Low Earth Orbiting LEO
Low Earth Orbit (LEO) satellites are in an orbit about 400 to 800 miles above the Earth’s surface - far below geostationary (GEO) satellite orbit. Orbits lower than this are not stable, and will decay rapidly because of atmospheric drag.
A LEO satellite orbits a local horizon in approximately 20 minutes. The orbiting periods range anywhere from 90 minutes to two hours, at approximately 17,000 mph. LEOs are considered to have no delay. A LEO system must use a satellite-to-satellite hand-off to maintain communications and are best for short, narrowband communications. Once one satellite moves out of the area a new one will move in.
There are two types of LEOs - little LEOs and Big LEOs.
Big LEO
Big LEOs are used for technology devices such as high-speed, high-bandwidth data communications, and video conferencing. They carry voice and high-speed data services. They are aimed at data communications and real-time voice into hand-held devices. Big LEO can also offer global services, which are also subject to regulatory requirements. There have been five Big LEO already licensed. The five Big LEO are Iridium, Global Star, Odyssey, ICO Global Communications, Teledesic.
Little LEO
Little LEOs are required to offer non-voice services for example vehicle tracking, environmental monitoring and two-way data communication. A little LEO is a constellation of small, low-earth orbiting satellites, used for short, narrowband communications. Little LEOs are a small, low-cost, class of satellites. LEOs are to use a spectrum allocated between 137-138MHz for space-to-Earth downlinks, and 148-150.05 MHz for Earth-to-space uplinks. ORBCOMM is a Little LEO that Stevens uses for environemental monitoring application.
ORBCOMM
ORBCOMM, a leading LEO system used in data collection and environemtal monitorng, provides global coverage with 30 satellites, and is capable of sending and receiving tow-way alphanumeric packets of data anywhere in the world. Stevens is ORBCOMM’s value added reseller (VAR) or a.k.a. Message Server for environmental monitoring applications.
The Users communicators (SCs) pass data messages to and from Gateway Control Centers (GCC) over ORBCOMM satellites . Messaging traffic flows between the satellites and a GCC through tracking stations called Gateway Earth Stations (GESs) that connect with satellites as they pass overhead. The GCC than route messages to third party value added service providers such as Stevens Water Monitoring Systems, Inc. Stevens collects, processes, stores and delivers the data message to the user via Internet, e-mail or dedicated linesWhen a satellite is not connected to a GES, it can still support SC messaging in a store-and-forward mode.
There are currently 12 GES facilities on four continents, maintaining satellite-GCC connectivity and near-real-time messaging capabilities for users throughout much of the world.
The ORBCOMM telemetry system for environmental monitoring is easy to set up and the users only need to buy a modem and establish an account with a value added reseller, such as Stevens Water Monitoring Systems, Inc. to use the system. Stevens was the first company to transmit environmental monitoring data over the ORBCOMM system.
Advantages to LEO Satellite Telemetry
- Global applications in developing a remote environmental monitoring communication system.
- Two-way communications
- Easy to set up and low maintenance costs
- Low profile, non-directional whip heliz antenna
- Easy access to data
- No FCC or other governmental agency requirements for data transmission
- Coverage in very remote areas
- Data is proprietary
- Stevens is able to quickly detect any problems with transmission of data
- Systems verifies that data has been transmitted which minimizes risk of missing data.
- Event notification on line, by pager, etc.
- Lower power transceivers compared to GEO transmitters.
- LEO Transceivers hardware system is lower in cost than a GEO transmitter system.
Disadvantages to LEO Satellite Telemetry
- Monthly service fee which could be expensive with frequent transmissions of data.
- Power outage at GCC would shut down the communication server, which could delay transmission of data to end user until power is restores. However, no data is lost.
- LEO satellites have a much shorter life span (five to eight years) than GEO satellites.