Is DSC worthwhile for handheld VHF given limitations

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
RV-Kayaker
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Is DSC worthwhile for handheld VHF given limitations

Postby RV-Kayaker » Sun Feb 18, 2018 12:28 pm

I may purchase two hand held VHF Marine Radios for use communicating between our kayaks, between nearby boats, and with Coast Guard in an emergency.

Is it worthwhile to spend the extra money for a DSC capable handheld radio given the limited distance the signal will travel due to the low transmission power of 5 or 6 watts, small antenna, and virtually no antenna height riding just above the water surface?

Another serious concern is the battery drain by the DSC and GPS seeing as how we are out kayaking for the day. Tthere is also the additional complexity to deal with and the limitation of changing MMSI.

I am looking for a handheld with 6-watts, battery time of at least 10 hours, floating with IPX 8 rating (or minimum IPX7), and strobe as being the most important, next to the DSC of course. Preferably, it will have NOAA alerts. I'm considering Uniden or Standard Horizon because they have a wider selection of handhelds suitable for kayaking with long battery life.

We mainly kayak in the Ocean and the Gulf waters around the Florida Keys and inland water ways when traveling throughout Canada and USA. We don't go more than two or three miles out from the mainland.

Any help would be appreciated. Thanks in advance!

jimh
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Re: Is DSC worthwhile for handheld VHF given limitations

Postby jimh » Sun Feb 18, 2018 1:07 pm

About Time-to-first-fix

Portable VHF Marine Band radios ( "hand held" radios) that have DSC capability are required to have their own internal GNSS receiver. As you properly mention, the GNSS receiver needs electrical power to be able to receive. The power consumed by the GNSS receiver is small compared to the power needed for transmitting, but it can be a factor in battery life.

One solution to the power consumption of the radio is to keep the radio turned off until needed. This strategy is fine for many uses, but in a distress alert situation, there is a particular concern about the GNSS receiver.

All GNSS receivers take some time after being powered-on to produce a valid position fix. The time required can vary greatly, depending on the particular GNSS receiver, its design, and its basis for operating. The essential data needed by a GNSS receiver to acquire a fix is:

--to know the current time and date
--to know its approximate position
--to know what satellites are in view
--to find the signals from those satellites and lock onto three or more of them to develop a pseudo-range solution to them
--to know the orbital position of those satellites (called their ephemerides) with high precision

When a receiver has all this data it can, from trilateralization of the psuedo-ranges to the satellite positions, find its position relative to the satellite inertial reference frame, and then, by translating that positoin to an earth-centered earth-fixed reference frame, find its position on earth.

If a GNSS receiver when powered-on knows none of those data, the time to first position fix can be as long as 12-minutes or more. (This is because the navigation message is sent very slowly at 50-bps from the satellites, and the total message takes at least 12-minutes to send.)

The most modern GNSS receivers incorporate very crafty, very-low-power-consumption clock and memory, and often can retain the current time and date. Some receivers can also retain old ephemerides and then compute a rough present ephemeris for the satellites.This gives the receiver the satellites in view. Knowing what satellites are in view reduces the time to search for their signals. As soon as one satellite is acquired, its navigation message will begin to be received, leading to more accurate data about satellite position.

I mention all this because in a distress alert situation when you have just turned on the portable DSC radio, the radio may not have a GNSS fix and may take a while to get one.

The solution to this problem is to let the GNSS receiver in the portable radio continue to run, even when the radio is switched off. This will, of course, consume electrical power, so the operating time available will be steadily decreasing, even though the radio is switched off. But the GNSS receiver will be running all the time, so it should have a fix immediately upon powering on the rest of the radio

When considering which DSC-rated portable radio to get, I recommend you carefully consider the GNSS receiver, its time-to-first-fix specification, and whether or not the radio provides for the GNSS receiver to continue to run when the radio is switched off.

