Have you ever wondered why it sometimes takes your GPS module 10-20 minutes to get a GPS fix? This post will explain why.
Each satellite sends a message every 30 seconds. This message consists of two main components;
Ephemeris data, used to calculate the position of each satellite in orbit
Almanac , which is information about the time and status of the entire satellite constellation.
Only a small portion of the Almanac is included in a GPS message. It takes 25 messages (12.5 minutes) to get the full Almanac. The full Almanac is needed before a GPS fix can be obtained. This is Time To First Fix (TTFF).
TTFF is a measure of the time required for a GPS receiver to acquire satellite signals and navigation data, and calculate a position solution (called a fix).
The above happens during a cold start, this is when the GPS module has been off for some time and has no data in its memory. A full Almanac download is required to get TTFF. If the GPS module has clear line of sight to all satellites, the shortest time for TTFF is 12.5 minutes.
In a warmstart scenario, the GPS module has valid Almanac data, is close to its last position (100km or so) and knows the time within about 20 seconds. This approximate information helps the receiver estimate the range to satellites. The TTFF for a warm start can be as short as 30 seconds, but is usually just a couple of minutes.
A receiver that has a current almanac, ephemeris data, time and position can have a hot start. A hot start can take from 0.5 to 20 seconds for TTFF.
Smarts phones use Assisted GPS (aGPS), this allows them to download the Ephemeris data and Almanac over the cell network which greatly reduces the TTFF.
The BerryGPS-IMU was used to capture the GPS coordinates as well as “attitude”. No external antenna was needed as the BerryGPS-IMU includes an internal antenna.
The “attitude” would include values such as pitch, roll, direction. Some of this data you can see annotate in the image below.
Other programs can use some of this data to plot the image on a map and even show the direction of the camera at the time the image was taken. A good example of this is seen in GeoSetter
The cap has the BerryGPS-IMU sitting on top of the visor, with the Raspberry Pi sitting under the viso. Some holes where made in the visor to allow connectivity between the BerryGPS-IMU and Raspberry Pi.
We also created a basic camera mount out of 3mm laser cut acrylic. M2.5 Nylon screws were used to hold everything in place.
Navit can be installed without a GPS connected to your Raspberry Pi, but you will not be able to use the real-time turn by turn navigation. You will however be able to browse maps. If you are not going to use a GPS, you can skip to the next step.
As we are using the BerryGPS-IMU, we will be following the guide in the link below. As most GPS modules use serial to communication, this guide can be followed for other GPS modules.
The images below shows how we have connected the BerryGPS-IMU to the Raspberry Pi 3 whilst it is in the SmartPi Touch case.
If you plan on testing this out in your car, you need to be mindfully of where you place your BerryGPS. In my setup and I have placed it in the air vent as shown below, and BerryGPS gets a good strong signal.
If you are using an external antenna, then there is no need to worry about where your BerryGPS is placed.
BerryIMU also works great with Windows IoT Core on the Raspberry Pi.
Our Git repository contains the source files needed to get the BerryIMU up and running on Windows IoT.
The code will print out the following values to the screen;
Raw values from the gyroscope, accelerometer and magnetometer.
Accelerometer calculated angles.
Gyro tracked angles.
Fused X and Y angles.
Tilt compensated heading.
Connecting BerryIMU to a Raspberry Pi
BrryIMU can connect via the jumper cables to the Raspberry Pi as shown below;
Or BerryIMU can sit right on top of the GPIO pins on a Raspberry Pi. The first 6 GPIOs are used as shown below.
Get the Code
Download the BerryIMU code for Windows IoT from our GIT repository. The files you need are under the WindowsIoT-BerryIMU folder.
You will need to download the entire git repository as GIT doesn’t allow downloading individual folders.
Once downloaded, double-click the file WindowsIoT-BerryIMU.sln to open up the project in Visual Studio.
About the code
The project code outputs all of the needed values to the screen and a complementary filter is used to fuse the accelerometer and gyroscope angles.
We have a number of guides already documented on how to get the BerryIMU working with the Raspberry Pi. http://ozzmaker.com/berryimu/ These are based on Raspbian, however the principals and math are the same for Windows Iot.
The final values which should be used are the fused X &Y angles and the tilt compensated heading.
The sensor on the BerryIMU is the LSM9DS0 and all the I2C registers for this sensor can be found in LSM9DS0.cs
The main code can be found in MainPage.xaml.cs
A complementary filter is used to fuse the angles. Is summary, the complementary filter trusts the gyroscope for short periods and trusts the accelerometer for longer periods;
Changing how much trust is given for each of the sensors can be changed by modify the complementary filter constant at the start of the code.
const float AA = 0.03f; // Complementary filter constant
The loop speed is important as we need to know how much time has past to calculate the rotational degrees per second on the gyroscope.
A time delta is set at the start of the code.
const int DT = 100; //DT is the loop delta in milliseconds.
This is then used to specify a new timer method.
periodicTimer = new Timer(this.TimerCallback, null, 0,DT);
Here you can see where DT is used to keep track of the gyroscope angle. You can also see it in the above calculation for the complementary filter.
