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Satellites required for a three-dimensional GPS fix

Aviation Mechanic Airframe (AMA) practice question 1 of 26 on Communications, Light Signals, and Runway Lighting Systems. ACS element AM.II.I.K12 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA GPS receiver has measured the signal travel time from three satellites and produced a latitude/longitude position. How many satellites in total must it use in order to also compute altitude?

  1. AThree satellites
  2. BFour satellites
  3. CSix satellites
Show the answer and explanation

Correct answer: B. Four satellites

  • A — incorrect Three satellites give only the two dimensional position expressed in latitude and longitude coordinates.
  • B — correct Incorporating the distance to a fourth satellite allows altitude above the surface of the earth to be calculated, producing a three dimensional fix.
  • C — incorrect Six is the number of separate orbital planes in which the NAVSTAR satellites are placed, not the number of signals needed for altitude.

BACKGROUND

The GPS receiver measures the time it takes for a signal to arrive from transmitting satellites. Since radio waves travel at 186,000 miles per second, the distance to each satellite can be calculated, and the intersection of three such ranges provides a two dimensional position expressed in latitude/longitude coordinates. By incorporating the distance to a fourth satellite, the altitude above the surface of the earth can be calculated as well, which results in a three dimensional fix. Additional satellite inputs refine the accuracy of the position. The satellites are positioned so that at any place on earth at any one time at least four will be a minimum of 15 degrees above the horizon, with typically five to eight in view.

STUDY TIP

Count dimensions, not satellites: three ranges fix you on a surface in two dimensions, and each added range buys one more dimension.

KEY POINTS

  • Three satellite ranges give a two dimensional latitude/longitude position.
  • A fourth satellite adds altitude, producing a three dimensional fix.
  • At least four satellites are always at least 15 degrees above the horizon anywhere on earth.

SOURCES

  • ama-airframe/ch11.txt "Global Positioning System (GPS)"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.