GPS Receiver

Owned by Kiran Surendran
2021-08-22
2 min read

References

 

What does the receiver do? How do they work?

GPS receivers are used to give your exact position and time from almost anywhere on Earth for free. They use a constellation of satellites and ground stations launched and ran by the US military. At any time, there are at least 24 active satellites orbiting over 12,000 miles above the Earth though the positioning of these satellites are constructed in such a way that the sky above your location will always contain at most 12 satellites.

These 12 visible satellites will transmit information back to Earth over a radio frequency (ranging from 1.1 to 1.5 GHz). The data sent down from each satellite contains information such as the time on the (extremely accurate) atomic clock onboard the satellite, the satellite’s orbital position and arrival times at different points in the sky. If the GPS receiver’s antenna can see at least 4 satellites, it can accurately its position and time (this is called a lock or a fix). This is done by measuring the time required for the signal to travel from satellite to the receiver.

Calculating position and time

After calculating the distance from 3 or more satellites and their location, then the receiver can calculate its location by using the Trilateration method (see references).

The most common way GPS data is spit out by a GPS module is using the NMEA standard over a serial interface.

 

What specs/metrics should you be looking at when selecting a GPS receiver?

  • Accuracy

    • Highly dependent on where you are in the world, what time of day, weather etc.

    • Most modules can get down to +/-3m but getting to sub meter or centimeter accuracy gets very expensive

  • Size

    • In general, the smaller the receiver → the smaller the antenna → lower accuracy

  • Update Rate

    • The update rate of a GPS module is how often it recalculates and reports its position

    • The standard for most devices is 1Hz, though UAVs and other flying/fast vehicles require faster update rates to stay on track. Note that faster update rates means more NMEA sentences flying out of the module which could overwhelm the FMU.

  • Power Requirements

    • GPS modules can use a lot of power due to their computational load

    • The antennas will use an amplifier that draws extra power

  • Number of Channels

    • Since module doesn’t know which satellites are in view, the more frequencies that you can check at once the faster you’ll find a fix

    • After a lock/fix is achieved, some modules will shut down the extra blocks of channels to save power

  • Antennas

    • Chunk of ceramic on top of the module, which is finely trimmed to pickup the GPS L1 frequency of 1.57542 GHz.

    • Make sure you point the antenna to the sky!

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