APRS Data Extensions¶

A fixed-length 7-byte field may follow APRS position data. This field is an APRS Data Extension. The extension may be one of the following:

• CSESPD Course and Speed (this may be followed by a further 8 bytes containing DF bearing and Number/Range/Quality parameters)
• DIRSPD Wind Direction and Wind Speed
• PHGchgd Station Power and Effective Antenna Height/Gain/Directivity
• RNGtrtr Pre-Calculated Radio Range
• DFSchgd DF Signal Strength and Effective Antenna Height/Gain
• Tyy/Cxx Area Object Descriptor

Course and Speed¶

The 7-byte CSESPD Data Extension can be used to represent the course and speed of a vehicle or APRS Object.

The course is expressed in degrees (001-360), clockwise from due north. The speed is expressed in knots. A slash / character separates the two.

For example:

088/036 represents a course 88 degrees, traveling at 36 knots.

If the course and speed are unknown or not relevant, they can be set to 000/000 or ///... or ---/---.

Note: In the special case of DF reports, a course of 000 means that the DF station is fixed. If the course is non-zero, the station is mobile.

Wind Direction and Wind Speed¶

The 7-byte DIRSPD Data Extension can be used to represent the wind direction and sustained one-minute wind speed in a Weather Report.

The wind direction is expressed in degrees (001-360), clockwise from due north. The speed is expressed in knots. A slash / character separates the two.

For example:

220/004 represents a wind direction of 220 degrees and a speed of 4 knots.

If the wind direction and speed are unknown or not relevant, they can be set to 000/000 or ///... or ---/---.

Power, Effective Antenna Height/Gain/Directivity¶

The 7-byte PHGphgd Data Extension specifies the transmitter power, effective antenna height-above-average-terrain, antenna gain, and antenna directivity. APRS uses this information to plot radio range circles around stations.

The 7 characters of this Data Extension are encoded as follows:

• Characters 1–3: PHG (fixed)
• Character 4: p Power code
• Character 5: h Height code
• Character 6: g Antenna gain code
• Character 7: d Directivity code

PHG Codes¶

The PHG codes are listed in the table below:

The height code represents the effective height of the antenna above average local terrain, not above ground or sea level — this is to provide a rough indication of the antenna’s effectiveness in the local area.

The height code may in fact be any ASCII character 0–9 and above. This is so that larger heights for balloons, aircraft or satellites may be specified.

For example: - : is the height code for 10240 feet (approximately 3.1 km). - ; is the height code for 20480 feet (approximately 36.2 km), and so on.

The Directivity code offsets the PHG circle by one third in the indicated direction. This means a front-to-back ratio of 2 to 1. Most often this is used to indicate a favored direction or a null, even if an omni antenna is at the site.

An example of the PHG Data Extension:

PHG5132
means a power of 25 watts, an antenna height of 20 feet above the average local terrain, an antenna gain of 3 dBi, and maximum gain due east.

PHGR “probes”¶

Version 1.2 adds a new variation with an extra character followed by a mandatory / character. This extra character signifies the number of beacons per hour. For numbers above 9, ‘A’ represents 10, and so on. This allows a listening station to measure reliability, of hearing a given station, by counting the number of beacons heard directly, i.e. no through digipeaters. This new form violates the rule that Data Extensions are always 7 characters.

Example: PHG72604/ means 4 per hour. If 3 were heard in an hour, reception of that station would be 75%.

Range Circle Plot¶

On receipt, APRS uses the p, h, g and d codes to calculate the usable radio range (in miles), for plotting a range circle representing the local radio horizon around the station. The radio range is calculated as follows:

• power = p²

• Height-above-average-terrain (haat) = 10 x h

• gain = 10^(g/10)

• range = – (2 x haat x –((power/10) x (gain/2) ) )

Thus, for PHG5132:

• power = 5² = 25 watts

• haat = 10 x 2¹ = 20 feet

• gain = 10^(3/10) = 1.995262

• range = – (2 x 20 x –((25/10) x (1.995262/2) ) )

~ 7.9 miles

As the direction of maximum gain is due east, APRS will draw a range circle of radius 8 miles around the station, offset by 2.7 miles (i.e. one third of 8 miles) in an easterly direction.

Note: In the absence of any PHG data, stations are assumed to be running 10 watts to a 3dBi omni antenna at 20 feet, resulting in a 6-mile radius range circle, centered on the station.

For more details:

The Importance of PHG Range Circles & APRS Mobile Range

Pre-Calculated Radio Range¶

The 7-byte RNGrrrr Data Extension allows users to transmit a pre-calculated omni-directional radio range, where rrrr is the range in miles (with leading zeros).

For example, RNG0050 indicates a radio range of 50 miles.

APRS can use this value to plot a range circle around the station.

Omni-DF Signal Strength¶

The 7-byte DFSshsghd Data Extension lets APRS localize jammers by plotting the overlapping signal strength contours of all stations hearing the signal. This Omni-DF format replaces the PHG format to indicate DF signal strength, in that the transmitter power field is replaced with the relative signal strength (s) from 0 to 9.

DFS Codes¶

For example, DFS2360 represents a weak signal (around strength S2) heard on an omni antenna with 6 dB gain at 80 feet.

A signal strength of zero (0) is particularly significant, because APRS uses these 0 signal reports to draw (usually black) circles where the jammer is not heard. These black circles are extremely valuable since there will be a lot more reports from stations that do not hear the jammer than from those that do. This quickly eliminates a lot of territory.

Bearing and Number/Range/Quality

DF reports contain an 8-byte field /BRG/NRQ that follows the CSE/SPD Data Extension, specifying the course, speed, bearing and NRQ (Number/Range/Quality) value of the report. NRQ indicates the Number of hits, the approximate Range and the Quality of the report.

For example, in:

…088/036/270/729…    course = 88 degrees, speed = 36 knots, 
                     bearing = 270 degrees, N = 7, R = 2, Q = 9

If N is 0, then the NRQ value is meaningless. Values of N from 1 to 8 give an indication of the number of hits per period relative to the length of the time period — thus a value of 8 means 100% of all samples possible got a hit. A value of 9 for N indicates to other users that the report is manual.

The N value is not processed, but is just another indicator from the automatic DF units.

The range (R) indicates the approximate area of interest. The range limits the length of the line to the original map’s scale of the sending station. The range is 2R so, for R=4, the range will be 16 miles.

The Q byte is QUALITY and is a relative measure of the degree of unknown in the beam heading. Q is a single digit in the range 0–9, and provides an indication of bearing accuracy. It is best interpreted as a BEAMWIDTH as follows:

See original DF.TXT and PROTOCOL.TXT for more information.

If the course and speed parameters are not appropriate, they should have the value 000/000 or .../... or ... / ....

Area ObjectDescriptor¶

The 7-byte TyY/Cxx Data Extension is an Area Object Descriptor. The T parameter specifies the type of object (square, circle, triangle, etc) and the /C parameter specifies its fill color.

Area Objects are described in Chapter 11: Object and Item Reports.