GPS

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GPS stands for Global Positioning System and is a navigation system owned by the US government. A GPS receiver is the actual device used in cars, phones, and satellites which uses this system to determine their location. Many receivers are actually compatible with multiple Global Navigation Satellite Systems (GNSS). GPS is the most common in North America (and many other places) and is often used as the default when discussing GNSS. A GNSS system works by having a series of satellites in orbit broadcasting information down to the earth. This information includes things like the satellites position and highly accurate time data. When a GPS receiver gathers this information from multiple satellites it can use this to calculate its own location based on the time it takes for each transmission to reach it. The more satellites which it receives a signal from the more accurate the position data will be.

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GPS Guide: OEM 719

For CSDC-4 the team purchased an OEM719 GPS receiver from Novatel. Since the CSDC-4 satellite was not launched the GPS will be used for CSDC-5 as well. Software and the hardware connections for this GPS were already completed with CSDC-4 and will also be adopted for CSDC-5. This GPS guide can also be found here.

Why the OEM719

How it works

The global positioning system works through a network of 30 satellites orbiting at 20,000km. The network was developed and is controlled by the US government. Each GPS satellite transmits its position and the current time at constant intervals. This data is received by a GPS receiver via an antenna. The GPS receiver requires a signal from at least 3 GPS satellites to calculate its position. This information is then sent out in a specific format that must be decoded and processed.

GPS Signal Types

Four GPS signal types exist for civilian use:

  • L1 C/A: A legacy GPS signal that uses “coarse/acquisition” code (rather than precision code used for the military).
  • L2C: Only on newer satellites, provides better accuracy, acts a redundant signal.
  • L5: “Safety of Life” secure and robust signal for life critical systems such as plane landings
  • L1C: New civilian use signal on 10 satellites launching from 2017 onwards.

The use of multiple signals can improve accuracy and reduce chance of errors.

Navigation Message

The message received from each satellite contains three main pieces of information:

  1. The GPS date, time and the satellites “health” status.
  2. The ephemeris: Orbital information of satellite for position calculation. (Valid for <4 hours)
  3. The almanac data: Status data for parts or all of GPS satellite network. (Valid for < 180 days)

Each message consists of 1,500 bit long “frames” each consisting of 5 “subframes” 300 bits long. Each subframe contains 10 words 30 bits long and require 6 seconds to transmit.

Satellites are uniquely identified by a serial number called space vehicle number (SVN) and with a space vehicle identifier (SV ID) and pseudorandom noise number (PRN number) which uniquely identifies the ranging codes that a satellite uses.

Communicating

The following are a few different methods of communication with the GPS.

UART

“Universal asynchronous receiver-transmitter” is a configurable asynchronous serial communications format. Asynchronous SC means the data stream contains sync information via start and stop signals rather than through a common clock.

The receiver handles data in 6 potential formats: Abbreviated ASCII, ASCII, Binary, RTCMv3, NOVTELX, and NMEA. We will be attempting to use NMEA.

NMEA

A standard for serial-data networking of marine electronic devices. It serves as a simple format for GPS data which contains all the major information required. Data in the NMEA Format can be read using the TinyGPS library. NMEA Standard log information can be found on page 594 of the OEM7 Commands and Logs Reference Manual. A useful library is tiny gps.

SPAN

NovAtel's Synchronous Position, Attitude and Navigation (SPAN®) technology brings together two very different but complementary positioning and navigation systems namely Global Navigation Satellite System (GNSS) and an Inertial Navigation System (INS) Requires a GPS, IMU and GNSS antenna


You can communicate with the receiver using NovAtel Web User Interface, NovAtel Connect or a terminal emulation program.

Remote Terminal

One method of communicating with the receiver is through a remote terminal. To communicate with the terminal, the receiver requires only the RX, TX and GND lines be used. Handshaking is not required but is an option. Ensure the terminal’s communications set up matches the receiver’s RS-232 settings.

Computer

A computer can emulate a remote terminal as well as provide the added flexibility of supporting multiple command batch files and data logging storage files. Use any standard communications software package that emulates a terminal to establish bidirectional communications with the Chapter 3 OEM7 Receiver Operation OEM7 Installation and Operation User Manual v3A 52 receiver. Examples include NovAtel Connect and PuTTY. All data is sent as raw 8-bit binary or ASCII characters.

COM Port to use RS-232 or RS-422

COM1 of the OEM729 communicates using RS-232 protocol by default, but can be configured to use RS-422 protocol. The SERIALPROTOCOL command is used to select RS-232 or RS-422 for the COM port.


The data transfer rate determines how fast information is transmitted. Take for example a log whose message byte count is 96. The default port settings allows 10 bits/byte (8 data bits + 1 stop bit + 1 framing bit). It therefore takes 960 bits per message. To get 10 messages per second then requires 9600 bps. Also remember that even if you set the bps to 9600, the actual data transfer rate may be lower and depends on the number of satellites being tracked, data filters in use, and idle time. It is therefore suggested that you leave yourself a margin when choosing a data rate. Can operate at data transfer rates as low as 300 bps, this is not recommended. For example, if several data logs are active (that is, a significant amount of information needs to be transmitted every second) but the bit rate is set too low, data overflows the serial port buffers, causing an error condition in the receiver status that results in lost data.

This means we can go lower than recommended to save small amounts of power if needed.

Novatel Offer several position algorithms that can be used with the receiver card.