The GPS Observer: Glonass Joins the 21st Century
When discussing Global Navigation Satellite System (GNSS) constellations, you will often hear “Glonass is similar to GPS.” Not quite. In fact, GPS and Glonass are mirror images of each other. Simply put, every GPS satellite broadcasts on the same two frequencies, but each satellite has unique codes. Every Glonass satellite broadcasts the same codes, but each satellite has two unique frequencies. GPS has code division multiple access (CDMA) signals, and Glonass has frequency division multiple access (FDMA) signals.
However, as reported in Inside GNSS magazine, Glonass and the other major GNSS constellations have started to follow GPS in adopting the CDMA signal structure. Let’s examine the difference between the two and how this can affect Glonass signals.
CDMA vs. FDMA
In my library, I have a book that might be called the bible of GPS. It’s a two-volume set titled Global Positioning System: Theory and Applications. The following information is based on Chapter 3, “GPS Signal Structure and Theoretical Performance,” by one of the book’s editors, James J. Spilker Jr. of Stanford Telecom in Sunnyvale, Calif.
The GPS signal falls into the broad category known as spread-spectrum signaling. Spread-spectrum signaling has the capability to provide a form of multiple-access signaling called code division multiple access (CDMA) wherein multiple signals can be transmitted in exactly the same frequency channel with limited interference between users if the total number of user signals is not too large. This multiple-access capability is important to GPS because a user receiver may simultaneously receive 10 GPS signals from 10 different satellites.
The simplest example of this is the hand-held receiver used by the general public. It only receives the L1 frequency, but if 10 satellites are visible, the receiver has 10 channels and can receive the signal from each satellite, calculate the pseudorange from the unique code of each satellite and give a position accurate to approximately +/-5 meters. To the best of my knowledge, there are few if any Glonass-only receivers that can operate like the GPS receivers for the general public, at least outside of Russia.
An alternative multiple-access technique is frequency division multiple access (FDMA). Selected for the Glonass navigation satellites, FDMA has the advantage of truly uncorrelating the civil signal by offsetting the carriers in frequency.
This approach occupies a larger bandwidth from a different code bandwidth, which is a disadvantage Glonass developers diminished by operating the civil signal at roughly half the clock rate of the GPS signal. However, decreasing the C/A-code clock rate provides a somewhat lesser accuracy. It also affects the differential GPS (DGPS) receivers that use GPS and Glonass. For example, last August, I attended a lecture in Houston where the speaker discussed the positioning processes for oil rigs in the Gulf of Mexico. He said that if firms that use DGPS for applications like offshore positioning use certain Glonass satellites with GPS, the positional accuracy is decreased. That’s because these folks are using the Russian civil code, which has half the clock rate. He went on to say that his company has to be careful in selecting which Glonass satellites are used.
Another disadvantage of the frequency-division approach is that the receiver would have to operate with several frequency offsets if several satellites were to be tracked simultaneously.
For surveying applications, most GPS receiver manufacturers have developed receivers that can receive signals from both the GPS and Glonass constellations. These are generally expensive dual-frequency receivers with a price range in the thousands of dollars. GPS, Galileo and Compass (the proposed Chinese GNSS) all want to have user receivers--not the expensive dual-frequency receivers--that can be used for navigation, search and rescue, 911 emergency and like applications, which is much easier to do with the CDMA signal structure. They can’t build a small, inexpensive receiver when each satellite has a different frequency.
Glonass-K and Other Improvements
The new generation of Glonass satellites, Glonass-K, is scheduled to be launched in late 2010. Like GPS launches, Glonass launches are notoriously delayed. Figure 1 on page 46 shows the different Glonass satellites in a Glonass modernization table. Figure 2 above shows Glonass specifications. At the present time, surveyors are observing Glonass and Glonass-M satellites.
The Glonass specifications shown in Figure 2 will change. The new M satellites now have improved atomic clocks. Additionally, there is a plan underway for the expansion of the ground control network including the establishment of new monitoring stations outside of Russia’s borders. The current Russian ground control segment can only handle 24 satellites, but they want to be able to handle a minimum of 30. (The GPS ground control network can handle 31 satellites and is also in the expansion phase.) One difference with Glonass-K will be an additional FDMA signal at the L3 frequency; for security reasons, the Russian authorities want to keep the FDMA signal structure on one frequency.
While Glonass has joined the 21st century, it isn’t yet up to par with GPS. If everything goes as planned, the United States will launch the first Block IIF satellite in the first half of 2009. Like Glonass-K, it will have a third frequency, but it will be a civil frequency named L5. Also, once launched, the modified Block IIR-M satellite will broadcast some test data on the L5 frequency (its scheduled launch in June 2008 didn’t occur). Even so, it will be years before there are enough satellites on orbit to make L5 useable. The same can be said for the L2C signal on the Block IIR-M satellites. But L2C will be available to users before L5.
I want to repeat that the lower clock rate on the Glonass civil signal is not a problem with survey-grade carrier-phase receivers. Russia admits that Glonass positioning and timing accuracy are currently about seven years behind that of GPS. And when they refer to positioning accuracy, they mean code point positioning accuracy. In surveying, we do relative positioning, not point positioning.
1- “Russia Approves CDMA Signals for Glonass, Discussing Common Signal Design,” Inside GNSS, April 28, 2008. www.insidegnss.com/node/648.
2- Global Positioning System: Theory and Applications. American Institute of Aeronautics and Astronautics Inc. Washington, D.C., 1996.