Introduction
This project deals with the effects of radio frequency (RF) interference on the Global Positioning System (GPS). After a brief introduction of electromagnetic spectrum and interference, the next topic is the autocorrelation function and how GPS uses it to obtain range data to the satellite. This then leads to a short section on how interference affects the receiver’s ability to get a position fix. The final section is an experiment that tries to determine how television signals from a transmitting tower can interfere with a GPS receiver. This project deals with only the effect of interference on the C/A code since the P code is much more jam resistant
Radio frequency encompasses the whole of the electromagnetic spectrum. Although there is no beginning or end to the electromagnetic spectrum, the “usable” part is divided up and classified according to frequency. The classification ranges from extremely low frequency (ELF) around 100 Hz to Gamma rays with frequencies greater than 1020 Hz. Figure 1 shows give a graphical representation of the electromagnetic spectrum along with some other useful information. RF energy is constantly being
Figure 1 The Electromagnetic Spectrum
emitted by various intentional and unintentional sources. Any kind of wireless signal propagation like television, cellular phones, and radio emit RF energy on different frequencies. Also, power converters, signal-cables, and other electronic devices emit unintentional RF energy. Usually, these unintentional RF sources are shielded to prevent them from interfering with the intentional signals.
The frequencies that GPS uses are derived from their on-board rubidium or cesium atomic oscillator that is oscillating at a frequency of 10.23MHz. This is used to produce the two carriers, L1 and L2, operating at 1575.4 MHz and 1227.6 MHz respectively. The data is modulated on to these two carriers and transmitted so that anyone with a receiver can locate their position anywhere in the world. Every GPS satellite transmits on the same frequency and the signals are able to not interfere with each other by using spread spectrum technology, where the transmitting signal is spread over a wide band. However, the satellites are approximately 20,000 km away from the GPS receiver, so the signal is very weak and susceptible to interference.