Advantages and Disadvantages of a Low Earth Orbit

The low earth orbit (LEO) offers a number of advantages over the geostationary orbit, and at least one serious disadvantage.

Delay. One-way delay to a GEO satellite is budgeted at 125 ms; one-way up and down is double this value of 250 ms. Round-trip delay is about 0.5 s. Delay to a typical LEO is 2.67 ms and round-trip delay is 4 x 2.67 ms or about 10.66 ms. Calls to/from mobile users of such systems may be relayed still again by conventional satellite services. Data services do not have to be so restricted on the use of "handshakes" and stop-and-wait ARQ as with similar services via a GEO system.

Higher Elevation Angles and "Full Earth Coverage." The GEO orbit provides no coverage above about 80° latitude, and it gives low angle coverage of many of the world's great population centers because of its comparatively high latitude. Typically, cities in Europe and Canada face this dilemma. LEO satellites, depending on orbital plane spacing, can all provide elevation angles >40°. This is particularly attractive in urban areas with tall buildings. Coverage would only be available on the south side of such buildings in the Northern Hemisphere with a clear shot to the horizon. Properly designed

TABLE 18.7 Performance Summary of INMARSAT-4 Satellite Communication Subsystem

Repeater

Transparent bent-pipe repeater utilizing a digital channelizer, providing 630 x 200-kHz channels on each of the forward and return links, with a digital L-band beamformer allowing the generation of different types of beams. The basic L-band coverage relies on around 200 narrow spot beams (1.2°, 3-dB beamwidth), 19 wide spot beams (4.5°, 3-dB beamwidth), and one global beam. All power amplifiers are solid state and utilize iinearizers.

Service link options

C-L, L-C, C-C, L-L and navigation

Antennas

Separate transmit and receive for C-band and single antenna for L-band. Single L-band center-fed navigation antenna provides global coverage. Focal plane array, offset-fed, deployed unfurlable reflector for L-band transmit/receive, producing the global and spot beam coverages. C-band horns provide global coverage. Single navigation antenna

G/T

L-band = 10 dB/K for narrow spot beams = 0 dB/K for wide spot beams = -10 dB/K for global beam

C-band =-11 dB/K

e.i.r.p.

L-band

67-dBW aggregate e.i.r.p. for narrow spot beams. Can be continuously shared with wide spot beams and global beam.

C-band

31 -dBW/poiarization

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Responses

  • vincenza
    What would 40 tons of i beam do if droped from a low earth orbit?
    1 year ago
  • Marybeth Kimbrough
    What are the disadvantages of low earth orbit satillites?
    6 days ago
  • curtis
    What is the advantages of lower earth orbit?
    6 days ago

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