SSR & DME Quiz

Secondary Surveillance Radar and Distance Measuring Equipment
1. What is the primary purpose of Secondary Surveillance Radar (SSR) compared to Primary Radar?
a) To detect weather patterns
b) To obtain identification and altitude information from aircraft
c) To measure aircraft speed more accurately
d) To provide navigation guidance to pilots
2. In SSR systems, what is the function of the transponder on the aircraft?
a) To passively reflect radar signals
b) To receive interrogation signals and transmit coded replies
c) To measure distance to ground stations
d) To communicate with other aircraft
3. What are the typical interrogation frequencies used in SSR systems?
a) 1030 MHz for transmission and 1090 MHz for reception
b) 1090 MHz for transmission and 1030 MHz for reception
c) 1030 MHz for both transmission and reception
d) 1090 MHz for both transmission and reception
4. What does DME measure?
a) The altitude of an aircraft
b) The speed of an aircraft relative to ground
c) The slant range distance between aircraft and ground station
d) The magnetic heading of an aircraft
5. In DME systems, how is distance calculated?
a) By measuring signal strength variations
b) By measuring the phase difference between transmitted and received signals
c) By measuring the time delay between interrogation and reply
d) By using Doppler shift calculations
6. What frequency band does DME operate in?
a) VHF (30-300 MHz)
b) UHF (962-1213 MHz)
c) L-band (1-2 GHz)
d) C-band (4-8 GHz)
7. What is the typical accuracy of DME systems?
a) ±0.1 nautical miles
b) ±0.5 nautical miles
c) ±1 nautical mile
d) ±5 nautical miles
8. What is Mode S in SSR systems?
a) A selective addressing system that allows direct communication with specific aircraft
b) A system for measuring aircraft speed
c) A backup mode for when primary radar fails
d) A military-only mode for classified operations
9. How does DME handle multiple aircraft interrogations?
a) By using different frequencies for each aircraft
b) Through time-division multiplexing and pseudo-random transmission delays
c) By limiting the number of aircraft that can use a DME station
d) Through code-division multiple access (CDMA)
10. What is a key limitation of DME measurements?
a) DME measures slant range, not horizontal distance to the station
b) DME cannot measure distances beyond 50 nautical miles
c) DME requires visual contact with the ground station
d) DME only works during daylight hours

Answers and Explanations

Question 1: Correct answer is b) To obtain identification and altitude information from aircraft
Unlike Primary Radar which only detects position through signal reflection, SSR actively interrogates aircraft transponders to obtain additional information like identity (Mode A/C) and altitude (Mode C).
Question 2: Correct answer is b) To receive interrogation signals and transmit coded replies
The transponder receives interrogation signals at 1030 MHz and transmits coded replies at 1090 MHz containing information such as aircraft identity and altitude.
Question 3: Correct answer is a) 1030 MHz for transmission and 1090 MHz for reception
SSR ground stations transmit interrogations at 1030 MHz and receive replies from aircraft transponders at 1090 MHz.
Question 4: Correct answer is c) The slant range distance between aircraft and ground station
DME measures the straight-line (slant range) distance between the aircraft and the ground station, which is slightly longer than the horizontal distance when the aircraft is at altitude.
Question 5: Correct answer is c) By measuring the time delay between interrogation and reply
DME calculates distance by measuring the time it takes for a signal to travel from the aircraft to the ground station and back, then converting this time to distance using the speed of light.
Question 6: Correct answer is b) UHF (962-1213 MHz)
DME operates in the UHF band between 962-1213 MHz, with 1 MHz channel spacing providing 252 possible channels.
Question 7: Correct answer is a) ±0.1 nautical miles
Modern DME systems typically have an accuracy of about ±0.1 nautical miles or better, though this can vary with distance from the station and other factors.
Question 8: Correct answer is a) A selective addressing system that allows direct communication with specific aircraft
Mode S (Select) provides selective addressing capabilities, allowing air traffic control to communicate with specific aircraft and exchange data, reducing frequency congestion.
Question 9: Correct answer is b) Through time-division multiplexing and pseudo-random transmission delays
DME uses a combination of time-division multiplexing and pseudo-random transmission delays to handle multiple aircraft, with each aircraft's transponder adding a random delay to its reply to avoid interference.
Question 10: Correct answer is a) DME measures slant range, not horizontal distance to the station
The primary limitation is that DME measures the straight-line distance (slant range), which is greater than the horizontal distance when the aircraft is at altitude. This difference becomes more significant at higher altitudes and closer distances to the station.