GPS Modernization: Capabilities of the New Civil Signals

2 Integrity Machines: Real Time Error Bounds
To enable a host of safety benefits, civil aviation organizations worldwide are deploying
GPS augmentation systems that improve and measure the performance of GPS in real time. One
of these is a local area augmentation system (LAAS) and each LAAS serves one airport. The
other is a wide area augmentation system (WAAS) that serves continental areas. The
International Civil Aviation Organization (ICAO) has standards for both of these architectures.
The ICAO documents refer to the LAAS as a ground based augmentation system (GBAS), and
they refer to the WAAS as a space based augmentation system (SBAS) for reasons that will soon
be apparent.
Both WAAS and LAAS are differential GPS systems. They measure GPS performance
with GPS receivers at known reference locations. These reference receivers compare their GPS
measurements to those that should exist at their known locations. The differences are converted
to corrections and error bounds that are broadcast to participating aircraft in real-time. The
corrections are applied by the avionics and improve the accuracy of GPS from ten meters to one
meter or better.
LAAS is a local area differential GPS system, because all reference receivers are placed
on the property of the airport to be served. LAAS corrections are nearly perfect for aircraft
approaching the subject airport. The LAAS corrections and error bounds are broadcast to the
approaching aircraft using a line-of-sight VHF transmitter that is also located on the airport
property. The broadcast system is referred to as a VHF data link or VDL. ICAO refers to these
systems as ground-based augmentation systems (GBAS), because the data link is a terrestrial
radio.
WAAS is a wide area differential GPS system. In contrast to LAAS, WAAS reference
stations span continental areas, and WAAS develops a four dimensional correction for each
satellite. One element of this 4-tuple corrects the satellite clock and the remaining three correct
the satellite ephemeris (location). WAAS also sends a grid of ionospheric errors for the region
spanned by the SBAS ground system. These corrections are valid across the area spanned by the
reference stations, and so they are broadcast to users through a geostationary satellite with a large
coverage footprint. Since satellites are used for the data link, ICAO refers to such systems as
space-based augmentation systems (SBAS). The geostationary satellite modulates the WAAS
data onto a signal that is in the GPS L1 band and resembles the GPS signal, and this signal is
synchronized to GPS time. Consequently, the geostationary satellite serves two purposes: it is a
data link and it can augment the normal suite of GPS ranging measurements.
Even though LAAS and WAAS develop corrections to improve accuracy, their truly
essential purpose is to broadcast real-time error-bounds. These bounds are called protection
levels (PLs) and must over bound the true position error under all conditions and in real time.
The pilot uses the current PL to determine whether a particular operation is safe. If the protection
level is smaller than the alarm limit (AL) required for a particular operation, then the pilot may
fly that procedure. If the PL fails to bound the error, then the pilot may attempt a landing that is
not safe and integrity has failed. On the other hand, the PLs cannot be too conservative -
otherwise the full capability of the system will not be utilized.
The essential purpose of LAAS and WAAS is to provide real time error bounds or
protection levels (PL) that meet the following requirement.
(
According to the top equation, the protection level (PL) must overbound the true error
experience by the user ( ) with a probability of 1-10-7. The second equation develops this basic
requirement. As shown, the protection level is based on the measurements that are currently
available from the reference receivers (z). This data is used to ascertain the probability that
certain specified hazards may be in force. Example hazards include an active ionosphere,
satellite signal failures, signal reflections near the receiving antennas, and radio frequency
interference. ( ) Pr i H z is the probability that hazard i is true given the current data. The PL
should be the minimum value that almost certainly overbounds the true error given that the
system must operate in the presence of the specified list of potential hazards, { } 1
I
i i H = .
Importantly, LAAS and WAAS are structured so that individual countries retain control
over the safety of navigation within their borders. Indeed, each government has responsibility for
safe navigation within its borders, and today they rely on in-country, terrestrial equipment to
execute this sovereign responsibility. With LAAS and WAAS, they still have a national
mechanism to monitor and override the signals that come from a shared international satellite
navigation system. This mechanism is under sovereign control, but conforms to international
standards, so that an aircraft need not carry multiple sets of avionics simply because it crosses
political boundaries. WAAS became operational in the United States on July 10, 2003. The
Europeans and Japanese will deploy similar systems within a few years. Ground-based
augmentation systems will come on-line in a similar time frame.
3 Using Signals of Opportunity to Harden GPS
GPS signals travel 20,000 kilometers from medium-earth orbit to the surface of the Earth,
and so the received signals are extremely weak. Consequently, they are vulnerable to radio
frequency interference (RFI) and signal obstructions. The performance of the receiver depends
on the received carrier power to noise density ratio, which is nominally given by
The received signal is amplified by the gain of the user’s receiving antenna, GR, to yield the
effective carrier power C. Even in the absence of RFI, the GPS signal must still compete with
natural noise received by the antenna and generated in the front end of the receiver. The external
noise is given by kTA, where k is Boltzmann’s constant, and TA is the temperature of the external
