31. May 2018 · Write a comment · Categories: Embedded Hardware · Tags: , , , ,

This blog is the sequel of the previous blog on Geo positioning system technologies. Here we will discuss more in detail about the important terms related to GPS receivers and guidelines for selecting suitable GPS receiver module for embedded system design.

GPS Receiver

Connection diagram for GPS module

GPS Module – Connection Diagram

The above picture depicts the typical connection diagram of GPS module with any host controller. There are multiple interface options available for a host controller to receive the NMEA data where UART, SPI and USB are most common. PPS signal is an output from GPS and it is discussed in detail in the upcoming sections. Most of the GPS modules have internal patch antenna but also supports external active antenna connection.


 NMEA (National Marine Electronics Association) data is the detailed output from any GPS receiver that includes the current location data of the receiver such as Latitude, longitude, altitude etc. This data is provided in a standard format to the user for compatibility with various manufacturers much like ASCII standard for digital computer characters.

Following is the example of a NMEA message from a GPS receiver,


All the NMEA message starts with a $ character where each field is separated by a comma.

“GP” in GPGGA – represent that it is a GPS position. For GLONASS, it will be GL instead of GP.

GPGGA is a basic GPS NMEA message and many other NMEA messages are also available providing similar or additional information beside GPS coordinates. There are 19 different NMEA messages and they are listed below,

  1. $GPBOD – Bearing, origin to destination
  2. $GPBWC – Bearing and distance to waypoint, great circle
  3. $GPGGA – Global Positioning System Fix Data
  4. $GPGLL – Geographic position, latitude / longitude
  5. $GPGSA – GPS DOP and active satellites
  6. $GPGSV – GPS Satellites in view
  7. $GPHDT – Heading, True
  8. $GPR00 – List of waypoints in currently active route
  9. $GPRMA – Recommended minimum specific Loran-C data
  10. $GPRMB – Recommended minimum navigation info
  11. $GPRMC – Recommended minimum specific GPS/Transit data
  12. $GPRTE – Routes
  13. $GPTRF – Transit Fix Data
  14. $GPSTN – Multiple Data ID
  15. $GPVBW – Dual Ground / Water Speed
  16. $GPVTG – Track made good and ground speed
  17. $GPWPL – Waypoint location
  18. $GPXTE – Cross-track error, Measured
  19. $GPZDA – Date & Time

181908.00 UTC Time stamp in hours, minutes and seconds.

3004.6040718 Latitude in DDMM.MMMMM format. Decimal places are variable.

N North latitude

07040.3900269 Longitude in DDMM.MMMMM format. Decimal places are variable.

W West Longitude

4 Quality Indicator. Other options are as follows,

0 = Fix not valid

1 = Uncorrected coordinate

2 = Differentially correct coordinate (e.g., WAAS, DGPS)

4 = RTK Fix coordinate (centimeter precision)

5 = RTK Float (decimeter precision)

13 Number of satellites used in the coordinate

1.00 denotes the HDOP (horizontal dilution of precision).

408.135 Altitude of the antenna

M unit of altitude (e.g. Meter or Feet)

29.200 geoidal separation. Subtracting this from the altitude of the antenna will provide the Height Above Ellipsoid (HAE)

M Unit of geoidal separation

0.10 age of correction

0000 correction station ID

*40 checksum

Time-To-First-Fix (TTFF)

 Another most important deciding feature of GPS receiver is the TTFF. TTFF of a receiver only decides how fast it can provide a valid NMEA data to the user. So the user must be very careful on the TTFF values while choosing the receiver. The TTFF values will be provided in the datasheet in seconds.

