Electronic Engineers Notebook

GPS

The Constellation

Signal Structure

Modernisation

Navigation

Links

The Global Positioning System (GPS) has become widely used for navigation and as a time standard. The system was designed during the 70s, principally as a military system. The first satellite was launched in 1978. The system became fully operational in 1995. As well as military signals, it also transmits unencrypted civil signals. The civil signals were originally intended as an aid to acquisition but have become extensively used for navigation themselves. They are also used in applications such as time synchronising the basestations in mobile phone networks.

The Constellation

Block IIR Satellite

The nominal constellation is 24 satellites arranged in 6 orbital planes (4 satellites per plane). Each plane has an angle of 55degrees relative to the equatorial plane. In practice more satellites are usually in orbit. The extra satellites are ready to replace any failures. New satellites are launched as needed to keep the constellation healthy. Currently satellites are achieving a life expectancy of over 10 years. The orbits are circular with an altitude of 20,200km (26,560km radius). The period is 12 hours which means, allowing for the rotation of the earth around the sun, they appear in the sky about 4 minutes earlier each day. The satellites have stationary ground tracks. The satellite speed is about 4km/s. There are almost always at least 4 satellites in view, more usually 6 to 8. There may be as many as 11 depending on location.

The operational control system (OCS) consists of a master control station near Colorado Springs (Schriever AFB) , 3 ground antenna stations (Ascension Island, Diego Garcia and Kwajalein) that transmit data and commands to the satellites using S-band and 5 monitoring stations. These are arranged around the equator (Ascension Island, Diego Garcia, Kwajalein, Hawaii, and Colorado Springs). Even though the satellites carry atomic clocks, drifts and errors still occur over time. Cesium clocks are accurate to about 1 part in 1014. The OCS measures these errors and calculates a clock correction to be added to the satellite data message.

Click to the the construction of a block IIR satellite (238k)

Signal Structure

GPS satellites currently transmit at two frequencies; 1575.42MHz (L1) and 1227.6MHz (L2). These are multiples of a standard 10.23MHz clock. The use of two well separated frequencies enables ionospheric group delay errors to be corrected for. The transmitted signal at L1 can be written as:

S(t) = P(t)D(t)cos(w 1 t) + AC(t)D(t)sin(w 1 t)

P(t) is the military P(Y) spreading code. This is a BPSK signal clocked at 10.23MHz. It is encrypted to prevent unauthorised use and also the spoofing of the military signal. Each satellite has a unique code.

D(t) is the 50bits/s data message. The data message is contained in a frame that is 1500bits long. It is split into 5 subframes, each of which carries system time and handover information for when the C/A code is used to acquire the P(Y) code. The first subframe carries the space vehicle's (SV's) clock correction and ionospheric propagation delay parameters. The second and third subframes describe the SV's ephemeris. The fourth subframe carries a message. The last subframe cycles through the almanacs of all the space vehicles providing their emphemeris, clock correction and health.

w 1 is the L1 angular frequency.

A gives the relative level of the C/A code relative to the P(Y) code. Generally the C/A code is around 3dB stronger.

C(t) is the clear/aquisition (C/A) spreading code. This is also a BPSK signal but clocked at 1.023MHz. It is a short code that repeats every 1023 bits (1ms). The spreading code is a known Gold code (different for each satellite) and is the signal used by civil GPS receivers. The Gold codes have autocorrelation and crosscorrelation sidelobes that are at least 24dB down.

The L2 signal has the same format except that currently the C/A code is not transmitted. However in an attempt to improve GPS for the civil user, new satellites will also transmit C/A code on L2.

Compared with the spreading ratios used in modern mobile communications systems, the spreading ratios of GPS are very large (20,460 or 43dB for C/A code compared with the data message). However the satellite signals are very weak. The satellite RF power at the transmit antenna is about 50W. This results in a minimum strength of the L1 C/A code signal at the receiver of -160dBW. Noise in a 2MHz bandwidth is around -136dBW assuming the receiver has a 5dB noise figure. The signal to noise ratio is thus around -24dB or 19dB after despreading to the data message. Assuming 10dB is needed to recover the data message this leaves only 9dB spare to counter interference or additional attenuation. The military code provides more spreading gain (53dB) but the signals are weaker (-163dBW minimum at L1, -166dBW minimum at L2).

