GPS World September 2006 - 2

INNOVATION | RTK Systems where is the single-difference operator; wind-up
in the carrier-phase measurements The single-difference between anten- is
the distance between the transmit and will be investigated in the near
future. nas observation equation for carrier-phase receive antenna phase
centers; is the Single-Difference Observable. The measurements can be
expressed as: integer ambiguity; is the tropospheric following equations
are useful for qualita- delay; is the ionospheric delay where i = + i Ni +
T - I i + + LB tive assessment of the phase wind-up effects I = f / f I ; f
and are the frequency and 2 2 + RHCPi + PCVi + MPi + i , i = 1 or 2, 2 1 2
1 in the single- and double-differenced car- wavelength of the
carrier-phase measure- rier-phase measurements. 1 ments, respectively; is
the receiver clock bias; LB is the antenna line bias; RHCP is the phase
wind-up; PCV is the antenna MANY ADVANCES in GPS technology have occurred
since the first test phase center variation; MP is the multi- satellite
was launched in February 1978. Perhaps the most significant for path
contribution to the carrier-phase meas- applications requiring very high
accuracies in real time was the develop- urement; is receiver system
noise; and i ment of the technique known as RTK, or real-time kinematic.
indicates the L1 or L2 signal. In RTK positioning and navigation, a
reference station transmits carrier- The first step in obtaining the phase
phase and pseudorange data over a radio link to one or more roving sta-
wind-up observables is highpass filtering tions. At a rover, the reference
station data is combined with the rover the single-differenced
carrier-phase meas- data, resolving carrier-phase ambiguities, and the
rover's position is deter- urements. In this case, we can remove con-
mined in real time. Either single- or dual-frequency GPS receivers can be
stant components such as the integer am- used, with the dual-frequency
systems biguity and reduce low-frequency typically affording faster
ambiguity resolu- components such as the tropospheric and tion and higher
positioning accuracies over ionospheric delay, the inter-frequency bias,
longer distances. the antenna line bias, the antenna phase- RTK systems,
in common with other center variation, and multipath in EQUA- techniques,
are susceptible to biases and TION 1 . Differential in the time domain
errors such as ionospheric and tropos- single-differenced carrier-phase
measure- pheric refraction along with line-of-sight- ments correspond to
the output of the dependent phase-measurement effects INNOVATION INSIGHTS
with Richard Langley highpass filter as: including multipath, antenna
phase-center variation, and carrier-phase phase wind-up. d i = d + d RHCPi
+ d i, i = 1 or 2, Phase wind-up can This latter phenomenon may not be
famil- 2 iar to all readers. It is a bias introduced into where d i
includes receiver noise and the significantly degrade carrier-phase
measurements by the rota- residuals of low frequency components. tion of a
GPS receiver's antenna. There is The second step in obtaining the phase
also a contribution from the rotation of a system performace. wind-up
observables is generating the geom- GPS satellite's antenna as it orbits
about etry-free combination of L1 and L2 carrier- the Earth. phase
measurements as: In developing an RTK-based vehicle navigation system at
the University of New Brunswick UNB , we have observed a few instances
where the d GF = d 1 - d 2 = d RHCPGF + d GF phase wind-up due to rotation
of the rover receiving antenna can signifi- 3 cantly degrade system
performance. In this month's column, we'll look at As the geometry-free
combination can carrier-phase wind-up, introducing three wind-up
observables that al- remove common components in L1 and lowed us to
perform qualitative assessments of its effects on the UNB L2 carrier-phase
measurements namely, RTK system. One motivation behind such an assessment
is to determine the tropospheric delay, the receiver clock whether or not
we need to proceed to the next step of implementing al- bias, and the
antenna line bias , the resid- gorithms to correct for the effects of
phase wind-up. I am joined by Dr. uals of low-frequency components in Don
Kim, the chief architect and developer of the UNB RTK system, and d GF
will be further reduced. graduate student Luis Serrano. The phase wind-up
observable will be ob- tained by the integration of EQUATION 3 .
Innovation features discussions about recent advances in GPS technology
and its applications as well By using the same type of antennas to re- as
the fundamentals of GPS positioning. The column is coordinated by Richard
Langley of the duce the antenna phase-center variation and Department of
Geodesy and Geomatics Engineering at the University of New Brunswick, who
welcomes careful site selection to reduce multipath , your comments and
topic ideas. and for short baseline situations to reduce GPS World |
September 2006 2 www.gpsworld.com         

GPS World September 2006

Table of Contents for the Digital Edition of GPS World September 2006

GPS World September 2006 - 1
GPS World September 2006 - 2
GPS World September 2006 - 3
GPS World September 2006 - 4
GPS World September 2006 - 5
GPS World September 2006 - 6
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