I have not made a comprehensive study of all DSC portable radios, so I cannot offer any really informed advice about which one to get. I am aware (somewhat) that the Standard-Horizon HX870 has some optional settings that can affect the GNSS receiver power setting, and that might have a significant influence on the time to first fix for that particular radio. Whether a similar feature is available in other radios, I cannot say.

Another factor affecting time to first fix is the number of "channels" a receiver has. When searching for signals, the more channels a receiver can simultaneously search with, the faster signals will be found.

ASIDE: we have become accustomed to GNSS receivers in devices like smartphones being able to get a very rapid time to first fix. This is due to the integration in the GNSS receiver of a system called A-GPS or Assisted Global Position System. Because a smartphone typically is in radio contact with a cellular telephone network, and because the cellular providers support A-GPS, the navigation message data, normally taking about 12-minutes to receive from the orbiting satellites via their L1 signal data payload at a rate of 50-bps, can be send over the cellular data at a rate of many megabits-per-second and received nearly instantaneously. This data tells the GNSS receiver all the satellite orbital data it needs for a position solution. The smartphone also has a rough idea where it is located by what cellular towers it can receive, so this gives the GNSS receiver a notion of where it is and what satellites are in view. And the cellular network also gives precise time data, so the receiver knows the data and time. By combining all these advantages, and by stunning advances in receiver design, it is actually possible now for the GNSS receiver in a smartphone to be powered off until a position fix is needed, then power on and get a fix in a few tenths of a second.

Unfortunately A-GPS is not available in a portable DSC radio. Its GNSS receiver will operate without assistance, in an autonomous mode. Time to first fix will be important.

ANECDOTE: I have a very old, first generation Magellan portable GPS receiver. It had not been powered on in many years. I put new batteries in it, and waited for a fix. Nothing happened. Finally I left it outside with a completely clear view of the sky for about 12-hours; it eventually got a fix.

jimh
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Re: Is DSC worthwhile for handheld VHF given limitations

Postby jimh » Sun Feb 18, 2018 1:21 pm

RV-Kayaker wrote:...We mainly kayak in the Ocean and the Gulf waters around the Florida Keys and inland waterways when traveling throughout Canada and USA. We don't go more than two or three miles out from the mainland.


In the USA, the RESCUE 21 system covers all the coastal areas of: the oceans; the Great Lakes; the Western Rivers (so-called). In those areas the design of the RESCUE 21 system is to be able to receive a signal of 1-second duration from a 1-Watt transmitter with an antenna about 6-feet above the sea. The implication of that design specification is that in a Kayak at three miles from shore with a 5-Watt portable radio your signal should be easily received by RESCUE 21.

RESCUE 21 coverage also overlaps the inland area, but by no means is every stretch of inland water covered. Consult RESCUE 21 coverage maps for guidance.

Canada has similar coverage of ocean coastal areas, their Great Lakes waters, and the St. Lawrence River system.

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Re: Is DSC worthwhile for handheld VHF given limitations

Postby jimh » Sun Feb 18, 2018 1:24 pm

RV-Kayaker wrote:...Is it worthwhile to spend the extra money for a DSC capable handheld radio...


Note that I deleted you criteria specifying the power and antenna of a DSC capable portable radio. All portable radios will suffer those limitations. I infer your question really is to assess the added cost of DSC in a portable radio compared to a portable radio without DSC, and then to assess its value in adding safety.

I have not surveyed the cost of portable radios, so I do not know what the cost differential is. In order to assess if the cost had sufficient worth, I would need to know what the cost differential is. Please give some explicit costs for portable radios with and without DSC.

On a general basis, I cannot imagine the added cost of DSC is so great that one would choose to forgo having that benefit, particularly if operating in areas with excellent USCG radio coverage. Also, in addition to sending your position in a distress alert, a portable radio with DSC will have a GNSS receiver that will give your position all the time. This certainly can be useful and valuable in normal use of the kayak, not just in a distress situation. This added value should also be considered when comparing costs and benefits.

