Introduction to u-blox A-GPS Solutions

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Introduction to u-blox A-GPS Solutions

GPS applications are becoming increasingly popular. Vehicle Navigation and PDA/PND portable navigation devices are widely used in the market. In addition to combining photos, audio and video playback, it also gradually enters pedestrian walking navigation from Vehicle Navigation, with more emphasis on Location-based
Service,
LBS), that is, the positioning function extends value-added services such as nearby restaurants, scenic spots, taxi/bus routes. In addition, the US E911 law requires that each mobile phone be equipped with a GPS function, in order to play a role in locating and searching in an emergency.

 

When the application of GPS continues to promote personal application, the traditional GPS Positioning method has encountered a lot of bottlenecks. Use Autonomous
Positioning) independent GPS device, must receive more than four satellite signals in the open sky with good signal conditions, and, the GPS receiver of the device must collect the complete GPS satellite orbit information before locating and computing. For the portable navigation application, the first positioning Time (Time
To first fix,
TTFF) it is too long, and users are often in the streets where buildings are everywhere, under the viaduct, or even in the indoor environment, because the signal receiving conditions in these places are very poor, it takes a long time for the user to wait for the first time to locate the target, and the user may not be able to succeed. In this case, through another set of network to obtain satellite information auxiliary Positioning method, that is, A-GPS (Aiding)
GPS) has become an inevitable trend in the development of GPS. As a result, Swiss u-blox has introduced two different forms of solutions in A-GPS, to help users shorten TTFF time, fast positioning.

 

GPS satellite information composition

 

Before in-depth discussion of A-GPS, we must first master the basic principle of general GPS autonomous positioning and the composition of satellite information, in order to understand the advantages of A-GPS. Currently, multiple positioning satellite systems are in operation in the sky, including the us gps system, Russia's GLONASS system under construction, and the EU's Galileo system under construction, among them, GPS in the United States is the mainstream in today's market, and it is also the system discussed in this article.

 

GPS is composed of 24 satellite groups running on six orbital planes. Each satellite continuously transmits satellite information about the satellite orbit, time, and various parameters, receiving such information is the key to the successful positioning of GPS terminals. Currently, GPS satellites have L1 and 1227.60MHz L2 carriers at 1575.42MHz respectively, and C/A code (C/
Code) and P code. Generally, we use L1 and C/A electronic codes, while L2 and P Codes are beautiful? The military is in use.

 

The satellite information carried on L1 is in Frame (Frame), and each Frame is 1500
Bits is divided into five Sub-frames. Its contents include Ephemeris data, elliptical parameters, and Almanac. See table 1. The star calendar is the precise orbital position of individual satellites. It is updated every hour and the validity of each update is about four hours. The longitude is the rough position and condition of all satellites on the orbit, it is updated every day and valid for several weeks.

 

Table 1: Composition of each frame in satellite information.

 

For a GPS terminal that does not have any effective positioning data, the most important thing is to collect the time data of the four satellites, respectively, and to calculate and locate the data correctly. Because the satellite uses 50
The bit/s (bps) rate is used to send signals. Therefore, it takes at least 18 seconds to synchronize the time for the four satellites to receive a complete calendar data, since each update of data requires 25 frames to transmit the updated shard data, it takes 12.5 minutes to complete the download.

 

Figure 1: satellite information composition and receipt time.

 

For a GPS terminal, whether the device has valid satellite information at startup determines the speed at which the device locates for the first time. There are three types of GPS start-up: Cold Start (Cold
Start, Warm Start, and Hot Start ).

 

1.
Cold start: If the GPS receiver is started without any data, it is called a cold start. As shown in, if the signal is good, the receiving signal is not interrupted, it takes at least 18 seconds to download the calendar. If the worst case occurs, you just miss the first bit of the first sub-frame, you need to re-download the sub-frame in the next cycle, in this way, it takes 36 seconds to download the calendar, and the GPS receiver can calculate the positioning data after the calendar data is downloaded. However, the download of the calendar subframe cannot be interrupted, if the signal is too weak to interrupt for a moment, you have to receive the Sub-frame from the beginning. This takes a longer time to download the calendar and takes a longer time to locate the sub-frame.