Any receiver can boot up in any one of the following three modes:

  1. Hot start
  2. Warm start
  3. Cold Start

The Time-To-First-Fix (TTFF) depends on the startup mode, with cold starts giving the longest TTFF. Following are the factoring affecting boot mode,

  1. Non availability of valid almanac and ephemeris data
  2. Level of incoming signals
  3. The unit is within 60 miles (100 Km) of location of previous fix
  4. Length of time since previous fix

Cold Start Mode

: Any receiver start in this mode when,

  1. Receiver has not been used for long time
  2. Moved several hundred kilometers
  3. Incoming signals are low or marginal. i.e. the predicted satellites are overhead of the receiver but cannot receive signals due to tall buildings, foliage etc.

Any of the above situation will make the receiver cannot predict which satellites are overhead. Then the receiver works with an internal list of satellites and tries to acquire each one in turn. This allows the receiver to discover the satellite which are in view and eventually establish a position. Normally the TTFF on cold start takes 2 to 4 minutes.

Warm Start Mode

: Any receiver start in this mode when,

  1. It has valid almanac data
  2. The current location of receiver is within hundred kilometers of the last fix location
  3. Receiver has been active in last three days and current time is known
  4. Ephemeris data that has not been stored or it has become stale
  5. Good signal strength and 4 or more satellites are visible

In this mode the receiver can predict which satellites are overhead but it needs to download the current Ephemeris data. TTFF for warm start mode is typically 45 seconds.

Hot Start Mode:

The receiver starts in this mode when the warm start conditions are met and,

  1. A fix has been established within the last 2 hours
  2. The receiver has stored valid ephemeris data for atleast 5 satellites

Receiver tracks the overhead satellites and needs to download minimum data to establish a position. TTFF for a hot start is typically 22 seconds. 


We have come across many startup modes for any receiver, but wonder how smartphone GPS units get a fix almost immediately?

They use Assisted-GPS (A-GPS) as a procedure of improving the TTFF or even allowing a fix in conditions where receiver might not be able to function.

A-GPS device will use a data connection available on the smartphone to contact an assistance server. This server will supply almanac and ephemeris data instead of waiting to receive them from the satellites. The server can also provide approximate location derived from the cell phone towers facilitating immediate fix.

PPS Signal

Most GPS receiver modules have an output called Pulse Per Second abbreviated as PPS. It is a digital output signal with much lower jitter than anything a MCU can do. PPS signal can be used          to time things very accurately at a precision in nanoseconds.

They are most commonly used to wake the MCU from deep sleep mode periodically at an interval of one second. In some applications, they are used to synchronize the system time and rectify the time drift due to the temperature effects of the RTC crystals. 

Selection guide for receiver

There is lot of options available while selecting a receiver module. Some of the main factors to be considered while selecting the receiver are as follows,

  1. Multi system support – Receiver module can be GNSS or GPS only. GNSS module provides simultaneous support to GPS, GLONASS, BeiDuo and Galileo systems. GNSS modules are better than GPS only modules and the cost of the GNSS module will be bit more comparative to the other.
  2. Size – Most important deciding factor for size constraint devices. Nowadays modules are getting very small but in general the antenna will also shrink to fit the module which will reflect in the lock time and accuracy.
  3. Number of channels – At a given time, there are so many satellites available in view but the number of channels a receiver module can track/acquire will affect the TTFF. The more a module can track/acquire many satellites, the faster it will find a fix.
  4. Update Rate – It is the time interval that how often a receiver module can recalculate and reports its position. The standard rate of a module is 1Hz (one report per second). Fast update rate means that there are more NMEA sentences coming out of the receiver module by which any microprocessor will be quickly overwhelmed trying to parse that much data.
  5. Power consumption – Another important factor that decides the success of a battery powered devices. There are many factors deciding the power consumption of a receiver module and in any case the typical power consumption should be low in few tenses of mA ranging between 25 to 30mA. Most of the receivers have various power saving modes which can be used during idle conditions.
  6. Antenna – Antenna defines the quality of the receiver module and it must be finely trimmed good enough to pick up the frequencies. Receiver modules available with a small patch antenna on top of it. It is made of ceramic. Some modules will also have dedicated antenna pin for connecting external active antenna. Receivers used in cars require external antenna support since the receiver module has to be placed inside the vehicle mostly connected to the OBD port which will be placed beneath the dashboard where a patch antenna will struggle to receive the signals.
  7. Accuracy – Varies between modules. Most modules can get it down to +/- 3m and sub meter or centimeter accuracy are also available but bit expensive.

