Satellite communication
A communications satellite is, in essence, a very tall repeater: it receives on one frequency, amplifies and retransmits on another. What changes everything is the altitude, and with it the coverage, the delay and the losses.
01The orbits
| Orbit | Altitude | Period | Round-trip delay | Use |
|---|---|---|---|---|
| LEO | 300 to 2000 km | 90 to 120 min | 5 to 40 ms | Earth observation, internet constellations, space station. |
| MEO | 2000 to 35,000 km | 2 to 12 h | 50 to 150 ms | Navigation: GPS at 20,200 km, Galileo, GLONASS. |
| GEO | 35,786 km | 24 h | ≈ 480 ms | TV, VSAT, meteorology. It looks fixed in the sky. |
At that altitude the orbital period is exactly one sidereal day. A satellite placed over the equator, moving in the same direction as the Earth's rotation, stays still relative to the ground. That is the reason a satellite TV antenna is aimed once and never moved again, and why all the antennas in a neighborhood point to the same spot in the sky.
The trade-off is delay: 36,000 km up plus 36,000 down, at 300,000 km/s, comes to about 240 ms in each direction. In a conversation you notice it, and on geostationary satellite internet you notice it even more, because every protocol exchange pays that delay.
A GEO sees roughly a third of the Earth's surface. With three, spaced 120° apart, the world is covered except for the polar regions, where the satellite sits too low on the horizon or cannot be seen at all. For those latitudes you need inclined orbits or LEO constellations.
02Space segment and ground segment
- Transponders: each one receives a slice of spectrum —typically 36 MHz—, shifts it in frequency and amplifies it. A satellite carries dozens.
- Antennas shaped to illuminate exactly the service region: the footprint.
- Power: solar panels and batteries for eclipses.
- Orbit and attitude control: thrusters that correct drift. When the fuel runs out, the service life ends, even if the electronics are still healthy.
- Hub stations: large antennas that uplink the content and control the satellite.
- User terminals: the dish antenna with its LNB, which amplifies with very little noise and converts the signal down to an intermediate frequency to carry it over coax to the receiver.
- VSAT: small two-way terminals, for connecting branch offices, ATMs or rural schools.
| Band | Frequency | Typical antenna | Characteristics |
|---|---|---|---|
| C | 4 / 6 GHz | 2 to 3 m | Very little affected by rain. Large antennas. |
| Ku | 12 / 14 GHz | 60 to 120 cm | The one used for home satellite TV. Heavy rain already affects it. |
| Ka | 20 / 30 GHz | 60 to 90 cm | Lots of capacity, spot beams. Very sensitive to rain. |
At 12 GHz, coax loses about 1 dB per meter. If the received signal —which is extremely weak— traveled that way all the way to the receiver, nothing would be left. That is why the LNB is mounted at the focus of the dish: it amplifies right there, with a very low noise figure, and only then converts the signal down to 950–2150 MHz, where the cable loses very little. It is the same principle that leads to putting the equipment at the top of the mast in a microwave link.
03The link budget
It is the same calculation as for a terrestrial radio link —see microwave links— but with enormous distances and with two quantities specific to the satellite environment.
Downlink from a GEO at an oblique distance of 38,000 km, at 12 GHz:
FSPL = 32.44 + 20·log(12,000 MHz) + 20·log(38,000 km) = 32.44 + 81.58 + 91.60 = 205.6 dB.
That is 205 dB: the power is divided by 1020. If the satellite transmits 100 W with a 30 dBi antenna —an EIRP of 50 dBW— about −155 dBW reaches the receiving antenna, that is 10−15.5 W. That this becomes television in a living room is the direct result of two things: a dish with 40 dBi of gain and an LNB with a noise figure under 1 dB.
- Azimuth: which way to look in the horizontal plane, measured from north. It depends on the longitude of the site and that of the satellite.
- Elevation: how far to raise the antenna. In the south of Argentina GEO satellites appear low; near the equator, almost at the zenith.
- Polarization or “skew”: the rotation of the LNB about its axis. If it is poorly adjusted, both polarizations come in mixed and interference appears.
- The practical procedure: take the three calculated values, adjust with the signal meter, and only then fine-tune in millimeters, because with a 2° beam a small movement costs several dB.
