What is gps fix

The full Almanac is needed before a GPS fix can be obtained. 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.

In a warm start scenario, the GPS module has valid Almanac data, is close to its last position km 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. When communicating with these receivers, the GPS can acquire a lock on the satellite more quickly as well as receive more accurate information. But AGPS is present in more devices than just cellphones; it's even available in cameras and some vehicles.

DGPS also uses ground or fixed GPS stations to determine the location, but differs in that it finds the difference between both the satellite and the ground location reading. These ground stations may be up to nautical miles from the receiver, and it is important to note that accuracy deteriorates the further you are from the ground station.

DGPS is accomplished by a ground station broadcasting a signal which dictates the error between the actual pseudorange and the measured pseudorange. This value is calculated by multiplying the speed of light by the time it takes the signal to travel from the satellite to the receiver.

WAAS holds a specific set of accuracy standards that ground station measurements must meet. Laterally and vertically, WAAS must be accurate to within 7. These ground stations send their measurements to master stations which send the corrections to WAAS satellites every 5 seconds or quicker. From the Satellite, a signal is broadcast back to the receivers on earth where the corrections are used to improve the GPS accuracy.

In some locations, WAAS is able to provide an accuracy of 1 meter lateral and 1. GPS data is displayed in different message formats over a serial interface. There are standard and non-standard proprietary message formats. The NMEA standard is formatted in lines of data called sentences. Each sentence contains various bits of data organized in comma delimited format i.

The data is separated by commas to make it easier to read and parse by computers and microcontrollers. This data is sent out on the serial port at an interval called the update rate. Most receivers update this information once per second 1Hz , but more advanced receivers are capable multiple updates per second.

Most GPS modules have a serial port , which makes them perfect to connect to a microcontroller or computer. Once a GPS module is powered, NMEA data or another message format is sent out of a serial transmit pin TX at a specific baud rate and update rate , even if there is no lock. It is common for the microcontroller to parse the NMEA data.

Parsing is simply removing the chunks of data from the NMEA sentence so the microcontroller can do something useful with the data. Instead of dealing with all of this text, the microcontroller can parse the GPGGA sentence and end up with only the altitude in meters. Once the microcontroller can grab the data needed, the information can be manipulated to create other interactions on the microcontroller.

Next, open a serial terminal program at the same baud rate of your GPS module. The GPS chipset contains a powerful processor that is responsible for the user interface, all of the calculations, as well as analog circuitry for the antenna. The chipset also allows for data to be sent to the GPS receiver to configure parameters like, update rate, baud rate, sentence selection, etc. In order to send commands over a serial interface to a GPS receiver, you will need a command set or reference manual.

Before diving too far into the command set for a given module, be sure to check with the vendor. Many chipset vendors provide software that allows you to easily communicate and configure the GPS module over a serial port.

These protocols contain the same information, but communicate using binary instead of ASCII for faster communication.

When communicating with a GPS receiver, most commands need to be terminated by a checksum. In most cases, you need to XOR each of your sentences. Here is a simple XOR online calculator.

We've got a page just for you! We'll walk you through the basics of how GPS works, the hardware needed, and project tutorials to get you started. Accuracy - How accurate is GPS? Overall, to get the best accuracy from your GPS, you must be in clear view of the sky and moving.

Each track represents a different type of GPS module. This is when the GPS module isn't moving. Once the module starts moving, the track is relatively accurate, and the GPS can 'guess' your track. However, notice on the approach to the Urban Canyon , which is in between two tall buildings, the accuracy can suffer. Remember, the GPS signals are being transmitted from satellites that are not necessarily over you head; some can be close to the horizon. Always keep in mind, GPS works best with a full view of the sky.

Antenna - Remember, that little GPS module is receiving signals from satellites about 12, miles away, not only above your head, but anywhere in the sky. For the best performance, you want a clear path between the antenna and most of the sky. Weather, clouds, snow storms, shouldn't affect the signal, but things like trees, buildings, mountains, the roof over your head, will all create unwanted interference and your GPS accuracy will suffer.

The smallest and most common form of antenna is the ceramic patch antenna. This antenna is low profile, inexpensive, and compact, but it has lower reception compared to other types of antennas. The page in question appears during the boot cycle until the unit has collected the coordinates and is able to report a "fix," i. Which page, please? GPS fix either means that it's found the GPS satellites, or that it has not only found them but calculated your position too.

Click to expand I had trouble finding it. I hasten to state the page. Page I don't think I'm helping either answering timtjf's questions, as if they were yours! Page 46 starts 3. Time-to-first-fix is likely to be 30 seconds to 2 minutes depending on satellite availability and the type of GPS receiver. Hot start - if the receiver has been off for, say, less than an hour time-to-first-fix will likely be seconds.

What does this all mean in practice? If the GPS has been recently used you should get a fix almost immediately. If it hasn't, put the GPS outside with a clear view of the sky and have a cup of tea.

If you have a GPS in a vehicle, it's better to wait for the unit to get a fix before driving off. Receiving ephemeris data for a satellite takes 30 seconds. If you momentarily interrupt the signal during that time the GPS it could take up to a minute more to get the ephemeris for that satellite as it has to start over.

If you drive in an area with tall buildings or other obstructions it may take a long time to get the ephemeris data, for four satellites, that is needed for the first fix. The accuracy of the position your GPS reports is influenced by a number of factors, such as the positions of the satellites in the sky, atmospheric effects, satellite clock errors and ephemeris errors etc. GPS units often show on the screen an accuracy figure, e. EPE on Garmin units.

Under ideal conditions, this may be 5, or even 3 metres. Manufacturers are vague on exactly how this figure is determined and it would be unwise to take this figure literally. You'll get a more realistic figure by looking in the specification section of your GPS receiver's user-manual.

The error in altitude will probably be at least twice the horizontal error. WAAS uses a network of ground-based reference stations.