RV-Kayaker
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Re: Is DSC worthwhile for handheld VHF given limitations

Postby RV-Kayaker » Sun Feb 18, 2018 8:42 pm

Please give some explicit costs for portable radios with and without DSC.


Here are some hand held VHF radios to provide an idea of pricing. The only one in a close price range with GPS/DSC is the standard Horizon HX870 and there is a rebate now closing the gap and making it an easier decision with all but the most cheap radios below $65 with the transmission power of 5 Watts. All the others with DSC, such as ICOM are above the $300 price point.

They all have Li-On batteries and monitor NOAA weather channels and sound weather alerts. The ICOM M73 does not Float but the others listed below do float with IPX8 ratings as deep as 5ft for 30 minutes. There are some Standard Horizon Radios that float and have similar features as the Uniden radios listed below but they are only 5 Watts. There are several radios with extended standby times of 15 hours or more but they are also only rated at 5 Watts. I would rather go with a 6 Watt radio and buy an additional Li-on battery for more standby time. However, now that I found the Standard Horizon HX870 at the GPS Store.com for $200 with free shipping and $40 mail in rebate, its more of an easy decision to get a radio with GPS/DSC.

66 channel WAAS GPS with DSC
Standard Horizon HX870 - 6Watt - 1800mAHr battery with 12 hour standby time - Floating with strobe, IPX8 (5FT/30 minutes) - $200 -$40 Rebate

The following radios do NOT have DSC
ICOM M73 - 6Watt - 2000mAHr battery with 16 hour standby time - IPX8 (5FT/30 minutes) Submersible (Does NOT float) - $200
Uniden MHS126 - 6Watt - 1150mAHr battery with 11 hour standby time - Floating with strobe, IPX8 (5FT/30 minutes) - $100
Standard Horizon HX210 - 6Watt - 1850mAHr battery with 10 hour standby time - Floating with strobe, IPX8 (5FT/30 minutes) - $100
Uniden Atlantis 270 - 1100mAHr battery with 10 hour standby time - Floating with Strobe, IPX7 (3FT/30 minutes) - $65

jimh
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Re: Is DSC worthwhile for handheld VHF given limitations

Postby jimh » Sun Feb 18, 2018 11:53 pm

I would discard the notion that a 6-Watt radio is going to be something much better than a 5-Watt radio. The difference is less than 1 dB.

I don't see much price difference at all. The HX-870 looks like a clear choice.

jimh
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Re: Is DSC worthwhile for handheld VHF given limitations

Postby jimh » Mon Feb 19, 2018 9:57 am

I should have mentioned the value of DSC in a distress situation. With a DSC radio and a properly registered MMSI, if you make a DSC Distress Alert broadcast, and if the USCG receives the transmission, they will know you are in distress, and they will also have a description of your vessel, have you name, and have a contact number, and they will know your position with very good accuracy. Passing all of that information via voice transmissions could take several minutes or longer, depending on how well the USCG can copy your signal and how clearly you are speaking.

Also, I would explore the features of the HX870 radio in greater depth. Some DSC radios include a navigation computer that can calculate the range and bearing to positions the radio receives via DSC from the radio's current position. This could be useful if you and your kayaking companions become separated. With DSC position reports and a navigation computer, you could know, for example, that a fellow kayaker is a certain number of miles away at a certain bearing. I don't know if the HX870 has that feature, but I would not be surprised to find it does.

The cost differential seems to be about $100. For that extra cost you get a GNSS receiver (well maybe just a GPS receiver), a DSC radio, and maybe a little navigation computer built-in. The is good value in those features for an extra $100. And, by the way, that seems like a great price. I had no idea the HX870 was being sold at that price.

