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Tech Note: What is Error Correction?

Error Correction is an available setting in all SATEL radio modems, which can help improve data transfer in cases of poor or unreliable communication. Referred to as FEC Mode (Forward Error Correction) in Configuration Manager, note that this setting refers to SATEL 3AS Error Correction and does not affect other compatibility modes, such as PacCrest. This setting can be turned ON or OFF both in Configuration Manager and directly on the front screen of your radio modem.

When Error Correction is enabled (ON), the radio modem automatically adds additional error correction information, which increases the amount of transmitted data by 30%. It is used by the receiving radio modem to correct erroneous bits, as long as the ratio of correct and erroneous bits is reasonable. The resulting benefit can be an improvement of up to 3dB sensitivity.

Error Correction improves the reliability of data transfer via the radio interface, especially in unfavorable conditions. The Error Correction function should be used when link distances are long or received signal is otherwise low due to poor propagation conditions or multi-path fading. It is also recommended to use Error Correction in case there are intermittent interferences on the radio channel.

The Error Correction function decreases data transfer throughput by approximately 30%. Though transfer delays are longer, Error Correction can be quite useful for the best data transmission quality in the scenarios described above.

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Tech Note: Speed vs Modulation

Few radio parameters can be as confusing as modulation and over-the-air speed, not only for the novice user, but also for the experienced. Here are a few comments that hopefully help to shine some light on the subject:

Over-the-Air Speed

Although Port Speed and over-the-air speed have some relation to each other, they are totally separate settings.

While the Radio Port speed and GPS receiver Port speed must match exactly (for example GPS = 9600 bps & Radio = 9600bps), to allow the normal transfer of data over the connecting cable, the speed over-the-air cannot be set by simply changing the Port speed. It’s actually a fixed value which depends on both bandwidth and modulation.

In the US, the most common bandwidth today is 12.5kHz. So if your radio uses any of the following modulations:

– SATEL 4FSK, PacCrest 4FSK, PacCrest FST at 12.5kHz,

the speed over the air is fixed at 9600bps.

Whereas if you are using the following modulations:

– PacCrest GMSK, or Trimtalk 450S at 12.5 kHz, the speed over the air is fixed at 4800bps.

If a half-speed modulation type such as GMSK is used, the Base radio Transmitter is on-the-air for twice the length of time respect to a faster modulation. This means higher battery consumption and more overheating of the radio itself.

Modulation

Modulation can be an even more complex subject and it has a major influence on both the speed over the air and the actual range of the radio equipment.

The most common modulation today is 4FSK (4 Frequency Shift Keys). This means that the radio transmitter (base) transforms the NMEA data from the GPS into radio frequency variations that shift to 4 different points around its central working frequency. All the points must be contained within the 12.5kHz required by the FCC. This means that the TX will constantly “shift” to 4 frequency points, up to 6.25 kHz away from its central frequency, both to the right and to the left in the frequency spectrum.  A rough example: the Tx “shifts” from its central 450.000000 MHz to -> 450.006250 MHz and then back and forth from and to -> 449.9993750 MHz.  Each “shift” is interpreted as a data 1 or zero and the whole NMEA string of the GPS is recomposed in the rover.

Some equipment offer even higher over-the-air speed by using more complex types of modulation (ex. 8FSK, 16FSK). Some even use 32QAM, where not only frequency “shifts” but also “phase” variations are measured. SATEL offers an internal OEM module, already in production and used by major GPS manufacturers, with 8FSK and 16FSK modulation and over-the-air speed up to 14400bps.  This becomes useful when several constellations are received (for example GPS + GLONASS), which cause the radio to transmit more data. In order to be able to contain all data within 1 Hz (once a second) a higher over-the-air speed is desirable. The downside of higher modulation is that the receiver must work harder to detect very small variations, thus a stronger radio signal is required to counterbalance the environmental noise present in the air. Ultimately this means a decreased working range.

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Tech Note: Using a Repeater

Repeater Mode can be easily set up from the front screen of any SATEL radio (Main Setup>Additional>Repeater>select ON>Save changes). Many SATEL users stop there, however additional steps can be taken in order to prevent potential issues with your system. Some rather common issues we hear about when a repeater is in place are:

“I get a signal when near the base, but not when near the rover”

“I’m getting a signal, but not getting a fix”

“The rover sometimes hears both the base and repeater and it looks to be creating issues with my GPS”

The first scenario is likely an antenna issue; you will want to check your antennas and cables. The second scenario is likely due to parameters; you will want to go back through your settings and make sure parameters match. The last issue of an “echo” or double signal is a common problem you’ll experience with a repeater, especially in Precision Ag applications, or other scenarios in which the rover is moving. Some GPS are immune to the echo, while we’ve seen others get confused by the double signal. Here are some tips for fixing this common issue, depending on your licensing:

If You Have More Than 1 Frequency:

If in your licensing you have more than one frequency available for use, we recommend using separate TX/RX frequencies. For example, let’s say you have 469 MHz and 462 MHz available. You would set up the base to transmit at 469 MHz. You would set up the repeater to receive at 469 MHz (Main Menu>Radio Frequency>RX Frequency (change setting)) and to transmit at 462 MHz (Main Menu>Radio Frequency>TX Frequency (change setting)). Then you will set the rover to receive at 462 Mhz. This will ensure that even if the rover hears the base at times, it will not receive and interfere with the communication.

