GPS has one civil signal available on L1. The Galileo E1 signal is composed of three components centred on the same 1575.42MHz carrier frequency, two open signals and one secure signal, spread by different codes. There is the opportunity to exploit both open signals
The three components are each designed to provide a distinct service to the navigational user.
At an application level, the E1A signal will provide a wideband signal for superior precision, which is secured and unavailable for public use. The open E1B signal provides the navigation data e.g. ephemeris and satellite status. E1C is also an open signal, providing a pilot tone to help reception in low signal to noise environments. The E1B and E1C channels are often known as data and pilot respectively.
The signal definitions for the GIOVE-A signals are specified in the Interface Control Document (ICD) [Galileo Project Office 2007]. The signals transmitted by the GIOVE-A satellite are representative of the future Galileo signals with some minor differences.
The design of each GNSS signal has been driven by the aim of best navigational
performance, as GNSS reflectometry is an application distant from the intended use, then differences between the signals need to be reassessed. The most significant differences when moving from a GPS L1 receiver to GIOVE-A E1 are the modulation changes from Binary Phase Shift Keyed (BPSK) to Binary Offset Carrier (BOC), this causes the bandwidth to almost double. In addition the PRN code length is increased: the GPS C/A code is 1 ms long, GIOVE-A’s two open codes, E1B and E1C are 4 ms and 8 ms long respectively (the Galileo full operational constellation has been specified to use 4 ms on both E1B and E1C as well as some other differences [Galileo Project Office 2010]).
The broadcast signal 𝑠𝐸1(𝑡) is made up of the three sub-signals (E1A, E1B and E1C) and an intermodulation product, 𝑚𝐸1(𝑡).
𝑠𝐸1(𝑡) = √2 cos(2𝜋𝑓𝐿𝑡) ⋅ (𝑐𝐸1𝐵(𝑡) ⋅ 𝑠𝑐(𝑡) ⋅ 𝑑(𝑡) − 𝑐𝐸1𝐶(𝑡) ⋅ 𝑠𝑐(𝑡) ⋅ 𝑐𝑆(𝑡))
+ 𝑖 √2 sin(2𝜋𝑓𝐿𝑡) ⋅ (2𝑐𝐸1𝐴(𝑡) + 𝑚𝐸1(𝑡)) (4.25)
The carrier at frequency, 𝑓𝐿 = 1.57542 GHz, is modulated with a binary phase shift due to the modulation terms: 𝑐𝐿1𝐵(𝑡), 𝑐𝐿1𝐶(𝑡), 𝑠𝑐(𝑡), 𝑑(𝑡), 𝑐𝑠(𝑡) ∈ (−1,1).
The E1A component is a secure wideband signal where 𝑐𝐸1𝐴(𝑡) contains its own sub carrier, data terms and an unpublished code and is on the orthogonal carrier phase to the E1B and E1C codes. The intermodulation product, 𝑚𝐸1(𝑡), ensures a constant amplitude modulation before high power amplification on board the satellite.
The open components are modulated using different PRN codes, 𝑐𝐸1𝐵(𝑡) and 𝑐𝐸1𝐶(𝑡), which for Galileo are stored memory codes, with a sequence specific to each satellite. For the
E1A,B signals the sub carrier term, 𝑠𝑐(𝑡) provides the Binary Offset Carrier (BOC) sub-modulation. The sine phase BOC(1,1) sub carrier is used, which resembles that shown in the lower part of Figure 4.26. The effect of the sub-carrier is to broaden the bandwidth of the signal and thus improve the ranging resolution of the auto-correlation function.
Figure 4.26 BOC(1,1) modulation components. PRN code c(t) and subcarrier sc(t)
In addition to these high-rate modulations, the E1B component is also modulated by the data bit, 𝑑(𝑡) of 250 symbols per second. The pilot component, E1C, is modulated by a secondary code 𝑐𝑠(𝑡), which is a 25 chip long sequence that repeats every 200 ms.
The BOC(1,1) modulation has an auto-correlation function compared to GPS L1 is shown in Figure 4.27. The higher resolution is evident and beneficial in GNSS-R, however the large side-lobes would usually be considered a problem in radar signal designs.
Figure 4.27 Correlation function of the BOC(1,1) (blue) and BPSK (red) spreading codes.
The code lengths and chipping rates are reproduced here in Table 4.1 for reference, these are compiled from the GPS and GIOVE-A ICDs [GPS Directorate 2011; Galileo Project Office 2007].
Table 4.1 Comparison of GPS and GIOVE signals centred on 1.57542GHz. (Excluding secured signals)
The model of the GNSS-R reflection will be taken from Section 2.2.1 and modified to represent the E1 signal.
