Lieutenant Herbert Kröll explained the possible danger of partial or complete failure of GPS signals and its impact on military and civilian applications in the “Military Service Navigation War Report 12/2017”.
Sufficiently accurate location determination, navigation and time settings are the basic prerequisites for using most weapon systems. This can only be guaranteed by a functioning navigation system. The goal of navigation warfare is the enemy’s interference with the satellite’s location, navigation and weather information.
The navigation system or GNSS (Global Navigation Satellite System) now covers the entire earth. By receiving signals from navigation satellites and pseudolites (ground transmitters, such as satellites that send signals), they can be used to determine location and navigate on land, sea and air.
They were originally designed as military systems for weapon systems, warships or fighter jets to achieve global positioning and are now useful for everyone. The first satellite was launched in 1978 and was not officially put into use until 1995. At the same time, Russian “GLONASS”, Chinese “COMPASS” or IRNSS covering only India are also in service. Europe has been building its own system under the name “Galileo” for many years. Please also refer to the TD 2/2017 brochure (n°356).
The GPS term only includes the American system “NAVSTAR-GPS” (a navigation system that uses timing and telemetry-Global Positioning System). This system covering the world is the most famous and is also used in the European military. Therefore, the GPS navigation system is more important for navigation warfare than the European Galileo system.
In order to accurately determine the GPS location, several satellite signals need to be used.
Using GPS, the position of the receiving antenna on the earth can be determined by measuring the distance to several satellites orbiting the earth 20,000 kilometers. A single satellite is not enough to determine the location. Further distance measurement with other satellites is required. The spherical radiation of only three satellite signals will cause the intersection, so that the exact position can be determined.
However, the propagation of satellite signals in the vacuum of space is different from the propagation after entering the earth’s atmosphere. Therefore, the signal delay time must also be considered, otherwise it will cause a measurement error of several meters. For distance measurement, the GPS system time at the time of transmission is encoded in the satellite signal. Since the receiver’s clock is not fully synchronized with GPS time initially, this difference is used as other unknowns when determining the location. In order to determine three initially unknown position coordinates and a time constant, the distance to the fourth satellite is required. This means that the satellite navigation system basically consists of rooms, floors and users. As far as GPS is concerned, the space segment consists of 24 active satellites and 7 spare satellites, which orbit the earth in six orbits, with four satellites in each orbit, and their height is about 20,200 kilometers. For civilian use, the L1 frequency is transmitted at 1575.42 MHz, and for military use, the quantifiable frequency L2 is used at 1227.60 MHz. The name L represents the L frequency band from 1000 to 2000 MHz. Navigation data and standard codes (C/A codes) are sent on civilian frequencies, while encrypted and undisclosed precision codes (P codes) are sent on military frequencies.
Each of these two signals is composed of carrier, code and navigation data, and each satellite of this signal informs the receiver of its orbit data and the time of sending the signal. In order to distinguish which satellite the signal comes from, a special modulation method is used for GPS signals, which allows multiple data streams to be transmitted simultaneously within a certain frequency range. This means that each satellite can be distinguished by its code sequence.
Modern GPS receivers can use standard codes to determine the distance to the satellite with an accuracy of 2.93 m. The encrypted P/Y code with ten times the chip rate is only used for military purposes. When measuring the distance to the satellite, this can achieve an accuracy of about 30 cm. Since May 2, 2000, the time tampering (S/A) of artificial satellite transmission (causing the incorrect position determination of the civilian GPS receiver) has been disabled until further notice. Current positioning standards include civilian standard positioning service (SPS) and military precision positioning service (PPS). Navigation warfare will further improve safety standards.
Due to the limited power supply, the transmitting power of GPS satellites is similar to that of TV satellites, about 50 watts. As far as the frequency and the unknown direction of the transmitter are concerned, the large bandwidth of the receiver will lead to higher noise components, so that the useful signal is lost in the noise. Therefore, the signal must be multiplied by the receiver’s own code sequence to make it stand out from the noise again. In addition, interference may occur due to the multipath propagation of de-return signals reflected from the surrounding area. However, the correct antenna installation location and shielding can reduce this.
GPS Jammers
Using jammers, GPS signals can usually be successfully blocked or successfully blocked. Since the GPS signal used for navigation only touches the earth very weakly, only a weak interference signal is needed to interfere with the navigation system on the ground. Even a very small GPS jammer will jam the signal within ten meters. You can find enough plans on the Internet. This means that it is no longer possible to find a vehicle equipped with a GPS receiver, and the navigation system no longer functions.
In the case of pure navigation, problems that can be solved only by viewing the map may have significant consequences for military systems that rely on precise location, navigation, and time information. As a result, as part of network-centric operations, communication systems can no longer exchange data with each other. Weapon systems that obtain data from many sensors and rely on time information from GPS signals will no longer be available.