You might have heard about Global Positioning Service (GPS) Modernization, but what is it?
Credit: U.S. Air Force
GPS reached full operational capability in 1995. It consists of at least 24 satellites, distributed in six different orbits, circling the earth exactly twice a day. The original system transmitted signals on two frequencies, designed to support both commercial and military positioning receivers. There was a predicted total of 40,000 military GPS receivers.
Fast-forward to today, where more than 40,000 new cell phones with GPS capability are sold every week/month. GPS modernization tells that new satellites and ground stations will be added to the existing constellation. There will be three new civilian frequencies: L2C, L1C, and L5. The first two allow the correction of atmospheric effects which can degrade the accuracy of computed position. Signal processing improvements improve the ability to acquire and track the GPS satellites.
L5, called safety of life, is transmitted on a completely new frequency, 1176.45 MHz. Air Force Successfully Transmits and L5 Signal from GPS IIR-20(M) Satellite tells that the new frequency was first transmitted on 10 April, 2009. Not yet operational, the signal is being used to assist GPS developers in readying products to use the new capability. Because of international frequency agreements, if the frequency was not transmitted, it would have been reassigned to other uses. Operational use will be delayed until the Block IIF and Block III satellites.
The Industry Day presentations on L5 provides some of the design characteristics. (If you want the technical details of the waveform, see - ICD-705.) The System: L5 Arrives tells that the primary benefit will be reliability and integrity, very much needed for safety of life applications such as commercial airline navigation. It will be transmitted with higher power and with longer spread spectrum codes, making it much less susceptible to interference. The new frequency assignment permits correction of ionospheric-induced errors through frequency diversity.
GPS Modernization: Capabilities of the New Civil Signals explains that besides signal hardening, the system includes integrity machines. These are monitors to that provide error bounds to safety critical users. Airports are augmenting GPS with local and wide area differential GPS systems. Corrections and error bounds are transmitted to aircraft, providing accuracies better than 1 meter. The error bounds allow a pilot to determine whether a particular procedure/maneuver is safe. Thus the error bounds advise the pilot to avoid unsafe landings.
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