E2A: Satellite Operation

E2A covers the technical concepts behind amateur satellite communications: orbital mechanics and terminology, satellite hardware including linear transponders, satellite mode designators, frequency band identifiers, digital store-and-forward functions, and the antenna considerations required for reliable satellite operation.

Satellite operating combines orbital geometry, RF propagation, and transponder physics into a single discipline. The Extra class exam tests whether you can apply this knowledge precisely.

Orbital Terminology: Ascending Pass and Orbit Types

An ascending pass for an amateur satellite is one where the satellite travels from south to north. A descending pass moves from north to south. This terminology comes from the satellite's position relative to Earth's equatorial plane — the satellite is ascending toward higher latitudes during a south-to-north pass.

Two orbit types appear on the exam. A geostationary satellite orbits at approximately 35,786 km altitude above the equator and matches Earth's rotation rate exactly, causing it to appear to stay fixed in one position in the sky relative to a ground observer. A Low Earth Orbit (LEO) satellite moves rapidly across the sky, visible for only minutes per pass. A Highly Elliptical Orbit (HEO) satellite moves at varying speeds — very fast near perigee, very slow near apogee.

Keplerian Elements

Keplerian elements are a set of parameters that define the orbit of a satellite around the Earth. They describe the shape, size, orientation, and current position of the orbital ellipse with enough precision that tracking software can predict the satellite's position and visibility at any future time. Satellite tracking programs require up-to-date Keplerian elements (also called "Keps" or TLE — Two-Line Elements) to accurately point antennas and predict pass windows.

Satellite Mode Designators

The "mode" of an amateur radio satellite specifies its uplink and downlink frequency bands — not the orbit type, the satellite's orientation, or the modulation scheme. Mode designators use letters to identify the uplink and downlink bands. For example, "Mode U/V" indicates a UHF uplink and VHF downlink. The letters in a satellite mode designator specify the uplink and downlink frequency ranges — both pieces of information are encoded in the mode name.

Inverting Linear Transponders

A linear transponder receives a range of frequencies on the uplink, translates the entire passband to a different frequency, and retransmits it on the downlink — preserving the linear frequency relationships of all signals within the passband simultaneously. An inverting linear transponder does this with a mathematical inversion: it mixes the uplink signal with a local oscillator signal and transmits the difference product.

The mixing process itself is: the uplink signal is mixed with a local oscillator signal, and the difference product (not the sum) is what gets transmitted on the downlink. This is standard heterodyne translation applied to the entire passband at once.

Linear transponders can relay any type of signal within their passband — FM, CW, SSB, SSTV, PSK, packet, and any other mode. The transponder does not demodulate or remodulate; it simply frequency-translates and retransmits.

ERP Limits on Linear Transponders

When using a satellite with a linear transponder, operators should limit their effective radiated power (ERP) to avoid consuming too much of the transponder's capacity. A linear transponder amplifies and retransmits whatever it receives — if one station transmits at very high power, the satellite allocates more of its limited downlink power to that one signal, which reduces the downlink power available to all other users simultaneously using the transponder. The reason to limit ERP is to avoid reducing the downlink power to all other users — not to prevent out-of-band emissions, not to protect telemetry, and not to avoid interfering with terrestrial stations.

L Band, S Band, and Satellite Frequency Ranges

Amateur satellite operations use several microwave frequency bands identified by letter designators:

These are not the 2-meter and 70-centimeter bands (which are VHF and UHF, not microwave), and they are not related to sideband selection. The L and S band designators are standard radar and microwave frequency identifiers applied to the amateur bands in those ranges.

Digital Store-and-Forward

Some amateur satellites include digital store-and-forward capability. The purpose is to hold digital messages in the satellite for later download when the satellite passes over a ground station that can receive them. A user at one location transmits a message to the satellite; the message is stored onboard; later, when the satellite passes over the intended recipient's location, the recipient downloads the stored message. This allows communication between two ground stations that are never in simultaneous range of the same satellite — a significant capability for long-distance digital messaging.

Antennas for Satellite Operation

Two propagation effects challenge satellite antenna design:

Spin modulation occurs when a rotating satellite's antenna pattern sweeps past the ground station, causing the received signal strength to vary in sync with the satellite's spin rate.

Faraday rotation occurs as radio waves pass through the ionosphere — the plane of polarization rotates in a way that depends on the ionosphere's electron density and the wave frequency. This rotation is unpredictable and variable, making fixed linear polarization unreliable for satellite links.

The solution to both problems is a circularly polarized antenna. A circularly polarized antenna is equally responsive to any orientation of linear polarization, and it is not affected by Faraday rotation. It also handles spin modulation better than a linearly polarized antenna because the satellite's rotation does not cause the polarization mismatch that a linearly polarized antenna would experience.

E2A Practice Questions