Galileo Signal (GPS and Galileo Receiver) Part 4

Time and Clock Correction Parameters

As for GPS, Galileo has its own system time, called Galileo System Time (GST). Its starting epoch still has to be determined. GST consists of two parts: week number, WN, and time of week, TOW. The WN covers 4096 weeks and is then reset to zero. A week has 604,800 s and is reset at midnight between Saturday and Sunday. Hence GST is described as a 32-bit binary number split into the two parts just mentioned. Table 3.3 shows these parameters.

Let a signal be transmitted at timefrom satellite k, and let the same signal be received at timeat receiver i. Then the travel time is

TABLE 3.3. Galileo system time parameters

Parameter	No. of bits	Scale factor	Unit
WN	12	—	week
TOW	20	1	s

TABLE 3.4. Galileo clock correction parameters

Parameter	No. of bits	Scale factor	Unit
	14		s
	28		s
	18
	12

Knowing the travel timethis quantity can be converted to the so-called pseu-dorangeby multiplication with the speed of light c:

However, clocks do not work perfectly. So we introduce the receiver clock offsetand the satellite clock offset

The receiver clock offset has to be estimated from the observed pseudoranges while the satellite clock offset can be computed from

where t is the transmit time. The constantsare parameters transmitted according to Table 3.4.

The basic computational equations for time are

Additionally, a signal in space accuracy (SISA) parameter is planned. It is encoded as 8 bits.

An ionospheric correction service is planned. An effective ionization-level parameter is computed from three broadcast coefficients.

TABLE 3.5. GST to UTC conversion

Parameter	No. of bits	Scale factor	Unit
	32		s
	24		s/s
	8	1	s
	8	3600	s
	8	1	week
	8	1	week
	3	1… 7	day
	8	1	s

Conversion of GST to UTC and GPST

Compared to the present GPS, Galileo offers some advantages for the timing community. For example, data for real-time estimation of Universal Time Coordinated (UTC) are available. Likewise for the difference between GST and GPST. However, in case the user disposes over a combined GPS and Galileo receiver, it is likely that an estimate based on Equation (8.45) turns out to be more accurate.

The relation between GST and UTC is established via the time scale Temps Atom-ique International (TAI). UTC and TAI differ by an integer number of seconds. On January 1, 2003, the difference was

UTC is a uniform time scale, and it tries to follow variations in the Earth’s rotation rate; this is accommodated for by introducing leap seconds in the UTC. Consequently, this changes the difference between UTC and GST in steps of 1 s (see Table 3.5).

Let the estimated epoch time in GST, relative to the start of the week, be denoted byLetdenote the offset between GST and TAI at the timeThe time derivative ofis calledLet the difference between TAI and UTC beand the validity timefor the UTC offset parameters.

Leap seconds are always introduced on January 1 or/and July 1. The day number in the week in which the leap second is introduced is called DN. Days are counted from 1 to 7 (Sunday is 1) and is rounded to an integer.

The week number, modulo 256, in which DN falls is denotedFinally, the offset due to the introduction of a leap second atand DN is called

The following equations are in unit of s. We start by introducing the correction

TABLE 3.6. GST to GPST conversion

Parameter	No. of bits	Scale factor	Unit
	16		s
	12		s/s
	8	3600	s

 

We need to distinguish among three different cases:

The difference between GST and GPST is determined as follows. Letbe GST estimated by the user receiver; then the offset between GST and GPST at time tGal is

Table 3.6 describes the parameters concerned. 3.4.3 Service Parameters

The satellite identificationis a number between 1 and 128. A parameter, issue of data (IOD), identifies the set of data. This allows a receiver to compare batches of data received from different satellites. IOD is transmitted in each page of ephemeris and clock correction (9 bits) and almanac (2 bits).

Signal and data health status referring to the transmitting satellite is planned as well.

The six Keplerian elementsof any active satellite are contained in the almanac. The elements are given with less precision than the ephemeris. Clock correction parameters are given for computation of satellite clock offset

The almanac reference timerefers to the almanac reference week

TABLE 3.7. Almanac parameters

Parameter	No. of Bits	Scale Factor	Unit
	7	1	dimensionless
	24
	16		dimensionless
	16		semicircle
	24		semicircle
	16		semicircle/s
	24		semicircle
	24		semicircle
	15		s
	11
	5	—	dimensionless
	3	—	dimensionless
	2	—	dimensionless
	2	—	dimensionless
	8	4096	s
	8	1	week

Two parameters tell the satellite’s signal component health SVSHS and the satellite’s navigation data healthIn the almanac the applicable navigation data structure for each satellite is defined byThe IODA identifies an almanac batch unambiguously. The update rate being slow, two bits are sufficient. All parameters are described in Table 3.7.

The Received L1 OS Signal

Let the total received power be P, the transmission delay (traveling time) be the carrier frequency offset be(Doppler), and the received phase beThen the received L1 OS signal can be written as

The data channel and the pilot channel are denoted by d and p, respectively. The coefficientsare products of code sequences and subcarriers with sine phasing.

From the observationwe want to estimateThe first step is to find global approximate values ofwhich is called signal acquisition.

The second step is a local search forand possiblyIfis estimated,the search is called coherent signal tracking. If the carrier phaseis ignored, the search is called noncoherent signal tracking.

The purpose of code tracking is to estimate the travel timeand is done by means of a delay lock loop (DLL). For a coherent DLL we have

To demodulate the navigation data, a carrier wave replica must be generated. To track a carrier wave signal, a phase lock loop (PLL) often is used.

A final remark. This topic exposed most of the material relevant to code the L1 OS Galileo signal. However, GPS is also under continuous development— a very fortunate situation for the user. The planned civilian L5 GPS signal is described in ICD-GPS-705 (2002). Comparing the present topic, which is based on Anonymous (2005), with ICD-GPS-200 (1991) you realize that the European and American satellite navigation communities are each using their own lingos.