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Acquiring a signal is the process of determining the frequency and code phase (both relative to receiver time) when it was previously unknown. Code phase must be determined within an accuracy that depends on the receiver design (especially the tracking loop); 0.5 times the duration of code chips (approx. 0.489 μs) is a representative value.
The war also demonstrated the vulnerability of GPS to being jammed, when Iraqi forces installed jamming devices on likely targets that emitted radio noise, disrupting reception of the weak GPS signal. [140] GPS's vulnerability to jamming is a threat that continues to grow as jamming equipment and experience grows. [141] [142] GPS signals have ...
These units use the signal from both the L1 and L2 GPS frequencies. Even though the L2 code data are encrypted , the signal's carrier wave enables correction of some ionospheric errors . These dual-frequency GPS receivers typically cost US$10,000 or more, but can have positioning errors on the order of one centimetre or less when used in ...
Real-time kinematic positioning (RTK) is another approach for a precise GPS-based positioning system. In this approach, the determination of the range signal can be resolved to a precision of less than 10 centimeters (4 in). This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver.
Modern telecommunications networks use highly accurate primary master clocks that must meet the international standards requirement for long term frequency accuracy better than 1 part in 10 11. [4] To get this performance, atomic clocks or GPS disciplined oscillators are normally used.
Moving stations (portable or mobile) automatically derive their position information from a GPS receiver connected to the APRS equipment. [8] The map display uses these fields to plot communication range of all participants and facilitate the ability to contact users during both routine and emergency situations. Each position/object/item packet ...
The GPS alternatives rely on signals that can be measured locally (for instance, motion or magnetic fields as used in a compass), so a vessel can navigate even when GPS is unavailable or untrusted.
There are several ways to transmit a correction signal from base station to mobile station. The most popular way to achieve real-time, low-cost signal transmission is to use a radio modem, typically in the UHF Band. In most countries, certain frequencies are allocated specifically for RTK purposes.