A generic GNSS complex baseband signal transmitted by a given GNSS space vehicle can be described as
being the transmitting power, the navigation message data symbols, the bit period, the number of repetitions of a full codeword that spans a bit period, the codeword period, a chip of a spreading codeword of length chips, the transmitting chip pulse shape, which is considered energy-normalized for notation clarity, and is the chip period.
Particularizations of such signal structure for the different existing systems1 are described below.
Global Positioning System (GPS)
The Navstar Global Positioning System (GPS) is a space–based radio–navigation system owned by the United States Government (USG) and operated by the United States Air Force (USAF). GPS provides positioning and timing services to military and civilian users on a continuous, worldwide basis. Two GPS services are provided:
the Precise Positioning Service (PPS), available primarily to the military of the United States and its allies, and
the Standard Positioning Service (SPS) open to civilian users.
The most updated and authorized source is the Official U.S. Government website about GPS and related topics.
Defined in IS-GPS-2002, this band is centered at MHz. The complex baseband transmitted signal can be written as
where is the exclusive–or operation (modulo–2 addition), means modulo , means the integer part of , is the GPS navigation message bit sequence, transmitted at bps, s, s, , and is a rectangular pulse of a chip–period duration centered at and filtered at the transmitter. The precision P codes (named Y codes whenever the anti–spoofing mode is activated, encrypting the code and thus denying non–U.S. military users) are sequences of days in length.
Regarding the modernization plans for GPS, it is worthwhile to mention that there is a new civilian–use signal planned, called L1C and defined in IS-GPS-800D3, to be broadcast on the same L1 frequency that currently contains the C/A signal. The L1C signal will be available with first Block III launch, currently scheduled for May 3, 2017, and it will feature a Multiplexed Binary Offset Carrier (MBOC) modulation scheme that ensure backward compatibility with the C/A signal.
The L1C signal consists of two main components; one denoted to represent a pilot signal, without any data message, that is spread by a ranging code, and that is spread by a ranging code and modulated by a data message. The is also modulated by an SV unique overlay code, . The SVs could transmit intentionally “incorrect” versions of the respective ranging codes as needed to protect users from receiving and utilizing anomalous signals. These “incorrect” codes are termed non-standard (NSCP) and non-standard (NSCD). Non-standard codes are not for utilization by the users and, therefore, are not defined in IS-GPS-800D.
GPS signals spectra in L1. Source: Navipedia.
Defined in IS-GPS-2002, this band is centered at MHz. The complex baseband transmitted signal can be written as:
with the In–phase component defined as:
with an optional presence of the navigation message . For the Quadrature–phase component, three options are defined:
where ms and is a rectangular pulse of half chip–period duration, thus time–multiplexing both codes. The civilian long code is chips long, repeating every s, while the civilian moderate code is chips long and its repeats every ms. The CNAV data is an upgraded version of the original NAV navigation message, containing higher precision representation and nominally more accurate data than the NAV data. It is transmitted at bps with forward error correction (FEC) encoding, resulting in sps.
GPS signals spectra in L2. Source: Navipedia.
GPS L2C is only available on Block IIR–M and subsequent satellite blocks.
The GPS L5 link, defined in IS-GPS-7054, is only available on Block IIF and subsequent satellite blocks. Centered at MHz, this signal in space can be written as:
where ms and s. The L5I component contains a synchronization sequence , a –bit Neuman–Hoffman code that modulates each symbols of the GPS L5 civil navigation data , and the L5Q component has another synchronization sequence .
GPS signals spectra in L5. Source: Navipedia.
The nominal baseline constellation of the Russian Federation’s Global Navigation Satellite System (GLONASS) comprises GLONASS–M satellites that are uniformly deployed in three roughly circular orbital planes at an inclination of to the equator. The altitude of the orbit is km. The orbit period of each satellite is hours, minutes, and seconds. The orbital planes are separated by right ascension of the ascending node. Eight satellites are equally spaced in each plane with argument of latitude. Moreover, the orbital planes have an argument of latitude displacement of relative to each other. The current constellation status can be checked at the Russian Information and Analysis Center for Positioning, Navigation and Timing website.
The ground control segment of GLONASS is almost entirely located within former Soviet Union territory, except for a station in Brasilia, Brazil. The Ground Control Center and Time Standards is located in Moscow and the telemetry and tracking stations are in Saint Petersburg, Ternopol, Eniseisk, and Komsomolsk-na-Amure.
GLONASS civil signal–in–space is defined in GLONASS’ ICD 5. This system makes use of a frequency–division multiple access (FDMA) signal structure, transmitting in two bands:
- MHz and
where is the channel number. Satellites in opposite points of an orbit plane transmit signals on equal frequencies, as these satellites will never be in view simultaneously by a ground–based user.
