Orbital Spacing

First appeared in Tele-satellit magazine
© copyright 1997 MLE INC. All Rights Reserved.

Between now and the dawn of the new millennium, more than twenty new geostationary satellites will begin providing telecommunication services within Asia and the Pacific Rim. My own estimates suggest that there will be 30 percent growth in the number of C-band transponders and as much as an 85 percent increase in the number of Ku-band satellite transponders serving the region by the end of the year 2000. Finding a good orbital slot for all these new systems has become a major headache for the region's leading satellite manufacturers and operators.

Orbital Location	Satellite Name	C-band Pol.	Tprs	Bandwidth (MHz)	Power (Watts)	Ku-band Pol.	Tprs	Bandwidth (MHz)	Power (Watts)	BOL Year EOL Year
33 East	INTELSAT 805	Switchable	14	36/41/72	10 - 30	Linear	6	72/77		1998
60 East	INTELSAT 902	Circular	tbd	tbd	tbd	Linear	tbd	tbd	tbd	2000
62 East	INTELSAT 901	Circular	tbd	tbd	tbd	Linear	tbd	tbd	tbd	2000
68.5 East	PAS-7	Linear	14	36	50	Linear	30	36	100	1998
75 East	LMI-2
83 East	INTELSAT APR	Linear	18	36	4/10/50	Linear	3	72/77	20	1998
87.5 East	Chinastar-1	Linear	18	36/72	45	Linear	20	36/72	85/135	1998
88 East	ST-1	Linear	14	36	n/a	Linear	16	54	n/a	1998
91.5 East	Measat 3	Linear	tbd	tbd	tbd	Linear	tbd	tbd	tbd	1998
95 East	K-TV Hot Bird					Linear	30	n/a	n/a	1999
105.5 East	AsiaSat 3 (replacement)	Linear	28	36	55	Linear	16	54	138	1999
108 East	Telkom 1	Linear	32	n/a	n/a
110 East	BSAT-1B					Circular	4	27	106	1998
110.5 East	Sinosat	Linear	24	n/a	21	Linear	14	n/a	97	1997
116 East	Koreasat 3					Linear/ Circular	15	27/36	12/120	1998
120 East	Thaicom 4	Linear	25	36/72	n/a	Linear	11	36/54	n/a	2000
122 East	AsiaSat 4	tbd	tbd	tbd	tbd	tbd	tbd	tbd	tbd	1999
126 E.	LSTAR 1					Linear	16	33	n/a	1998
126 East	LSTAR 2					Linear	16	33	n/a	1999
139 East	Orion 3	Linear	10	36	n/a	Linear	23	27/54	n/a	1998
154 East	JCSat 6					Linear	32	27/36	60/90	2000
166 East	PAS-8	Linear	16	54/64	33	Linear	16	54/64	63	1998
tbd	M2A	Linear	52	27/36	20 to 240

There are several technical solutions, however, which satellite operators intend to implement to permit more efficient use of the available orbital resources. A few of today's satellite operators already have elected to use the available satellite spectrums more efficiently by collocating multiple communications satellite at the same orbital location. A prime example of this is the Astra satellite constellation. A total of six Astra satellites, each operating within a distinct segment of the 10.7 to 12.75 Gigahertz frequency band are collocated at an orbital assignment of 19.2 degrees east longitude. One major advantage of satellite collocation: a single fixed antenna can receive hundreds of digital TV and radio services from the satellite constellation without having to change its alignment.

Several satellite operators in the Asia/Pacific region are adopting collocation strategies of their own. In Thailand, for example, the Thaicom 2 and Thaicom 3 satellites are now collocated at 78.5 degrees east longitude, with Thaicom 2 operating ten transponders in the 3.7 to 4.2 Gigahertz frequency spectrum and Thaicom 3 operating twelve extended C-band transponders in the 3.4 to 3.7 GHz frequency range along with an additional twelve standard C-band transponders in the 3.7 to 4.2 Gigahertz frequency spectrum. Both Thaicom satellites also carry Ku-band transponders operating in the 12.25 to 12.75 GHz frequency range. Thaicom system operator Shinawatra Satellite intends to duplicate this collocation strategy in early 1998, when Thaicom 1 and Thaicom 4 will be collocated at 120 degrees east longitude. What's more, Thaicom 5 and Thaicom 6 satellites are now on the drawing boards. Expect the new pair of satellites to be collocated at one of the many other orbital slots for which Shinawatra Satellite has already filed.

The Insat satellite system also has adopted a collocation strategy for its Insat 2 series of spacecraft, under which two satellites with standard C-band transponders operating in the 3.7 to 4.2 Gigahertz range as well as six transponder using the 4.5 to 4.8 Gigahertz spectrum will eventually be collocated at India's three main orbital positions of 74, 83.5 and 93.5 degrees east longitude. Satellite TV service providers are also pressuring other system operators to collocate their satellites in order to create larger, more attractive satellite neighbourhoods.

In a meeting with AsiaSat in Hong Kong last year, program service providers urged Asia's premiere satellite operator to take this step. Other Asia/Pacific satellite operators with future collocation plans of their own include Measat (Malaysia), PanAmSat (United States) and L-STAR (Laos).

