Select Page

Columnist Marcelo Santos talks about the benefits expected from the insertion of the L5 signal in the global positioning system

Part of the modernization process of the Global Positioning System involves changing the structure of the signals transmitted by the satellites. A new signal will become available to the civilian community, referred to here as L5, with a frequency equal to 1176.45 MHz. At the same time, there will be a change in the modulation of the L2 signal, which will have included the C / A code, as already happens with the L1 signal. Within a few years, the satellites to be placed in orbit will already transmit the signals in this new structure.

Characteristics of the signals in terms of frequency and wavelength.

There are several reasons for updating the L2 signal, and the appearance of the new L5 signal. Notably two, related to navigation: redundancy of signals aimed at increasing availability and reducing the risks of interference, and correction of the effects of the ionosphere. The fact that the L5 signal is within the protected navigation band makes it more attractive to correct the ionosphere effect than the L2, to be modulated by the C / A code, since the latter is more susceptible to interference.

For the community of users linked to Geodesy and topography, which use observations about the phase of the signal itself, the third signal is interesting for another reason: the resolution of ambiguity. The ambiguity, for any satellite, corresponds to the number of entire cycles, unknown to the receiver, at the beginning of the observation session. Because it is incognito for the receiver, ambiguity must be calculated or resolved, if centimeter-level precision is desired, for example, in differential applications using the carrier phase (RTK). The resolution of ambiguity, in these cases, is facilitated by combining linearly (that is, adding or subtracting) phase observations at two frequencies. Two combinations, proposed in the late 1980s, found utility in resolving ambiguity, the so-called wide-lane, and narrow-lane combinations.

The characteristics of these combinations can be seen in the Table. It can be seen that the wide-lane combination results in a new signal with a longer wavelength than the L1 and L2 signals. This characteristic is very important: in general, the longer the wavelength, the faster the ambiguity resolution. The narrow-lane combination participates in a complementary way in the process, whose objective is, in general, the resolution of the L1 carrier ambiguity. There is another method of solving the ambiguity that uses a search process to determine, among a family of combinations, which one offers the best result according to some pre-established criterion, for example, the least-squares criterion. It can be seen that the wide-lane combination results in a new signal with a longer wavelength than the L1 and L2 signals. This characteristic is very important: in general, the longer the wavelength, the faster the ambiguity resolution. The narrow-lane combination participates in a complementary way in the process, whose objective is, in general, the resolution of the L1 carrier ambiguity.

There is another method of solving the ambiguity that uses a search process to determine, among a family of combinations, which one offers the best result according to some pre-established criterion, for example, the least-squares criterion. It can be seen that the wide-lane combination results in a new signal with a longer wavelength than the L1 and L2 signals. This characteristic is very important: in general, the longer the wavelength, the faster the ambiguity resolution. The narrow-lane combination participates in a complementary way in the process, whose objective is, in general, the resolution of the L1 carrier ambiguity. There is another method of solving the ambiguity that uses a search process to determine, among a family of combinations, which one offers the best result according to some pre-established criterion, for example, the least-squares criterion. The narrow-lane combination participates in a complementary way in the process, whose objective is, in general, the resolution of the L1 carrier ambiguity.

There is another method of solving the ambiguity that uses a search process to determine, among a family of combinations, which one offers the best result according to some pre-established criterion, for example, the least-squares criterion. The narrow-lane combination participates in a complementary way in the process, whose objective is, in general, the resolution of the L1 carrier ambiguity. There is another method of solving the ambiguity that uses a search process to determine, among a family of combinations, which one offers the best result according to some pre-established criterion, for example, the least-squares criterion.

A note regarding the word lane, which means “wavelength”. Knowing this, one can appreciate the meaning of the adjectives wide (“wide”) and lane (“narrow”), meaning the “widening” or “narrowing” of the wavelength, with respect to carriers L1 and L2, when wide-lane and narrow-lane combinations are formed, respectively.

The advent of Third Signal

The advent of the third signal (L5), whose frequency is close to the frequency of the original signals L1 and L2, allows two new combinations to appear, according to the Table: the combinations (L1-L5) and (L2-L5). These two combinations have a longer wavelength than those of signals L1 and L2, and the combination (L2-L5) has a wavelength of 5.86 meters, 5 meters longer than the wide-lane combination! The ambiguity is resolved sequentially, that is, the ambiguity of the combinations with longer wavelengths is resolved, one by one, up to that of any of the three primary signals (L1, L2 or L5).

Simulations made indicate the possibility of resolving ambiguity after collecting a small sequence of observations (or even a single one), over distances short enough to ignore the effect from the ionosphere. In addition to the ionosphere, there is another limiting factor in this strategy: the effect caused by multi-path. For long bases, little advantage comes from the third frequency.

Pseudo-distance observations are welcome, in this ambiguity resolution strategy, when amalgamating with the carrier’s phase observations, in a softening process. This process of combining code and phase was originally proposed by Hatch in the early 1980s, which is why it is sometimes referred to as the Hatch filter.

It is also interesting to note that the same author, Ron Hatch, then a scientist at Magnavox, already advocated the advantages that a third frequency would bring, at a time when one could not imagine the revolution that GPS would cause. And even if a third signal ever became available.

The advantages arising from the future new structure of GPS signals are remarkable. In short and real-time RTK surveys, it will be possible to immediately resolve the ambiguity, with consequent centimeter accuracy, practically when turning on the receiver. It is possible to foresee a possible advantage also in air navigation, in the approach of the aircraft: if there is a reference receiver at the airport, the distance between it and the aircraft will be small (short base), and just at the right time. Also for navigation in general, access to a new, protected signal will bring advantages, by correcting the delay caused by the ionosphere, and redundancy.