On Feb. 15, 2008, an Associated Press article appeared in major newspapers across the country announcing that the Pentagon was going to fire a Navy missile to destroy a broken U.S. spy satellite before it re-entered the atmosphere.1

The purpose of the mission, officials said, was to minimize the risk to humans from the satellite’s toxic fuel. Launched in December 2006, the spy satellite became uncontrollable almost immediately after it lost power and its central computer failed. It was in a polar orbit, which means the inclination (the angle the orbital plane makes with the Earth’s equator) was 90°. The satellite must have been in a highly elliptical orbit because, within two years, it was well below the altitude of a normal satellite.

The U.S. Navy successfully destroyed the satellite, according to a report I read on Feb. 21. The report stated, “Destroying the satellite’s onboard tank of about 1,000 pounds of hydrazine fuel was the primary goal, and a U.S. official earlier told NBC News that it ‘looks like the tank was hit.’”2

Figure 1. A June 2000 status report of GPS launches.

The Tie-in to GPS

All GPS satellites have onboard fuel tanks, probably filled with hydrazine fuel, that are used to “move” the satellite as needed. I know of two cases when satellites are moved: to change slots in an orbital plane and to correct attitude.

When launched, a GPS satellite is placed in one of six orbital planes inclined 55° to the equator. Once positioned on orbit, a satellite cannot be moved into another orbital plane, but it can be moved to another position in the same orbital plane. When this happens, a note appears in the daily U.S. Coast Guard GPS status report. (Other private firms and universities also publish status reports but not daily.) If the move is planned, it will be announced at least one day in advance.

In the late 1980s, people like me, who were surveying daily with GPS receivers, noticed when a satellite that was supposed to be operational was not. One particular time, I noticed a satellite that was not working, but the GPS status sheet for that day did not note that the satellite would be turned off. The following day, a note did appear in the status report noting that the satellite would be set unhealthy until further notice. Three weeks later, the status report said the satellite status would be changed to healthy and become operational. About three months after that, I had the chance to tour the GPS master control station at Falcon (now named Schriever) Air Force Base just east of Colorado Springs, Colo. The tour members were escorted into the control center by an Air Force lieutenant colonel, so I asked him about the satellite from three months earlier. He laughed and said it was an error on their part--they noticed an attitude change needed to be made, which is done by giving a shot of fuel to the thrusters. He used the word “psst” to describe holding down the control button for this operation. He said the control button was held down too long (“psssst”), and the satellite started to tumble. All transmissions were shut down, and it took about three weeks to get the satellites stabilized, the solar panels pointing toward the sun and antennae pointing toward Earth.

GPS satellites do fail but not on a regular basis. To date, 11 Block I, 28 Block II/IIA and 19 Block IIR/IIR-M satellites have been launched, and only two failed to attain orbit. Of the 56 satellites successfully launched, 31 are operational, which is the maximum permitted by the existing ground control segment. The 25 not in operation failed for a variety of reasons, but most failed long after their design life of seven and one-half years. One satellite on orbit today has been in operation since Aug. 30, 1991--almost 17 years.

Figure 1 on page 47 shows a status report downloaded from the U.S Coast Guard Web site on June 15, 2000, that details the status of Block I and the first eight satellites of Block II at that time. No Block I satellites were operational, and two of the Block II satellites were not operational. The “Nav Lost” column lists the date the satellite was decommissioned, and the “Reason” column tells why it failed. Four satellites failed due to clock problems, four due to stabilizing wheels, two due to the Emergency Positioning System (EPS) failing and one due to the Tracking, Telemetry & Communications (TT&C) system failing. Today, information like this is hard to find since the status reports list only the satellites that are operational. As mentioned in my last column (April 2008), there are three Block IIR-M satellites on the ground that will replace the next three satellites that are decommissioned. The decommissioned satellites, however, are still orbiting the Earth. In fact, the boosters that put the satellites on orbit may also be in orbit. There’s a tremendous amount of space junk in the sky.

Will these older decommissioned GPS satellites return to the Earth’s atmosphere as did the wayward spy satellite? No, not in our lifetime. The spy satellite was in a much lower orbit, perhaps lower than planned. If the orbital plane is highly elliptical so that the satellite is close to the Earth at one part of the orbit, the Earth’s gravitational attraction will eventually pull it into the atmosphere and it will burn and fall; this is what was about to happen with the spy satellite before it was destroyed. Some scientific satellites are planned to be in a low, highly elliptical orbit with a life span of about one year. But at approximately 12,000 miles, GPS satellites are much higher and their orbit, though elliptical, is near circular.

The higher the satellite is, the longer its orbital period will be. GPS satellites make one revolution around the Earth approximately every 11h 58m. Communication satellites make one revolution approximately every 23h 56m; they rotate at the same rate as the rotation of the Earth. The moon, which is an Earth satellite, has an orbital period of approximately 27.3 days. The space shuttle, in the past, has been in orbit where the orbital period was 90 minutes--about 150 miles above the Earth.

Figure 2. GPS satellites have a near circular orbit as shown on the right. Other satellites need a more elliptical orbit as shown on the upper left.

Satellite Launches and Orbits

A short treatise on launching satellites is probably in order to better understand these principles. Let’s start with the first basic law of motion developed by Johannes Kepler (1571-1630):

All planets are in an elliptical orbit about the Sun, with the Sun at one foci of the ellipse.

Although Kepler’s laws were derived from observations made on planets, the same law applies to satellites orbiting the Earth. Figure 2 shows two different ellipses. The ellipse on the lower right can be thought of as a near circular ellipse. To relate these to an orbit, a red dot is placed to show the position of the Earth and a green dot to show a satellite. This is Kepler’s first law. When a satellite is launched, say a GPS satellite from Florida, the first stage of a Delta II rocket takes the rocket assembly to approximately the desired height, about 12,000 miles. The first stage burns out and is ejected from the rest of the rocket; the remainder of the rocket coasts to its highest position and levels off. At that time, the second stage of the rocket ignites and sends the assembly horizontally at a high rate of speed. When it reaches the speed for the desired orbit, the satellite is ejected from the nose cone of the rocket. From then on, ground control takes over using telemetry to move the solar panels and correct the attitude, etc. When everything is correct, ground control activates the satellite to make it healthy.

GPS satellites have a near circular orbit similar to that shown in the lower right of Figure 2. For other types of satellites that need to be in a more elliptical orbit, the second stage of the rocket sends the assembly at a higher speed making the orbit look like that shown in the upper left of Figure 2.

Growing Constellations

We refer to GPS satellites as the GPS constellation. A friend of mine, a retired Air Force man who spent much of his career with Air Force satellites, tells me a constellation is defined as two or more satellites doing the same thing. The present GPS constellation is Block II (IIA, IIR, IIR-M and, in a year or so, IIF), and the next constellation will be Block III. Another constellation is Russia’s Glonass constellation. The European Union’s Galileo, however, has only one satellite on orbit at this time. China has satellites on orbit, but these are not part of the constellations used by North American surveyors. Because of this broad spectrum, you will notice me using the acronym GNSS (Global Navigation Satellite System) regularly in the future.