The following figure shows the spectra of the C/A and P(Y)-codes. Of course this is normally hidden by thermal noise. The actual GPS band is 24MHz wide so the tails should be attenuated by the satellite transmit filter. The centre frequency may also differ by up to 5kHz due to Doppler shift from the motion of the satellite.

GPS Spectra

GPS Spectra

Modernisation

The above figure also shows the presence of an M-code. This is a new military code. The first satellite able to transmit M-code is expected to launch in 2003. By 2008 there should be 18 satellites transmitting M-code which is enough for operational use. M-code is transmitted on two subcarriers at +/-10.23MHz of the centre frequency. Each subband carries a 5.115MHz spreading code. The military signal is thus moved away from the C/A-code, enabling it to be jammed without disrupting reception of the military signal. There are also plans to transmit additional M-code signals at a much higher power level to specific regions of the earth by using a more directional antenna at the satellite.

At the same time as the military service is modernised the civil service is also being upgraded. The C/A code will also be transmitted at L2 so that ionospheric correction becomes possible for the civil user. This also provides a source of redundancy. In addition a new, more accurate civil signal will be transmitted at L5 (1176.45MHz).

In order that GPS can be used in safety of life applications (e.g. landing aircraft), augmentation systems are being set up (WAAS in the US, EGNOS in europe, MSAS in Japan). These basically consist of geo-stationary satellites that provide additional ranging signals, integrity information on the GPS signals and differential correction data.

Navigation

The GPS receiver has four unknowns; its position in three dimensional space and precise time. The satellites are precisely aligned in time using atomic clocks and effectively transmit their position on the data link. If the GPS receiver is able to correlate on the signals from four satellites it is able to measure the relative ranges for each satellite from the relative timing of the correlation peaks. It then has four measurements and four unknowns and is able to calculate its position and the precise time. Signals from additional satellites help to reduce the error.

Let xi,yi,zi be the position of satellite i
Let x,y,z be the unknown receiver position
Then the estimated pseudorange (Ri) to satellite i is given by:

Ri2 = (xi-x)2 + (yi-y)2 + (zi-z)2 + b2

where b2 is the range error due to the offset between the satellite and receiver clocks

As well as pseudorange, the receiver measures the Doppler shift and the carrier phase to give the rate of change of pseudorange. This, when combined with knowledge of the satellite velocities, enables the receiver velocity to be calculated.

Sources of error include drift of the satellite clocks and special relativistic clock shifts. Clock drift is monitored and corrections supplied in the data message. The bulk of the relativistic effect is corrected for by running the satellite clocks very slightly slow. Both the ionosphere and the troposphere generate range errors. Measurements at two frequencies enable the ionospheric delays to be modelled and largely removed. Once the satellite signal reaches the earth it may reach the receive antenna accompanied by reflections (multipath). These spread the correlation peak or may even introduce multiple peaks.

The accuracy achieved will depend on the numher of satellites visible and their distribution over the sky. The 95% percentile of the error (worldwide) is 21m horizontal, 28m vertical and 200ns for the military service. The civil service, until recently, was deliberately degraded (selective availability) to 100m horizontal, 156m vertical and 340ns.

Links

http://www.laafb.af.mil/SMC/CZ/homepage/

The NAVSTAR GPS homepage at the JPO

http://www.navcen.uscg.mil/

US Coastguard Navigation Centre - system status

http://www.gpsy.com/gpsinfo/

Lots of links including links to GPS software

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

GPS information and links

http://www.cnde.iastate.edu/staff/swormley/gps/gps.html

GPS information and links


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Last updated 11 March, 2001

© 2001 Michael Wells