The GPS Observer
In the July 1996 issue of POB magazine, The GPS Observer column was “The Russian GLONASS Satellites.” Many changes have taken place since that time, so let me bring you up to date.
GLONASS—Then and NowThe former Soviet Union developed a satellite system similar in design to the U.S. Global Positioning System (GPS) called Global Navigation Satellite System (GLONASS). The Soviet constellation was to consist of 24 GLONASS satellites, the same as the GPS constellation. GLONASS, like GPS, is a military system. Unlike GPS, there were no premeditated measures for the precision dilution of navigation parameters. That means no Selective Availability (SA) or Anti-spoof (A/S). As we all know, SA was not removed from GPS until May 2, 2001.
I said in 1996 that interest in GLONASS would increase in the States since Ashtech (now Thales Navigation, Santa Clara, Calif.) had announced a single board receiver that integrates GPS and GLONASS. The Ashtech receiver has the ability to receive signals from both constellations. Since that time, other receivers have appeared on the market capable of integrating GPS and GLONASS, notably Topcon (Topcon Positioning Systems, Pleasanton, Calif.) and Javad. In my community firms using GPS/GLONASS receivers for real-time kinematics (RTK) are picking up three or four GLONASS satellites along with eight to 10 GPS satellites.
GLONASS versus GPSThere has been enough written about GLONASS that a comparison to GPS can be made. Without getting into signal structure and other technical matters, the similarities and differences between GPS and GLONASS are the following:
- Both systems broadcast on L-band frequencies. With GPS, the frequencies L1 and L2 are the same for all satellites: 1575 MHz for L1 and 1228 MHz for L2. With GLONASS, each satellite broadcasts on a different frequency: 1602 – 1615 MHz for L1 and 1246 – 1256 MHz for L2. The frequency ranges are close enough that a combined antenna can be used.
- Both systems broadcast codes. With GPS, the codes are unique for each satellite. All GLONASS satellites broadcast the same codes.
- GPS was scheduled to have 24 satellites in six orbital planes; there are 29 operational satellites at the time of this writing. GLONASS was scheduled to have 24 satellites in three orbital planes; there are seven operational satellites at the time of this writing. Three GLONASS satellites were launched Dec. 1, 2001. GPS orbital planes are denoted A, B, C, D, E and F. GLONASS orbital planes are given the numbers 1, 2 and 3.
- The time for one complete revolution of a GPS satellite is 11h 58m; the ground track repeats every sidereal day. GLONASS satellites are closer to the earth, making one revolution every 11h 15m. With GLONASS, the ground track repeats every 17 orbits, which is eight sidereal days.
- Block II and Block IIR GPS satellites are inclined 55o to the equator; GLONASS satellites are inclined 64.8o.
- The clock times for the two systems are different. GPS operates on Coordinated Universal Time U.S. Naval Observatory UTC (USNO). GLONASS did operate on Coordinated Universal Time Soviet Union UTC (SU); today it’s on UTC (Russia). I understand that all launch and usable dates are based on Moscow Time (Universal Time + three hours). All GLONASS satellites operate with cesium clocks; GPS satellites operate with a cesium clock or a rubidium clock.
- The geodetic coordinate systems are different. GPS works in the World Geodetic System of 1984 (WGS 84). The GLONASS coordinate system was SGS-85, but recent literature refers to the datum as PZ-90.
- The first GPS satellite was launched on Feb. 22, 1978; the first GLONASS satellite on Oct. 12, 1982.
- The satellite numbering systems are different. GPS used Pseudo Random Noise (PRN) numbers, which range from 1 to 40. GLONASS uses GLN numbers, which begin at 49. When I wrote the column in 1996, it was announced that they expected to launch 77, 78 and 79 that year. In October 2000, they launched 83, 84 and 85. On Dec. 1, 2001, they launched 86, 87 and 88.
“The Russian federation successfully launched three replacement satellites for its heretofore faltering GLONASS system on December 1. A Proton rocket placed two standard Uragan spacecraft and one Uragan-M, the next generation design with a longer orbit lifespan, and purported higher accuracy, into GLONASS Plane 2. When set operational, the new spacecraft will bring the number of operational satellites to nine, out of a nominal constellation of 24.
The launch was the first in more than a year for the economically strapped program. However, Russian space officials say they have the intention and the resources to slowly restore the system over the coming years. Officials associated with the program say they hope to have 10-12 operational satellites in place by 2003, and a complete constellation by 2007.”1
• GPS satellites are launched one at a time with a Delta II rocket from Cape Canaveral, Florida. As can be surmised from the previous paragraph, GLONASS satellites are launched three at a time by Proton launches from Baijkonur in Asia.
The Status of GLONASSWhat is the status of GLONASS? Russia says they have the resources to restore the system. In a previous issue of POB magazine, it was mentioned that China was giving financial support to GLONASS. As a conclusion, I can say that a GLONASS-only receiver may have problems with such a small constellation. But a GLONASS/GPS receiver can have from two to four additional satellites, and in the eyes of some users, that’s an advantage.
1 GPS World Magazine, January 2002.