Global Navigation Satellite Systems (GNSS) — Page 307, Lesson 305

We're starting a new topic now: Global Navigation Satellite Systems, or GNSS. This is the umbrella term for satellite-based navigation, and we're going to look at the signal structure of GPS first, then contrast it with GLONASS. Let's begin with the GPS signal. Each satellite broadcasts on two carrier frequencies. The L1 carrier is at 1575.42 MHz, and the L2 carrier is at 1227.6 MHz. Think of these as the radio waves that carry all the navigation information. Modulated onto these carriers are several components. First, there's the C/A code, which stands for Coarse/Acquisition code, running at 1.023 MHz. This is the civilian code. Then there's the P code, or Precise code, at 10.23 MHz, which is ten times the rate of the C/A code. And there's the navigation and system data message, which is transmitted at 50 Hz. Now, here's a critical point for you as a professional pilot: only the C/A code is available to civilian users. The P code is reserved for the US military, approved civilian users, and foreign military users, all at the discretion of the US Department of Defense. When anti-spoofing measures are implemented, the P code is designated as the Y code. The Y code is encrypted, so it's only available to users with the necessary decryption algorithms. Anti-spoofing means preventing someone from transmitting a fake signal to trick the receiver. The PRN codes—that's Pseudo-Random Noise codes—serve two vital functions. They provide satellite identification, so the receiver knows which satellite it's hearing, and they provide a timing function for the receiver to measure the satellite's range. That timing function is the basis of how the receiver calculates distance. Now, the navigation and system data message contains specific information. It includes the satellite's position, the satellite's clock time, the satellite's clock error, information on ionospheric conditions, and supplementary information. That supplementary information includes the almanac, which is the orbital parameters for all the satellites, satellite health—though that's P-code only—correlation of GPS time with UTC, and other command and control functions. GPS provides two services. The Standard Positioning Service, or SPS, uses the C/A code. The Precise Positioning Service, or PPS, uses both the C/A and P codes. So the difference between the two services is essentially which codes you have access to. Now let's shift to GLONASS, the Russian system. It also has an operational constellation of 24 satellites, but they're positioned in three orbital planes inclined at 65° to the equator. The orbital height is 10,313 nautical miles, which is 19,099 kilometers, giving an orbital period of 11 hours 15 minutes. Here's the key difference from GPS. In GLONASS, the codes are the same for all satellites, but each satellite uses different frequencies. The L1 frequency is incremental from 1602 MHz, and the L2 frequency is incremental from 1246 MHz. So GPS uses the same frequency for all satellites with different codes, while GLONASS uses different frequencies for each satellite with the same codes. That's a fundamental design contrast you need to remember. Let me show you the signal structure visually. This figure shows how the L1 and L2 signals combine through a mixer and a modulo-to-sum process. The L1 carrier at 1575.42 MHz carries the C/A code, the P code, and the navigation data. The L2 carrier at 1227.6 MHz carries the P code. The reason the two frequencies matter will come up when we discuss GNSS errors, but for now, understand that having two frequencies allows the receiver to correct for ionospheric delay, which is a major source of error. So to summarize what we've covered: GPS uses L1 and L2 carriers, with C/A and P codes, and the P code becomes the encrypted Y code under anti-spoofing. The PRN codes provide identification and timing. The data message carries satellite position, clock info, ionospheric data, and the almanac. And GLONASS differs by using the same codes on different frequencies per satellite, with a different orbital configuration.