 

2.
Warm Start: When the GPS receiver only has valid temperature data and does not move from the last time it was located, it is called a warm start. A typical example is that the receiver is shut down for more than two hours, but the last position, time, and period data are retained, which allows the receiver to predict the current position of the visible satellite, it is easier to capture satellites. However, if the GPS receiver needs to calculate the positioning information, it still needs to download the complete calendar data of more than four satellites, just like the cold start.

 

3.
Hot Start: When the GPS receiver is started, the start of the active calendar and calendar data is called Hot Start. A typical example is that the receiver does not shut down for more than two hours, and the internal clock of the receiver remains working during this period. During Hot Start, the receiver can also predict the satellite location and quickly capture and track satellite signals, therefore, it is not necessary to re-download the calendar data like cold start and warm start to quickly locate the data.

 

From the above description, we can see that in the case of cold start or warm start, the receiver needs to download the calendar data from the satellite. In the case of poor signal, it may take tens of seconds or even minutes. Is there any other way to solve this problem? Of course! Two different A-GPS technologies of u-blox can save a long time to download data from satellites. A-GPS refers to other ways to obtain the secondary data technology, with the A-GPS, GPS receiver does not need to download more than four satellite calendar data, it can be located immediately after the satellite signal is captured.

 

A-GPS Service

 

A general A-GPS system consists of a GPS Global Reference Network, a root server that publishes secondary data, and a receiver with A-GPS functionality. GPS global reference networks must establish a wide range of monitoring sites and continuously and accurately monitor the movement of satellites. It transmits the monitored satellite data to a high-performance root server, which predicts the future movement of the Satellite Based on the data. (International)
GNNS-Service) is such a network that continues to run globally.

 

The first step in the GPS receiver's operating program is to search for satellite signals and then receive the calendars before locating and tracking. If you can obtain satellite information in advance or download the calendar at a faster speed, the positioning speed can be accelerated. See figure 2 ). In this case, u-blox proposed two different A-GPS solutions to obtain auxiliary data, one is real-time through GSM, GPRS, CDMA or UMTS and other mobile communication systems, that is, the online A-GPS Method
Online); the other is using offline A-GPS
Offline), that is, pre-download of satellite data through a mobile network or directly from the Internet based on users' convenience, can play a secondary positioning role when needed. The following describes the features and differences between the two methods.

 

Figure 2: When a cold start occurs, a terminal with a calendar or differential period correction data can be quickly located.

 

1. Online A-GPS (buy now Online)

 

One method is the online method. The brand of u-blox's online method is buy now online.

 

A terminal with online A-GPS function, can communicate with mobile network by two interfaces, one is the Control Platform (Control plane), one is the User
Plane ). The former is the interface specification defined by different mobile systems for positioning assistance functions, where GSM/GPRS is RRLP, UMTS is RRC, CDMA is IS-801A. In addition to different interface specifications, different system service providers often establish their own control platform operating systems, although this can ensure better service quality, but the construction cost is high, users are also limited by system service providers.

 

Another interface system is the User platform. It uses a set of common interface specifications defined by the OMA organization, called SUPL (Secure User Plane
Location ). It packages RRC, RRLP, and other information into a consistent specification before sending it out, which is very similar to the TCP/IP architecture. Because of its high universality, the system construction cost is low, it is helpful for the promotion of A-GPS in mobile devices such as mobile phones.