Most communications satellites transmit using two orthogonal senses of polarization so that satellite operators can re-use the available satellite frequency spectrum twice from any given orbital assignment. Transponders with one sense of polarization are totally transparent to the second set of transponders using the opposite sense. Twice the number of transponders can therefore occupy the same amount of frequency spectrum. This is called "frequency re-use." Not all satellite systems, however, currently employ frequency re-use. For example, the older satellites in the Russian Statsionar system, which currently uses more than twenty geostationary orbital positions worldwide, do not employ this strategy. Moreover, most of the C-band satellites in the Statsionsar system do not even make full use of the 'standard' 3.7 to 4.2 Gigahertz frequency range.

The antiquated design of Russia's Statsionar satellite system consumes twice the number of orbital positions that would be needed if frequency re-use was employed at each orbital assignment. What's more, Russia would only need five orbital slots instead of seventeen if each Statsionar satellite employed frequency re-use of the entire available C-band spectrum. Hopefully the recently announced joint venture between Lockheed Martin and Intersputnik will result in the replacement of older Statsionar spacecraft with more modern designs that more efficiently use the available orbital and frequency resources.

Another strategy for achieving more efficient use of a single orbital position is to re-use the available frequency spectrum within multiple coverage areas that are physically isolated from each another. This is called "spatial isolation." The Intelsat satellite system has long employed spatial isolation to derive maximum benefit from each of its seventeen orbital assignments around the world.

In the C-band, for example, Intelsat VI, VII, and VIII series satellites can simultaneously transmit in four spatially isolated zone beams that target the Northeast, Northwest, Southeast and Southwest quadrants of the 42.4 percent of the earth's surface that any one satellite can see

from a given orbital assignment. Within the Ku-band frequency spectrum, Intelsat VII satellites can simultaneously transmit through three steerable Ku-band spot beams, with each beam targeting a different geographical location, or two beams covering identical locations while using orthogonal senses of polarization. By the 21st Century, other international and regional satellite operators may need to adopt this strategy to obtain access to the capacity that they will need to expand services in the C-band and Ku-band spectrums.

In the past, satellite manufacturers have been limited in the number of transponders that they can build onto any one platform by the amount of weight that the launch service provider can put in orbit. But that's about to change. Arianespace, for example, has developed a new Ariane 5 launch vehicle that can lift heavier payloads into orbit. What's more, leading satellite manufacturers are switching to new ion propulsion systems. Until now, all communications satellites have had to carry several hundred pounds of hydrazine gas to perform all the periodic station keeping maneuvers that a satellite has to undertake over its entire mission lifetime to keep the spacecraft anchored at its assigned orbital position. This added a significant amount to each satellite's launch weight. With the switch to ion propulsion, satellite designers are now able to increase the weight of each satellite's communications payload because of this reduction in station keeping fuel requirements.

In the United States, the Federal Communications Commission mandated 2 degree spacing between satellites back in the mid-1980s. This regulation made it possible for more satellite to serve the North American region than would have been possible under the previously existing three degree spacing environment. In 1997, Intelsat voluntarily switched to 2 degree spacing between its satellites located at 60, 62, 64 and 66 degrees east longitude over the Indian Ocean. Two degree spacing will not always be possible, however, especially between two satellites that intend to offer DTH services into small aperture receiving terminals.

In 1997, Measat and Intelsat were unable to resolve potential interference problems between the Measat 1 satellite at 91.5 degrees east longitude and the Intelsat K-TV hot bird set to launch to 95 degrees east longitude next year. Both satellites are designed to transmit DTH services into antennas as small as 60cm in diameter. The beam width of such a small antenna is so wide that the potential exists for interference between the two systems, even when the spacing between satellites is 3.5 degrees in longitude!

Until last year, the International Telecommunication Union (ITU) of the United Nations, which co-ordinates the registration of satellite frequencies and orbital positions on a global basis, did not have any regulatory powers over the national administrations filing for orbital slots. The ITU's 1997 World Radiocommunication Conference (WRC-97), however, adopted procedures governing the "due diligence" of existing and future satellite systems which file for frequency spectrum and orbital assignments. The new due diligence rules will require regular disclosure of the steps taken to implement new satellite systems, including the number of satellites actually under construction, the contractual date or dates of spacecraft delivery, the identify of the launch service provider and the contractual launch date.

Beyond the aforementioned steps, the ITU should also take a look at those existing satellite systems that mis-manage valuable orbital resources and establish procedures whereby these inefficient system would be required to implement more efficient satellite system designs. At the same time, the ITU will need to continue to protect the rights of developing countries so that each nation has guaranteed access to orbital resources whenever it is ready to launch domestic satellite communications systems.

On the technical side, one of the most promising developments now on the horizon is the promised introduction of Ka-band satellites in the early years of the 21st Century. These Ka-band spacecraft will generate a cellular-like grid of highly focused spot beams no more than 500 miles in diameter; each Ka-band satellite will be able to re-use the available frequency spectrum dozens of times from any given orbital location. What's more, the beam width of small aperture Ka-band receiving antennas will be so narrow that the interference potential between adjacent satellites will be greatly reduced. Best of all, the eventual introduction of inter-satellite links will allow operators to send signals directly from one satellite to the next in any fully integrated system. Signals that currently must be uplinked and downlinked two or more times to reach their final destinations will be able to go from source to destination without any intervening hop. This will eliminate the need to duplicate the feeds of international programmers on multiple satellites.