About Embien

Embien Technologies is a product engineering service provider with handsome experience on automotive product developments. Embien has various solutions for time to market developments for automotive domain including Sparklet Embedded GUI library for 2D or 2.5D or 3D Instrument clusters, Flint IDE for GUI prototyping and eStorm-B1 – Automotive grade BLE Module.

Geo positioning system or GPS has become more or less a norm for smart phones. Geo positioning system was first created for the navigation of defense vehicles in any part of world. But over the period of time, this system is being used in many other purposes outside defense and has proved itself to be a revolutionary technology in today’s world. Apart of the smartphone, most of the premium cars and commercial vehicle do have inbuilt GPS for fleet tracking, vehicle Telematics, and driver assistance.

Apart from such fleet navigation use cases, GPS are now being used for many applications such as locating nearby restaurants, hotels and gas stations and finds huge applications in tourism industry. Personal navigation devices also employ GPS technology.

Also most of the IoT/M2M applications use GPS modules. Some of them are as follows

  • Smart utility metering
  • Connected health and patient monitoring
  • Smart buildings
  • Security and video surveillance
  • Smart payment and PoS systems
  • Wearable devices etc

While the term GPS in general represents the technology, there are numerous systems being used to achieve this. In this blog, we will briefly describe about the various such Geo positioning systems and related concepts.

Geo Positioning System – Technology

Any geo positioning system uses about three to four satellites from more than a dozen of satellites orbiting in a group (satellite constellation) to provide autonomous geo-spatial positioning. These satellites transmit 1500 bits of data such as the satellite health, its position in space, propagation delay effects, constellation status, the time of information being sent, etc. This allows a small electronic receiver to determine its location in terms of latitude and longitude based on triangulation of the data obtained from at least three satellites. With four or more satellites, the receiver can also determine the 3D position, i.e. Latitude, longitude and altitude. In addition, a GPS receiver can provide information about the speed and direction.

Anyone with the GPS receiver can access the system. Since it is an open source and providing almost accurate 3D position, navigation and timing 24 hours a day, 7 days a week, all over the world, it is used in numerous applications even in GIS data collection, mapping and surveying.

Geo Positioning System – Types

At present there are many options available for geo positioning system each of them owned and operated by countries such as US, Russia, European Union, China, etc. They are as follows

NAVSTART GPS – GPS, Global Positioning System is a one among the various satellite navigation system designed and operated by the U.S. Department of defense. Official name of GPS is Navigational Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS).

GLONASS – Global Orbiting Navigation Satellite System, GLONASS developed by Russian, is an alternative to GPS and is the second global navigational system in operation providing global coverage with comparable precision. A GLONASS satellite design has various upgraded versions and the latest is GLONASS-K2 which is expected to operate in early 2018.

Galelio – Galelio is created by European Union with the aim to provide an independent high precision positioning system for European nations.

BeiDou – BieDuo Navigation Satellite System (BDS) is a Chinese satellite navigation system consisting of two separate satellite constellations BeiDuo-1 and BeiDuo-2. BeiDuo-1 is decommissioned and BeiDuo-2 also known as COMPASS offering services to customers in the Asia-Pacific region with a partial constellation of 10 satellites in orbit.

IRNSS – Indian Regional Navigation Satellite System also known as NAVIC (Navigation with Indian Constellation) is a regional satellite navigation system covering the Indian region extending 1500Km. This constellation is already in orbit and expected to operate in early 2018.

