GPS Signal

- notes and overview of the GPS signal transmitted by GPS satellites detailing the make-up of the signal and explanations of their functions.

GPS satellites transmit signals that are received by the GPS receivers on the ground. These signals are then decoded and enable the receivers to provide the position information required.

With limited power on the satellites, the signals transmitted are relatively low power, and in view of the bandwidth available, multiplexing techniques are used to provide access to all the signals that are available.

GPS signal basics

The GPS satellites transmit a variety of signals that are picked up by the GPS receivers. These signals are relatively complicated but enable the system to operate in a very efficient fashion.

There are two primary frequencies that are used for the transmission of the GPS signal - both signals are in the UHF portion of the frequency spectrum. Additional GPS signals are used or being proposed as summarised below:

  • L1 - 1575.42 MHz:   This GPS signal is used to provide the course-acquisition (C/A) and encrypted precision P(Y) codes. It is also used for the L1 civilian (L1C) and military (M) codes on the Block III satellites
  • L2 - 1227.60 MHz:   This signal is used to carry the P(Y) code, as well as the L2C and military codes on the Block IIR-M and later satellites
  • L3 - 1381.05 MHz:   This frequency is used to carry information regarding any nuclear detonation (NUDET) event detected.
  • L4 - 1379.913 MHz:   This signal is being studied for use with additional ionospheric correction. This would considerably improve the accuracy.
  • L5 - 1176.45 MHz:   This GPS signal is being proposed for use as a civilian safety-of-life (SoL) signal.

The GPS signal uses a CDMA spread-spectrum technique to allow all the satellites to use the same frequencies without mutual interference.

Note on CDMA:

CDMA, Code Division Multiple Access, is a multiple access scheme used by many 3G cellular technologies, and other forms of wireless technology. It uses a process called Direct Sequence Spread Spectrum where spreading codes are used to spread a signal out over a given bandwidth and then reconstituting the data in the receiver by using the same spreading code. By supplying different spreading codes to different users, several users are able to utilises the same frequency without mutual interference.

Click on the link for a CDMA tutorial

Using the CDMA based signal low rate message data is encoded with a high-rate pseudo-random number, PRN, sequence that is different for each satellite - this spreads the signal over a wide bandwidth.

In order to decode the signal the correlate or decode the signal, the receiver must be aware of the PRN codes for each satellite.

There are two codes that are used:

  • C/A code:   The C/A code on the GPS signal is the one used for general or Civilian Access. This code is transmitted at 1.023 million chips per second, Mcps.
  • P code:   The P code is the precision code that can only be accessed by the US military. The P code transmits at a rate of 1.023 Mcps.

Both the C/A and P codes provide time of day information to the receiver.

These two codes are modulated onto the GPS signal. The C/A code is only carried by the L1 signal, whereas the P code is carried by both L1 and L2.

GPS signal data

The data carried by the GPS signal contains three types of data:

  • Pseudo-random code:   This is an identification or ID code that identifies which satellite is transmitting the information. It is possible to view this information on many SatNav systems.
  • Ephemeris data:   The almanac data on the GPS signal is used to carry information about the status of the satellite it also carries the current date and time which is used in the calculations for determining the position of the GPS receiver. This data is updated every two hours and is normally valid for four hours.
  • Almanac data:   the almanac data elements of the GPS signal provide information about the position of the satellite - orbit information about the satellite transmitting the information and all other satellites in constellation. This data is updated every 24 hours.

GPS transmitted data

Data transmitted in the GPS signals is split into frames to provide structure, and allow the receivers to be able to know where the beginning and end of messages are so that they can synchronise with the incoming signals and decode the data correctly.

A complete message is contained within a frame consisting of 15000 bit, transmitted at a rate of 50 bits per second which takes 30 seconds to complete. It starts transmission exactly on the minute or half minute as determined by the atomic clock on each satellite.

Each frame is then further subdivided into five sub-frames, equal in length which take six seconds to transmit and contain 300 bits.

Each sub-frame then contains ten words of 30 bits which take 0.6 seconds to transmit. Data required to be transmitted within the overall GPS signal is transmitted in set frames as detailed in the table below.

The GPS signal frames are split into sub-frames. In turn these have a specific structure to enable the system to be able to identify the data and keep the timing, etc.

GPS Data Frame and Subframe Structure
GPS Data Frame and Subframe Structure

It can be seen that the sub-frames are split into two sections:

  • Header:   In turn, the header itself is also split, and again into two sections:

    • Telemetry Word, TLM:   The GPS Telemetry Word, TLM is used to provide start information for the sub-frame - effectively providing data synchronisation. The GPS Telemetry Word is 24 bits long and it includes parity information. The GPS TLM starts with an 8 bit preamble for recognition for the receiver. This is then followed by 16 bits of reserved data which includes the ending 6 bit checksum or parity.

      A receiver will look for the pre-amble which marks the beginning of the new sub-frame. To confirm this, the receiver takes the reserved data, creates a parity, and checks to see if this corresponds with the last 6 bits of the TLM. If this does not match, the receiver again looks for the next preamble.
    • Handover Word, HOW:   The GPS Handover Word, HOW, again comprises two sections. The first 17 bits of this part of the header are given over to providing Time of Week, TOW. This is used to ensure that the GPS receiver is fully synchronised to the satellite time.

      The next 7 bits of the GPS TLM contain general sub-frame data. This consists of the sub-frame ID - indicating which one of the five sub-frames this one is. It also contains a reserved alert flag and an Anti-Spoofing flag.

      The alert flag is used to indicate to the receiver if the satellite may be giving an inaccurate measurement.

      The last six bits within the GPS HOW are again used for parity checking. Before the receiver stores data, it performs a further check to ensure that it is reading the header and not a very similar bit pattern. If the parity check does not match, it then moves on to look for the next preamble.
    Each sub-frame has a separate header so that a receiver can stare meaningful reception in the middle of a broadcast, and not have to wait up to 30 seconds for the next cycle.
  • Data:   Although the headers for each of the sub-frames are identical in format so that the receivers can identify which GPS sub-frame is being received and perform the relevant parity checks, the data sections of the sub-frame are completely different.

    Sub-frame Information carried within sub-frame
    1 Satellite clock, GPS time. This sub-frame contains the week number, time within the week as well as the health of the satellite.
    2 - 3 Ephemeris data.
    4 - 5 Almanac data.

By Ian Poole

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