04Applications
Direct-to-home TV: a single satellite feeds millions of antennas. It is the case where the economics work best, because the cost does not depend on how many receive.
Connectivity where nothing else reaches: farms, offshore platforms, rural schools, backup for critical links, emergency communications.
GPS and its equivalents: the receiver measures the time of flight from several satellites and triangulates. With four satellites it solves position and clock.
Hundreds or thousands of low satellites, moving fast, with ground antennas that track them electronically. The advantage is the low delay —tens of milliseconds instead of half a second—, which makes video calls and online gaming usable. The cost is complexity: satellites have to be replaced constantly and handovers between them coordinated continuously.
05In the lab
With the coordinates of the school and the orbital position of an in-service satellite over South America, calculate azimuth, elevation and polarization. Verify the result with a pointing app and, if an antenna is available, check it in the field.
Mount a satellite TV dish with its LNB, aim it with a signal meter and record how the level varies when it is moved one degree in azimuth and in elevation, and when the LNB is rotated. Document the curve: it is the radiation pattern of the antenna, measured with the satellite as the source.
With a GPS module connected to a microcontroller, read the NMEA sentences and extract position, number of satellites and the signal-to-noise ratio of each. Compare reception outdoors, next to a building and inside the classroom. It is the most accessible way to work with a real satellite link.
With the published data of a satellite —EIRP over the area— calculate the power received by a 90 cm antenna and estimate the carrier-to-noise ratio. Compare with the reading reported by the receiver. A reasonable match between calculation and measurement is the goal of the exercise.
06Common errors
| Symptom | Usual cause |
|---|---|
| Signal strong but quality poor | Aimed at a neighboring satellite, or polarization poorly adjusted: energy comes in but mixed. |
| Drops out in rain | Attenuation in the Ku or Ka band. With a thin margin, a storm takes the link down. |
| It worked and stopped working | The antenna was moved by wind, or water got into the LNB connector. |
| Satellite internet is “slow” even though the bandwidth is enough | It is the delay, not the speed: half a second of round trip affects every protocol that waits for an acknowledgment. |
| No signal anywhere in the sky | Elevation calculated wrongly, or the antenna is blocked by a tree or a building in the direction of the satellite. |
| Brief outage every day at the same time | Solar interference: twice a year, the Sun passes behind the satellite and saturates the receiver with its noise. |
07Self-assessment
Why does a geostationary satellite look still?
Because at 35,786 km above the equator its orbital period equals the Earth's rotation period: it turns at the same rate as the ground beneath it.
How long does the signal take to go to a GEO and back?
About 480 ms: around 240 ms in each direction, covering 36,000 km at the speed of light.
What advantage do LEO constellations have?
The low delay, tens of milliseconds, which makes interactive applications usable. In exchange, they require many satellites and constant handovers.
What does a transponder do?
It receives a slice of spectrum, shifts it in frequency and amplifies it to retransmit it toward the Earth. Different uplink and downlink frequencies are used so that it does not feed back on itself.
Why is the LNB mounted at the focus of the antenna?
Because at 12 GHz coax loses a great deal. By amplifying with low noise right at the focus and converting the frequency down to 950–2150 MHz, the signal can travel the cable to the receiver without being lost.
Advantages and disadvantages of the C band compared with Ku.
The C band is hardly affected by rain, but it needs antennas of 2 to 3 m. Ku allows antennas of 60 to 120 cm, which is why it reached homes, but it suffers in heavy storms.
Calculate the free-space loss at 36,000 km at 4 GHz.
FSPL = 32.44 + 20·log(4000) + 20·log(36,000) = 32.44 + 72.04 + 91.13 = 195.6 dB.
What does the G/T ratio measure?
The quality of the complete receiving system: the antenna gain minus the noise of the system, in dB/K. It is used to compare earth stations independently of their details.
What three values are needed to point a satellite antenna?
Azimuth, elevation and polarization. All three are calculated from the coordinates of the site and the orbital position of the satellite.
How many satellites does a GPS receiver need, and why?
Four: three to solve the three position coordinates and one more to correct the error of its own clock, which is not as precise as the atomic clocks on the satellites.