Differential GPS - Understanding GPS: Principles and Applications - page 419

tion time of at least 1 hour was driven by the need to have sufficient movement in
the GPS satellite constellation to allow the carrier-cycle integer ambiguities to be
resolved. Another key consideration was the overall lack of GPS satellites, which
eliminated the use of redundant measurements for resolving the carrier-cycle
ambiguities. In this pioneering work, the ambiguity function method was used for
determining the integer-cycle ambiguities.
Once the level of accuracy using GPS interferometric techniques was estab-
lished, it became a natural desire to improve the efficiency of their application. The
technique of kinematic surveying came into being as a result. Here, through use of a
known survey point and an existing baseline, the carrier-cycle ambiguities are first
determined. One technique that can be used to do this quickly is an antenna swap,
wherein GPS data is collected for several minutes at each end of the baseline, the
receivers/antennas are then exchanged without losing SV lock, and another period
of GPS data is collected. Several minutes of GPS data, with epoch times on the order
of 10 seconds, are required during each occupation period to collect sufficient data
to resolve the ambiguities. Four (and preferably more) SVs yielding improved satel-
lite geometry are required to accomplish this. Subsequent to the antenna swap, one
receiver/antenna is moved to each of the points making up the survey. Generally, the
receiver/antenna at the known survey point becomes the control point (base station)
for the survey and the other becomes the rover. Following a 1- to 2-minute occupa-
tion of each survey point, the rover is returned to its initial starting location to pro-
vide data for closure of the overall survey. In all instances, it remains necessary to
have continuous track on a minimum of the same four (but preferably more) GPS
satellites. The GPS data is postprocessed, and the survey results are calculated. For
baselines of up to 10 km, the effects of the ionosphere are minimal and centimeter-
level accuracies can be expected. There are variations on the static and kinematic
surveying methods, but generally the resulting accuracies remain at or near the cen-
timeter level. Furthermore, it is the kinematic method that allows for the extension
of GPS interferometric techniques to the near-static or low-dynamic environment
mentioned previously.
Nowadays, with a complete GPS constellation of 24 SVs and the availability of
low-noise receivers that can track both the L1 and L2 P(Y) codes, it has become pos-
sible to resolve the carrier-cycle ambiguities without the need for either the
presurveyed baseline or an initial period of GPS data collection (e.g., the antenna
swap procedure). The term applied to this technique is on the fly. Implicit in this
approach is differential, carrier-phase integer-cycle ambiguity resolution. As a rule,
the base station broadcasts either differential corrections or raw measurement data
over a datalink, and the rover computes its position relative to the base station by
combining its own measurements with the information received over the datalink.
Such an implementation reduces the dependence on postprocessing and permits the
user to know immediately whether or not the survey is progressing in a successful
manner. In most instances, the base station is located at a precisely known surveyed
point; thus, the rover can determine its absolute position (i.e., latitude, longitude,
and elevation), since it has calculated the baseline vector between it and the base sta-
tion. Accuracies on the order of 10 cm ( /2 at L1) are achievable in near real time

with the rover in motion. With longer occupation times, the accuracy will increase
as multipath tends to average out.

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