16.1.2 Low-earth-orbiting satellites

  

Figure 16.1: Example of a polar-orbiting LEO constellation. This figure was generated using the SaVi software package from the geometry center at the University of Minnesota.

sat-constellation

Polar orbiting satellite systems, such as Iridium and the proposed Teledesic system, can be modelled in . In particular, the simulator supports the specification of satellites that orbit in purely circular planes, for which the neighboring planes are co-rotating. There are other non-geostationary constellation configurations possible (e.g., Walker constellations)- the interested user may develop new constellation classes to simulate these other constellation types. In particular, this would mainly require defining new intersatellite link handoff procedures.

The following are the parameters of satellite constellations that can currently be simulated:

• Basic constellation definition Includes satellite altitude, number of satellites, number of planes, number of satellites per plane.
• Orbits Orbit inclination can range continuously from 0 to 180 degrees (inclination greater than 90 degrees corresponds to retrograde orbits). Orbit eccentricity is not modeled. Nodal precession is not modeled. Intersatellite spacing within a given plane is fixed. Relative phasing between planes is fixed (although some systems may not control phasing between planes).
• Intersatellite (ISL) links For polar orbiting constellations, intraplane, interplane, and crossseam ISLs can be defined. Intraplane ISLs exist between satellites in the same plane and are never deactivated or handed off. Interplane ISLs exist between satellites of neighboring co-rotating planes. These links are deactivated near the poles (above the ``ISL latitude threshold'' in the table) because the antenna pointing mechanism cannot track these links in the polar regions. Like intraplane ISLs, interplane ISLs are never handed off. Crossseam ISLs may exist in a constellation between satellites in counter-rotating planes (where the planes form a so-called ``seam'' in the topology). GEO ISLs can also be defined for constellations of geostationary satellites.
• Ground to satellite (GSL) links Multiple terminals can be connected to a single GSL satellite channel. GSL links for GEO satellites are static, while GSL links for LEO channels are periodically handed off as described below.
• Elevation mask The elevation angle above which a GSL link can be operational. Currently, if the (LEO) satellite serving a terminal drops below the elevation mask, the terminal searches for a new satellite above the elevation mask. Satellite terminals check for handoff opportunities according to a timeout interval specified by the user. Each terminal initiates handoffs asynchronously; it would be possible also to define a system in which each handoff occurs synchronously in the system.

The following table lists parameters used for example simulation scripts of the Iridium^16.1 and Teledesic^16.2 systems.

 

Table 16.1: Simulation parameters used for modeling a broadband version of the Iridium system and the proposed 288-satellite Teledesic system. Both systems are examples of polar orbiting constellations.

	Iridium	Teledesic
Altitude	780 km	1375 km
Planes	6	12
Satellites per plane	11	24
Inclination (deg)	86.4	84.7
Interplane separation (deg)	31.6	15
Seam separation (deg)	22	15
Elevation mask (deg)	8.2	40
Intraplane phasing	yes	yes
Interplane phasing	yes	no
ISLs per satellite	4	8
ISL bandwidth	25 Mb/s	155 Mb/s
Up/downlink bandwidth	1.5 Mb/s	1.5 Mb/s
Cross-seam ISLs	no	yes
ISL latitude threshold (deg)	60	60