The signal input to the receiver has been distorted by the reflection off the ocean, and is buried in noise. This shall be modelled as a set of E1 signal sources (𝑠𝐸1(𝑡) from Equation (4.25)), each distributed over the surface so that they have some phase, 𝜙𝑖, Carrier frequency, 𝑓𝐿, Doppler shift, 𝑓𝐷,𝑖, propagation delay 𝜏𝑖, and amplitude 𝐴𝑖
𝑠𝑟𝑥(𝑡) = ∑ (𝐴𝑖∙ 𝑠𝐸1(𝑡 − 𝜏𝑖) ∙ exp(𝑗2𝜋(𝑓𝐿+ 𝑓𝐷,𝑖) + 𝜙𝑖))
𝑖=0,1…
+ 𝑛(𝑡) (4.26)
To bring the signal out from the dominant noise term, n(t), the signal needs to be mixed down from the Doppler shifted carrier frequency, (𝑓𝐿+ 𝑓𝐷,𝑖), and de-spread by correlating with an internal replica 𝑠𝐸1. For a navigation receiver, the replica cannot be produced as in Equation (4.25) for the full 𝑠𝐸1 signal, due to the unknown data bits 𝑑(𝑡) and not necessarily a
synchronised secondary code 𝑐𝑠(𝑡).
The receiver can correlate the combined PRN code and sub-carrier for the B and C channels separately. This forms the following two correlation results for a ray of the reflected signal,
𝑤𝐸1𝐵(𝑡′, 𝑓′) = 1
Code length (chips) Data symbol
𝑤𝐸1𝐶(𝑡′, 𝑓′) = 1
𝑇𝑐𝑜ℎ∫𝑇𝑐𝑜ℎ𝑠𝑟𝑥(𝑡)𝑐𝐸1𝐶(𝑡 − 𝑡′)𝑠𝑐(𝑡
0
− 𝑡′) exp( 𝑗2𝜋(𝑓𝐿+ 𝑓𝐷− 𝑓′)𝑡 + 𝜙) 𝑑𝑡 + 𝑛𝑤,𝐶 (4.28)
using a coherent integration time of 𝑇𝑐𝑜ℎ and trial delay and Doppler of 𝑡′ and 𝑓′
respectively. The post-correlation noises are separated out as 𝑛𝑤,𝐵 and 𝑛𝑤,𝐶 for convenience.
Performing these correlations for a range of trial delays 𝑡’ and Doppler frequencies 𝑓′, builds up a map of the distortion caused by the reflection and resulting in a coherent DDM as discussed for the GPS signals in Section 2.3.1. Recovery of the complete signal power from E1 will be necessary if a GNSS-R receiver is to use Galileo transmitters in addition to GPS to increase remote sensing coverage.
Focusing in on the modulation terms of the E1B correlation: 𝑠𝑟𝑥(𝑡)𝑐𝐸1𝐵(𝑡 − 𝑡′) 𝑠𝑐(𝑡 − 𝑡′) and expanding out the received signal,
(𝑐𝐸1𝐵(𝑡 − 𝜏) ⋅ 𝑠𝑐(𝑡 − 𝜏) ⋅ 𝑑(𝑡 − 𝜏) − 𝑐𝐸1𝐶(𝑡 − 𝜏) ⋅ 𝑠𝑐(𝑡 − 𝜏) ⋅ 𝑐𝑆(𝑡 − 𝜏))
⋅ 𝑐𝐸1𝐵(𝑡 − 𝑡′) 𝑠𝑐(𝑡 − 𝑡′) (4.29)
𝑐𝐸1𝐵(𝑡 − 𝜏) ⋅ 𝑠𝑐(𝑡 − 𝜏) ⋅ 𝑑(𝑡 − 𝜏) ⋅ 𝑐𝐸1𝐵(𝑡 − 𝑡′) ⋅ 𝑠𝑐(𝑡 − 𝑡′)
−𝑐𝐸1𝐶(𝑡 − 𝜏) ⋅ 𝑠𝑐(𝑡 − 𝜏) ⋅ 𝑐𝑆(𝑡 − 𝜏) ⋅ 𝑐𝐸1𝐵(𝑡 − 𝑡′) ⋅ 𝑠𝑐(𝑡 − 𝑡′) (4.30)
Inside the integration, we can use the relation that as the E1A, E1B and E1C codes are
different pseudo-random bi-phase sequences, so that for all replica delays, 𝑡′ and propagation delays 𝜏:
〈𝑐𝐸1𝐵(𝑡 − 𝜏)𝑐𝐸1𝐶(𝑡 − 𝑡′)〉 ≅ 0
〈𝑐𝐸1𝐵(𝑡 − 𝜏)𝑐𝐸1𝐴(𝑡 − 𝑡′)〉 ≅ 0
〈𝑐𝐸1𝐶(𝑡 − 𝜏)𝑐𝐸1𝐴(𝑡 − 𝑡′)〉 ≅ 0
This causes the E1B correlation of Equation (4.30) to simplify down to, 𝑐𝐸1𝐵(𝑡 − 𝜏) ⋅ 𝑠𝑐(𝑡 − 𝜏) ⋅ 𝑑(𝑡 − 𝜏) ⋅ 𝑐𝐸1𝐵(𝑡 − 𝑡′) ⋅ 𝑠𝑐(𝑡 − 𝑡′)
(4.31)
The replica E1B signal does not include the data, 𝑑(𝑡), as the bit phase is unknown to the receiver, so it remains outside the correlation result: 𝑑(𝑡 − 𝜏) ∙ 𝑤𝐸1𝐵(𝑡′, 𝑓′). Equivalently for
the E1C cross-correlation, the phase of the secondary code remains unknown, so is left outside the result: −𝑐𝑠(𝑡) ∙ 𝑤𝐸1𝐶(𝑡′, 𝑓′).