Using your GPS Receiver with DTS: What You Need to Know – DTS Help Center

Purpose:

When using a GPS Receiver with DTS Software and Hardware, there are several important technical specifications you need to know about your receiver in order to properly capture and decode the GPS information. This article will provide information on what technical aspects you need to know about your receiver as well as provide some basic breakdown information on what each of these technical specifications mean/are used for.

NOTE: DTS equipment is not designed to handle GPS sent over a CAN bus interface. Only GPS which is in Serial 
Format (RS-232 or RS-422) can be processed.

Important Technical Specifications of Your GPS Receiver:

There are a number of different GPS receivers available on the consumer market, each with their own specifications and capabilities. The most important technical specifications you need to know about your GPS Receiver in order to use it with DTS Software and Hardware are the following:

These settings can typically be found in the Technical Specifications table(s) in the User Manual of your GPS Receiver. However, if they are not there, you may need to read through/use a digital search to find them in the manual.

Power Source Information:

When working with a GPS Receiver, you need to ensure that you can provide the required input power and current for the unit to work properly. As DTS DAS have a limited output power and current range -- hardware dependent, please see your DAS's datasheet for more information -- you need to ensure that you can either run the GPS Receiver off of your DAS's Analog Channel Excitation (SLICE Micro/Nano/IP68, SLICE PRO SIM, and SLICE6) or off of the external power pin on a SLICE6 AIR system. If your GPS Receiver requires more power and/or current draw than available on the DAS outputs, then you will need to provide and external power source such as a battery that will power your GPS Receiver.

Baud Rate, Update Rate, Parity Setting, Start Bit, Stop Bit, and Data Bit Settings:

The Baud Rate, Parity, Start Bit, Stop Bit, and Data Bit settings of your GPS Receiver are important to understand so that you can properly configure your DTS DAS to capture and decode the GPS Sentence(s) being transmitted to the DAS.

Baud Rate

Baud Rate is the number of bits transmitted per second. Baud Rate for GPS systems can vary widely and, depending on your model, may actually be configurable via your system's 'Configuration Utility' (if available). In order for the DTS software to properly capture and decode your GPS data, it needs to know how fast the data is coming into the unit.

Important notes regarding GPS NMEA Standards and Baud Rate recommendations:
The current standard NMEA 0183 version 3 recommends a baud rate of at least 4800.  However, NMEA 0183 
is being phased out by some industries in favor of NMEA 2000, which has a baud rate of 250k.
Configurable GPS units may have Baud Rate options as low as 1200.

Capturing GPS on an Analog Channel: Baud Rate vs Sample Rate

When using an Analog Channel on a DTS DAS to capture GPS data, your DAS's Sample Rate must be at least 4 times (4x) your GPS's Baud Rate in order to allow for proper decoding of the GPS serial stream. For example, a GPS Baud Rate of 1200 would require a minimum Sample Rate of 4800 samples per second (5,000 is the closest software setting available). For a Baud Rate of 4800, the minimum Sample Rate would be 19,200 samples per second (20,000 Sps).

As you can see, this means that the higher the GPS Receiver Baud Rate, the faster the DTS DAS must sample; thus reducing the amount of time your can sample your signal before the DAS's memory is full. Keep this in mind, when planning to capture GPS data on an Analog Channel.

Update Rate

Update Rate is how frequently the GPS Receiver calculates and updates its location information. The most common Update Rate for GPS units is 1Hz -- meaning that the unit will calculate its location information and send it out once every second. However, more and more GPS Receivers are becoming available with alternate update rates -- most commonly available are 5Hz and 10Hz models. This means that the GPS units will transmit their newly-calculated location information x-times every second, where x is the number of Update Rate in Hertz. This is important because it means that the GPS data being sent is being sent x-times per second, thus meaning that each transmission must fit into 1/x * Baud Rate. Failure to ensure that the number of bits being sent in each sentence is less than 1/x of the Baud Rate means that sentences will overrun each other, thus corrupting your GPS data stream.