//Calculate the angles from the gyro
gyroXangle += rate_gyr_x * DT / 1000;
gyroYangle += rate_gyr_y * DT / 1000;
gyroZangle += rate_gyr_z * DT / 1000;
The calculations in the code are based on how the BerryIMU is orientated. If BerryIMU is upside down, then some of the angles need to be reversed. It is upside down when the skull logo is facing up(or to the sky).
If it is upside down, set the below value to true. Otherwise, set it to false.
Our BerryIMU GIT repository has been updated with code for the Teensy, specifically Teensy 3.6.
Now you can have access to an accelerometer, gyroscope, compass, temperature and pressure sensor on your Teensy.
Here you can see the angles displayed using the Serial Plotter in the Arduino IDE which is connected to a Teensy 3.6.
4 web pages are created:
“/” – home page which is used to display the gauges.
“/chart.html” – Is used to display chart.
“/table.html” – Is used to display the data in a table.
“/data.json“- Is used by the home page to update the the gauge values.
The below diagram shows how to connect a BerryIMU to an ESP8266 microcontroller, in this case it is the Adafruit Feather Huzzah .
Both GPS modules use the M10478-A2 from Antenova, which is a high quality GPS module which is able to track 22 satellites and has an internal antenna. This means no external antenna is needed if the module has clear access to sky. Both feature a SuperCap to store ephemeris data for up to four hours. This and many more features are included.
Both have been specifically designed for the Raspberry Pi Zero, however they will work with any version of Raspberry Pi.
The BerryGPS-IMU also includes all the components found on the BerryIMU. And is compatible with the existing code in our repository. The BerryGPS-IMU present a lot of sensors in a very, very small package.
The ESP8266 is another good microcontroller which can be used with the BerryIMU.
The ESP8266 is small and includes Wifi.
In this guide we will setup of the ESP8266 to provide a web page which we can then use to read the accelerometer, gyroscope and compass values from the BerryIMU. We will also force this webpage to refresh every 1 seconds.
The ESP8266 Arduino core to program our ESP8266. This allows you to use the Arduino IDE to program and upload to the ESP8266.
We have used the Adafruit Feather Huzzah and the Sparkfun Thing Dev board in this guide. Both are excellent boards with included USB to serial converters. Just plug in and upload. This guide can also be used with other ESP8266 boards, just take note of the pins used.
The code can be found here . Download the entire Git repo. The code for this guide can be found under the directory ESP8266-BerryIMU/BerryIMU_ESP8266_simple_web/ The file you load into the Arduino IDE is BerryIMU_ESP8266_simple_web.ino.
This guide will only cover the specific to the ESP8266. There is another guide here http://ozzmaker.com/berryimu/ which covers the code used to calculate the angles and heading from the BerryIMU.
The first thing to do is update the code with your wireless network settings.
Further down you can see where we define the web server and what port to listen on
There is then a function called handleroot(). This is what builds the web page and sends it to the client when the client requests it E.g. When a web browser requests for a page.
Looking at the line which contains the meta tag, you can see where the refresh timer is set to 1 seconds.
I have also hilighted the variables which store the angles and heading from the BerryIMU.
//Create webpage with BerryIMU data which is updated every 1 seconds
server.sendContent("HTTP/1.1 200 OK\r\n"); //send new p\r\nage
("<html><head><meta http-equiv='refresh' content='1'</meta>"
"<h3 style=text-align:center;font-size:200%;color:RED;>BerryIMU and ESP8266</h3>"
"<h3 style=text-align:center;font-size:100%;>accelerometer, gyroscope, magnetometer</h3>"
"<h3 style=text-align:center;font-family:courier new;><a href=http://ozzmaker.com/ target=_blank>http://ozzmaker.com</a></h3><hr>");
("<h2 style=text-align:center;> Filtered X angle= " + String(<strong><span style="color: #ff0000;">CFangleX</span></strong>));
("<h2 style=text-align:center;> Filtered Y angle= " + String(<span style="color: #ff0000;"><strong>CFangleY</strong></span>));
("</h2><h2 style=text-align:center;> Heading = " + String(<span style="color: #ff0000;"><strong>heading</strong></span>));
("</h2><h2 style=text-align:center;> Tilt compensated heading = " + String(<strong><span style="color: #ff0000;">headingComp</span></strong>));
Within setup(), we define what pins are used for I2C to communicated with the BerryIMU.
The first value is the SDA pin and the second specifies the SCL pin. Any pin on the ESP8266 can be used for I2C.
Wireless is then enabled and then we try and connect to the wireless network. The IP address is then printed to the serial console.
// Wait for connection
while (WiFi.status() != WL_CONNECTED)
Serial.print("Connected to ");
//Print IP to console
Serial.print("IP address: ");
And finally, the web server is started and match on root of the web server and then run handleroot().
You need to add the below line in the main loop to handle web requests.
server.handleClient(); //Handler for client connections