The U.S Global Positioning System (GPS) is a satellite based navigation
system that provides worldwide coverage. Typical use of the system includes
positioning and timing. However, it can be used for other applications such
as GPS bistatic radar, which in turn can be used for altimetry by calculating
the delay between the direct and the reflected signal. It can also be used as
a passive system (no user transmitted Radio Frequency, RF, energy) for remote
sensing. The basic principle for GPS bistatic radar is that by using two
receivers, one for direct and one for ground reflected GPS signals,
differential processing can be performed to extract more information than
just the position. By analysing the shape and the relative power of the
reflected signal, information about the reflection surface can be acquired.
To perform this kind of science with GPS, normal receivers can not be used
since they lack the flexibility and low-level output that is critical for
this kind of processing. The receiver for the reflected signal requires many
correlators, in order to see a wide time window, while a typical GPS receiver
only has two correlators per channel. Having multiple correlators also gives
other advantages, with a narrow spacing there is less tracking jitter and
better correlation peak resolution. A larger number of Automatic Gain Control
(AGC) bits, 4 or more, than what is typically used for position, one or two,
is also good in order to increase resolution.
If a sampling receiver is used, in other words a receiver that outputs
Intermediate Frequency (IF) samples, very large data files are created during
each experiment. Processing these files in software can be very
time-consuming so there is a need for hardware acceleration.
To implement a large number of correlators for an experimental real-time
receiver or processing module there is only one option and that is Field
Programmable Gate Array (FPGA) design. This design is based on the Wildcard
II from Annapolis Microsystems, it is a PCMCIA card with a FPGA that conn...
