GPS Q&A
By Francis Yuen

Q. I have heard that GPS signals are pretty complicated and contain some kind of navigation message. Can you briefly explain how a GPS receiver uses this data to calculate its position? G.I. Boulder, Colo.

A. Similar to FM radio stations, GPS satellites broadcast their signal over a sinusoidal carrier in two radio frequencies: 1575.42 and 1227.60 Megahertz (MHz); these two frequencies are generally known as L1 and L2 respectively.
      As the requirement of the signal structure is to allow the GPS receiver to distinguish between different satellites, the signal must be modulated so that each satellite would broadcast a distinct message sequence. A specially designed method was therefore developed and named the "pseudorandom noise" codes (PRN).
      There are two groups of PRN codes - the coarse/acquisition (C/A) code and the precision (P) code. The only differences between the two are the length of the code sequence and the rate of transmission. In essence, C/A code has a sequence of 1,023 bits long which repeats itself once every millisecond. P-code, however, has an extremely long sequence of bits which repeats itself once every 266 days.
      Within the C/A and P codes, there is a section containing the satellite navigation message. This message includes parameters which define the GPS week number, age of data, ionospheric model, UTC data, antispoof flag, health of satellites, clock and orbital parameter (also known as broadcast ephemeris), etc. By decoding these parameters, the GPS receiver is able to estimate the availability and location of the GPS satellites.
      To calculate the distance (or range) between the GPS antenna and the satellite, a GPS receiver generates the same PRN sequence for a specific satellite and compares the time (t) between the received GPS signal from the satellite and the time the GPS receiver generated that same signal. Since we know the speed in which the GPS signal travels (i.e. speed of light, c), according to our high school mathematics, distance equals speed multiplied by (t x c), we are able to obtain an estimated "range." Since the clock on board the GPS satellite is so much more accurate (and expensive!) than the one used in the GPS receiver, there exists a clock offset (error). Hence, the range in which we just calculated contains this inherent clock error that must be corrected - this range is generally known as the "pseudorange."
      Essentially, by knowing the pseudoranges and the positions of all the satellites being tracked using orbital parameters, the GPS receiver will then be able to compute the unknown antenna position on Earth.

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