GPS speed is the speed of movement of a receiver, computed from the signals of positioning satellites. It is expressed in metres per second (m/s) or in km/h. On a smartphone, FizziQ displays this speed in real time and records it: it is the right instrument for movements lasting several seconds or minutes, such as walking, running, cycling or a train ride.
Discover FizziQ
How to measure it in class
FizziQ displays GPS speed among the measurements of the GPS instrument, alongside latitude, longitude, altitude and the estimated accuracy.
Steps:
- Open FizziQ, select the “GPS Speed” instrument and check outdoors, standing still, that the value stays close to zero.
- Walk at a steady pace and read the speed: a normal walk sits around 1.4 m/s, or 5 km/h.
- Run then stop abruptly: observe the reaction time of the measurement, tied to the receiver’s rate of about one reading per second.
- Record a full trip and plot the speed curve: identify accelerations, plateaus and stops.
- Compare GPS speed with a stopwatch measurement over a measured distance: v = d/Δt, and discuss the differences.
Scientific activities on this topic
Possible extensions with FizziQ: compare your average and instantaneous speed over a trip, or read the speed of a bus or a train and compare it with the posted limits.
Learn more
How the receiver computes speed
One might think speed is obtained by dividing the distance between two successive positions by the elapsed time. Receivers do better: they exploit the Doppler effect of the satellite signals. The receiver’s motion slightly shifts the frequency received from each satellite, and this shift gives the speed directly, without going through positions. As a result, GPS speed is often more precise than the position itself, with an uncertainty of about 0.1 m/s in good conditions.
What GPS speed actually measures
It is the speed of movement over the ground, updated about once per second. This slow rate distinguishes it from the accelerometer, which delivers hundreds of readings per second but does not give speed directly. GPS speed is therefore ideal for steady, prolonged movements, and ill-suited to brief phenomena: a 3-second sprint is described by only 3 points. At rest, a slight noise can remain; indoors, the measurement is unreliable, for lack of proper satellite signals.
Instantaneous and average speed
The displayed value is a quasi-instantaneous speed, averaged over about a second. The average speed of a trip is computed over the total distance covered: the two only coincide for uniform motion, which makes it an excellent support for kinematics lessons.
Orders of magnitude: walking: 1.4 m/s (5 km/h); running: 3 to 5 m/s; cycling: 5 to 10 m/s; high-speed train: up to 90 m/s (320 km/h); typical GPS speed uncertainty: about 0.1 m/s in favourable conditions; update rate: about 1 reading per second.
Formula
The average speed over a trip follows from the distance covered:
v = d / Δt
Where:
- v: average speed (m/s)
- d: distance covered (m)
- Δt: duration of the trip (s)
Useful conversion: 1 m/s = 3.6 km/h.
The receiver itself exploits the Doppler shift of the frequency received from each satellite, proportional to the relative velocity:
Δf / f = v_r / c
where v_r is the receiver-satellite relative velocity along the line of sight and c the speed of light.
Application examples
- Check that a walk or a run is indeed uniform motion, and measure its speed.
- Plot the speed curve of a bus trip and spot stops and accelerations.
- Compare instantaneous and average speed over the same route.
- Study the climb rate of an aircraft by combining GPS speed and altitude.
- Estimate a distance covered by integrating the speed over time.
FAQ
Q: Why doesn’t the displayed speed react immediately? A: The receiver only delivers about one reading per second, averaged over that duration. A sudden change of pace therefore only shows up with one to two seconds of delay: it is the instrument’s main limitation.
Q: Is GPS speed reliable at low pace? A: Below about 0.5 m/s, the measurement becomes noisy and can display a small phantom speed at rest. For a slow walk, it is better to average over a timed route.
Q: Does GPS speed account for climbs and descents? A: The displayed value corresponds to the speed over the ground. On a steep slope, the actual speed along the slope is slightly higher; the difference remains small for ordinary slopes (a 2% to 20% slope only changes the speed by 0.02% to 2%).
Related concepts
GPS geolocation - Instantaneous vs average speed - GPS altimeter - Doppler effect - Accelerometer - Reference frame