The GPS Observer: GPS in 2008 and beyond.
March 31, 2008
The GPS constellations are now more than 30 years old. The first satellite, launched on Feb. 22, 1978, was in the Block I constellation. It was designated SVN 1/ PRN 4 and remained operational for 21.9 months. A total of 11 Block I satellites were launched from 1978 to 1985; one of the 11 failed to attain orbit. Since 1989, all satellites launched are designated as Block II.
To date, four different types of Block II satellites have been launched: II, IIA (Advanced), IIR (Replenishment) and IIR-M (Military). The first Block II satellite was launched on Feb. 14, 1989; the first IIA on Nov. 29, 1990; the first operational IIR on July 23, 1997; and the first IIR-M on Sept. 26, 2005. In early 2009, a fifth type is scheduled to be launched, Block IIF. The Block I and the first of the Block II satellites are no longer operational and have been taken out of service.
The GPS Constellation TodayMuch progress has been made to the GPS constellation since 1978. Another satellite was launched on Dec. 21, 2007. It was the fifth Block IIR-M satellite put on orbit and the second in 2007. Designated GPS IIR-18 (M), the satellite was placed in slot C1 and replaces Space Vehicle Number (SVN) 36 which, in turn, will replace SVN 37. The satellite became operational on Jan. 2, 2008, a record time from launch to operational status.
That’s the purpose of the Block IIR satellites. When an older satellite, usually a IIA satellite, starts to fail, it is replaced by a IIR-M satellite. Of the original 21 Block IIR satellites, 17 are on orbit and three are on the ground. (The first failed to attain orbit.) Of those three on the ground, plans call for launching two this year.
In all, there are 31 operational GPS satellites. The GPS Operational Control Segment of the U.S. Air Force is only able to control 31 satellites at a time, although it has plans to upgrade the ground segment of the system to increase the number of satellites that can be tracked.
Configuration of Satellites on OrbitFigure 1 shows the configuration of the Block II and IIA satellites--the first operational satellites. At press time, there were 14 operational IIA satellites. The first 12 Block IIR satellites had the same configuration.
The C/A code on L1 is the civil code used by the civilian community. These days it seems that just about everybody owns a “code receiver.” The C/A code generates pseudoranges, and when four or more satellites are visible at one time, the receiver calculates a position that, on average, is accurate to +/-10 meters--not bad for an investment of less than $200. The C/A code works well, but by today’s standards it’s old technology; this same code was on the first GPS satellite launched in 1978.
I’m sure you have heard the expression “if it ain’t broke, don’t fix it.” That’s how GPS was treated for 27 years. Then, in 2005, after years of deliberation, the Air Force agreed to the development of a modern civil code--L2C. The new code is positioned on the L2 carrier of the Block IIR satellites, as shown in Figure 2 on page 47. These satellites are designated Block IIR-M.
L2CThere are five satellites broadcasting the L2C code. Several manufacturers of survey-grade receivers sell units than can pick up L2C.
Unlike code receivers used by the general public, receivers sold to surveyors are carrier-phase receivers that work in pairs. In these receivers, the pseudorange gets the approximate distance to the satellite, and the receiver also measures the phase delay. Using sophisticated software, the ambiguities in the carrier waves are determined and, with the phase delay known, the distance to the satellite is determined to a high accuracy. Using this technology, two receivers observing at the same time can determine the difference in position to an accuracy approaching subcentimeter.
L2C will allow the receivers to determine pseudoranges at times where the C/A code can’t, and the signal-to-noise ratio will be better. But, according to the technical literature, the satellites in operation still need the C/A code. If a receiver is an L2C receiver only, significantly more effort is required to acquire the signal than an L1 C/A code receiver−more than 200 times higher in the case of hardware and more than 500 times in the case of software. Also, if both L1 and L2C are tracked, the power consumption of a receiver will double. However, in a dual-frequency receiver, the signal may be acquired by L1 C/A first, and then using a handover process, it can track L2C using a phase-locked loop.1 This gives the modern L2C signals slightly better tracking performance than the L1 C/A code by itself. For survey-grade receivers, a more accurate pseudorange will make it easier to resolve carrier ambiguities.
Block IIFIf there are only three Block IIR-M satellites left on the ground, what’s next? A new block of satellites called Block IIF (Follow-on) that are designed with three frequencies: L1, L2 and L5. Figure 3 is my rendition of the configuration. L5 is a civil frequency, not a civil code. I have seen it described as the third civil signal (L5) – BPSK(10). The full operational capability (FOC) of the Block IIF constellation is expected to be reached around 2015.
The IIF program has had technical difficulties from Day 1. The program originally called for 15 satellites with the first to be launched in 2005. The program is now showing 12 satellites with the first launch early in 2009. However, according to an ITT press release published Dec. 20, 2007, testing of L5 will begin with the launch of the next IIR-M satellite. The press release states: “GPS satellite navigation payload provider ITT Corp. has delivered a new demonstration payload for a modernized GPS Block IIR (GPS IIR-M) satellite to satellite builder Lockheed Martin for integration on the spacecraft and final system-level testing.”2 The demonstration payload will temporally transmit a third civil signal.
The signal, located on the L5 frequency (1176.45 MHz), will comply with international radio frequency spectrum requirements. Future generations of GPS spacecraft will include an operational third civil signal to improve the accuracy and performance capabilities of the system. The spacecraft with the demonstration payload, known as SVN 09, is one of the final three Block IIR-M satellites planned for launch this year, according to ITT. I noticed that the Air Force is now saying there will be three launches in 2008, whereas an earlier announcement called for two launches. Also, I noticed that the Air Force did not describe the code on L5. It sounds to me that, as with Galileo, the Air Force needs to have L5 operational before worrying about a code.
In addition to a $215 million IIF development contract, Boeing holds about $1.3 billion in contract awards to build the first nine IIF satellites. The Air Force also expects to order another three satellites from Boeing.3
Plans for More GPS Satellites?There will be a new group of satellites named Block III. The Air Force isn’t saying much about this since the first Block IIF hasn’t been launched yet. What I have heard is the first launch of Block III is scheduled for 2013. Block III will be in three orbital planes (Block II is in six planes), and the present C/A code will be replaced by a new code named L1C. Much is being written about this code but, as the Air Force says, let’s wait until Block IIF gets off the ground.
When sources talk about dates of 2013 and 2015, you don’t have to worry about the future of GPS. During the early days of Block II, the Air Force used the expression “Initial Operational Capability.” It meant that FOC hadn’t been attained but the satellites could be observed. Block IIF hasn’t been launched, but when it has, the Air Force will announce an IOC, probably after seven or more satellites are on orbit. You may have to upgrade older receivers after a period of time. Let’s wait and see what the receiver manufacturers tell us.
References1 Dempster, Andrew, “Correlators for L2C, Some Considerations,” Inside GNSS, September 2006.
2 “ITT Delivers IIR-M Payload Featuring Third Civil Signal,” GPS World Daily, Dec. 20, 2007.
3 “360 Degrees: USAF Announces More IIF Delay,” Inside GNSS, September 2006, 17.