GPS RECEIVER TRACKING PERFORMANCE UNDER IONOSPHERIC

SCINTILLATION CONDITIONS

*S. H. Skone*

# *Department of Geomatics Engineering*

*University of Calgary*

*2500 University Dr. NW*

## Calgary, AB

*T2N 1N4*

####### Summary

GPS receiver tracking performance can be degraded during periods of
enhanced ionospheric activity, where small-scale scintillation effects
(phase and amplitude variations) are observed in the high latitude
auroral region and the low latitude equatorial anomaly region. During
periods of intense scintillation, the availability of carrier phase
observations may be limited through loss of signal lock, with a
significant impact on precise positioning applications. Such effects
have a larger impact on the L2 tracking performance, where codeless and
semicodeless technologies are employed to extract the encrypted L2
signal.

The tracking performance of a given receiver depends not only on the
magnitude of scintillation activity observed, but also on the receiver
tracking capabilities. Recent research has shown that tracking
performance can vary significantly between receivers, under identical
scintillation conditions. In this presentation, the impact of
scintillation effects on various receiver tracking capabilities is
presented. A comparison of receiver technologies is conducted, using
both codeless and semicodeless GPS receivers. Performance comparisons
are established and interpreted with respect to GPS network applications
and GPS availability at solar maximum. The frequency and magnitude of
receiver tracking errors is also examined on a regional basis, for both
the high latitude and low latitude regions.

### 1. Introduction

Ionospheric scintillations are rapid random fluctuations of the phase
and field strength (amplitude) of an RF signal. Such effects arise from
RF propagation through regions where small-scale irregularities in
electron density exist. The rapid phase variations (referred to as phase
scintillations) cause a Doppler shift in GPS signals, which may exceed
the bandwidth of the phase lock loop (PLL) - resulting in a loss of
phase lock. Amplitude fades can cause the GPS signal-to-noise-ratio
(SNR) to drop below receiver threshold, resulting in loss of code lock.
These effects have a larger impact on tracking loops employing codeless
and semicodeless technologies (to extract the encrypted L2 signal)
versus full code correlation. In particular, codeless and semicodeless
tracking loops experience losses of 27-30 dB and 14-17 dB, respectively,
with respect to full code correlation, and are therefore more
susceptible to the effects of amplitude fading. The L2 PLL also employs
a narrower bandwidth (≈1 Hz, compared with ≈15 HZ for L1) to eliminate
excess noise, and is therefore more susceptible to phase scintillations.

Scintillation effects are strongest in the equatorial (±10-20°
geomagnetic latitude), auroral (65-75° geomagnetic latitude), and polar
cap (\> 75° geomagnetic latitude) regions. This paper focuses on high
latitude auroral and equatorial scintillations, which arise from
different physical phenomena. The auroral scintillations result from
geomagnetic or magnetospheric substorms, where energetic electrons
precipitate at altitudes of 100-150 km, resulting in optical emissions
("Northern Lights"). Such auroral intensifications have times scales on
the order of 15 minutes and, for intense events, multiple
intensifications can take place over a period of hours (Rostoker 1991).
The auroral oval can expand several degrees equatorward during such
events (i.e. over Northern Europe, Northern United States), and the
magnitude and frequency of substorms are functions of the solar cycle.

A feature of the equatorial ionosphere is the equatorial, or Appleton,
anomaly. This anomaly consists of two maxima in electron density,
located approximately 15° north and south of the magnetic equator. The
daily equatorial anomaly generally begins to develop near 0900-1000
local time, and reaches its maximum development at 1400-1500 (Huang and
Cheng 1991). In periods of solar maximum, however, the anomaly may peak
at ≈2100 local time, and patches of plasma depeletions (or "bubbles")
form in this postsunset region. Small-scale irregularities form within
these patches, becoming a source of intense scintillation effects.

The magnitude and frequency of such scintillation effects are
well-correlated with the solar cycle, which is expected to peak
mid-2000. Equatorial scintillations are strongest at solar maximum
during the months Oct.-Nov. and Feb.-Mar., while high latitude
scintillations are generally associated with specific storm events -
activity which tends to peak several years before and after the solar
maximum. The largest, most intense, high latitude events generally occur
several years after the solar cycle maximum (i.e. 2001-2003).

### 2. Analysis and Results

In order to establish the impact of scintillations on GPS receiver
tracking performance in the high latitude and equatorial regions, and
establish comparisons for different receiver tracking technologies,
several analyses were conducted. Two tests, and relevant results, are
briefly described in this Section.

