I learned about the existence of an interesting radio navigation aid, the the AN/ARQ-25 UHF ranging system used, inter alia, on the AC-119G Shadow gunship. This was a kind of a navigation interrogator, which cooperated with the AN/TRQ-26 ground stations. According to the information on the web, the system was manufactured by Sperry and used six predetermined preset frequencies. The control unit of the ARQ-25 can be seen below.
It appears that the system was capable of determining the distance between the aircraft and the ground station in lieu of TACAN or DME (when not available), and provided simultaneous two-way duplex voice communication between the pilot and the ground station. The ARQ-25 allowed making approach at a predetermined angle providing the alternative Glide Slope and ADF functions when dedicated ILS (GS/LOC) and NDB equipment was not available on the landing strip. The ADF function of the ARQ-25 was most likely slaved to the ARA-25 UHF DF equipment, which was standard on military aircraft. The maximum range of the ARQ-25 interrogations was below 60 NM, with final approach from within 6 NM.
To sum up, the ARQ-25 looks like a deep upgrade to the Rebecca Mk 7 or Mk 8 DME approach system developed by the UK. Its signals were difficult to intercept and direction find by the adversary due to their short duration. The ARQ-25 substituted TACAN and VOR/ILS and ADF when not available. Since the system used UHF airband, it was rather reserved for military use only.
My questions are:
It appears that the system was capable of determining the distance between the aircraft and the ground station in lieu of TACAN or DME (when not available), and provided simultaneous two-way duplex voice communication between the pilot and the ground station. The ARQ-25 allowed making approach at a predetermined angle providing the alternative Glide Slope and ADF functions when dedicated ILS (GS/LOC) and NDB equipment was not available on the landing strip. The ADF function of the ARQ-25 was most likely slaved to the ARA-25 UHF DF equipment, which was standard on military aircraft. The maximum range of the ARQ-25 interrogations was below 60 NM, with final approach from within 6 NM.
To sum up, the ARQ-25 looks like a deep upgrade to the Rebecca Mk 7 or Mk 8 DME approach system developed by the UK. Its signals were difficult to intercept and direction find by the adversary due to their short duration. The ARQ-25 substituted TACAN and VOR/ILS and ADF when not available. Since the system used UHF airband, it was rather reserved for military use only.
My questions are:
- Which aircraft apart from the AC-119 were fitted with the ARQ-25 as well?
- In which years was the ARQ-25 used and when was the system eventually retired?
- Which duplex frequencies were used by the ARQ-25 system?
- Given that many landing strips in SEA were not fitted with TACAN or VOR/ILS, was the TRQ-26 ground equipment present on all of them, or only on selected ones?
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