Figure 1. How the different components of the GPS signal are combined. After Langley (1990)

The PRN codes are unique for each satellite and the correlation between any pair of codes is very low. This allows all satellites to share the same carrier frequency.

There are basically two methods to deny civilians full use of the GPS system:

• Selective Availability (SA) - adding noise to the clock and ephemeris in the navigation message (SA has been turned off in 2000)
• Anti-spoofing (AS)- the P-code is encrypted (Y-code) and available to military users only.

Ref. — Langley R. (1990), Why is the GPS signal so complex?, GPS World, May/June, p. 56.

(After Hoffmann-Wellenhof et al. (1997), GPS: Theory and Practice, 4^th Ed., Springer.)

2. GPS observables

The pseudorange. The GPS receiver measures the distance (pseudorange) between the satellite and the antenna by measuring the time the signal takes to propagate from the satellite to the receiver. The pseudorange is this time offset multiplied by the speed of light.

The pseudorange is biased by the lack of time synchronization between the clock in the GPS satellite and the clock in the GPS receiver. Other bias effects include the ionosphere and troposphere delay, multipath and receiver noise. The equation for the pseudorange observable is

where p is the pseudorange, r is the geometric range to the satellite, c is the speed of light, dt and dT are the offsets of the satellite and receiver clock from the GPS time, d_ion and d_trop the delays imparted by the ionosphere and troposphere and e _p represents the effect of multipath and receiver noise. The receiver coordinates are hidden in the geometric range r .

Figure 2. How the pseudorange is measured, after Langley (1998), in GPS for Geodesy , p. 151.

Carrier phase. A more precise observable than the pseudo-range is the phase of the received carrier with respect to the phase generated by an oscillator in the GPS receiver. The difference between the received carrier and the receiver generated one is called the carrier beat phase. The problem is that the GPS receiver cannot distinguish one cycle of a carrier from another. The receiver measures the fractional phase, and keeps track of changes to the phase. The initial phase is undetermined, or ambiguous, by an integer number of cycles N.

If we convert the carrier beat phase into an equivalent distance by multiplying by the carrier wavelength l, we get

which is very similar to the pseudorange expression, the major difference being the presence of the ambiguity term .

Linear combinations. We can form what are known as between-receivers (or between- satellites) differences to obtain new observable with significantly reduced errors.

Figure 3. Linear combinations. After Langley (1993).

• The between-receivers single difference (two different receivers tracking the same satellite) - eliminates the satellite clock offset

• The between-satellites single difference (one receiver tracking two satellites) - eliminates the receiver clock offset

• The double difference (we can difference either the between receivers or the between-satellite difference pairs) - eliminate both the receiver and satellite clock offset

• The LC (ionosphere free) combination. The linear combination of the L1 and L2 phase measurements reduces the effect of the ionosphere, but may amplify other sources of error

• Wide-lane and narrow-lane combinations (applied for ambiguity resolution)

Ref. — Langley R. (1993), The GPS observables, GPS World, April, p. 52.

Orbital Errors /Clock Bias/Measurement Noise: As mentioned earlier, GPS signals contain information about ephemeris (orbital position) errors, and about the rate of clock drift for the broadcasting satellite. The data concerning ephemeris errors may not exactly model the true satellite motion or the exact rate of clock drift. Distortion of the signal by measurement noise can further increase positional error. The disparity in ephemeris data can introduce 1-5 meters of positional error, clock drift disparity can introduce 0-1.5 meters of positional error and measurement noise can introduce 0-10 meters of positional error (see Table 6.2).

Signal propagation: The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from "Signal Speed x Time" will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space.

Multipath: A GPS signal bouncing off a reflective surface prior to reaching the GPS receiver antenna is referred to as multipath. Because it is difficult to completely correct multipath error, even in high precision GPS units, multipath error is a serious concern to the GPS user.

Selective Availability (turned off in January 2000): Ephemeris errors should not be confused with Selective Availability (SA), which is the intentional alteration of the time and ephemeris signal by the Department of Defense. SA can introduce 0-70 meters of positional error. Fortunately, positional errors caused by SA can be removed by differential correction.

Dilution of Precision (DOP). The UERE is mapped into the computed position by a geometrical factor called DOP. The DOP is a mathematical function involving the relative coordinates of the receiver and the satellite and can be easily computed for a particular satellite arrangement. The more spread out the satellites are in the sky, the smaller the DOP value. A typical value for the horizontal dilution of precision (HDOP), assuming that a receiver is processing the signals of 4 satellites only, is 2.0.

Ref. - Langley, R. B. (1997), The GPS error budget. GPS World , Vol. 8, No. 3, pp. 51-56.