Parity Setting

Parity is a form of error checking meant to ensure signal validity after transmission. The Parity Setting for your GPS Receiver, once selected, will apply to all transmissions from your GPS Receiver. There are several Parity Settings available, though the most-common, when used at all, is Even.

None → No error checking is present
Odd → Total number of binary '1's is odd
Even → Total number of binary '1's is even
Mark → Parity bit is always 1, regardless of number of binary '1's in the transmitted signal
Space → Parity bit is always 0, regardless of the number of binary '1's in the transmitted signal

Start Bit, Stop Bit, and Data Bits

The total number of bits transmitted that represents a 'single ASCII character' is the sum of the Parity Bit, Data Bits, Start Bit, and Stop Bit. The most common length for GPS transmission for this combination is 10: 1 Parity, 8 Data, 1 Stop.

Start and Stop Bits are used to indicate the beginning and end, respectively, of the 'single character' within the transmitted data. While Start and Stop bits are not always used, there is no clear indication for when a character ends and another begins without at least one of these bits present.

The Data Bits value indicates the number of bits holding the actual binary value for the character being transmitted. For example, the 8-bit binary string '0110 0001' represents the ASCII character 'a', while the 8-bit binary string '0100 0001' represents the ASCII character 'A'.

1PPS Capability:

1PPS means 1-Pulse-Per-Second and is a precision time-tracking signal available on some GPS Receivers that allows for a more-accurate delineation of NMEA sentence(s) as well as a quick-reference for 1-second time intervals. When possible, it is recommended to pair your GPS Receiver output with a 1PPS signal from the same GPS Receiver so as to allow for the most-accurate timing possible.

Using 1PPS and GPS Data on an Analog Channel:

When capturing 1PPS and GPS Data on Analog Channels for the purposes of decoding, it is important to note that the 1PPS signal and GPS Data signals should be captured on separate Analog Channels on the same DAS unit. If multiple stacks/units are used, you can use one channel per-stack to capture the 1PPS signal so as to allow for better time-alignment across stacks; it is not necessary to capture GPS Data on more than one unit in a multi-stack configuration, unless those units are not traveling together.

NMEA Sentence(s):

GPS Receivers output data in the form of NMEA sentences, each of which contains a set of information outlined by the Nation Marine Electronic Association (NMEA). There are currently 58 sentence types, each of which provide specific GPS information, and each of which is designated by a 5-letter acronym. The most-common NMEA sentence is GPGGA, which stands for the Global position system fixed data.

A good source for NMEA Sentence information can be found here:  
http://aprs.gids.nl/nmea/

It is rare that a single NMEA sentence contains all desired GPS information needing to be captured. While the GPGGA sentence is the minimum recommendation to use, it is usually paired with at least one other sentence. The sentences which are most-commonly paired with GPGGA are as follows:

GPGLL -- Geographic position - latitude/longitude
GPGSA -- GNSS DOP and active satellites
GPGSV -- GNSS satellites in view
GPRMC -- Recommended minimum specific GNSS data
GPVTG - Course over ground and ground speed

NMEA sentences are comma-delineated with each subsection representing a specific data type/value.

Example: GPGGA Sentence Breakdown

A GPGGA sentence capture is seen below:

$GPGGA → Sentence Type Identifier (always denoted by a '$' sign
UTC time (hh:mm:ss format) → 23:33:10
Latitude → 3345.4723
Latitude direction (N/S) → N
Longitude → 11805.3319
Longitude direction (E/W) → W
Quantity/Fix Indicator (indicated validity of sentence based on satellite fix conditions) → 2
Number of satellites in view → 08
Horizontal dilution of precision → 1.0
Altitude of antenna → 27.0
Altitude of antenna units of measure (M/F) → M (meters)
Geoidal separation (subtract from Altitude to arrive at Height Above Ellipsoid (HAE)) → -35.1
Geoidal separation units of measure (M/F) → M (meters)
Age of correction (if any) → blank
Correction station ID (if any) → blank
Checksum → *43
- Bitwise exclusive OR (XOR) of ASCII codes of all characters in the sentence between the '$' character and the '*' (not inclusive)