Two kind of signals are transmitted: a standard precision (SP) and an obfuscated high precision (HP) signal. The complex baseband transmitted signal can be written as
where s, s, and . The navigation message is transmitted at bps. Details of its content and structure, as well as the generation of the code, can be found in GLONASS’ ICD 5. The usage of the HP signal should be agreed with the Russian Federation Defense Ministry, and no more details have been disclosed.
GLONASS signals spectra in L1. Source: Navipedia.
Beginning with the second generation of satellites, called GLONASS–M and first launched in 2001, a second civil signal is available using the same SP code than the one in the L1 band.
The use of FDMA techniques, in which the same code is used to broadcast navigation signals on different frequencies, and the placement of civil GLONASS transmissions on frequencies close to MHz, well above the GPS L1 band, have complicated the design of combined GLONASS/GPS receivers, particularly low–cost equipment for mass–market applications.
Future plans of modernization are intended to increase compatibility and interoperability with other GNSS, and include the addition of a code–division multiple access (CDMA) structure (and possibly BOC modulation) beginning with the third civil signal in the L3 band ( MHz). Russia is implementing the new signals on the next–generation GLONASS–K satellites, with a first space vehicle successfully launched on February 26, 2011.
On July 2, 2013, a Russian Proton-M rocket carrying three GLONASS–M navigation satellites crashed soon after liftoff today from Kazakhstan’ Baikonur cosmodrome.
GLONASS signals spectra in L2. Source: Navipedia.
The nominal Galileo constellation comprises a total of operational satellites (plus active spares), that are evenly distributed among three orbital planes inclined at relative to the equator. There are eight operational satellites per orbital plane, occupying evenly distributed orbital slots. Six additional spare satellites (two per orbital plane) complement the nominal constellation configuration. The Galileo satellites are placed in quasi–circular Earth orbits with a nominal semi–major axis of about km and an approximate revolution period of hours. The Control segment full infrastructure will be composed of sensor stations, control centers, Mission Uplink stations, and TT&C stations. The current constellation status can be checked at the European GNSS Service Centre website.
Galileo’s Open Service is defined in Galileo’s ICD6, where the following signal structures are specified:
This band, centered at MHz and with a reference bandwidth of MHz, uses the Composite Binary Offset Carrier (CBOC) modulation, defined in baseband as:
where the subcarriers are defined as
and MHz, MHz are the subcarrier rates, , and . Channel B contains the I/NAV type of navigation message, , intended for Safety–of–Life (SoL) services:
In case of channel C, it is a pilot (dataless) channel with a secondary code, forming a tiered code:
with s and ms. The and primary codes are pseudorandom memory code sequences defined in Galileo’s ICD6 [Annex C.7 and C.8]. The binary sequence of the secondary code is . This band also contains another component, Galileo E1A, intended for the Public Regulated Service (PRS), that uses a BOC modulation with cosine–shaped subcarrier, MHz, and s. The PRS spreading codes and the structure of the navigation message have not been made public.
Galileo signals spectra in E1. Source: Navipedia.
Intended for the Commercial Service and centered at MHz, this band provides with pilot and data components
where is the C/NAV navigation data stream, which is modulated with the encrypted ranging code with chip period s. Codes and primary codes and their respective lengths, and , have not been published. The secondary codes for the pilot component, , are available in Galileo’s ICD6. The receiver reference bandwidth for this signal is MHz.
This band also contains another component, Galileo E6A, intended for PRS. It uses a BOC modulation with cosine–shaped subcarrier, MHz, and s. The PRS spreading codes and the structure of the navigation message are not publicly available.
Galileo signals spectra in E6. Source: Navipedia.
Centered at MHz and with a total (baseband) bandwidth of MHz, its signal structure deserves some analysis. The AltBOC modulation can be generically expressed as
where is the single side–band subcarrier, is the subcarrier frequency, stands for the conjugate operation, and and are QPSK signals. The resulting waveform does not exhibit constant envelope. In case of Galileo, the need for high efficiency of the satellites’ onboard High Power Amplifier (HPA) has pushed a modification on the signal in order to make it envelope–constant and thus use the HPA at saturation. This can be done by adding some inter–modulation products to the expression above, coming up with the following definition:
where the single and product side–band signal subcarriers are
The signal components are defined as
where ms and s. Channel A contains the F/NAV type of navigation message, , intended for the Open Service. The I/NAV message structures for the E5bI and E1B signals use the same page layout. Only page sequencing is different, with page swapping between both components in order to allow a fast reception of data by a dual frequency receiver. The single subcarrier and the product subcarrier are defined as:
with a subcarrier frequency of MHz.
Plotting the power spectrum of the carriers for (see Figure below), we can see that the QPSK signal defined above is shifted to MHz, while is shifted to MHz.