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Re: Is DSC worthwhile for handheld VHF given limitations

Postby jimh » Mon Feb 19, 2018 10:40 am

To clarify a feature of the HX870 radio I alluded to earlier;

In the set-up of the HX870 the user can configure the power settings for the GPS receiver. Using the user-interface, navigate as follows:
  • from normal operation, press MENU key; a menu is displayed;
  • in menu select the choice SETUP with the UP/DWN arrows and then press SELECT
  • in menu select the choice GPS SETUP with the UP/DWN arrows and then press SELECT
  • in menu select the choice GPS ON/OFF with the UP/DWN arrows and then press SELECT
  • in menu select from options ON / OFF / INT at PWR OFF with the UP/DWN arrows and then press ENTER
  • press CLR to return to normal operation

Setting to OFF shuts off the GPS receiver all the time.

Setting to ON powers on the GPS receiver when the radio power is on.

If you select the option INT at PWR OFF (and here INT means INTERMITTENT), the HX870 will operate as follows: when the radio power is switched to OFF, the GPS receiver will activate itself every 15-minutes and obtain a position fix.

This means if you power-on the radio it will have a GPS position that won't be more than 15-minutes old. And the GPS receiver will have reasonably fresh data about satellite ephemerides, know its present position, and know the date and time with very good accuracy. This will reduce its time to a new fix to a minimum.

This would be a good setting to use when you are away from power for charging the radio battery but want very fast GPS fix to occur. If you don't need the fastest possible GPS fix, then set the power option to just ON. If you don't need GPS fixes at all, set to OFF.

The specification for Time-to-first fix for the GPS receiver in the HX870 is:
    Cold start = 1-minute typical
    Warm start = 5-seconds typical
My interpretation of the combination of the GPS ON/OFF setting to be INT at PWR OFF and the specification for warm start time to first fix means that when operating the radio in this mode the GPS receiver will have a very accurate position solution almost immediately when you turn on the radio power.

RV-Kayaker
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Re: Is DSC worthwhile for handheld VHF given limitations

Postby RV-Kayaker » Mon Feb 19, 2018 3:24 pm

WOW!. Ok. I have a lot to learn. Even more than I thought. I was not aware of all those capabilities regarding the GPS. I especially like the power saving option for the Intermittent GPS fix.

And now reading the manual, I see there is a capability to keep track of other DSC units and even plot a position to one (at least the HX870 - have to read the manual more) described by the section 10.7.1; "Transmitting a Position Request to Another Vessel" to track other vessels by sending/receiving position reports and the ability to navigate to that position described in 10.8.3; "Navigating to the Reported Position". There is also a group call function using the DSC: 10.6 "Group Call" for up to 20 group programmed MMSI's

The Brochure also describes a "GM (Group Monitor) using DSC Group Position Call" which may be what I read in the manual and they increased it from 9 to 20 or its year another "advanced function":
This new ADVANCED feature allows you to continuously track and display position information of up to 9 pre-selected vessels.
Their position information is displayed on the compass page relative to your location on the display.

NOTE: The HX870 can only be programmed with 1 MMSI #. It must be reset by the factory if another MMSI # is assigned to it.

Thanks for all your help. Now I have a better understanding DSC / GPS / RESCUE 21 and VHF radio capabilities with GPS & DSC. I will go out and buy 2 of these Standard Horizon HX870 radios.

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Re: Is DSC worthwhile for handheld VHF given limitations

Postby jimh » Mon Feb 19, 2018 4:59 pm

DSC Position Poll or Request is a great asset. Here is an example:

We planned to meet another boating friend up on Lake Superior at the Apostle Islands. We trailered the boat up there--a very long drive from SE Michigan--got the boat launched, and then made a DSC Position Poll to our friend's boat's MMSI. His DSC radio replied with their current position. Bingo--we knew right where they were. We headed out in that direction, and later made contact with them on voice.

Re the GPS ON/OFF settings: the owner's manual is not completely clear about them. The information I gave you above should supplement what the manual says and explain it a bit better.