Microwave Antenna
Precise GPS positioning depends upon an antenna for receiving the
satellite signals, which has a well-defined and reproducible phase center.
We must know the exact spatial offset between the surveyed monument
reference point and this phase center. The actual antenna is a small
electrical element beneath the plastic dome in the center. The concentric
metal rings which make the antenna look like a "UFO" serve to minimize the
antenna's susceptibility to reflected signals coming in from the sides.
Such "multipath" signals reflected from buildings, fences, or other nearby
objects would otherwise give erroneous satellite range measurements.
Tripod
Microwaves (carrying GPS data) can be blocked or absorbed by trees,
passing trucks, people, walls, and so forth. Any solid structure above
the horizon of the antenna potentially blocks the GPS signal. To improve
reception, we often raise the antenna above the height of surrounding
obstacles using the tripod. The heavy-duty surveyor's tripod holds the
antenna motionless and allows the antenna to be raised or lowered. The
tribrach allows the tripod to position the antenna very precisely over the
survey mark and level with the horizon.
Tribrach
The tribrach is the link between the antenna and the tripod. The
bottom of the tribrach attaches to the top of the tripod. An attachment
on the top of the tribrach allows the antenna to be attached. There is a
kind of "bombing sight" on the tribrach that puts a crosshair over the
image of whatever is directly under the tripod. By aligning this
crosshair with the survey mark, the tribrach can be centered above the
mark precisely. A bubble level is built into the top of the tribrach.
Adjusting the tripod until the bubble is aligned inside the painted circle
levels the tribrach and the attached antenna.
The antenna cable carries the signal received by the antenna to the GPS
receiver. Since the signals from the antenna are easily corrupted, it is
important to make the cable an ideal waveguide to the extent that is
possible. When coiling and uncoiling the antenna cable, we are careful
not to twist it or allow it to bend sharply. Water absorbs microwave
radiation, and keeping the cable sockets dry is vital.
The GPS receiver is the heart of the system; it is the device which
converts the amplified raw microwave signals into meaningful digital
information. Ordinary consumer GPS receivers contain hardware and
software which detects and identifies the data streams from any satellites
that are above the local horizon, and calculates the pseudorange distances
to the satellites. This in turn, is used to calculate the receiver's
position on the Earth's surface, with a precision comparable to the 300
meter wavelength of the C/A code modulation. However the receiver used in
this research achieves higher precision by also coherently recording the
phase of the 1.5 GHz carrier wave. Since its wavelength is only about 20
cm, this phase information allows positioning with sub-centimeter
precision. Our receivers display in real time information about the
satellites they are tracking and the approximate position, however the
precise millimeter-level results are not obtained until weeks later, after
computer post-processing of the data.
The receiver is typically powered by two 12-V car batteries. When
fully charged, these batteries provide sufficient power so that data can
be collected for about three days. Of all the equipment used in this
project, these batteries are by far the most unwieldy and difficult to
transport. For sites that are not directly accessible by car, smaller
batteries supplemented by solar panels can be substituted.
Solar panels are used to charge the 12V batteries during the daylight
hours. The panels are not used to power the receiver directly. Since
data are collected over a 24 hour period, the panels charge the batteries
during the day, and the batteries power the receiver unassisted at night.
The solar panels are used when to assure that the batteries will not fail
unexpectedly during an unmonitored data session, and also when smaller,
lower power batteries are used on a site because of logistical
considerations.
The spike mount circumvents the tripod and tribrach, allowing the
antenna to be mounted on a spike that rests on the survey mark. Using the
spike mount reduces the complexity and error invovled in set up and makes
the antenna less obvious to vandals. This setup is just as stable as the
tripod mount. A "spike mount" consists of a spike, which aligns the
antenna directly above the survey point, and two adjustable legs that
allow the antenna to be leveled with the horizon. A site description will
always note whether the tripod or the spike mount is more appropriate.
When the tripod is set up, the three legs are hammered into the ground. Often the ground is too hard for the legs to penetrate, or too soft to hold the tripod immobile. In these cases, Plaster of Paris is used on the base of each leg to stabilise the tripod and immobilise it. The legs of the spike mount are also frequently insufficiently stabilised; Plaster of Paris can be used to attach the legs of the spike mount directly to the survey mark.