Power spectrum of single and product side-band subcarriers signals for , normalized to the power of at . The modified AltBOC modulation can be well approximated by two QPSK signals apart, with negligible contribution of the crossed terms around its center frequency.1
Thus, we can bandpass filter around and get a good approximation of a QPSK signal, with very low energy components of , , and :
The same applies to , allowing an independent reception of two QPSK signals and thus requiring considerably less bandwidth than the processing of the whole E5 band.
Galileo signals spectra in E5. Source: Navipedia.
People’s Republic of China was also concerned with the importance of an accurate (and independent) navigation and timing satellite system.
According to the China National Space Administration, in a communicate dated on May 19, 2010, the development of the system would be carried out in three steps:
- 2000 – 2003: China built the BeiDou Satellite Navigation Experimental System, also known as BeiDou-1, consisting of 3 satellites. It offered limited coverage and applications and nowadays is not usable.
- by 2012: regional BeiDou navigation system covering China and neighboring regions.
- by 2020: global BeiDou navigation system.
The second generation of the system, officially called the BeiDou Satellite Navigation System (BDS) and also formerly known as COMPASS or BeiDou-2, will be a global satellite navigation system consisting of geostationary satellites and non–geostationary satellites. The geostationary satellites will be located at E, E, E, E and E. Non–geostationary satellites will be in medium–Earth orbit (MEO) and inclined geosynchronous orbit. Global coverage is planned by 2020. The ground segment will consist of one Master Control Station, two Upload Stations and Monitor Stations.
After the first geostationary satellite (located at E) was launched on October 31, 2000, a second satellite (located at E) and a third satellite (located at E) were launched on December 21, 2000 and May 25, 2003, respectively. The first geostationary satellite, COMPASS–G2, was launched on April 15, 2009.
On December 27, 2012, the Chinese government released the first version of BeiDou’s Interface Control Document (ICD), a 77-page document that included details of the navigation message, including parameters of the satellite almanacs and ephemerides that were missing from a “test version” of the ICD released exactly one year before. One year later cersion 2.0 was released, and version 2.1 followed in November 20167. The wonderful Navipedia keeps track of BeiDou status.
On December, 2012, the China Satellite Navigation Office released the official logo of the BeiDou system, the design of which incorporates the yin/yang symbol reflecting traditional Chinese culture, dark and light blue coloration symbolizing, respectively, space and Earth (including the aerospace industry), and the Big Dipper (a pattern of stars recognized on Earth’s night sky which star components are the seven brightest of the constellation Ursa Major) used for navigation since ancient times to locate the North Star Polaris and representing the first navigation device developed by China.
It also appeared that China intended to discontinue use of COMPASS as the English name for BeiDou. During the press briefing about publication of the ICD, Ran Chengqi, director of China Satellite Navigation Office, said the English designation will henceforth be BeiDou Navigation Satellite System with the abbreviation BDS.
Beidou’s Interface Control Document version 2.1 describes the Inphase component of the Beidou B1 link. The nominal frequency of B1I signal is 1561.098 MHz.
The chip rate of the B1I ranging code, is 2.046 Mcps, and the length is 2046 chips.
C. Fernández-Prades, L. Lo Presti, E. Falleti, Satellite Radiolocalization From GPS to GNSS and Beyond: Novel Technologies and Applications for Civil Mass–Market, Proceedings of the IEEE. Special Issue on Aerospace Communications and Networking in the Next Two Decades: Current Trends and Future Perspectives. Vol 99, No. 11, pp. 1882-1904. November 2011. DOI: 10.1109/JPROC.2011.2158032. ↩ ↩2
Global Positioning System Directorate Systems Engineering & Integration, Interface Specification IS-GPS-200H: Navstar GPS Space Segment/Navigation User Interfaces, Sept. 2013. ↩ ↩2
Global Positioning System Directorate Systems Engineering & Integration, Interface Specification IS-GPS-800D: Navstar GPS Space Segment/User Segment L1C Interface, Sept. 2013. ↩
Global Positioning System Directorate Systems Engineering & Integration, Interface Specification IS-GPS-705D: Navstar GPS Space Segment/User Segment L5 Interfaces, Sept. 2013. ↩
Global Navigation Satellite System GLONASS. Interface Control Document. Navigational radiosignal in bands L1, L2. Edition 5.1, Moscow, Russia, 2008. ↩ ↩2
European GNSS (Galileo) Open Service Signal In Space Interface Control Document, Version 1.2, Nov. 2015. ↩ ↩2 ↩3
BeiDou Navigation Satellite System Signal In Space Interface Control Document. Open Service Signal (Version 2.1). China Satellite Navigation Office, November 2016. ↩