Satellite Based Augmentation System (SBAS)

All the above systems are autonomous and governed by the respective countries. Other than autonomous systems, other regional augmented systems are available that run with the aid of other autonomous satellites. These augmentation systems will provide reference signals (Signal in Space- SIS) via satellites to the receivers including correction information with the objective of increasing the accuracy of the position. In addition to the accuracy they also help to maintain the reliability and availability of the navigation system. The whole system is known as SBAS (Satellite Based Augmentation System) and satellite providing the SIS signal are known as SBAS GEO satellites. Some of them are as follows,

GAGAN – GPS-Aided Geo Augmented Navigation – It is the implementation of SBAS by Indian government. It supports pilots to navigate in the Indian airspace by an accuracy of 3m.

QZSS Quasi Zenith Satellite System is a project governed by Japanese government and operated in order to receive the US operated GPS in the Asia-Oceania regions with Japan as a primary focus.

Other commonly available SBASs are WAAS (US), EGNOS (EU) and MSAS (Japan).


The above mentioned satellite systems such as global, regional and augmented systems are integrated together to form Global Navigation Satellite System, GNSS. It is a standard term for satellite navigation systems providing autonomous geo spatial positioning with global coverage. It is a satellite system that is used to pinpoint the geographic location of a user’s receiver anywhere in the world. Three GNSS systems are currently in operation: the United States’ Global Positioning System (GPS), the Russian Federation’s Global Orbiting Navigation Satellite System (GLONASS) and the Europe’s Galileo.

Most degrading factor of a receiver, i.e. Line of Sight degradation can be solved with the GNSS system due to its accessibility to multiple satellites and if one satellite system fails, GNSS receivers can pick up signals from other system.

Navigation messages

Any satellite in the constellation will transmit a detailed set of information such as each satellite position, network to receiver called the navigation messages. Following are available in the navigation message, 

  1. Date and time together with the satellite status and an indication of its health 
  1. Almanac data – Contains coarse orbit and status information of all the satellites in the constellation. It allows the GPS receiver to predict which satellites are overhead, shortening acquisition time. Almanac data can be received from any of the satellites. The receiver must have a continuous fix for approximately 15 minutes to receive a complete almanac data. Once downloaded it is stored in the non volatile memory.
  1. Ephemeris data – Contains precision correction to the almanac data necessary for the receiver to calculate the position of the satellite. It is continuously updated every 2 hours and so ephemeris data of a deactivated receiver will become stale after 3 to 6 hours.

Time-To-First-Fix (TTFF)

For a receiver to get a fix, it needs a valid almanac, initial location, time and ephemeris data. When a receiver is switched ON, it requires some time delay for the first fix. This delay depends on how long since the stored data’s being used. The time delay is commonly termed as Time To Fist Fix, TTFF and it is one of the main factor for receiver selection.

About Embien

Embien Technologies is a leading provider of embedded design services for the Automotive, Semi-conductor, Industrial, Consumer and Health Care segments. Embien has successfully designed and developed many products with GPS for various domains such as Wrist wearable based tracker device for healthcare, Vehicle Telematics device for automotive, Data acquisition/logger devices for industry etc.

In the past, electronic devices in vehicles are connected via point to point wiring systems. Automotive manufacturers started using more and more electronics in vehicle, which resulted in bulky wire harnesses that were heavier and expensive too. Then they introduced a specialized internal communication network called vehicle bus that interconnects electronic devices inside a vehicle. Vehicle bus reduced the wiring cost, weight and complexity.

At present there are several types of network types and protocols used in vehicles by various manufacturers. Most common vehicle bus protocols includes,

  1. CAN
  2. LIN
  3. FlexRay
  4. MOST
  5. DC-BUS
  6. IEBUS
  7. J1850
  8. ISO 9141-1/-2
  9. D2B – Domestic Digital Bus
  10. VAN

Among the various bus protocols, CAN emerged as the standard in-vehicle network and it became the international standard known as ISO 11898. Bosch originally developed the Controlled Area Network (CAN) and it has been adopted by the automotive industry. Several higher level protocols have been standardized on CAN such as CANopen and DeviceNet which are commonly used for industrial communications. CAN is also adopted specifically for classes of vehicles such as J1939 for commercial vehicles and ISO11783 for agricultural vehicles.