noise. The receiver’s internal noise is given by kT0FLNALcable/filter, where T0 is a reference
temperature of 290 K, FLNA is the noise figure of the low noise amplifier (LNA), and Lcable/filter is
the loss of the cable and filter that precede the LNA. This noise density is denoted N0, which is a
power per unit bandwidth, and typical values are around –201 dBW/Hz.
If man-made signals other than GPS are being received, then we have radio frequency
interference (RFI) and the denominator contains an additional term I0, which is also a power per
unit bandwidth. If an obstruction attenuates the signal, then the numerator of our signal to noise
ratio suffers, and the received signal power is reduced by Lblock. The signal to noise ratio is
reduced to
( )
block R block R
0 0 0 0 LNA cable/filter 0
S S
A
L G P L GP C
N N I kT kT F L I
= =
+ + +
(4)
In general, I0 depends on the range to the source of the RFI and the details of the
propagation path. Figure 1 approximates the signal to noise ratio (C/N0) assuming that the RFI
transmitter is radiating 1 mW/MHz, and that their is no blockage (Lblock=1). As shown, the signal
to noise ratio suffers out to distances of ten kilometers.
Figure 1: Received Signal to Noise Ratio With RFI. A modest radiation of 1
mW/MHz can degrade GPS SNRs to ranges of 1 to 10 kilometers depending on
the propagation path from the RFI source to the GPS victim receiver.
Signal obstructions pose a similar challenge. More and more GPS users find themselves
in environments that block satellite signals. For example, cell phone users want to be able to
automatically communicate their position when they dial 911 to report an emergency. However,
they want this capability even if they are downtown or indoors. Under open sky, the C/N0 for
GPS satellites range from 40 to 50 dB-Hz. Downtown, C/N0 values range from 15 to 40 dB-Hz.
Indoors, even the strongest C/N0 is probably weaker than 25 dB-Hz (Van Roy, et.al.).
These twin challenges, RFI and signal obstructions, have motivated a large family of
counter-measures.
Civil aviation operations that use GPS have specified procedures for escaping the area
troubled by RFI. Others use inertial guidance to help escape or perhaps complete the operation.
Some simply require backup navigation aids such as Loran-C or distance measuring equipment
(DME).
Military applications of GPS frequently include beam steering antennas that selectively
null out interfering signals without appreciably degrading the signal strength from the GPS
satellites. Some of these advanced antennas leverage the direction of arrival of the offending
signal and other discriminate based on the polarization of the interfering wave. Such antennas are
called null-steering or controlled radiation pattern antennas (CRPAs). These antennas are a
subject of their own, and the interested reader is referred to Compton (1988), Applebaum (1976)
and Widrow (1967). Other military receivers incorporate inertial measurements to reduce their
tracking bandwidths and thus reduce the impact of RFI. The Joint Precision Approach and
Landing System (JPALS) will use both beam steering and inertial aiding.
Consumer applications may well augment GPS with range measurements to television
stations or to cell phone base stations. (Rabinowitz and Spilker, 2003). They also benefit from
GPS assistance data that is communicated by the cell phone signals. (Agarwal, et.al. 2002, Enge,
Fan and Tiwari, 2001; Taylor and Sennott, 1984; and Syrjarinne, 2001.) The assistance data
improves GPS coverage in urban areas by supplanting the fragile navigation message. The
replacement navigation message is most welcome, because signal blockages wreak havoc with
the navigation messages from the satellites. The cell phone network also delivers estimates of the
satellite Doppler, which is used to decrease the signal search area and thereby improves signal
sensitivity by enabling the mobile receiver to use longer averaging times. Finally, the cell phone
carrier can be used to frequency lock the local oscillator in the mobile receiver, thus improving
its stability and further reducing the Doppler search.
4 New Civil Signals
In the near future, GPS will provide an expanded signal set for both military and civil
users. Figure 2 shows the signals that are available today along with the new signals. The current
signals are described in Section 4.1, and the new signals at L2=1227.60 MHz and L5=1176.45
MHz are described in Sections 4.2 and 4.3 respectively. This paper focuses entirely on the civil
signals and capabilities. We spend little time with the old or new military signals.
Figure 2: New & Old GPS Signals: Power Spectrum. In 2000, GPS satellites
radiate C/A-coded and Y-coded signals at 1575.42 MHz, and a Y coded signal at
1227.60 MHz. By 2015, they will radiate civil signals at 1575.42, 1227.60 and
1176.45 MHz. They will also radiate new military signals at 1575.42 and 1227.60
MHz.
4.1 Today’s Civil Signal
The spectra of today’s GPS signals are shown in Figure 2. The signal from the kth satellite
can be described mathematically as follows:
(As shown, each satellite sends three rather similar signals. Any of these can be described
as the product of four terms: an amplitude 2P ; the navigation data, D(t); a spread spectrum
code, x(t) or y(t); and the radio frequency (RF) carrier cos 2 πft + θ ( ) or sin 2 πft + θ ( ). This
hierarchy is shown in Figure 3.
Carrier at 1575.42 MHz (L1)
1227.60 MHz (L2)
19 cm (L1)
300 m
Code at 1.023 Mcps (C/A)
Figure 3: Hierarchy of GPS Signals Showing Relationship Between the
Carrier, Code and Navigation Data. The C/A coded signal on 1575.42 MHz
is used as an example.

Article Source: http://www.articlesboard.com

1. N. Agarwal, J. Basch, P. Bechmann, P. Bharti, S. Casadei, A. Chou, P. Enge, W. Fong, N. Hathi, W. Mann, A. Sahai, J. Stone, J. Tsitsiklis and B. Van Roy, “Urban GPS: Algorithms for GPS Operation Indoors and Downtown,” to be published in GPS Solutions, 2002 2. A. Applebaum, “Adaptive Arrays,” IEEE Transactions on Antennas and Propagation, vol. AP-24, no. 5, pp. 585-598, 1976 3. F. Bauregger, “Novel Anti-Jam Antennas for Airborne GPS Navigation,” PhD dissertation, Stanford University Dep

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