Choosing NMEA Sentences to Use:

When configuring your GPS Receiver to transmit sentences, it's important to know what data you need: time/date, speed, lat/long, data validity check, satellite communication check and number of satellites used, etc. However, just as important, is ensuring that the number of sentences you select can be transmitted in the available time. For GPS Receivers with a 1Hz Update Rate, that means that all sentences must be transmitted in less than 1 second. For GPS Receivers with a 5Hz Update Rate, each set of sentences must transmit in less than 200msec in order to avoid overrun/corruption. When using 1PPS in conjunction with your GPS NMEA sentences, all transmissions of your GPS Receiver at its set Update Rate must take place between the confines of two 1PPS pulses. Failure to ensure these conditions are met will result in overrun/corrupted data which will be unusable. This means that you mean need to 'pick and choose' your sentences carefully. It's also important to note that some sentences are longer than others, thus taking more bits to transmit.

A helpful guide for the most-commonly used GPS sentences and the specific data they carry is below:

For other data types/needs, review the full list of NMEA sentences on the recommended reference site.

Configuring Your DTS Software and Hardware with your GPS Receiver:

Configuring your GPS Receiver to work with your DTS DAS will depend on the software suite and method of capture being used. There are two methods to capture GPS Data using DTS DAS:

Analog Channel Capture
When capturing GPS Receiver data and/or 1PPS using a DTS DAS Analog Channel, there are several requirements:
- Each signal captured must use its own Analog Channel. IE. GPS Serial Data on one Analog Channel and 1PPS on a separate channel.
  - For multi-stack/chained systems, NMEA should be captured on only one unit in the system.
  - 1PPS should be captured on one channel per stack.
- Software Suite:
  - DAS Configuration/Setup: SLICEWare or DataPRO
  - GPS Sentence Decoding: GPS Accessory Kit App/Matlab Code
```
NOTE: The GPS Accessory Kit Application only provides summaries and CSV outputs 
for GPGGA and GPRMC sentence types, though all sentences are available in output 
file for self-decoding.

Access to this article requires special permissions. If you require access, 
please Submit a Request on the DTS Help Center and one of our Technical Support 
personnel will be happy to assist you. 
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- Minimum Sample Rate: 4 * Baud Rate
- Power Source: Depending on GPS Receiver, you may need an external battery/power source. See the datasheet for your DTS hardware for information on Excitation Voltage and Current output limits for Analog Channels.
Connection Information:

* For GPS Receivers that can use +Excitation from your DTS DAS for power, this would connect to +EX.
SLICE6 AIR UART/1PPS Capture
When using the UART and/or 1PPS ports on a SLICE6 AIR unit to capture GPS data, you will require the following:
- Software Suite: DataPRO 4.0 or later
- Recording/Streaming Limitations: SLICE6 AIR can currently either Record-in-Place data (not including UART) or stream data. It cannot do both at the same time.
- Power Source: Depending on the GPS Receiver being used and its Power Source requirements, you may be able to use the SLICE6 AIR power output pins to power the GPS Receiver. See the SLICE6 AIR datasheet for information on the power output capabilities of the unit. For GPS Receiver units that exceed the SLICE6 AIR capabilities, you will need an external power source to power your GPS Receiver.
When using the UART and/or 1PPS capture on a SLICE6 AIR unit, please see the following article on the DTS Help Center for guidance on setting up the software to work with your GPS unit: SLICE6 AIR | GPS Software Setup
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NOTE:  Access to this article requires special permissions.  If you require access, please 
Submit a Request on the DTS Help Center and one of our Technical Support personnel will be 
happy to assist you. 
```