In this blog, we will describe in detail about the CAN protocol as an in-vehicle network.

CAN Bus – Basics

CAN bus is an inexpensive, robust vehicle bus standard designed for multiple CAN device communications with one another without a host computer. CAN is also called as multi-master serial bus and the CAN devices on bus are referred to as nodes. Two or more nodes are required on the CAN network to communicate. All nodes are connected to each other via a two wire bus (CAN H and CAN L) and the wires are 120ohms nominal twisted pairs. Termination resistor commonly 120 ohms is must in each node in order to suppress the reflections as well as return the bus to its recessive or idle state.

Following is the block diagram of the CAN bus architecture,

CAN bus architecture

Architecture of CAN bus

Each node in the CAN bus requires the following

  1. Transceiver – It converts the data from the CAN controller to CAN bus levels and also converts the data from CAN bus levels to suitable level that the CAN controller uses.
  2. CAN controller – They are often an integral part of the microcontroller that handles framing, CRC etc.
  3. Microcontroller – It decides what the received messages mean and what messages it wants to transmit.

The transceiver drives or detects the dominant and recessive bits by the voltage difference between the CAN H and CAN L lines. The nominal dominant differential voltage is between 1.5V to 3V and recessive differential voltage is always 0V. CAN transceiver actively drives to the logical 0 (dominant bits) voltage level and the logical 1 (recessive bits) are passively returned to 0V by the termination resistor.  The idle state will always be in the recessive level (logical 1).

Individually, CAN H will always be driven towards supply voltage (VCC) and the CAN L towards 0V when transmitting a dominant (0). But in practical case, supply voltage (VCC) or 0V cannot be reached due to transceiver’s internal diode drop. CAN H/L will not be driven when transmitting a recessive (1) where the voltage will be maintained at VCC/2.

The following figure depicts the block diagram and real time capture of the CAN signals.

Voltage level of CAN bus lines

CAN Bus – Voltage Levels


Capture of CAN bus signals using DSO

Real time capture of CAN bus signals

CAN Physical Layers – Types

CAN has different physical layers which classifies the certain aspects of the CAN network such as signaling scheme, cable impedance, maximum data rates, electrical levels, etc. Following are the most commonly used physical layers,

1. High Speed CAN

High speed CAN is implemented with two wires and allows communication at data rate up to 1Mbits/s. It is also named as ISO 11898-2. Antilock brake system, engine control modules, emission systems uses high speed CAN.


CAN with Flexible Data rate is the next generation of high speed CAN communication with improved standards for higher data rates. CAN FD overcome the bandwidth limitation problems by allowing data rates higher than 1Mbits/s while also increasing the support of payloads up to 64 bytes in a single message.

3. Low speed/fault-tolerant CAN

Low speed/fault tolerant CAN networks are also implemented with two wires and can communicate at a data rate of up to 125Kbits/s with fault tolerant capabilities. They are also named as ISO 11898-3 and found in devices where wires that have to pass through the doors of the vehicle which have light stress that is inherent to opening and closing a door.

4. Single wire CAN

Single wire CAN interface have lower data rate up to 33.3Kbits/s and also named as SAE-J2411. The devices that do not require high performance like seat and mirror adjuster use Single wire CAN interface.

Transceivers are available for each type of CAN physical layer and it is one of the important criteria while choosing the transceiver. Many semiconductor manufacturers provide CAN transceivers including NXP semiconductors, Texas instruments, STMicroelectronics, Maxim Integrated, Infineon Technologies and Linear Technology.

Apart from generic CAN transceivers, there are special purpose transceivers such as galvanically isolated transceivers that are used for providing the isolated interface between a CAN protocol controller and the CAN bus. For galvanic isolated design, there is a need for isolated power supply and hence the design becomes more complex and costlier.

Transceivers with option for direct interface with the microcontroller are also available which reduces the requirement of external buffer ICs for voltage compatibility.

