Differential GPS - Understanding GPS: Principles and Applications - page 449

gation data contains anomalies, a measurement quality monitor (MQM) to detect
anomalies in the measurements such as pseudorange steps, a multiple reference con-
sistency check (MRCC) to check the consistency of the corrections among the LGF
RRs, and a sigma monitor (SM) to check the nominal error characteristics of the
LGF.

LGF Antennas, Airport Pseudolites, and Data Broadcast
The presence of ground multipath at the LGF could introduce large errors in the air-
borne position and velocity computations. To mitigate the error due to ground
multipath, antennas can be designed that limit the multipath error. One example is
the integrated multipath limiting antennas [67]. To increase the availability of
LAAS, ranging sources may be added to the LGF such as airport pseudolites (APLs)
[68]. APLs transmit a GPS-like signal that can be processed by the RRs and aircraft
avionics in a similar fashion as the GPS signals [68]. Having a ranging source at the
ground significantly improves the geometry and therefore vertical accuracy. The
communications link used to transmit corrections from the LGF to the LAAS avion-
ics is a very high frequency data broadcast (VDB).

8.6.1.4 Precise Point Positioning
A technique referred to as precise point positioning (PPP) has emerged in the last
decade that provides decimeter-level position accuracies over very broad geo-
graphic regions. In PPP systems, the user's receiver does not use the clock and
empheris data broadcast by the GPS satellites. Instead, the user employs precise
clock and satellite position estimates computed and provided by an external net-
work such as the one organized by the International GPS Service (see Section
8.6.2.2). Here, we characterize PPP techniques as wide-area code-based DGPS,
although

others have classified PPP techniques in its category apart from

stand-alone and differential GPS.
A typical implementation of PPP is described in [69]. IGS clock and orbit data
(see Section 8.6.2.2) is used by a single semicodeless, dual-frequency GPS receiver at
an arbitrary location. In addition to estimating its three-dimensional position in
ECEF coordinates and clock bias (e.g., the standard four estimated parameters—see
discussion in Section 2.4.2), the receiver also estimates the zenith tropospheric path
delay (assumed to be the same for all visible satellites) and the bias between the
pseudorange and carrier-phase measurement for each visible satellite (modeled as
arbitrary real numbers). A sequential filter (e.g., a Kalman filter) is employed to esti-
mate these parameters iteratively as new measurements are obtained. To obtain
decimeter-level positioning results, the user equipment must account for a number
of error sources that are negligible for most other stand-alone or differential GPS
applications. These error sources include:

• Satellite antenna lever-arm: Most often, in orbit determination, it is the loca-
tion of the satellite center of mass that is estimated, not the satellite's antenna
phase center. The satellite antenna lever arm is the vector difference between
these two locations. The lever arm is over 1m in magnitude for the GPS Block
II/IIA satellites.