APEX Beacons follow the same format as an APRS beacon but with a few additions. A beacon packet ban be identified because its AX.25 “TO” field will be set to “APRS”, and the first character of the payload which varies depending on the specific type of beacon packet; details can be found in the APRS specification. First off, the position is preferred to be encoded using mic-e compression, however it may be encoded as plain text as well. APEX clients must always naively interpret both plain text and mic-e but may transmit either.
APEX beacons should always include the PHG designator even for mobile stations. This is critical for APEX to calculate packet routing so if it is omitted the packet will not be considered APEX compliant. The PHG specification was documented in the original APRS specification as the following.
POWER-HEIGHT-GAIN: This optional field replaces the CSE/SPD fields with a report of transmitter power, antenna height-above-average-terain and antenna gain. APRS uses this to plot radio range circles around all stations. The following details the format to be used in the BText of a TNC dedicated as an APRS digipeater: !DDMM.mmN/DDDMM.mmW#PHG5360/WIDE...(identifying comments)... | | | | |||| |_____ makes station show up green | | | | ||||________ Omni (Direction of max gain) | | | | |||_________ Ant gain in dB | | | | ||__________ Height = log2(HAAT/10) LAT LONG | | |___________ Power = SQR(P) | |_____________ Power-Height-Gain identifier * |_______________ # is symbol for digipeater As you can see by the integers in the PHG string, there are only 10 possible values for each of these fields as follows: DIGITS 0 1 2 3 4 5 6 7 8 9 Equation ------------------------------------------------------------------- POWER 0, 1, 4, 9, 16, 25, 36, 49, 64, 81 watts SQR(P) HEIGHT 10,20,40, 80,160,320,640,1280,2560,5120 feet LOG2(H/10) GAIN 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 dB DIR 0,45,90,135,180,225,270, 315, 360, . deg (D/45) The DIRECTIVITY field offsets the PHG circle by one third in the indicated direction. This means a front to back range of 2 to 1. Most often this is used to indicate a favored direction or a null even though an OMNI antenna is at the site. Note that 0 means OMNI and 8 means 360 or a NORTH offset. HEIGHTS are ABOVE-AVERAGE TERRAIN! Not above ground or sea level. Your antenna may be at 1000 ft above sealevel and be on a 100 foot tower. But if you go out 10 miles in all directions and find that the average elevation is 1200 feet, then your height-above-averag-terain is less than ZERO!!!!
Obviously for a mobile station some of the fields, such as height omni, are usually 0. also the WIDE parameter is no longer used in the current specification and instead this is where the comment goes. So the current APRS and APEX format for beacons looks more like this:
The standard also dictates the content of the beacon comment, which is no longer customizable (instead users can use the status packet for that). The current suggestion is to follow the European standard, with some small modifications. The European standard has the following beacon comment format.
G/D R-I-R 24H
The various fields are defined in the European standard as follows
Beacon Comment – Service Code ----features---- G/D Gate & Digi available -/D Digi only G/- Gate only ----connectivity---- R-I-R Radio Internet Radio connection R-I Radio Internet only R Radio only / no internet i.e. Digi/p ----time table---- H24 24 hours operation H12 except night hours HX variable times / on request HN night times
The additions to the European standard defined by APEX includes a connectivity of “I-R” to specify stations which will port internet packets over the radio but will not gate traffic into the internet. It also adds an additional field to the end of the comment having the form C# where # specifies the rate at which packets were received since the last beacon was sent out. The rate is specified as average packets per minute. The value for # may have multiple digits and should not be prefixed with 0. This information is used to assist the APEX routing algorithms to automatically route packets around areas of high congestion.
The APEX standard for the beacon comment would, therefore, be the following.
Beacon Comment – Service Code ----features---- G/D Gate & Digi available -/D Digipeater only G/- Internet Gate only -/- Neither a gate, nor a digipeater ----connectivity---- R-I-R Bidirection traffic between radio and internet gateway R-I Radio traffic reported to internet only. I-R Internet traffic emited into radio network only. R Radio only / no internet ----time table---- H24 24 hours operation H12 except night hours HX variable times / on request HN night time operation only ----congestion level---- C## congestion level, ## is average packets per minute
According to the APEX standard the beacon comment on an APEX compliant beacon packet would look something like this:
G/D R-I-R H24 C30
The following is an example of a complete, compliant, APEX beacon including PHG specifier and coordinates:
!/:=i@;N.G& --PHG5360/G/D R-I-R H24 C30