Delay Mapping Receiver

A typical GPS receiver acquires a signal through the combination of a delay-lock loop for code tracking and a frequency- or phase-lock-loop for tracking the Doppler-shifted carrier. The code-tracking loop is closed so as to achieve maximum cross-correlation between the incoming code and a local replica of the PRN code.

NASA's Garrison and Katzberg developed a receiver to study GPS surface reflections by modifying a software-configurable GPS receiver to measure signal postcorrelation power at the series of discrete code-delay steps and Doppler frequencies. The modified GPS receiver and its application for sea-state sensing won the GPS World Applications Contest VI, published in the August 1998 GPS World Showcase issue. The new concept is in some sense the inverse of conventional GPS receiver designs because it seeks to ascertain the power at specific code delays as opposed to determining the code delay that maximizes the correlation power.

This "delay-Doppler-mapping receiver" concept consists of an array of correlator pairs. Each of these accumulates the results of the correlation between a locally generated PRN code replica (at some defined delay and Doppler frequency) and the in-phase and quad-rature components of a down-converted, digitized reflected signal. Each of these correlator pairs can be thought of as range "bins" or "gates" in conventional radar systems. The postdetection power in each bin is computed from the sum of the squares of the in-phase and quadrature components (I2+Q2). The receiver can then average this postdetection power to reduce the noise in the waveform samples.

For the receiver, Garrison and Katzberg used a commercially available GPS-development kit, which included a 12-channel correlator chip fed by two radio-frequency (RF) front ends. They employed a zenith-oriented RHCP antenna mounted on top of an aircraft fuselage to receive the direct signal through one RF front end. A nadir- oriented LHCP antenna located on the bottom of the fuselage received the reflected signal through the other RF front end. Each front end had an independent automatic gain control, ensuring that the variance of the sampled intermediate-frequency data (which, before detection, is dominated by white noise) maintains a predetermined average value.

This receiver has been used in aircraft remote-sensing experiments since the summer of 1997. It has also flown aboard a scientific balloon launched from Wallops Island, Virginia, in which waveform data was collected from an altitude of 25 kilometers. The existing receiver uses only one Doppler frequency, which corresponds to the direct signal and generates a one-dimensional map of the waveform dependence on code delay.