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Time and Position Formats

Time Formats

APRS timestamps are expressed in three different ways:

  • Day/Hours/Minutes format
  • Hours/Minutes/Seconds format
  • Month/Day/Hours/Minutes format

In all three formats, the 24-hour clock is used.

Day/Hours/Minutes (DHM) format is a fixed 7-character field, consisting of a 6-digit day/time group followed by a single time indicator character (Z or Y). The day/time group consists of a two-digit day-of-the-month (01–31) and a four-digit time in hours and minutes.

Times can be expressed in zulu (UTC/GMT) or local time. For example:

  • 09234Z is 2345 hours zulu time on the 9th day of the month.
  • 09234Y is 2345 hours local time on the 9th day of the month.

It is recommended that future APRS implementations only transmit zulu format on the air.

Note: The time in Status Reports may only be in zulu format.

Hours/Minutes/Seconds (HMS) format is a fixed 7-character field, consisting of a 6-digit time in hours, minutes and seconds, followed by the time-indicator character. For example:

  • 234517Z is 23 hours 45 minutes and 17 seconds zulu.

Note: This format may not be used in Status Reports.

Month/Day/Hours/Minutes (MDHM) format is a fixed 8-character field, consisting of the month (01–12) and day-of-the-month (01–31), followed by the time in hours and minutes zulu. For example:

  • 10092345 is 23 hours 45 minutes zulu on October 9th.

This format is only used in reports from stand-alone “positionless” weather stations (i.e. reports that do not contain station position information).

Use of Timestamps

When a station transmits a report without a timestamp, an APRS receiving station can make an internal record of the time it was received, if required. This record is the receiving station’s notion of the time the report was created.

On the other hand, when a station transmits a report with a timestamp, that timestamp represents the transmitting station’s notion of the time the report was created.

In other words, reports sent without a timestamp can be regarded as real-time, “current” reports (and the receiving station has to record the time they were received), whereas reports sent with a timestamp may or may not be real-time, and may possibly be (very) “old”.

Four APRS Data Type Identifiers specify whether or not a report contains a timestamp, depending on whether the station has APRS messaging capability or not:

No APRS Messaging With APRS Messaging
(Current/real-time) Report without timestamp !
(Old/non-real-time) Report with timestamp /

Stations without APRS messaging capability are typically stand-alone trackers or digipeaters. Stations reporting without a timestamp are generally (but not necessarily) fixed stations.

Latitude Format

Latitude is expressed as a fixed 8-character field, in degrees and decimal minutes (to two decimal places), followed by the upper case letter N for north or S for south.

Latitude degrees are in the range 00 to 90. Latitude minutes are expressed as whole minutes and hundredths of a minute, separated by a decimal point.

For example:

4903.50N is 49 degrees 3 minutes 30 seconds north.

In generic format examples, the latitude is shown as the 8-character string ddmm.hhN (i.e. degrees, minutes and hundredths of a minute north).

Longitude Format

Longitude is expressed as a fixed 9-character field, in degrees and decimal minutes (to two decimal places), followed by the upper case letter E for east or W for west.

Longitude degrees are in the range 000 to 180. Longitude minutes are expressed as whole minutes and hundredths of a minute, separated by a decimal point.

For example:

07201.75W is 72 degrees 1 minute 45 seconds west.

In generic format examples, the longitude is shown as the 9-character string ddmm.hhW (i.e. degrees, minutes and hundredths of a minute west).

PositionCoordinates

Position coordinates are a combination of latitude and longitude, separated by a display Symbol Table Identifier, and followed by a Symbol Code. For example:

4903.50N/07201.75R

The / character between latitude and longitude is the Symbol Table Identifier (in this case indicating use of the Primary Symbol Table), and the R character at the end is the Symbol Code from that table (in this case, indicating a “house” icon).

A description of display symbols is included in Chapter 20: APRS Symbols. The full Symbol Table listing is in Appendix 2.

Position Ambiguity

In some instances — for example, where the exact position is not known — the sending station may wish to reduce the number of digits of precision in the latitude and longitude. In this case, the mm and hh digits in the latitude may be progressively replaced by a (space) character as the amount of imprecision increases. For example:

  1. _N represents latitude to nearest 1/10th of a minute.

  2. __N represents latitude to nearest minute.

  3. ___N represents latitude to nearest 10 minutes.

  4. ____N represents latitude to nearest degree.