#### 2.1 Impact of high latitude scintillations on GPS networks

![](media/image1.wmf){width="4.316666666666666in"
height="3.0388888888888888in"}A substorm event took place August 27,
1998, during which auroral disturbances were observed in all high
latitude local time sectors. The storm effects continued over a 24-hour
period, with the strongest effects observed during the interval 0-6 UT.
During this event, 0.20 Hz dual frequency observations were available
from two regional GPS networks: the SATREF network in Norway, and the
SWEPOS network in Sweden (Figure 1). The SATREF stations were equipped
with Trimble 4000 SSi (codeless) receivers and the SWEPOS stations were
equipped with Ashtech Z-12 (semicodeless) receivers.

**Figure 1. Reference stations in the SATREF and SWEPOS networks.**

Figure 2 illustrates the receiver tracking performance for each station
within the two networks, in terms of the percentage of corrupt L2 phase
observations (i.e. cycle slips, missing observations), for elevation
angles \> 15 degrees. It is evident that receiver tracking performance
is significantly degraded for the codeless (SATREF) versus semicodeless
(SWEPOS) receivers. This is primarily due to the large number of missing
observations for the codeless receiver, during reacquisition periods. It
is also observed that receiver performance is degraded more for the
higher latitude stations (Alesund and Trondheim -- latitudes \> 62°).
This is due to the increased number of satellite-receiver lines-of-sight
through the auroral region, for stations at higher latitudes. L1
tracking performance was not affected significantly during this event.

![](media/image2.wmf){width="3.001388888888889in"
height="2.8645833333333335in"} ![](media/image3.wmf){width="3.16875in"
height="2.8118055555555554in"}

**Figure 2. Percentage of L2 cycle slips and missing phase observations
for stations in the SATREF and SWEPOS networks.**

###### 2.2 Equatorial scintillations

![](media/image4.wmf){width="3.375in" height="3.3333333333333335in"}A
long-term study of equatorial scintillations was conducted, using data
from reference stations in the Brazilian RBMC geodetic network (Figure
3). This network operates in the equatorial anomaly region and network
data were available over the period 1998 -- April 2000, allowing a
comprehensive study of seasonal and solar cycle variations in receiver
tracking performance. The RBMC stations are equipped with Trimble
4000SSi receivers, and data were available at 15 s sample intervals.
Phase tracking statistics (L2) are shown in Figure 3.

**Figure 3. L2 phase tracking statistics for the Brazilian network,
during the period 1998-April 2000, in the local time sector 2000-2300;
and corresponding locations of the Brazilian stations.**

In Figure 3, a clear seasonal variation is observed, with peak
degradations in tracking performance during the months September-March.
These variations correspond to well-established seasonal variations in
the magnitude of equatorial scintillations (Aarons 1982). Percentages of
corrupt L2 observations are also significantly increased in late 1998,
versus early 1998 -- this is evidence of the solar cycle dependence.
Stations BRAZ and VICO are generally affected to a larger extent than
PARA and IMPZ. This is due to the higher number of satellite-receiver
lines-of-sight through the peak anomaly region for stations BRAZ and
VICO. In March 2000, however, stations VICO and PARA have the largest
percentage of corrupt L2 phase observations, due to a southward shift in
the anomaly peak. The spatial distribution of tracking errors during
March 2000 is shown in Figure 4. The region bounded by 15-25° S
latitude, and 2000-2300 local time, contains the
![](media/image6.wmf){width="4.75in" height="3.661111111111111in"}most
intense scintillation effects. Statistics for corrupt L1 phase
observations are on the order of 1-5 percent during this period.

**Figure 4. Average percentage of corrupt L2 phase observations for
March 2000, in latitude and local time.**

**3. Conclusion**

Degradations in receiver tracking performance (L2 signal) are observed
in both the high and low latitude regions. High latitude tracking
performance is degraded during auroral substorm events, with greater
impact at latitudes above 62° for codeles, versus semicodeless,
receivers. The frequency and magnitude of such events is expected to
peak in the years following solar maximum (2001-2003). Receiver tracking
performance in the equatorial region exhibits clear seasonal variations,
and a dependence on the solar cycle. Based on observed statistics, and
predictions of a solar maximum in mid-2000, it is anticipated that the
high levels of degraded tracking performance will continue in the
equatorial region until early 2002. Stations located close to the
anomaly peak are affected to a larger extent -- this region spans
approximately 1000 km N-S, covering a significant portion of the
Brazilian regional network.

### References

Aarons, J (1982) Global morphology of ionospheric scintillations. Proc
of the IEEE 70(4): 360-378

Huang Y-N, Cheng K (1991) Ionospheric disturbances at the equatorial
anomaly crest region during the March 1989 magnetic storm. J Geophys Res
96(A8): 13953-13965

Rostoker G (1991) Some observational constraints for substorm models. In
Kan JR, Potemra TA, Kokubun S Iijima T (ed) Magnetospheric Substorms.
Geophysical Monograph 64, AGU, Washington, D.C.