If You Do Not Have More Than 1 Frequency:

The alternative option, if you do not have more than one frequency available to use in your application, is to set up addressing. This is easily done through the programming software on your PC, using either Configuration Manager or SaTerm. The set up would look like this:

Base = Address 0001 0001

Repeater = Receive Address 0001 0001, Transmit Address 0002 0002

Rover = Address 0002 0002

As above, this also ensures that the base will not interfere with the rover, even if a double signal can be heard at different points.

Other Common Issues:

Another common issue in repeater set ups relates to modulation settings. Using a slower protocol, such as GMSK, is not recommended on a repeater set up because of the length of the data stream. The data string nowadays are so long that it can create a back up of data; the repeater may still be transmitting to the rover as the base is trying to send the next string. This can be resolved by using a faster protocol.

Set up should also be considered; we have seen numerous situations in which the repeater is placed too close to the base, instead of near the rover. You want to make sure the repeater and rover are as close as possible in order to ensure the best signal.

Double repeater set ups are much more complicated, especially when the rover is moving. Addressing, as described above, is recommended in this scenario.

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Tech Note: Overheating

Even if the summer heat has passed, the heat dissipated by a 35 Watt radio can be an issue any time of year. It is perfectly normal for a 35W transmitter to generate enough heat to be very uncomfortable to the touch. Any transmitter of any major brand consumes energy equal to about three times the radiated power. In the case of a 35W transmitter, that is about 105 Watts. As a comparison, a 100 Watt light bulb cannot be touched when it is on because it is too hot to be handled. Something similar happens to a radio, although there are various steps you can take to decrease this large amount of heat.

The first precaution the manufacturer takes is that a radio of 35W power is equipped with a heat-sink. Note that the heat-sink, normally mounted on the back of the unit, is only effective if air circulates freely without obstructions, and circulates vertically. A radio mounted horizontally will heat up much more than one whose flow of air can freely flow from bottom to top.

A potential source of overheating can be in RTK equipment working with multiple constellations, thus requiring longer strings of data to be transmitted each second. This means that the transmitter will stay on for a longer period of time, generating more heat. The solution is a higher over-the-air speed, which will in turn reduce the transmission time. For example, a base radio sending RTK corrections to a rover utilizing a GMSK protocol will generate almost twice as much heat as a base radio using a faster protocol like 4FSK (at the same bandwidth). The difference can be dramatic.

Another undesirable source of heat is the so-called “reflected power”. All the RF power of the radio should reach the antenna and should be radiated in the form of electromagnetic energy. If some of it returns back to the radio, the radio will heat up considerably more, not to mention that the working range will be reduced. In order to prevent reflected power, it’s very important to use a good RF cable with no damage and to make sure that the antenna is in good working condition without connector oxidation or other visible damage. Also, many users who utilize a steel whip antenna may not be aware that this type of antenna requires “tuning” to the right operating frequency. Normally such antennas are sold at a length which is optimal for the lowest frequency of its range. When a higher frequency is used, the operator should then cut it to the right length, as listed in the antenna instructions. Many users disregard this “fine tuning”, thus somewhat reducing their working range and increasing the heat (however only marginally).

Even if all the above is taken care of, when a radio is left operating in full sunshine on a hot day, it will inevitably heat up. SATEL radios have an internal protection that prevents damage once the radio reaches 176°F (80°C). The radio will continue to operate at reduced power, and the screen will flash to alert the operator.

Your best course of action if you believe heat has become a problem, especially in the summer or in warmer regions: reduce the power of the transmitter from 35W to 25W. It may be counter-intuitive, but this reduction of power (about 30%) will not affect the range very much. In fact, the range will decrease by about 15%, with the potential of saving you from a lot of downtime.

