GPS, being the pioneer of all Global Navigation Satellite Systems (GNSS), is the only system that has remained fully operational for civilian use for more than two decades. The number of civilian applications has exploded in recent years and its popularity is undisputed. Other GNSS have had issues (GLONASS during the end of the 90s and the 00s) or are still incomplete (Galileo, BeiDou). This helps explain the fact that a large percentage of the population is familiar with the term “GPS”, but not so for the other GNSS: GLONASS, Galileo, BeiDou. However, these other systems are expected to catch up in a few years so GPS must maintain its edge and remain competitive.
Since the beginning of GPS, technical and performance improvements have been regularly implemented in each new generation (called “Block”) of GPS satellites launched. However, due to the relatively long lifespan of the satellites (between 8 and 20 years), and the fact that satellites are replaced progressively (one or two per year on average), the effects of the performance improvements are gradual and often go unnoticed. During the first generations of GPS, most improvements concerned satellites and the equipment inside them (improvement in clock performance, improvements in satellite autonomy and design, etc.). However, beginning with Block IIR-M, whose first satellite was launched in 2005, new GPS signals have been added. These new signals have several improvements, which, from the user perspective, are interesting because they enable enhanced positioning accuracy and reliability. Before introducing these new signals, let me give you a brief summary of currently used (legacy) GPS signals.
Image: Block IIR-M
Legacy Signals: L1 C/A, L1P(Y) and L2P(Y)
From the beginning, GPS satellites have transmitted three main signals: one civilian signal (L1 C/A), and two encrypted military signals (L1P(Y) and L2P(Y)). As this implies, only one signal is available to civilian users. Military signals are transmitted in two different frequency bands: L1 (1575.42 MHz) and L2 (1227.60 MHz) and are encrypted. Having two bands is highly beneficial in improving accuracy and convergence time, since the undesired ionospheric effects can be greatly reduced. Since the 90s, designers of Geodetic-grade receivers have managed to access the encrypted L2P(Y) signal through a technique called “semi-codeless tracking”, and thus offer all the benefits of having two frequencies to civilian users. However, on the one hand, geodetic-grade receivers have a much higher price tag compared with low-end receivers, and, on the other hand, the techniques used to access the encrypted L2P signal introduce a slight signal degradation.
As I previously mentioned, the benefits of receiving a second GPS signal in another frequency band are quite significant. With the ever-growing demand of civilian GPS applications, the U.S. Department of Defense (DoD) decided to include a new civilian signal on the L2 frequency band called L2C, beginning with block IIR-M satellites. Instead of just using a signal identical to legacy L1 C/A and transmitting it on L2, the L2C signal was redesigned to provide several technical improvements when compared with the L1 C/A signal. The L2C uses more sophisticated and modern modulation techniques, which offer the following advantages when compared with the legacy signal:
- Increased sensitivity or tracking threshold, which translates into an improvement in maintaining the tracking of the signal in unfavourable conditions, such as with obstructions or even indoors.
- Greater cross-correlation between signals, which enables a stronger tolerance to interference and multipath.
Since the L2C is an unencrypted signal intended for civilian use, it is expected that even non-expensive, non-geodetic grade receivers will be able to receive and track the L2C signal.
Even though the L2C signal looks promising, we still have to wait a few years before being able to take advantage of it. As of July 2015, out of 30 operational satellites, only 15 transmit L2C. It is expected that all satellites will transmit the L2C signal by 2020. Additionally, even though the first L2C transmitting satellite was launched in 2005, Block IIR satellites began transmitting usable navigation messages (updated daily) on December 31, 2014. However, according to the U.S. government’s GPS Web page, “L2C should continue to be considered pre-operational and should be employed at the user’s own risk.” Once the full constellation of GPS satellites transmits the L2C signal, “The U.S. government encourages all users of codeless/semi-codeless GPS technology to plan on using the modernized civil signals by December 31, 2020, as P(Y) may change after that date.”
After Block IIR-M, the next generation of satellites, Block IIF, offers a third civilian signal on another frequency band, the L5 band (1176.45 MHz). As opposed to the L2 band, this radio band is reserved exclusively for aviation safety services, and, as such, is expected to be favoured by civilian users over L2 in the future. The first Block IIF satellite was launched in 2010, and, as of July 2015, 8 Block IIF satellites are in operation. The L5 band will be entirely used for civilian signals and free of military signals. Thus, a more powerful signal, with higher bandwidth, when compared with L1 and L2, is transmitted in L5. The main advantages of the L5 signal are:
- the possibility to track weaker signals through a longer code sequence and better cross-correlations between codes. Additionally, the transmitted power is higher.
- better immunity to interference with its greater bandwidth, and thus greater spectrum spreading.
- ionospheric effect attenuation using an L1/L5 combination, similar to the L1/L2 combination.
- instantaneous or near-instantaneous ambiguity resolution for quicker fixed solution convergence given the use of a triple frequency (L1/L2/L5) signal combination.
For a full L5 transmitting satellite constellation, we will have to be more patient than with the L2C full constellation. Whereas the latter is expected by 2020, the former will take several additional years.
Compared with modern signals L2C and L5 in operation, the L1 C/A signal is less performant. It is necessary to upgrade it to be on par with the modernized signals. This is where the next generation of GPS satellites, Block III, comes into play. This block of satellites will include another modernized civil signal, the L1C, which will eventually replace the legacy L1 C/A signal. The characteristics of the L1C signal are similar with those of L2C and L5. The design and manufacturing of some of Block III satellites have already been completed, but no launch has taken place yet. First launch is expected during 2016. However, given the current pace of satellite replacement (which can change depending on economic and political unforeseen circumstances), a complete GPS constellation with L1C signals should take at least another decade.
The Bottom Line
Since the beginning of the new millennium, GPS professionals have impatiently been waiting for all new GPS signals to be fully deployed. Although it will not magically resolve all our current woes such as performance degradation in obstructed areas or long convergence time for a fixed solution, I believe that GPS modernization looks very promising.
Legend: Spectrum Use of Legacy and New GPS Signals.
a) Legacy signals: L1 C/A, L1 P(Y) and L2 P(Y). Note that the civilian signal uses the I (in-phase) component and the military signals use the Q (quadrature) component so that they can share the same band.
b) New signals since Block IIR-M: L2C and since Block IIF: L5. Note that there are also new military signals (M) in L1 and L2. Also note that the L5 signal uses both components, I and Q.
c) New signal since Block III: L1C. Note that this signal uses a type of modulation (BOC) that enables it to share the band and in-phase/quadrature component with legacy and military signals.
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