The system operates in the 12/14 Ghz bands. The downlink data rate is between 5 Kbps and 15 Kbps while the uplink is between 55 bps and 165 bps. The system is used extensively for alphanumeric messaging and on-board sensor reading for trucking fleets.
Little LEOs are intended to be relatively small and inexpensive satellites that provide low cost, low data rate, two-way digital communications (but not voice) and location positioning to small, handheld terminals. The frequency allocations are in the VHF band below 400 MHz. The advantages of little LEOs are their small size and relatively low costs. Systems include Leosat, Orbcomm, Starnet, and Vitasat. For example, the Orbcomm system requires 34 satellites for reliable, full world coverage, and provides 2400 bps on the uplink and 4800 bps on the downlink.
Big LEOs are larger, more expensive satellites that provide voice communications as well as moderate to high speed data communications (56 Kbps). Proposals include Aries, Ellipso, Globalstar, Iridium, and Odyssey. Frequency allocations are above 1 GHz. For example, Motorola’s Iridium system will offer worldwide cellular phone service from 66 satellites placed in 6 polar orbits.
Of the commercially available GEO systems, the Hughes DBS1 system provides several unique communications capabilities. Beyond its current ability to distribute digital video, it is ripe for data communications experimentation and pilot applications development. DBS has the potential for multi-megabit per second transmissions, although latencies are high (i.e., greater than 500 ms for the downlink alone).
However, the system presents considerable challenges for applications development. Unlike the other mobile satellite systems, DBS is not intended to be used as a two-way system over the satellite segment; the data communications uplink is provided through wireline networks such as the public switched telephone system (PSTN) and Internet gateways. The DBS is a highly asymmetric communications system that spans hybrid links (i.e., satellite downlink, wired uplink). This makes DBS a particularly attractive technology for wide-area data distribution or asymmetric data access, in which more information can profitably be broadcast on the downlink than on the uplink.
Notes:
1DBS – direct broadcast satellite – спутник прямого вещания.
8. Read the text and write 10 questions on its content. Types of questions:
1. What is the (nature, difference, process, advantage, importance, role, etc.) of …?
2. What is referred to as …?
3. What is used as …?
4. Where do we use …?
5. What function do the satellite systems play?
9. Match the parts to complete the sentences.
| 1. Little LEOs are intended 2. Newer generations of Inmarsats are intended 3. Geosynchronous systems have several advantages 4. DBS is ripe 5. GEO systems have the disadvantages of round trip latencies that | a) in terms of long satellite life and wide area coverage by a small number of satellites. b) for data communications experimentation and pilot applications development. c) exceed a half a second and poor coverage; d) to be used with smaller, less expensive terminals. e) to be relatively small and inexpensive satellites that provide low cost, low data rate, two-way digital communications. |
10. Arrange the following statements as the text goes.
1. DBS is a particularly attractive technology for wide-area data distribution.
2. LEOs need more frequent replacement and provide adequate coverage.
3. The OmniTRACS is used extensively for alphanumeric messaging and on-board sensor reading for trucking fleets.
4. The advantages of little LEOs are their small size and relatively low costs.
5. Motorola's Iridium system will offer worldwide cellular phone service from 66 satellites placed in 6 polar orbits.
6. Newer generations of Inmarsats are incorporating digital techniques for use with smaller, less expensive terminals.
7. These systems’ frequency allocations are above 1 GHz.
11. Express your attitude to the importance of the Hughes DBS system.
12. Make a short summary of the text in a written form. The phrases given below will help you.
1. The paper deals with … .
2. The author attempts to provide … .
3. There exist … .
4. Of prime interest to the reader will be the consideration of … .
5. It is true that … .
6. In conclusion … is (are) given.
Part B
13. Look through the text and note the title which conveys the contents most of all.
1. Satellite Based High Bandwidth Data Broadcast.
2. New Architectures that Provide Services Direct to the Consumer.
3. Future US Military Satellite Communication Systems.
4. Different Generations of Telecommunication Systems.
14. Divide the text into paragraphs. Find 1 or 2 sentences which can be omitted as inessential in each paragraph.
Text B
It is believed that the elements of successful DBS1 applications include the abilities to (1) exploit high bandwidth downlinks, (2) take advantage of the broadcast nature of the channel, and (3) hide the considerable latencies encountered in using the system. These three issues are addressed in the following paragraphs. A unique aspect of the DBS system is its potential for a very high bandwidth downlink. This makes DBS particularly appropriate for disseminating massive amounts of information. One way that the system could be used is to provide a distribution for large collections of information, such as directories, catalogs, software updates, or other digital library objects that could be locally cached on subscriber systems for better low latency access. The downlink is broadcast-based. Many subscribers can pick packets from the broadcast data stream simultaneously. Thus a broadcast satellite is of consider value as a way to implement a “community information system.” For certain kinds of frequently requested information, it should be possible to combine information requests from multiple users. In effect, we trade some latency in order to combine more common results to be broadcast to users, thus more efficiently using the broadcast channel. Furthermore, if request combining can be effectively exploited, the system would be able to support a larger user population within a given amount of available bandwidth. Satellite systems always face significant latencies, but these may be even larger in the DBS system because of the hybrid uplink path. Given the high downlink bandwidth, aggressive prefetching could be cheap operation, and we can trade downlink bandwidth in order to reduce the number of expensive uplink transactions. Alternately, other strategies could be pursued to trade uplink transactions for bandwidth whenever possible. For example, when fetching multiple images in a World Wide Web HTML2 document, these can be combined into a single multi-image fetch rather than multiple image by image fetches. This is an example of a strategy that can be of equal value in wireline, WLAN3, or satellite networks: the strategy dramatically reduces the number of TCP4 transactions needed to fetch an image intense document, which can lead to significantly higher latencies even in networks with symmetric bandwidths. These and other mechanisms must be developed to demonstrate their ability to be effectively used in constructing new kinds of information dissemination applications well suited for satellite networks. These will also be of value in other wireless network environments.
Notes:
1DBS – direct broadcast satellite – спутник прямого вещания.
2HTML – HyperTexl Markup Language – язык гипертекстовой разметки.3WLAN – Wireless Local Area Network – беспроводная сеть передачи данных.
4TCP – transmission control protocol – протокол управления передачей данных.
15. Write out key words and phrases revealing the contents of the text.
16. Ask questions to the text making an outline of it.
17. Sum up the text using the key words, phrases and the outline.
18. Say which facts presented in the text you’ve already been familiar with.
Part C
19. Read the title of the text and say what information is presented in it.