Quiz Question

One of a continuing series

GPS.gov

GPS.gov

Last week’s Quiz Question concerned GPS (Global Positioning System). A major issue with GPS is the variable caused by ionospheric propagation delay. Free electrons in the ionosphere slow down the signal from the satellite to the receiver, and this effect varies from day to day and with location about the Earth. Since operation of GPS relies on knowing how long (within a nanosecond) it takes the radio signal to reach the receiver, it’s critical to compensate for the ionospheric delay.

Receivers able to receive the L2 (encrypted) channel can compensate for the ionospheric delay as described in last week’s Quiz Question. Without accurate compensation the position error can be off in the order of 15 meters. When the ionospheric delay is properly compensated, the error can be reduced to the order of a few inches.

GPS has been available to the public for over 20 years, and people like farmers use GPS navigation to plow their fields. Without decoding the L2 signal they can they compute the location of their tractors to a few inches? This is done with the use of Differential GPS (DGPS), and it works this way.

Set up a GPS receiver at a known point. Use another receiver on the tractor to navigate with a few inches accuracy. Neither receiver is able to decode the L2 signal. How does DGPS accomplish this accuracy?

Post your answer as a comment below.

Update and solution

Without researching into how DGPS is implemented, I will explain a possible method. The location of the reference receiver is known. This allows it to compute the ionospheric delay. Since the other GPS receivers (for example, on the farmer’s tractor) are nearby, the ionospheric delay is the same for them. If the active GPS receivers can receive the ionospheric delay from the reference receiver, they can compensate for it and obtain their location within a few inches.

The compensation for ionospheric delay is detailed in the GPS Interface Control Document, publicly available on the Internet. Here is the link:

http://www.gps.gov/technical/icwg/ICD-GPS-200C.pdf

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One thought on “Quiz Question

  1. Pingback: Quiz Question | Skeptical Analysis

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