The Effects of Time Delay of Low-Earth-Orbit Satellite Communication

Nam Bo
Department of Electrical and Computer Engineering
California State Polytechnic University
Pomona, California

I. Introduction

There are several types of orbits used by satellites in space. The most used orbit type is the geostationary satellite(GEO). It occupies an orbital position 36,000 km above the Earth, and remains in a stationary position about the Earth because it is orbiting at the same speed as the Earth [1]. The world’s major existing telecommunications and broadcasting satellites fall into this group. A non-geostationary type orbit for satellites is the Medium Earth Orbit (MEO), which typically provides mobile telephone services. They are located 10,000~20,000 km from the Earth and rotate with respect to the Earth’s surface either in a prograde or retrograde type polar orbit[2]. Another non-geostationary type orbit is the Low Earth Orbit (LEO), providing mainly mobile data service. It occupies an orbital position 700~1500 km above the Earth. They also rotate in a prograde or retrograde type polar orbit. LEOs come in two basic categories: first, the so-called "Big LEOs" offer narrow band voice, data, paging and fax services, the operating frequency of "Big LEO" is in the range above 1GHz. Second, the "Little LEOs" are similar in concept to Big LEOs but provide less ambitious services, its operating frequency is below 1GHz. They offer only narrow band store-and-forward data, making them similar to two way paging system[3]. The last type of orbit is the Molniya type, this orbit is named after a class of Russian communication satellites; the name means "lighting" or "news flash". It is characterized by a highly eccentric, elliptical orbit with a period of 12 hours and a critical
angle of inclination of 63.4 degree. At this angle there is no rotation relative to the Earth. They are mostly used for special communications above the arctic circle[4].

Digital store-and-forward communication via LEO satellite is a method for non-real-time communication of digital information. The original ground station sends a digital message to the LEO satellite. It will store the message in an on-board storage system, and the destination ground station later will retrieve the message from the satellite. Between the storage and retrieval of a message, the LEO satellite moves around its orbit and the Earth rotates on its axis. This movement changes the satellite's communication footprint, bring it to different area of the Earth. A single satellite in a highly inclined LEO can relay messages to ground stations anywhere in the world. This global coverage results from the Earth's rotation beneath the satellite and the satellite's orbital motion. These two motions combine to bring every point on Earth within the satellite’s view at least twice each day and night. In contrast to a GEO satellite, which provides instantaneous coverage with a limited, stationary footprint, a LEO satellite can offer a moving footprint to provide global coverage[5].

By using a single satellite and low cost communication equipment, a store-and-forward system can create a low cost of the satellite communication system. A drawback to a single LEO satellite system is that the message delivery is delayed. The original ground station must wait for the satellite to come into the range before it can upload a message. The message must store on-board in the satellite until the destination ground station comes into the footprint. The time delay problem cause by a satellite can be found out by the following equations.

Fig 1-1 Earth coverage by a satellite at altitude h and a central angle g

For a given central angle g, the slant range at the edge of coverage is
d = (RE ^2 + r^2 - 2RE*r *cosg)^1/2 (1-1)
with the r = RE + h , the central angle g is given by
cos(q+g) = (RE/r) * cosq = cosq/(1 + h / RE ) (1-2)
the minimum angle of elevation q is given by
cosq = (r/d)*sing (1-3)
the actual period of the satellite is given by
t = ((4*p^2*r^3)/m)^1/2 (1-4)
the apparent period of the satellite is given by
tapp = (1440*t)/(1440-t) (1-5)
the time taken from satellite-rise to satellite-set is given by
tpass =(g / 180°)*tapp (1-6)
If the elevation angle q = 10° given, the central angle g from equation (1-2) will be g = 18.44°.
By equation (1-4), the actual period of a satellite will be