 

Using online A-GPS, different methods will affect the positioning efficiency. The first factor is network connection speed, which is closely related to the service quality of mobile operators and the location of users. Both CDMA and GSM/GPRS Protocols define the Minimum Operating Performance Standard for A-GPS mobile phones: the standard definition of CDMA is
C.S0036-0 (TIA 916), GSM/GPRS is 3GPP TS
25.171. The maximum start time (the longest TTFF) required by CDMA is within 16 seconds, while that required by GSM is 20 seconds. At present, various solutions are committed to meeting this requirement, with the u-blox unknown now
For example, Online can meet the standard requirements to provide high-quality Online services.

 

The second factor is related to the downloaded satellite data. When more useful information is obtained, the faster the positioning speed is. For example
Time), it can greatly shorten the positioning Time; this is because the satellite is moving very fast (800 meters per second), GPS Time helps to grasp the exact location of the satellite. GPS time can also be divided into rough GPS time (Coarse
GPS time) and Precise GPS time (Precise GPS time), the former positioning time is offered for 30 seconds, and the latter only takes a few seconds.

 

When a terminal that supports A-GPS is started, it will receive satellite signals from the sky at the same time, and connect the base station of the mobile network through the user platform (such as GPRS, the Base Station connects to servers that obtain global reference network data over the Internet. The GPS terminal usually downloads data from the server, including calendar, calendar, location, time, and satellite health status, in addition to the calendar, other data is optional. The data does not need to be stored in the memory of the GPS receiver or system, and the data is updated every time the connection is started. Online
See figure 3 for the service architecture for the A-GPS ).

 

Figure 3: Online A-GPS service architecture.

 

2. Offline A-GPS (coming now Offline)

 

The other method is the offline method. The brand of u-blox's offline method is buy now.
Offline, u-blox owns the patent ownership of this technology. Before use, the GPS terminal first obtains auxiliary data from the server through a mobile network or the Internet. These data are usually pre-estimated satellite or satellite orbit data, when they are stored, the connection to the server can be interrupted. When the next GPS receiver starts, the stored data is used to calculate the current track data to help navigation and positioning.

 

In this case, the receiver does not need to wait until all the data is downloaded from the satellite to start computing. It can quickly start navigation. The effectiveness of the auxiliary data is related to the data provider. It can be maintained for about ten days to two weeks. However, the accuracy of the provided location will decrease over time, and the accuracy will be the highest a few days after the download, the longer the time, the lower the accuracy. Therefore, it is best to maintain data updates frequently.

 

The accuracy of satellite orbit prediction is also closely related to the professional capabilities of data providers. If the satellite region is provided directly, because it only provides the rough location of all satellite orbit, there is an error of about 3-5 kilometers between the satellite orbit and the actual satellite orbit. if this data is directly used for positioning, the calculated position will be offset a lot. Therefore, professional data providers predict and correct satellite orbit using astronomical and gravity models. The AlmanacPlus Technology proposed by u-blox can use the Differential transform (Differential)
Almanac Correction Data) to improve the accuracy of satellite orbit to 10-50 meters, see (figure 4 ).

 

Figure 4: track prediction accuracy is improved by AlmanacPlus differential Deviation Correction data.

 

As shown in figure 5, mobile terminals with offline A-GPS functionality communicate with standard Mirror or Proxy servers through TCP/IP protocols, to obtain secondary satellite data copied to this server. This Mirror/Proxy Server also uses the standard HTTP protocol
Server) to obtain compressed satellite information data, while the root server data comes from the global reference network such as IMG.

 

Figure 5: Offline A-GPS service architecture.

 

Compared with offline A-GPS, online A-GPS has always been positioned with the current star, so it can get better accuracy. However, the validity of the calendar is short and must be updated at any time, and it is easily limited by the connection time and quality of the mobile communication system. In contrast, the offline A-GPS does not need to spend time downloading satellite orbit data, nor is it subject to the limitations of the base station coverage, coupled with the positioning does not need to stay online at any time, therefore, it can save a lot of Internet access fees and is a very convenient Positioning Solution. Baidu now of u-blox
In addition to the AlmanacPlus mentioned above to provide more accurate information, Offline has been available for 14 days. At present, there are other Offline solutions on the market, the Data Validity period is about 5.
To 10 days.