The level of ambiguity specified in the latitude will automatically apply to the longitude as well — it is permissible but not necessary to include any characters in the longitude.

For example, the coordinates:

  1. __N/07201.75W-

represent the position to the nearest minute. That is, the hundredths of minutes of latitude and longitude may take any value in the range 00–99.

Thus the station may be located anywhere inside a bounding box having the following corner coordinates.

  • North West corner: 49 deg 3.99 mins N, 72 deg 1.99 mins W
  • North East corner: 49 deg 3.99 mins N, 72 deg 1.00 mins W
  • South East corner: 49 deg 3.00 mins N, 72 deg 1.00 mins W
  • South West corner: 49 deg 3.00 mins N, 72 deg 1.99 mins W

Ambiguity Plots: Since not all positions in APRS are known to the same precision, a significant attribute of all APRS symbols is the provision to show the four ranges of ambiguity of 0.1, 1, 10 and 60 nautical miles. Notice how the object named GUESSED above is not shown with a symbol, but with a circle. This is because this station is transmitting or entered with a 1/10th mile ambiguity with his position. The station's symbol does show up at large area scales, but once you zoom the map below the scale at which the size of the ambiguity is larger than the "size" of the symbol, then the symbol disappears so that there can be no misinterpretation by the person viewing that there is precision where there is not. These ranges of ambiguity are conveyed just like a written position is conveyed. If a position is only known to the nearest degree (60 nm), then only the degrees are sent. If the position is only known to the nearest minute, then only the degrees and integer minutes are transmitted, and so on down to Tenths of a minute. If all digits are known, then the position conveys the full precision inherent in APRS which is to the nearest 100th of a nautical mile or about 60 feet.

Ambiguity in the Spec: There has been lots of confusion over Position Ambiguity caused by the poor wording in the spec that can be incorrectly interpreted as implying a truncation of digits and a lengthy box of ambiguity. It is not a truncation and it is not a box. The position field in APRS is a string field, not a numeric field. One should place in that field by inclusion only the digits the sender wants the receiver to use. Further the spec implies that this results in a box of ambiguity. This is wrong, because it would imply vastly different sizes of imprecision at the equator and at the poles. It is clear that the intent of position ambiguity was a range in nautical miles, since Ambiguity was defined in the LATITUDE field only, where the digits do correspond to Nautical Miles. And they do give the same circular area everywhere on earth.

Default Null Position

Where a station does not have any specific position information to transmit (for example, a Mic-E unit without a GPS receiver connected to it), the station must transmit a default null position in the location field. The null position corresponds to 0° 0' 0" north, 0° 0' 0" west.

The null position should be include the U symbol (unknown/indeterminate position). For example, a Position Report for a station with unknown position will contain the coordinates __0000.00N00000.00W__.

Maidenhead Locator (Grid Square)

An alternative method of expressing a station's location is to provide a Maidenhead locator (grid square). There are four ways of doing this:

  • In a Status Report — e.g. IO91SX/- (/– represents the symbol for a “house”).
  • In Mic-E Status Text — e.g. IO91SX/G (/G indicates a “grid square”).
  • In the Destination Address — e.g. IO91SX. (obsolete).
  • In AX.25 beacon text, with the T APRS Data Type Identifier — e.g. [IO91SX] (obsolete).

Grid squares may be in 6-character form (as above) or in the shortened 4-character form (e.g. IO91).

NMEA Data

APRS recognizes raw ASCII data strings conforming to the NMEA 0183 Version 2.0 specification, originating from navigation equipment such as GPS and LORAN receivers. It is recommended that APRS stations interpret at least the following NMEA Received Sentence types:

  • GGA Global Positioning System Fix Data
  • GLL Geographic Position, Latitude/Longitude Data
  • RMC Recommended Minimum Specific GPS/Transit Data
  • VTG Velocity and Track Data
  • WPL Way Point Location

Altitude

Altitude may be expressed in two ways:

  • In the comment text.
  • In Mic-E format.

Altitude in Comment Text

The comment may contain an altitude value, in the form /A=aaaaaa, where aaaaaa is the altitude in feet. It must contain exactly 6 digits. For example: /A=001234. The altitude may appear anywhere in the comment.

Note: Although not in the official standard, many applications also recognize negative values in the form /A=-aaaaa. It must contain exactly 5 digits after the minus sign.

Altitude in Mic-E format

The optional Mic-E status field can contain altitude data. See Chapter 10: Mic-E Data Format.