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How to Utilize SATEL’s AES-128 Encryption Feature

For stronger protection of your data over the air, certain models of SATEL modems now include AES-128 encryption capabilities. This feature is available on the following models (please note new part numbers):

  • SATELLINE-EASy Pro, p/n YM6813
  • SATEL EASy-Proof, p/n YM6485
  • SATELLINE-EASy with display, p/n YM6565
  • SATELLINE-M3-TR1, p/n YM6315

How to Set Up Encryption

AES-128 encryption can be set up through Configuration Manager (to download Configuration Manager, please go to Support>Software on our website). Set up must be done through a PC; set up cannot be completed through the keypad and display on the radio. To set up through Configuration Manager, connect your radio to the PC, open Configuration Manager, and follow the following steps:

  • Go to the Modem Settings tab
  • Click on the Misc Settings sub-tab
  • In the Encryption box, set OTA Encryption to ON
  • Under Password enter your password of choice. NOTE: make sure your password is memorable or kept in a safe place, as there is no way to retrieve your password once it is entered (if you forget or lose the password, a new password will need to be set for all radios in your system).
  • Click Generate & Save New Keys
  • Repeat for all other radios in the system. NOTE: As a safe check across radios, the key hash will match on all radios with matching password. The key hash is NOT the password, but instead is an indicator: if the key hash is different on one radio in your system, the password was entered incorrectly and should be redone.

Note that the LCD does not include any information or indication of the encryption or settings. Encryption support can be verified from the Firmware version. In Configuration Manager, when you go to Radio Modem Info, under Firmware Version it will list the version number, followed by “ENCRYPTION” or “CRYPT” or “CRYPT SURV”.

Will a new radio with encryption work with my current radios without encryption?

Radios with the AES-128 encryption feature are fully compatible with equivalent SATEL radio modem products when the encryption is set to OFF (Note: the factory default for the encryption feature is always OFF).

Does encryption work in Survey Mode?

CRYPT SURV firmware is available on our website (go to Support>Firmware). You can update/downgrade the firmware version between CRYPT and CRYPT SURV versions without issue.

Can my current equipment be upgraded?

You cannot update your standard firmware SATELLINE radio modems to the AES-supporting firmware in the field. Your equipment can be upgraded by the Satel factory. Please contact us for lead times and pricing.

Who can I contact for support?

Please contact us at support@satelusa.com or 408-973-1740.

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Tech Note: Radio Power vs. Distance

Myth: Doubling the power will double the distance of my transmitter.

It is a common misconception that in order to achieve twice the distance in a radio link, you can simply double the power. The reality is that in order to double the distance, 4 times the Tx power is needed. This rule comes from a simple geometric formula: the propagation of radio waves expands in the shape of a sphere, and the surface of a sphere (and thus power density) changes proportionally to the square of the radius (in this case the radius = distance).

To put it simply, if with a 1 Watt transmitter you can reach 1 mile, in order to reach 2 miles you will need a 4 Watt transmitter. Of course the distance depends not only on the power of a transmitter but also–and very much so–on the antenna gain and height, the quality of the receiver and on the obstacles in the terrain.

A very useful way to calculate radio links parameters is using dB’s. In this case, mathematics show that doubling the power is the equivalent to increasing the power by 3 dB. It can also be demonstrated that doubling the distance is the equivalent of an increase by 6dB.

Seen below is a table of equivalence between Watts and dBm.

1 mWatt  =  0 dBm    
1 Watt     =  30 dBm     2 Watt   =  33 dBm       4 Watt   =  36 dBm
10 Watt   =  40 dBm    20 Watt =  43 dBm     40 Watt =  46 dBm

Want to receive more Tech Notes, product information and other Satel USA updates? Email us at info@satelusa.com to be added to our quarterly mailing list.

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New firmware and software releases

New software and firmware has been posted to our site:

Satel Configuration Manager 1.5.5: Supports the M3-TR8 module and new TR3/4/8 firmware revisions. Also includes minor bugfixes. Download from our software page.

SATELLINE M3-TR3/TR4/TR8 firmware version 2.1.1.5: Supports M3-TR8 module, fixes miscellaneous minor bugs. Download from our firmware page. Note: Requires Configuration Manager 1.5.5 or higher due to changes in firmware internal structure.

SATELLAR 1/2/10/20DS Radio Unit (RU) firmware version 5.5.0.24: Minor bug and stability fixes. Download from our firmware page.

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Introducing the SATEL-LP line of wireless I/O

We are proud to offer a new product line of wireless I/O communications. The SATEL-LP line of radios and snap-on I/O modules are simple to configure (no programming or software needed), and will save time and money versus running wiring conduits. 900 MHz and 2.4GHz radio modules ease logistics, as no licence is needed.

These systems can operate in 3 modes:

  • Wire-in, wire-out
  • Wireless I/O to Modbus (control LP I/O modules with Modbus, without any extra hardware)
  • Wireless serial RS-232 and RS485, providing a simple link between PLCs or RTUs

A wide variety of I/O modules are available for digital in/out, analog in/out and more. See the linked brochure below, or check out the product pages for the SATEL-LP24 (2.4GHz radio), SATEL-LP9 (900MHz radio), or the full line of SATEL-LP expansion modules. Contact us for more information.

SATEL-LP Brochure