Enhancing the Code to Measure GPS Signal Strength – Essentials of GPS

Most GPS device vendors employ simple RSSI as a measure for GPS signal strength. RSSI can be defined as the measurement of strength at which a radio signal is received. It is a generic metric to measure GPS signal strength. The computations in RSSI are done at the Intermediate Frequencies, that is, once the radio signal passes through the Down Convertor in the RF Front of the GPS receiver. The output is an integer indicating the DC analog level of the GPS signal strength. The values of the output range from 0 to 255 that can be represented in a single bit. However, the single bit output leads to less accuracy of GPS device. However, these cheap methods compensate heavily on the accuracy of the GPS devices.

Hence, while creating better code to measure GPS signal strength, it is not only the RSSI that is taken into account. The competition among different GPS vendors has lead to consider all possible factors that help optimize the measurements. Some of these factors are briefed in the next section.

Though phase modulation of the data onto radio signals offers better stability, other factors to be considered to measure GPS signal strength are: Signal strength of the satellite transmitter; Attenuation in the transmitter; transmission gain; the distance between the transmitting antenna and receiving antenna; the gain of the receiver antenna; attenuation in the receiver; signal tracking technique used; and finally, the thermal and internal noise of transmitter and receiver. The problem of signals from different satellites intersecting is also a factor to be considered when implementing a code to measure GPS signal strength. Most GPS vendors leave out the attenuation caused by the atmosphere, as it almost negligible.

The next section further details the essential GPS signal "transmission and reception theorems" considered by the different GPS vendors to create better codes to measure GPS signal strength.

To understand the theorems better, let us use symbols for different components that help create a code to measure GPS signal strength. Assuming the GPS signal strength to be S and the noise level to be N, the basic formula to measure GPS signal strength is S/N. Taking the radio waves that carry the data to the GPS receiver as C, it has been proved that S is dependent on C. If the carrier waves (C) face obstructions, you never get S. This is a case of GPS Blackout. Another theorem says that S is constant throughout the journey while C degrades continuously as it enters the atmosphere.

The possibility of S getting corrupted is only when different signals intersect to superimpose C with more than one S, which causes S (the actual data) to be corrupted. However, technology has intelligent GNSS receivers that can identify erroneous data in order to discard it instead of forwarding it to the code for processing. Finally, the last theorem says that the bandwidth of common receivers has a degrading affect on C/N and S/N. Hence, the GPS vendors use a minimum bandwidth of 1Hz so that both C/N and S/N are almost bandwidth independent. This offers ease of coding to measure GPS signal strength more precisely.

Summary: As such, the current GNSS network of 31 satellites offers signals at two frequencies: L1 for civilians (1575.42 MHz, unencrypted) and L2 for defense only (1227.6 MHz, highly encrypted). Data is transmitted at 50Hz. The signal strength received on the standard L1 C/A (course/acquisition) code is -130dBm while on L1 P (Precision) code is -133dBm under clear sky (outdoors). The indoor L1 C/A ranges from 140 to 150dBm.