Satellites send information to Earth by turning digital data into electromagnetic radio or microwave signals that can travel through space. This matters because weather images, GPS timing, television broadcasts, internet links, and scientific measurements all depend on reliable satellite communication. A satellite is not just a camera or relay in orbit, but a networked computer system with sensors, processors, antennas, and error checking.
Ground stations receive the signals, decode them, and pass the data into terrestrial networks.
Key Facts
- Electromagnetic waves travel in space at about c = 3.00 x 10^8 m/s.
- Wave relation: c = fλ, where f is frequency and λ is wavelength.
- A bit rate of R bits/s sends N bits in time t = N/R.
- Free space path loss increases with distance and frequency, so higher or farther links need more antenna gain or power.
- Modulation encodes bits onto a carrier wave by changing amplitude, frequency, phase, or a combination of these.
- Error detection and correction add extra bits so receivers can find or fix some errors caused by noise.
Vocabulary
- Carrier wave
- A steady electromagnetic wave that is modified to carry digital information.
- Modulation
- The process of changing a carrier wave so it represents data bits.
- Ground station
- An Earth-based antenna and computer system that sends commands to satellites and receives their data.
- Latency
- The delay between sending a signal and receiving it, mainly caused by distance and processing time.
- Packet
- A formatted block of digital data that includes payload information and control information such as addresses or error checks.
Common Mistakes to Avoid
- Treating a satellite signal like a sound wave is wrong because satellite signals are electromagnetic waves and do not need air to travel through space.
- Ignoring signal travel time is wrong because even light-speed signals take measurable time to cross thousands of kilometers.
- Assuming stronger frequency always means better communication is wrong because higher frequencies can suffer more path loss and may be more affected by rain or atmosphere.
- Confusing raw sensor data with transmitted packets is wrong because the satellite computer usually compresses, formats, checks, and schedules data before transmission.
Practice Questions
- 1 A satellite is 36,000 km above a ground station. Estimate the one-way signal travel time if the signal travels at 3.00 x 10^8 m/s.
- 2 A satellite transmits a 48 megabit image at a data rate of 12 megabits per second. How long does the transmission take, not including overhead?
- 3 Explain why a satellite link uses both modulation and error correction instead of simply sending unprotected 1s and 0s directly.