The GNSS integer ambiguities: estimation and validation
The GNSS integer ambiguities: estimation and validation
The GNSS integer ambiguities: estimation and validation
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Table 2.1: Signal <strong>and</strong> frequency plan for modernized GPS.<br />
frequency b<strong>and</strong> frequency [MHz] civil precise military<br />
L1 1575.42 C/A P M<br />
L2 1227.60 C/A P M<br />
L5 1176.45 I + Q<br />
<strong>The</strong> principle of GPS is measuring distances between the GPS satellites <strong>and</strong> a receiver<br />
antenna. <strong>The</strong> ranging signals are transmitted on two carriers centered at the L1 <strong>and</strong><br />
L2 frequency b<strong>and</strong>s. Two types of Pseudo R<strong>and</strong>om Noise (PRN) codes are modulated<br />
on these carriers. Only the C/A code on the L1 b<strong>and</strong> is freely available for civil users.<br />
<strong>The</strong> P-codes are much more precise but are only available for military users; civil users<br />
can just access the encrypted P-codes, referred to as P(Y)-codes. <strong>The</strong> encryption of the<br />
P-codes is called Anti-Spoofing.<br />
In the future a third frequency will be added, namely the L5 frequency. <strong>The</strong> codes<br />
modulated on the L5 carrier will be freely available. Furthermore, a civil code on L2 will<br />
become available, <strong>and</strong> military M-codes will be transmitted on L1 <strong>and</strong> L2. Table 2.1<br />
gives an overview of the available signals.<br />
2.1.2 GLONASS<br />
<strong>The</strong> Russian equivalent to GPS is the GLObal NAvigation Satellite System (GLONASS).<br />
Like GPS, the nominal constellation consists of 24 satellites but with an orbital inclination<br />
of 64.8 o <strong>and</strong> an orbital radius of 25,500 km. GLONASS also transmits on the L1 <strong>and</strong><br />
L2 frequency b<strong>and</strong>s, but each satellite transmits its signals with a different frequency<br />
offset.<br />
<strong>The</strong> complete GLONASS constellation of 24 operational satellites has only been available<br />
for a short time in 1996. Since then the number of satellites decreased <strong>and</strong> not enough<br />
replacements were launched. It is planned that GLONASS should become completely<br />
operational with a complete constellation in the period 2007-2022, <strong>and</strong> it is planned to<br />
transmit on a third frequency as well in the b<strong>and</strong> 1190-1212 MHz, including integrity<br />
information.<br />
2.1.3 Galileo<br />
<strong>The</strong> Galileo satellite constellation will consist of 30 satellites, including three active<br />
spares. <strong>The</strong> satellites are divided over three orbital planes using the Walker 27/3/1<br />
constellation plus the spares. <strong>The</strong> inclination will be 56 o <strong>and</strong> the orbital radius 29,600<br />
km (Salgado et al. 2001; Z<strong>and</strong>bergen et al. 2004).<br />
Galileo will transmit on four frequency b<strong>and</strong>s. Table 2.2 gives an overview of the signals<br />
<strong>and</strong> the services for which they are available (Erhard 2002).<br />
6 <strong>GNSS</strong> observation model <strong>and</strong> quality control