CAN terminologies

Following are some CAN bus terminologies that are useful to understand how CAN bus communication works. 

1. CAN Frame and fields descriptions

Devices in CAN network send data in packets called frames. Following image depicts frame format,

CAN Protocol frame format

Frame Format of CAN protocol

SoF – Start of Frame bit – indicates the beginning of a message with a dominant (logic 0) bit

Arbitration ID – identifies the message and indicates the message’s priority. Frames come in two formats — standard, which use an 11-bit arbitration ID, and extended, which uses a 29-bit arbitration ID

IDE – Identifier Extension bit – This bit allows differentiation between standard and extended frames

RTR – Remote Transmission Request bit – This bit is used to differentiate a remote frame from a data frame. A logic 0 (dominant bit) indicates a data frame. A logic 1 (recessive bit) indicates a remote frame

DLC – Data length code – It indicates the number of bytes the data field contains

Data Field – contains 0 to 8 bytes of data and up to 64 bytes of data for CAN-FD (Flexible Data rate)

CRC – Cyclic Redundancy Check – The CRC field is used for error detection. It contains 15-bit cyclic redundancy check code and a recessive delimiter bit.

ACK – Acknowledgement slot – any CAN controller that correctly receives the message sends an ACK bit at the end of the message. The transmitting node checks for the presence of the ACK bit on the bus and reattempts transmission if no acknowledge is detected

2. Bus Arbitration

Arbitration is the process in which two or more CAN controller agrees on who is to use the bus. It is of very important for the really available bandwidth for data transmission and this is the base for CAN bus communication. Arbitration process is performed over the arbitration ID.

3. Bit stuffing

Bit stuffing is a practice used to guarantee enough edges in the NRZ (Non-Return to Zero) bit stream to maintain synchronization. After five identical and consecutive bit levels have been transmitted, the transmitter will automatically inject (stuff) a bit of the opposite polarity into the bit stream. Receivers of the message will automatically delete (destuff) such bits. If any node detects six consecutive bits of the same level, a stuff error will be flagged.

How CAN communication works?

As mentioned early, CAN is a Peer-to-Peer network in which there is no master that controls the transmission between nodes. When any CAN node is ready to transmit data, it should undergo a process called message arbitration. In this process, CAN node will check to see if the bus is idle and starts the transmission once it is idle. This will also trigger other CAN nodes in the bus and hence results in two or more nodes starting a message at a same time which results in a conflict. The conflict is resolved in the following methods,

  1. The transmitting node monitors the bus while they are sending data
  2. If any node detects a dominant level (logical 0) while sending a recessive level itself, it will fail in the arbitration process and quits immediately will start acting as a receiver.
  3. This arbitration process is performed while sending the arbitration ID field of the CAN frame and at the end, only one transmitter is left on the bus i.e. the node with the highest priority (lowest arbitration ID) will pass the arbitration.
  4. Then the node which has won the arbitration will continue message transmission as if nothing had happened.
  5. Other receiving node can decide if a message is relevant or if it should be filtered using a combination of hardware and software filters.
  6. This process is continuous and other nodes will transmit their messages when the bus has become available.
Bit Wise Arbitration process

CAN protocol – Bit Wise Arbitration

With this blog, we have covered all the basics of CAN communication including the physical layers, Data link as well as arbitration mechanisms. In the upcoming blog, we will explain more on software perspective about configuring a CAN controller for operation and handling message flow.

About Embien

Embien Technologies is a leading provider of product engineering services for the Automotive, Semi-conductor, Industrial, Consumer and Health Care segments. Embien has successfully executed many projects like Android based Auto infotainment system, GUI for TFT based instrument clusters, vehicle tracking devices, etc. Embien also offers a set of solutions such automotive grade BLE module (eStorm-B1), CAN to BLE gateway, CAN to RS232/RS485 gateway, LIN to BLE gateway, Sparklet GUI library that can be used to shorten automotive product development costs and time significantly.