Global NavIgatIon SatellIte Systems (GNSS)
Engineer's Guide
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GNSS is a general term for satellite constellation-based radio-navigation systems. The most well-known of such systems are Global Positioning System (GPS), governed by the USA (the one we use every day on our smartphones).
But there is more to know about them. How does it work? Why there is a need for it in our modern systems? What are the current state and future developments?
HISTORY
Although the principles of the technology date older(i.e. LORAN, Omega), the first experiments started in the 1960s in the USA for military purposes. In 1978 the first NAVSTAR (aka GPS) satellite was launched and 24 were launched and became operational by 1993. With the US Navy's attempts, the DoD decided to employ and fully invest in GPS systems. Initially, the system was only used for military purposes. After the year 1983, GPS became publicly available. When it became publicly available, the USA introduced Selective Availability (SA) to GPS signals that limit accuracy to ~100m to prevent tactical enemy usage. After 2000, SA was lifted and <10m accuracy performance became possible for general use. After 2018, the L5 signal is also introduced and up to 30cm accuracy is commercially available today.
There are multiple GNSS around. China's BEIDOU(first launch in 2000), Russia's GLONASS(first launch in 1982), EU's GALILEO(first launch in 2005), Japan's QZSS(first launch in 2010), and Korea's KPS(first launch in 2027 planned) to name a few.
THEORY
WORKING PRINCIPLE
All aforementioned satellite-based navigation systems work in a similar fashion. Here is how they work;
The process is called Trilateration. Imagine you are in an area that you have a map of and want to know where exactly you are(you have nothing but the map). And you can see 3 features by eyes which are also printed onto the map.
What you would do is, estimate the distance between 3 features and yourself to get a possible location area. On the 2nd figure, notice the estimation mistakes and relative uncertainty.
credit: nasa.gov
GNSS systems also use a similar technique to locate your precise position. There are satellites orbiting the earth that we precisely know their position (will be explained how). And we can measure the distance between us using RF signals that carry time information(will also be explained). The below figure explains the basic principle. 'R' values are calculated by using signal delays. If you only have 1 satellite signal, then you have only one R. This means, you can be anywhere on a big circle with 'R' radius. If you have only 2 satellites, 2 possible locations are the intersections of the two circles. For the location estimation -at least a confidence interval-, 3 satellites are required for trilateration, to calculate point location(cf. trilateration). But in the real world, we need at least 4 satellites 1 of which provide time sync. 1 extra satellite is also needed for usable accuracy to compensate for defects caused by "pseudo ranges".
In 1μs light
BURAYA TAŞINAN SİNYAL İLE NASIL MESAFE HESAPLANDIĞI (SAYISAL ÖRNEK HESAP YAP)
HANGİ UYGUNUN NE OLDUĞU
UYDULARIN OZİSYONUNU NASIL BİLDİĞİMİZİ
How do we know where exactly are the satellites?
hata miktarları ve performans ölçüleri hesaplamaları (2drms vs reliability makalesi). Hatalar nneye bağlı olarak oluyor, performansı ne etkiliyor, bazı ülkelerinki diğerleride iye daha hassas vs. theory of relativity(pseudo range vs)
GNSS AUGMENTATION TECHNIQUES-
DGPS, Local augmentation services such as......., RTK
ENGINEER'S PERSPECTIVE
For the sake of simplicity, the technology and details of the GNSS are explained based on "GPS" specifically. All the other systems have similar technology and system solutions but the service details and frequencies differ among constellations.
TECHNOLOGY & ARCHITECTURE
From the System of Systems architecture point of view, there are four segments in the GPS namely, Space, Control, Ground, and User Segments.
Space Segment
Orbits, angles, how 24/7 we can reach a satellite signal(;kaç uydu, kaç yörünge vs)
The nominal
specifications of the GPS satellites are as follows:
· Life goal: 7.5 years
· Mass: ~1 tonne (Block IIR: ~2 tonnes)
· Size: 5 meters
· Power: solar panels 7.5 m2 + Ni-Cd batteries
· Atomic clocks: 2 rubidium and 2 cesium
GPS currently provides two levels of service: Standard Positioning Service (SPS) which uses the coarse acquisition (C/A) code on the L1 frequency, and Precise Positioning Service (PPS) which uses the P(Y) code on both the L1 and L2 frequencies.
Also, L1 frequency carries "Navigation Messages"
The GPS navigation message contains four main parts: GPS time, satellite health, ephemeris, and the almanac.
Time
ATOMİC SAAT rubidyum?
Health
Ephemeris
Almanac
Hot start cold start
Satellites
constellations, numbers, SATELLİTE FUNCTİONS(ATOMİC SAAT rubidyum?, anten vs.)
Services, Codes
P(Y), SA, AS
Frequencies
L1,L2,L5 ....
Control Segment
Control Segment at frequency 1783.74 Mhz.
Uplink sigal The orbit information on board the satellite is
updated every hour.
Ground Segment
DGPS FREQ
User Segment
Receivers:
ortak alıcılar, karıştırmaya dirençl alıcılar
Antennas:
crpa multiband, single band
Datum
CONTEMPORARY STATE
Current Systems
L1, L2, L5 , GLONASS, BEIDOU, GALILEO VS ORTAK ALICIMEVZULARI
Civilian Use
WATCH, mobile phone, cars, .... buraya alıcı ve komple sistem fotoları
Military Use
SAASM CRPA
Services
S/A A/S
credit: novatel.com
FUTURE
GPS with quantum theory, q bits etc.
SYSTEMS ENGINEERING CONSIDERATIONS
Although they can be utilized stand-alone, GNSS systems provide essential time and position information to interfaced systems.
Here are a few tips for system engineers;
In the maritime environment, radar and ECDIS - the primary navigation systems for a safe voyage - heavily rely on the real position provided by GNSS.
Inertial Navigation Sytems also use GNSS position and time for initialization and comparison of the calculated and real positions when needed.
Particularly military systems require precise time info(1PPS). It is used for Time Division Multiple Access (TDMA) applications, frequency hopping, cryptographic equipment, and Time of Validity (TOV) info (for sensor data). All mission-critical systems need to have a way of time synchronization to be integrated with each other.
Typically, "Data Distribution Systems" receive and deliver 1PPS and position-related GNSS messages to linked systems on complex platforms.
The information of position and time would be interrupted in the event of GNSS failure. To avoid this, you may think about redundancy concepts.
You may consider;
2 different GNSS receivers with separate antennas,
An Inertial Navigation System (INS) that can continue to calculate the position when the signal is lost,
Providing GNSS receivers inside the critical systems so that the 'main GNSS' failure can be tolerated.
Additionally, you may think about getting an atomic watch (Time System) so that, interfaced systems can still receive accurate 1PPS information in the event of GNSS failure.
Network systems(Time servers etc.) may keep local time and can continue to feed basic time information unless a power cut.
The following NMEA messages are typically supported and fed by GNSS receiver systems:
ZDA
RMN
....
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