Prototype Experiment
The purpose of the prototype experiment is to collect test
data to validate the geodedic technique. The test is to be conducted
at a site on an ice sheet that has previoulsy been mapped with ground-penetrating
radar, so that the depth to the basal reflector is known.
Monstatic (Vertical Incidence) Tests: The transmiting and receiving antennas are colocated.
- Signal to Noise Test: The antenna is placed at a single site, and a variety of
different carrier frequencies (50 MHz - 1 GHz) are used. The data are used to understand
the length of time needed to achieve a 10:1 signal-to-noise ratio.
- Can firn reverberations be recognized?
- Can the basal reflector be recognized?
- Can velocity dispersion be measured?
- Antenna Placement Repeatability Test: The antenna is removed from the site and then
re-emplace, with every attempt made to completely duplicate its original position. The
data are used to estimate the loss of coherence associated with antenna re-emplacement.
- Is antenna coupling a problem?
- If so, what field techniques can be used to minimize this problem?
- Spatial Coherence Test: Data is collected, the antenna is moved a distance, dx, and
data are collected again. A variety of dx's, in the 1-1000 m interval, are used to estimate
the spatial coherence of the signal.
- Is the basal reflection more coherent than the firn reflections?
- Does the data suggest that the coherence is being controlled by scatters
in the firn (or elsewhere)?
- Firn Disturbance Test: Data is collected, the antenna is removed, the firn is
disturbed (compacted) by "steamrolling", and then the antenna re-emplaced and a comparison
data set is collected.
- How well can the change in the firn be determined?
- How large is the error in dterming the change in basal reflector depth
(which should be exactly zero).
- Internal Standard Test: A metal cylinder a few cm in diameter is emplaced in the
firn at a depth of a few meters (via a borehole). The antenna is operated a few
meters away.
- Do diffractions from the cylinder provide a useful reference signal
that can be used as an "internal standard" to reduce re-emplacement
errors?
- How far from the antenna should the cylinder be?
- Polarization Test: Data is collected with the transmitter sending out a "north"
polarization, and the receiver listing to first "north" and then "east" polarizations.
The measurements are repeated, but with the transmitter sending out an "east" polarization.
The measurements are repeated at a variety of different carrier frequencies.
- Can dielectric anisotropy be detected?
- If so, in what depth ranges is it the most important?
Bistatic (Wide-angle) Tests: The transmitting and receiving antennas are separated by 9000-10000 m.
- Timing Stability Test: Atomic clocks are used to measure the GPS-timing stability
under actual Arctic field conditions.
- Basal Reflector Coherence Test: Data is acquired for several transmitter-receiver offsets,
with the transmitter at a fixed location and the receiver in the 9+dx range, with dx varying from
1-1000 m interval. The receiver locations should be the same as in the monstatic test, above,
to allow comparison with vertical incidence data.
- How coherent is the basal reflector?
- Does the data suggest that the coherence is being controlled by scatters
in the firn or on the basal surface?
- Traveltime Variability Test: The data from the previous test are used to estimate
basal reflector traveltime as a function of range.
- How smooth is this traveltime curve?
- Can small-scale features be accounted for by changes in firn structure?