Global Positioning System Reference
In-Depth Information
Another family of discriminator-based DLL variants proposed for GNSS is the so-called
Double-Delta (
) technique, which uses more than 3 correlators in the tracking loop
(typically, 5 correlators: two early, one in-prompt and two late) Irsigler & Eissfeller (2003). The
ΔΔ
ΔΔ
technique offers better multipath rejection in medium-to-long delay multipath Hurskainen
et al. (2008), McGraw & Braasch (1999) in good Carrier-to-Noise density ratio (C/N
0
). Couple
of well-known particular cases of
technique are the High Resolution Correlator (HRC)
McGraw & Braasch (1999), the Strobe Correlator (SC) Garin & Rousseau (1997), Irsigler &
Eissfeller (2003), the Pulse Aperture Correlator (PAC) Jones et al. (2004) and the modified
correlator reference waveform Irsigler & Eissfeller (2003), Weill (2003). One other similar
tracking structure is the Multiple Gate Delay (MGD) correlator Bello & Fante (2005), Bhuiyan
(2006), Fante (2003), Fante (2004), where the number of early and late gates and the weighting
factors used to combine them in the discriminator are the parameters of the model, and can be
optimized according to the multipath profile as illustrated in Hurskainen et al. (2008). While
coping better with the ambiguities of Binary Offset Carrier (BOC) correlation function, the
MGD provides slightly better performance than the nEML at the expense of higher complexity
and is sensitive to the parameters chosen in the discriminator function (i.e., weights, number
of correlators and correlator spacing) Bhuiyan (2006), Hurskainen et al. (2008).
ΔΔ
ΔΔ
Another tracking structure closely related to
technique is the Early1 / Early2 (E1/E2)
tracker, initially proposed in Dierendonck & Braasch (1997), and later described in Irsigler
& Eissfeller (2003). In E1/E2 tracker, the main purpose is to find a tracking point on the
correlation function that is not distorted by multipath. As reported in Irsigler & Eissfeller
(2003), E1/E2 tracker shows some performance improvement over
ΔΔ
technique only for very
short delay multipath for GPS L1 Coarse / Acquisition (C/A) signal.
Another feedback tracking structure is the Early-Late-Slope (ELS) Irsigler & Eissfeller (2003),
which is also known as Multipath Elimination Technique (MET) Townsend & Fenton
(1994). The simulation results performed in Irsigler & Eissfeller (2003) showed that ELS is
outperformed by HRC with respect to Multipath Error Envelopes (MEEs), for both Binary
Phase Shift Keying (BPSK) and Sine BOC(1,1) (SinBOC(1,1)) modulated signals.
A new multipath estimation technique, named as A-Posteriori Multipath Estimation (APME),
is proposed in Sleewaegen & Boon (2001), which relies on a-posteriori estimation of the
multipath error tracking. Multipath error is estimated independently in a multipath estimator
module on the basis of the correlation values from the prompt and very late correlators.
According to Sleewaegen & Boon (2001), the multipath performance of GPS L1 C/A signal
is comparable with that of the Strobe Correlator: slight improvement for very short delays
(i.e., delays less than 20 meters), but rather significant deterioration for medium delays.
In Phelts & Enge (2000a), a fundamentally different approach is adopted to solve the problem
of multipath in the context of GNSS. The proposed technique, named as Tracking Error
Compensator (TrEC), utilizes the multipath invariant properties of the received correlation
function in order to provide significant performance benefits over nEML for narrow-band
GPS receivers Phelts & Enge (2000a), Phelts & Enge (2000b).
One of the most promising advanced multipath mitigation techniques is the Multipath
Estimating Delay Lock Loop (MEDLL) Nee (1992), Nee et al. (1994), Townsend et al.
(1995) implemented by NovAtel for GPS receivers. MEDLL is considered as a significant
evolutionary step in the receiver-based attempt to mitigate multipath. It uses many correlators
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