 

Whether it's an online A-GPS or an offline A-GPS, all GPS receivers manufactured by u-blox are supported at the factory, and the related program has been hardcoded inside the chip, no special program is required.

 

A-GPS Terminal System Planning

 

In terms of mobile terminal GPS system design, mobile phone manufacturers need to adopt different architectures: one is to use a single chip, which integrates the RF and baseband functions of GPS, it helps manufacturers reduce costs and installation sizes; the other is the use of RF and baseband independent chipset mode, which gives manufacturers greater design flexibility, however, System Engineers must be able to adjust the performance of the overall system, which is highly challenging in design. The other is to integrate the baseband function into the app processor or Baseband Chip of the mobile phone, however, this method will occupy a large amount of computing resources of the main processor.

 

In terms of online A-GPS, the system planning must take into account the positioning operation subject at that end. If the Mobile terminal is responsible for computing, it is called the MS-based mode (MS is Mobile
Station). If the locating operation is performed by the network server and sent back to the terminal, it is called the MS-received mode. The MS-based mode returns to the Independent Computing status after obtaining the auxiliary network data. The operation is simple, but it has high requirements on the computing resources of the terminal system.

 

MS-encoded Ed is more complex. The terminal must transmit the received satellite signal data to the network server, and then return the location information to the terminal after the server calculates the result, although this operation can reduce the computing load of the terminal and perform more complex operations to obtain a more precise location, it has high requirements on the quality of the connected network, because the positioning results will be lost once the connection is dropped.

 

In the system architecture, the master processor and GPS receiver use UART, SPI or
I? C and other standard interfaces for communication. In offline mode, data downloaded from the server is usually stored in non-volatile Flash.
In EPROM, according to different algorithms, GPS receivers and memory communication methods can be divided into two types: one is that the receiver directly communicates with Flash
EPROM communication: first, the receiver communicates with the memory through the main processor. The former requires an additional Flash
EPROM memory, which only needs to use the memory of the main processor, thus saving design costs and space. In addition, the design of A-GPS solutions on mobile terminals, will hope not to cause too much burden on the main processor. Using the explain now solution of u-blox, all the operation work is handed over to the GPS receiver. Therefore, you do not need to customize the work for the CPU, which is easier to design.

 

Figure 6: Architecture of configuring a separate Flash memory for the GPS receiver.

 

Figure 7: architecture in which the GPS receiver acquires stored data through the master processor.

 

In terms of memory demand, the online A-GPS has extremely low requirements for memory capacity, u-blox's limit now
Online, the size of Data downloaded each time is only 1-3 kb; for offline A-GPS solution then now
For Offline, the download of one day's Satellite Prediction data requires only 10 KB, and the 14-day prediction data is about 90 KB.

 

Mobile terminals with A-GPS functions can also support both online and offline modes. When the terminal is started, the internal GPS receiver will automatically check
When the valid satellite data in the EPROM cannot be found for the available calendar data, it uses the offline mode to assist in the positioning operation with the corrected seek data. Once the terminal obtains an effective star duration through the mobile network, the system will use it to replace offline data for more accurate positioning results. Similarly, when the calendar becomes invalid, the system switches back to the offline mode.

 

Summary

 

The u-blox company launched the faster now A-GPS to obtain satellite-assisted data through mobile networks or the Internet, and to achieve faster or more accurate positioning in a flexible online or offline manner, and more reliable positioning and navigation quality. This practice can also be very simple integration into China Mobile's current SUPL technology, can also work independently of SUPL alone, to help open up the A-GPS in the mobile terminal market, make GPS a new killer app for mobile phones. Finally, A-GPS technology can not only be applied in mobile phones, it can also be applied in all GPS products with communication functions, and will eventually become an indispensable technology in GPS applications. For details, see: www.u-blox.com.

 

Author: Liu Wei, Yan Zhihong

U-blox