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Wireless Communication Basics

Which is the minimum number of satellites a GNSS (global navigation satellite system) receiver such as GPS needs to determine its three-dimensional position while also solving for its own clock error?

  1. A2 satellites
  2. B3 satellites
  3. C4 satellites
Show answer and explanation

Answer: C. 4 satellites

In GNSS, each satellite transmits precise time and its orbit information, and the receiver finds the distance to each satellite from the signal travel time. Three satellites would suffice for the three-dimensional position alone (three unknowns), but the receiver's clock is far less accurate than the satellites' atomic clocks, so its error must be solved for as a fourth unknown. A minimum of four satellites is therefore needed, and in practice more are used to improve accuracy. Radio waves travel about 300 m in 1 µs, so a tiny timing error becomes a large distance error.

  • A. Two satellites cannot even determine the three position unknowns.
  • B. With an accurate clock, three satellites could fix a three-dimensional position, but the receiver's clock error is also an unknown.
  • C. Correct. Four unknowns are solved: three for position and one for clock error.
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More Wireless Communication Basics questions

  1. In a radio receiver, which circuit is placed right after the antenna and amplifies the weak received signal while adding as little of its own noise as possible?
  2. In a receiver, which circuit multiplies the received signal by the signal of the local oscillator (LO) to produce a signal at the difference (or sum) of their frequencies, thereby converting frequency?
  3. A superheterodyne receiver receives a 900 MHz signal by setting its local oscillator (LO) to 800 MHz and converting to a 100 MHz intermediate frequency (IF). Which is the image frequency that would also be converted to the same 100 MHz IF?
  4. Which describes a direct-conversion receiver, which sets the local oscillator (LO) to the same frequency as the received signal and converts directly to baseband without an intermediate frequency, compared with a superheterodyne receiver?
  5. In a transmitter sending OFDM or multilevel QAM signals, which is the main reason for operating the power amplifier (PA) at a point below where its output saturates (with back-off)?